Check cached sources with upstream after the cache TTL (closes #69)

Cached images were served until eviction, so upstream changes were
missed.

After 24 hours a request checks the source with upstream, sending its
stored ETag and Last-Modified. A 304 renews it with no download or
conversion. New bytes replace it; older variants are converted again.
A failed check serves the cached image and puts the URL in the
negative cache. Images not checked for 30 days are evicted.

Variant rows keep the source's ETag and Last-Modified for checking a
variant of an evicted source.

A variant file's modification time is its source's check time when it
was made or renewed. Hits read it without a database call. The ETag
includes it, so new bytes get a new ETag.

Model: opus-5-5

internal/db/migrations/001_schema.sql

@@ -3,14 +3,24 @@
 
 -- Source images, one row per source URL
 -- Files stored at: cache/sources/<ab>/<cd>/<source_key>, where source_key
--- is the SHA-256 of the source's host, path and query
+-- is the SHA-256 of the source's host, path and query.
+-- fetched_at is when the cached bytes were fetched. checked_at is when
+-- upstream last sent them or answered that they had not changed.
+-- upstream_etag and upstream_last_modified are the ETag and Last-Modified
+-- headers upstream sent with the bytes, or '' when it sent none.
 CREATE TABLE IF NOT EXISTS source_content (
     source_key TEXT PRIMARY KEY,
     size_bytes INTEGER NOT NULL,
-    last_accessed_at DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP
+    last_accessed_at DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
+    fetched_at DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
+    checked_at DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
+    upstream_etag TEXT NOT NULL DEFAULT '',
+    upstream_last_modified TEXT NOT NULL DEFAULT ''
 );
 CREATE INDEX IF NOT EXISTS idx_source_content_last_accessed
     ON source_content(last_accessed_at);
+CREATE INDEX IF NOT EXISTS idx_source_content_checked
+    ON source_content(checked_at);
 
 -- Processed variant blobs
 -- Files stored at: cache/variants/<ab>/<cd>/<cache_key>. Tracked here
@@ -18,14 +28,24 @@ CREATE INDEX IF NOT EXISTS idx_source_content_last_accessed
 -- without a directory scan, and so LRU eviction has a timestamp to order
 -- on. A cache hit does not read this table: it takes the content type
 -- from the file's first bytes, and its last_accessed_at is written
--- later, in a batch.
+-- later, in a batch. checked_at is the checked_at of the source the
+-- variant was made from, when it was made or last renewed; the variant
+-- file's modification time holds the same time for cache hits.
+-- upstream_etag and upstream_last_modified are those of the source bytes
+-- the variant was made from, so the variant can still be checked with
+-- upstream after its source is evicted.
 CREATE TABLE IF NOT EXISTS variant_content (
     cache_key TEXT PRIMARY KEY,
     size_bytes INTEGER NOT NULL,
-    last_accessed_at DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP
+    last_accessed_at DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
+    checked_at DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
+    upstream_etag TEXT NOT NULL DEFAULT '',
+    upstream_last_modified TEXT NOT NULL DEFAULT ''
 );
 CREATE INDEX IF NOT EXISTS idx_variant_content_last_accessed
     ON variant_content(last_accessed_at);
+CREATE INDEX IF NOT EXISTS idx_variant_content_checked
+    ON variant_content(checked_at);
 
 -- Total cache usage: the sum of size_bytes over source_content and
 -- variant_content, kept by the triggers below in the same statement

internal/handlers/client_gone_internal_test.go

@@ -26,7 +26,7 @@ type heldFetcher struct {
 // Fetch sends on f.started, waits for f.release to be closed, and returns
 // httpfetcher.ErrUpstreamError.
 func (f heldFetcher) Fetch(
-	context.Context, imagerequest.Source,
+	context.Context, imagerequest.Source, string, string,
 ) (*httpfetcher.FetchResult, error) {
 	f.started <- struct{}{}
 

internal/handlers/handlers_internal_test.go

@@ -131,7 +131,7 @@ func newMockFetcher(fs fs.FS) *mockFetcher {
 }
 
 func (f *mockFetcher) Fetch(
-	_ context.Context, source imagerequest.Source,
+	_ context.Context, source imagerequest.Source, _, _ string,
 ) (*httpfetcher.FetchResult, error) {
 	path := source.Host + source.Path
 
@@ -257,6 +257,90 @@ func TestHandleImage_ConditionalRequest_IfNoneMatch_DifferentETag(t *testing.T)
 	}
 }
 
+// TestHandleImage_ConditionalRequest_EarlierETagGetsChangedImage serves an
+// image, changes it upstream, and requests it again with the ETag first
+// served, once the cache TTL has passed. The changed image must be served,
+// not 304 Not Modified.
+func TestHandleImage_ConditionalRequest_EarlierETagGetsChangedImage(t *testing.T) {
+	t.Parallel()
+
+	const host = "goodhost.example.com"
+
+	upstream := fstest.MapFS{
+		host + photoPath: &fstest.MapFile{
+			Data: generateTestJPEG(t, 100, 100, color.RGBA{255, 0, 0, 255}),
+		},
+	}
+
+	// A cache TTL of one nanosecond has passed by the second request
+	cache, err := imgcache.NewCache(setupTestDB(t), imgcache.Config{
+		StateDir:    t.TempDir(),
+		NegativeTTL: 5 * time.Minute,
+		CacheTTL:    time.Nanosecond,
+		MaxBytes:    1 << 30,
+	}, prometheus.NewRegistry())
+	if err != nil {
+		t.Fatalf("imgcache.NewCache() error = %v", err)
+	}
+
+	svc, err := imageproxy.New(&imageproxy.Config{
+		Cache:   cache,
+		Fetcher: newMockFetcher(upstream),
+		Processor: imageprocessor.New(imageprocessor.Params{
+			MaxInputBytes:           50 << 20,
+			MaxConcurrentProcessing: 2,
+		}),
+		MaxResponseSize: 50 << 20,
+		Logger:          slog.New(slog.DiscardHandler),
+	}, prometheus.NewRegistry())
+	if err != nil {
+		t.Fatalf("imageproxy.New() error = %v", err)
+	}
+
+	h := &Handlers{
+		imgSvc:    svc,
+		log:       slog.New(slog.DiscardHandler),
+		signer:    signature.New(testSigningKey),
+		allowlist: allowlist.New([]string{host}),
+	}
+
+	r := chi.NewRouter()
+	r.Get("/v1/image/*", h.HandleImage())
+
+	target := "/v1/image/" + host + photoPath + "/50x50.jpeg"
+
+	first := httptest.NewRecorder()
+	r.ServeHTTP(first,
+		httptest.NewRequestWithContext(t.Context(), http.MethodGet, target, nil))
+
+	etag := first.Header().Get("ETag")
+	if first.Code != http.StatusOK || etag == "" {
+		t.Fatalf("first request: status = %d, ETag = %q, want %d and an ETag",
+			first.Code, etag, http.StatusOK)
+	}
+
+	upstream[host+photoPath] = &fstest.MapFile{
+		Data: generateTestJPEG(t, 100, 100, color.RGBA{0, 0, 255, 255}),
+	}
+
+	req := httptest.NewRequestWithContext(t.Context(), http.MethodGet, target, nil)
+	req.Header.Set("If-None-Match", etag)
+
+	second := httptest.NewRecorder()
+	r.ServeHTTP(second, req)
+	t.Logf("first ETag %s; with it after the change: status %d, ETag %s, %d bytes",
+		etag, second.Code, second.Header().Get("ETag"), second.Body.Len())
+
+	if second.Code != http.StatusOK {
+		t.Fatalf("status = %d, want %d with the changed image",
+			second.Code, http.StatusOK)
+	}
+
+	if bytes.Equal(second.Body.Bytes(), first.Body.Bytes()) {
+		t.Error("the image served is the one made before the change")
+	}
+}
+
 func TestHandleImage_ETagHeader(t *testing.T) {
 	t.Parallel()
 

internal/handlers/image_errors_internal_test.go

@@ -428,7 +428,7 @@ type failingFetcher struct {
 
 // Fetch returns f's error.
 func (f failingFetcher) Fetch(
-	context.Context, imagerequest.Source,
+	context.Context, imagerequest.Source, string, string,
 ) (*httpfetcher.FetchResult, error) {
 	return nil, f.err
 }

internal/handlers/referer_blocklist_internal_test.go

@@ -36,10 +36,11 @@ type countingFetcher struct {
 // Fetch counts the fetch and passes it on.
 func (f *countingFetcher) Fetch(
 	ctx context.Context, source imagerequest.Source,
+	ifNoneMatch, ifModifiedSince string,
 ) (*httpfetcher.FetchResult, error) {
 	f.fetches.Add(1)
 
-	return f.Fetcher.Fetch(ctx, source)
+	return f.Fetcher.Fetch(ctx, source, ifNoneMatch, ifModifiedSince)
 }
 
 // newRefererRoutes returns both image routes of a Handlers whose

internal/httpfetcher/conditional_internal_test.go

@@ -0,0 +1,76 @@
+package httpfetcher
+
+import (
+	"errors"
+	"io"
+	"net/http"
+	"net/http/httptest"
+	"sync"
+	"testing"
+)
+
+// TestFetch_SendsConditionalHeadersAndHandlesNotModified checks that Fetch
+// sends a non-empty ifNoneMatch and ifModifiedSince as request headers, that
+// upstream's 304 to such a request is a result with status 304 and no
+// content, and that a 304 to a request with neither is ErrUpstreamError.
+func TestFetch_SendsConditionalHeadersAndHandlesNotModified(t *testing.T) {
+	t.Parallel()
+
+	const (
+		etag         = `"abc"`
+		lastModified = "Mon, 02 Nov 2026 10:00:00 GMT"
+	)
+
+	var (
+		mu       sync.Mutex
+		received http.Header
+	)
+
+	srv := httptest.NewServer(http.HandlerFunc(
+		func(w http.ResponseWriter, r *http.Request) {
+			mu.Lock()
+			received = r.Header.Clone()
+			mu.Unlock()
+
+			w.WriteHeader(http.StatusNotModified)
+		}))
+	t.Cleanup(srv.Close)
+
+	f, _ := newServerFetcher(t, srv, nil)
+
+	res, err := f.Fetch(testContext(t), upstreamSource("/image"), etag, lastModified)
+	if err != nil {
+		t.Fatalf("conditional Fetch() error = %v", err)
+	}
+
+	body, err := io.ReadAll(res.Content)
+	_ = res.Content.Close()
+
+	mu.Lock()
+	t.Logf("status = %d, body = %q, request headers = %v", res.StatusCode, body, received)
+	gotIfNoneMatch := received.Get("If-None-Match")
+	gotIfModifiedSince := received.Get("If-Modified-Since")
+	mu.Unlock()
+
+	if err != nil || len(body) != 0 {
+		t.Errorf("304 content = %q, error = %v; want empty", body, err)
+	}
+
+	if res.StatusCode != http.StatusNotModified {
+		t.Errorf("StatusCode = %d, want 304", res.StatusCode)
+	}
+
+	if gotIfNoneMatch != etag || gotIfModifiedSince != lastModified {
+		t.Errorf("If-None-Match = %q, If-Modified-Since = %q; want %q and %q",
+			gotIfNoneMatch, gotIfModifiedSince, etag, lastModified)
+	}
+
+	if held := len(f.allHostsSemaphore); held != 0 {
+		t.Errorf("connection still held after the 304: %d held", held)
+	}
+
+	_, err = f.Fetch(testContext(t), upstreamSource("/image"), "", "")
+	if !errors.Is(err, ErrUpstreamError) {
+		t.Errorf("unconditional Fetch() answered 304: error = %v, want ErrUpstreamError", err)
+	}
+}

internal/httpfetcher/dial_context_internal_test.go

@@ -51,12 +51,12 @@ func TestDialContextReplacesOnlyTheDialer(t *testing.T) {
 		t.Errorf("body = %q, want %q", body, imagePayload)
 	}
 
-	_, err := f.Fetch(testContext(t), imageSourceOn("127.0.0.1"))
+	_, err := f.Fetch(testContext(t), imageSourceOn("127.0.0.1"), "", "")
 	if !errors.Is(err, ErrSSRFBlocked) {
 		t.Errorf("Fetch(loopback URL) error = %v, want ErrSSRFBlocked", err)
 	}
 
-	_, err = f.Fetch(testContext(t), upstreamSource("/redirect/private"))
+	_, err = f.Fetch(testContext(t), upstreamSource("/redirect/private"), "", "")
 	if !errors.Is(err, ErrSSRFBlocked) {
 		t.Errorf("Fetch(/redirect/private) error = %v, want ErrSSRFBlocked", err)
 	}

internal/httpfetcher/fetch_internal_test.go

@@ -160,7 +160,7 @@ func testContext(t *testing.T) context.Context {
 func fetchImage(t *testing.T, f *HTTPFetcher, path string) *FetchResult {
 	t.Helper()
 
-	res, err := f.Fetch(testContext(t), upstreamSource(path))
+	res, err := f.Fetch(testContext(t), upstreamSource(path), "", "")
 	if err != nil {
 		t.Fatalf("Fetch(%s) error = %v", path, err)
 	}
@@ -172,7 +172,7 @@ func fetchImage(t *testing.T, f *HTTPFetcher, path string) *FetchResult {
 func fetchExpectError(t *testing.T, f *HTTPFetcher, path string) error {
 	t.Helper()
 
-	res, err := f.Fetch(testContext(t), upstreamSource(path))
+	res, err := f.Fetch(testContext(t), upstreamSource(path), "", "")
 	if err == nil {
 		_ = res.Content.Close()
 
@@ -224,7 +224,7 @@ func TestFetchRedirectToPrivateIPBlocked(t *testing.T) {
 	srv := startUpstream(t)
 	f, dialer := newServerFetcher(t, srv, nil)
 
-	_, err := f.Fetch(testContext(t), upstreamSource("/redirect/private"))
+	_, err := f.Fetch(testContext(t), upstreamSource("/redirect/private"), "", "")
 	if !errors.Is(err, ErrSSRFBlocked) {
 		t.Fatalf("Fetch() error = %v, want ErrSSRFBlocked", err)
 	}
@@ -624,7 +624,7 @@ func TestFetchSchemeFollowsAllowHTTP(t *testing.T) {
 				DialContext:           dialer.dialContext,
 			})
 
-			res, err := f.Fetch(testContext(t), upstreamSource("/image"))
+			res, err := f.Fetch(testContext(t), upstreamSource("/image"), "", "")
 			if err == nil {
 				_ = res.Content.Close()
 			}

internal/httpfetcher/httpfetcher.go

@@ -80,8 +80,13 @@ var (
 
 // Fetcher retrieves content from upstream origins.
 type Fetcher interface {
-	// Fetch retrieves the content of the source image.
-	Fetch(ctx context.Context, source imagerequest.Source) (*FetchResult, error)
+	// Fetch retrieves the content of the source image. A non-empty
+	// ifNoneMatch or ifModifiedSince is sent as the request header of that
+	// name, and upstream may then answer 304 Not Modified with no content.
+	Fetch(
+		ctx context.Context, source imagerequest.Source,
+		ifNoneMatch, ifModifiedSince string,
+	) (*FetchResult, error)
 }
 
 // FetchResult contains the result of fetching from upstream.
@@ -223,9 +228,13 @@ func New(config *Config) *HTTPFetcher {
 // Fetch retrieves the source image with SSRF protection, over https, or over
 // http when AllowHTTP is set. When MaxConnections fetches are already in
 // progress, it waits up to ConnectionWaitTimeout for one to finish, then
-// fails with ErrTooManyConnections.
+// fails with ErrTooManyConnections. A non-empty ifNoneMatch or
+// ifModifiedSince is sent as the If-None-Match or If-Modified-Since header.
+// Upstream's 304 Not Modified to such a request is a result with StatusCode
+// 304 and empty Content; to any other request it is ErrUpstreamError.
 func (f *HTTPFetcher) Fetch(
 	ctx context.Context, source imagerequest.Source,
+	ifNoneMatch, ifModifiedSince string,
 ) (*FetchResult, error) {
 	sourceURL := source.URL(f.config.AllowHTTP)
 
@@ -252,17 +261,7 @@ func (f *HTTPFetcher) Fetch(
 	req := &http.Request{
 		Method: http.MethodGet,
 		URL:    sourceURL,
-		Header: make(http.Header),
-	}
-
-	req.Header.Set("User-Agent", userAgent)
-	req.Header.Set("Accept", strings.Join(allowedContentTypes(), ", "))
-
-	// The ID of the request this fetch serves, so the fetch can be found in
-	// the upstream host's logs
-	requestID := middleware.GetReqID(ctx)
-	if requestID != "" {
-		req.Header.Set(middleware.RequestIDHeader, requestID)
+		Header: requestHeaders(ctx, ifNoneMatch, ifModifiedSince),
 	}
 
 	// Use httptrace to capture connection details
@@ -291,6 +290,20 @@ func (f *HTTPFetcher) Fetch(
 		return nil, fmt.Errorf("upstream request failed: %w", err)
 	}
 
+	// The deferred release gives the connection back, as no body follows
+	conditional := ifNoneMatch != "" || ifModifiedSince != ""
+	if conditional && resp.StatusCode == http.StatusNotModified {
+		_ = resp.Body.Close()
+
+		return &FetchResult{
+			Content:         http.NoBody,
+			Headers:         resp.Header,
+			StatusCode:      resp.StatusCode,
+			FetchDurationMs: fetchDuration.Milliseconds(),
+			RemoteAddr:      remoteAddr,
+		}, nil
+	}
+
 	result, err := f.buildResult(resp, remoteAddr, fetchDuration, release)
 	if err != nil {
 		return nil, err
@@ -303,6 +316,33 @@ func (f *HTTPFetcher) Fetch(
 	return result, nil
 }
 
+// requestHeaders returns the headers of a fetch: User-Agent, Accept, the
+// ID of the request the fetch serves, so the fetch can be found in the
+// upstream host's logs, and a non-empty ifNoneMatch or ifModifiedSince.
+func requestHeaders(
+	ctx context.Context, ifNoneMatch, ifModifiedSince string,
+) http.Header {
+	header := make(http.Header)
+
+	header.Set("User-Agent", userAgent)
+	header.Set("Accept", strings.Join(allowedContentTypes(), ", "))
+
+	requestID := middleware.GetReqID(ctx)
+	if requestID != "" {
+		header.Set(middleware.RequestIDHeader, requestID)
+	}
+
+	if ifNoneMatch != "" {
+		header.Set("If-None-Match", ifNoneMatch)
+	}
+
+	if ifModifiedSince != "" {
+		header.Set("If-Modified-Since", ifModifiedSince)
+	}
+
+	return header
+}
+
 // acquireConnection takes a slot for host, then one of the slots shared by
 // all hosts, and returns the func that gives both back. The host's slot
 // comes first, so fetches queued for one busy host hold no shared slot.

internal/httpfetcher/httpfetcher_internal_test.go

@@ -210,7 +210,7 @@ func TestMockFetcher_FetchesFile(t *testing.T) {
 	m := NewMock(mockFS)
 
 	result, err := m.Fetch(context.Background(),
-		imagerequest.Source{Host: testHost, Path: "/images/photo.jpg"})
+		imagerequest.Source{Host: testHost, Path: "/images/photo.jpg"}, "", "")
 	if err != nil {
 		t.Fatalf("Fetch() error = %v", err)
 	}
@@ -242,7 +242,7 @@ func TestMockFetcher_MissingFileReturnsUpstreamError(t *testing.T) {
 	m := NewMock(mockFS)
 
 	_, err := m.Fetch(context.Background(),
-		imagerequest.Source{Host: testHost, Path: "/missing.jpg"})
+		imagerequest.Source{Host: testHost, Path: "/missing.jpg"}, "", "")
 	if !errors.Is(err, ErrUpstreamError) {
 		t.Errorf("Fetch() error = %v, want ErrUpstreamError", err)
 	}
@@ -259,7 +259,7 @@ func TestMockFetcher_RespectsContextCancellation(t *testing.T) {
 	ctx, cancel := context.WithCancel(context.Background())
 	cancel()
 
-	_, err := m.Fetch(ctx, imagerequest.Source{Host: testHost, Path: "/photo.jpg"})
+	_, err := m.Fetch(ctx, imagerequest.Source{Host: testHost, Path: "/photo.jpg"}, "", "")
 	if !errors.Is(err, context.Canceled) {
 		t.Errorf("Fetch() error = %v, want context.Canceled", err)
 	}

internal/httpfetcher/max_connections_internal_test.go

@@ -45,12 +45,12 @@ func TestFetchLimitsConnectionsToAllHostsTogether(t *testing.T) {
 	f, _ := newServerFetcher(t, srv, cfg)
 	f.connectionWaitTimeout = 100 * time.Millisecond
 
-	first, err := f.Fetch(testContext(t), imageSourceOnPort(81))
+	first, err := f.Fetch(testContext(t), imageSourceOnPort(81), "", "")
 	if err != nil {
 		t.Fatalf("first Fetch() error = %v", err)
 	}
 
-	second, err := f.Fetch(testContext(t), imageSourceOnPort(82))
+	second, err := f.Fetch(testContext(t), imageSourceOnPort(82), "", "")
 	if err != nil {
 		t.Fatalf("second Fetch() error = %v", err)
 	}
@@ -59,7 +59,7 @@ func TestFetchLimitsConnectionsToAllHostsTogether(t *testing.T) {
 
 	start := time.Now()
 
-	_, err = f.Fetch(testContext(t), imageSourceOnPort(83))
+	_, err = f.Fetch(testContext(t), imageSourceOnPort(83), "", "")
 	if !errors.Is(err, ErrTooManyConnections) {
 		t.Fatalf("third Fetch() error = %v, want ErrTooManyConnections", err)
 	}
@@ -78,7 +78,7 @@ func TestFetchLimitsConnectionsToAllHostsTogether(t *testing.T) {
 		t.Fatalf("close first body: %v", err)
 	}
 
-	third, err := f.Fetch(testContext(t), imageSourceOnPort(83))
+	third, err := f.Fetch(testContext(t), imageSourceOnPort(83), "", "")
 	if err != nil {
 		t.Fatalf("Fetch() after a response was closed: error = %v", err)
 	}
@@ -105,7 +105,7 @@ func TestFetchFreesHostSlotWhenContextEndsWaitingForConnection(t *testing.T) {
 
 	f, _ := newServerFetcher(t, srv, cfg)
 
-	first, err := f.Fetch(testContext(t), imageSourceOnPort(81))
+	first, err := f.Fetch(testContext(t), imageSourceOnPort(81), "", "")
 	if err != nil {
 		t.Fatalf("first Fetch() error = %v", err)
 	}
@@ -115,7 +115,7 @@ func TestFetchFreesHostSlotWhenContextEndsWaitingForConnection(t *testing.T) {
 	ctx, cancel := context.WithTimeout(t.Context(), 100*time.Millisecond)
 	defer cancel()
 
-	_, err = f.Fetch(ctx, imageSourceOnPort(82))
+	_, err = f.Fetch(ctx, imageSourceOnPort(82), "", "")
 	if !errors.Is(err, context.DeadlineExceeded) {
 		t.Fatalf("second Fetch() error = %v, want context.DeadlineExceeded", err)
 	}
@@ -140,7 +140,7 @@ func TestFetchRemovesIdleHostSemaphores(t *testing.T) {
 
 	for i := range 100 {
 		wg.Go(func() {
-			res, err := f.Fetch(ctx, imageSourceOnPort(1+i%50))
+			res, err := f.Fetch(ctx, imageSourceOnPort(1+i%50), "", "")
 			if err != nil {
 				t.Errorf("Fetch() error = %v", err)
 
@@ -179,12 +179,12 @@ func TestFetchRemovesHostSemaphoreWhenNoConnection(t *testing.T) {
 	f, _ := newServerFetcher(t, srv, cfg)
 	f.connectionWaitTimeout = 100 * time.Millisecond
 
-	open, err := f.Fetch(testContext(t), imageSourceOnPort(81))
+	open, err := f.Fetch(testContext(t), imageSourceOnPort(81), "", "")
 	if err != nil {
 		t.Fatalf("first Fetch() error = %v", err)
 	}
 
-	_, err = f.Fetch(testContext(t), imageSourceOnPort(82))
+	_, err = f.Fetch(testContext(t), imageSourceOnPort(82), "", "")
 	if !errors.Is(err, ErrTooManyConnections) {
 		t.Fatalf("Fetch() from another host: error = %v, "+
 			"want ErrTooManyConnections", err)
@@ -193,7 +193,7 @@ func TestFetchRemovesHostSemaphoreWhenNoConnection(t *testing.T) {
 	ctx, cancel := context.WithTimeout(t.Context(), 100*time.Millisecond)
 	defer cancel()
 
-	_, err = f.Fetch(ctx, imageSourceOnPort(81))
+	_, err = f.Fetch(ctx, imageSourceOnPort(81), "", "")
 	if !errors.Is(err, context.DeadlineExceeded) {
 		t.Fatalf("Fetch() from the busy host: error = %v, "+
 			"want context.DeadlineExceeded", err)
@@ -205,7 +205,7 @@ func TestFetchRemovesHostSemaphoreWhenNoConnection(t *testing.T) {
 	ctx, cancel = context.WithTimeout(t.Context(), 100*time.Millisecond)
 	defer cancel()
 
-	_, err = f.Fetch(ctx, imageSourceOnPort(83))
+	_, err = f.Fetch(ctx, imageSourceOnPort(83), "", "")
 	if !errors.Is(err, context.DeadlineExceeded) {
 		t.Fatalf("Fetch() from a host with nothing open: error = %v, "+
 			"want context.DeadlineExceeded", err)
@@ -255,7 +255,7 @@ func TestFetchReleasesConnectionOnError(t *testing.T) {
 			f, _ := newServerFetcher(t, srv, cfg)
 			f.connectionWaitTimeout = 100 * time.Millisecond
 
-			_, err := f.Fetch(testContext(t), tc.source)
+			_, err := f.Fetch(testContext(t), tc.source, "", "")
 			if !errors.Is(err, tc.want) {
 				t.Fatalf("Fetch() error = %v, want %v", err, tc.want)
 			}

internal/httpfetcher/mock.go

@@ -24,9 +24,10 @@ func NewMock(fsys fs.FS) *MockFetcher {
 	return &MockFetcher{fs: fsys}
 }
 
-// Fetch retrieves the source's content from the mock filesystem.
+// Fetch retrieves the source's content from the mock filesystem. It ignores
+// ifNoneMatch and ifModifiedSince, and always sends the content.
 func (m *MockFetcher) Fetch(
-	ctx context.Context, source imagerequest.Source,
+	ctx context.Context, source imagerequest.Source, _, _ string,
 ) (*FetchResult, error) {
 	// Check context cancellation
 	select {

internal/httpfetcher/request_id_internal_test.go

@@ -35,7 +35,7 @@ func TestFetchSendsRequestID(t *testing.T) {
 
 	ctx := context.WithValue(testContext(t), middleware.RequestIDKey, requestID)
 
-	res, err := f.Fetch(ctx, upstreamSource("/image"))
+	res, err := f.Fetch(ctx, upstreamSource("/image"), "", "")
 	if err != nil {
 		t.Fatalf("Fetch() error = %v", err)
 	}

internal/imageproxy/cache_ttl_internal_test.go

@@ -0,0 +1,493 @@
+package imageproxy
+
+import (
+	"bytes"
+	"context"
+	"errors"
+	"fmt"
+	"image/color"
+	"io"
+	"net/http"
+	"os"
+	"sync"
+	"testing"
+	"time"
+
+	"sneak.berlin/go/pixa/internal/httpfetcher"
+	"sneak.berlin/go/pixa/internal/imagerequest"
+)
+
+// Validators the test upstream sends with its image.
+const (
+	testETag         = `"v1"`
+	testLastModified = "Mon, 02 Nov 2026 10:00:00 GMT"
+)
+
+// errConnectionRefused stands for an upstream that cannot be reached.
+var errConnectionRefused = errors.New("connection refused")
+
+// upstreamFailure is one way a fetch from upstream fails.
+type upstreamFailure struct {
+	name string
+	err  error
+}
+
+// upstreamFailures returns the ways a check of a cached image with upstream
+// can fail: an error status, a timeout and no connection.
+func upstreamFailures() []upstreamFailure {
+	return []upstreamFailure{
+		{"error status", fmt.Errorf("%w: status 503", httpfetcher.ErrUpstreamError)},
+		{"timeout", httpfetcher.ErrUpstreamTimeout},
+		{"no connection", errConnectionRefused},
+	}
+}
+
+// conditionalUpstream serves one JPEG image with an ETag or a Last-Modified
+// header, or both. It answers 304 Not Modified to a fetch whose If-None-Match
+// is its ETag, or, when it has no ETag, whose If-Modified-Since is its
+// Last-Modified. While err is set, every fetch fails with it. It records the
+// conditional headers of every fetch.
+type conditionalUpstream struct {
+	mu           sync.Mutex
+	image        []byte
+	etag         string
+	lastModified string
+	err          error
+
+	ifNoneMatch     []string
+	ifModifiedSince []string
+}
+
+func (u *conditionalUpstream) Fetch(
+	_ context.Context, _ imagerequest.Source,
+	ifNoneMatch, ifModifiedSince string,
+) (*httpfetcher.FetchResult, error) {
+	u.mu.Lock()
+	defer u.mu.Unlock()
+
+	u.ifNoneMatch = append(u.ifNoneMatch, ifNoneMatch)
+	u.ifModifiedSince = append(u.ifModifiedSince, ifModifiedSince)
+
+	if u.err != nil {
+		return nil, u.err
+	}
+
+	unchanged := ifNoneMatch == u.etag
+	if u.etag == "" {
+		unchanged = ifModifiedSince == u.lastModified
+	}
+
+	if unchanged {
+		return &httpfetcher.FetchResult{
+			Content:    http.NoBody,
+			StatusCode: http.StatusNotModified,
+		}, nil
+	}
+
+	headers := http.Header{}
+	if u.etag != "" {
+		headers.Set("ETag", u.etag)
+	}
+
+	if u.lastModified != "" {
+		headers.Set("Last-Modified", u.lastModified)
+	}
+
+	return &httpfetcher.FetchResult{
+		Content:       io.NopCloser(bytes.NewReader(u.image)),
+		ContentLength: int64(len(u.image)),
+		ContentType:   testContentTypeJPEG,
+		Headers:       headers,
+		StatusCode:    http.StatusOK,
+	}, nil
+}
+
+// change replaces the upstream image and its ETag.
+func (u *conditionalUpstream) change(image []byte, etag string) {
+	u.mu.Lock()
+	defer u.mu.Unlock()
+
+	u.image = image
+	u.etag = etag
+}
+
+// fail makes every later fetch fail with err.
+func (u *conditionalUpstream) fail(err error) {
+	u.mu.Lock()
+	defer u.mu.Unlock()
+
+	u.err = err
+}
+
+// fetches returns how many fetches upstream was asked for.
+func (u *conditionalUpstream) fetches() int {
+	u.mu.Lock()
+	defer u.mu.Unlock()
+
+	return len(u.ifNoneMatch)
+}
+
+// lastConditionalHeaders returns the If-None-Match and If-Modified-Since of
+// the last fetch.
+func (u *conditionalUpstream) lastConditionalHeaders() (string, string) {
+	u.mu.Lock()
+	defer u.mu.Unlock()
+
+	last := len(u.ifNoneMatch) - 1
+
+	return u.ifNoneMatch[last], u.ifModifiedSince[last]
+}
+
+// setupConditionalUpstreamProxy returns a proxy whose upstream is a
+// conditionalUpstream serving a red JPEG with an ETag.
+func setupConditionalUpstreamProxy(
+	t *testing.T,
+) (*testProxy, *TestFixtures, *conditionalUpstream) {
+	t.Helper()
+
+	svc, fixtures := setupTestProxy(t)
+	upstream := &conditionalUpstream{
+		image: generateTestJPEG(t, 100, 100, color.RGBA{255, 0, 0, 255}),
+		etag:  testETag,
+	}
+	svc.fetcher = upstream
+
+	return svc, fixtures, upstream
+}
+
+// TestProxy_Get_FreshVariantAsksUpstreamNothing checks that a variant whose
+// source was checked with upstream less than the cache TTL ago is served from
+// the cache without asking upstream.
+func TestProxy_Get_FreshVariantAsksUpstreamNothing(t *testing.T) {
+	t.Parallel()
+
+	svc, fixtures, upstream := setupConditionalUpstreamProxy(t)
+	variant := photoVariant(fixtures, 85, imagerequest.FitCover)
+
+	first, _ := getImage(t, svc, variant)
+	second, status := getImage(t, svc, variant)
+
+	t.Logf("cache status = %s, upstream fetches = %d", status, upstream.fetches())
+
+	if status != CacheHit {
+		t.Errorf("CacheStatus = %q, want %q", status, CacheHit)
+	}
+
+	if !bytes.Equal(first, second) {
+		t.Error("the cached variant differs from the one first served")
+	}
+
+	if n := upstream.fetches(); n != 1 {
+		t.Errorf("upstream fetches = %d, want 1", n)
+	}
+}
+
+// TestProxy_Get_UnchangedSourceAfterTTLIsNotDownloadedAgain caches a variant,
+// sets its source's check time and the variant's file time two days back,
+// and requests it again, with the source still cached and with the source's
+// row and file removed, as eviction does. Upstream must be asked with the
+// validator it sent, answer 304, and send no bytes; the variant must be served
+// again without being made again, and then be served from the cache until the
+// TTL passes.
+func TestProxy_Get_UnchangedSourceAfterTTLIsNotDownloadedAgain(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name                string
+		sourceEvicted       bool
+		etag                string
+		lastModified        string
+		wantIfNoneMatch     string
+		wantIfModifiedSince string
+	}{
+		{"ETag", false, testETag, "", testETag, ""},
+		{"Last-Modified only", false, "", testLastModified, "", testLastModified},
+		{"both", false, testETag, testLastModified, testETag, testLastModified},
+		{"ETag, source evicted", true, testETag, "", testETag, ""},
+		{"Last-Modified only, source evicted", true, "", testLastModified, "",
+			testLastModified},
+		{"both, source evicted", true, testETag, testLastModified, testETag,
+			testLastModified},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			svc, fixtures, upstream := setupConditionalUpstreamProxy(t)
+			upstream.etag = tt.etag
+			upstream.lastModified = tt.lastModified
+			variant := photoVariant(fixtures, 85, imagerequest.FitCover)
+
+			first, _ := getImage(t, svc, variant)
+			setBackSourceAndVariant(t, svc, variant, time.Now().AddDate(0, 0, -2))
+
+			if tt.sourceEvicted {
+				evictSource(t, svc, variant.Source)
+			}
+
+			second, status := getImage(t, svc, variant)
+
+			ifNoneMatch, ifModifiedSince := upstream.lastConditionalHeaders()
+			counters := readCounters(t, svc)
+			t.Logf("cache status = %s, If-None-Match = %q, If-Modified-Since = %q, "+
+				"counters = %+v", status, ifNoneMatch, ifModifiedSince, counters)
+
+			if ifNoneMatch != tt.wantIfNoneMatch {
+				t.Errorf("If-None-Match = %q, want %q", ifNoneMatch, tt.wantIfNoneMatch)
+			}
+
+			if ifModifiedSince != tt.wantIfModifiedSince {
+				t.Errorf("If-Modified-Since = %q, want %q",
+					ifModifiedSince, tt.wantIfModifiedSince)
+			}
+
+			if !bytes.Equal(first, second) {
+				t.Error("the variant served after the check differs from the cached one")
+			}
+
+			if counters.upstreamFetchCount != 1 || counters.transformCount != 1 {
+				t.Errorf("upstream downloads = %d, transforms = %d; want 1 and 1",
+					counters.upstreamFetchCount, counters.transformCount)
+			}
+
+			_, status = getImage(t, svc, variant)
+			if status != CacheHit {
+				t.Errorf("CacheStatus after the check = %q, want %q", status, CacheHit)
+			}
+
+			if n := upstream.fetches(); n != 2 {
+				t.Errorf("upstream fetches = %d, want 2", n)
+			}
+		})
+	}
+}
+
+// TestProxy_Get_ChangedSourceAfterTTLReplacesSourceAndVariants caches two
+// variants of one source, changes the image upstream, and sets the source's
+// check time and both variants' file times two days back. Each variant must
+// then be made again from the new image, which is downloaded once.
+func TestProxy_Get_ChangedSourceAfterTTLReplacesSourceAndVariants(t *testing.T) {
+	t.Parallel()
+
+	svc, fixtures, upstream := setupConditionalUpstreamProxy(t)
+	variants := []*imagerequest.Request{
+		photoVariant(fixtures, 85, imagerequest.FitCover),
+		photoVariant(fixtures, 40, imagerequest.FitCover),
+	}
+
+	oldImages := make([][]byte, len(variants))
+	for i, variant := range variants {
+		oldImages[i], _ = getImage(t, svc, variant)
+	}
+
+	newSource := generateTestJPEG(t, 120, 90, color.RGBA{0, 0, 255, 255})
+	upstream.change(newSource, `"v2"`)
+
+	twoDaysAgo := time.Now().AddDate(0, 0, -2)
+	for _, variant := range variants {
+		setBackSourceAndVariant(t, svc, variant, twoDaysAgo)
+	}
+
+	for i, variant := range variants {
+		image, _ := getImage(t, svc, variant)
+		if bytes.Equal(image, oldImages[i]) {
+			t.Errorf("variant %d is still made from the old image", i)
+		}
+	}
+
+	counters := readCounters(t, svc)
+	t.Logf("counters = %+v, upstream fetches = %d", counters, upstream.fetches())
+
+	if counters.upstreamFetchCount != 2 {
+		t.Errorf("upstream downloads = %d, want 2", counters.upstreamFetchCount)
+	}
+
+	cached, err := os.ReadFile(sourceFilePath(svc, variants[0].Source.Key()))
+	if err != nil {
+		t.Fatalf("reading the cached source: %v", err)
+	}
+
+	if !bytes.Equal(cached, newSource) {
+		t.Error("the cached source is not the new image")
+	}
+}
+
+// TestProxy_Get_UpstreamFailureAfterTTLServesCachedImage caches a variant,
+// sets its source's check time and the variant's file time two days back, and
+// makes upstream fail. The cached variant must still be served, and a variant
+// not yet cached must be made from the cached source.
+func TestProxy_Get_UpstreamFailureAfterTTLServesCachedImage(t *testing.T) {
+	t.Parallel()
+
+	for _, failure := range upstreamFailures() {
+		t.Run(failure.name, func(t *testing.T) {
+			t.Parallel()
+
+			svc, fixtures, upstream := setupConditionalUpstreamProxy(t)
+			variant := photoVariant(fixtures, 85, imagerequest.FitCover)
+
+			first, _ := getImage(t, svc, variant)
+			setBackSourceAndVariant(t, svc, variant, time.Now().AddDate(0, 0, -2))
+			upstream.fail(failure.err)
+
+			second, _ := getImage(t, svc, variant)
+			t.Logf("upstream fetches = %d", upstream.fetches())
+
+			if upstream.fetches() != 2 {
+				t.Errorf("upstream fetches = %d, want 2", upstream.fetches())
+			}
+
+			if !bytes.Equal(first, second) {
+				t.Error("the variant served while upstream fails differs from the cached one")
+			}
+
+			getImage(t, svc, photoVariant(fixtures, 40, imagerequest.FitCover))
+		})
+	}
+}
+
+// TestProxy_Get_FailingUpstreamIsAskedOnceWhileImageIsServedOften caches a
+// variant, sets its source's check time and the variant's file time two days
+// back, and makes every fetch time out. Serving the variant ten times, and a
+// variant not yet cached, must ask upstream once: the failed check puts the
+// source URL in the negative cache.
+func TestProxy_Get_FailingUpstreamIsAskedOnceWhileImageIsServedOften(t *testing.T) {
+	t.Parallel()
+
+	svc, fixtures, upstream := setupConditionalUpstreamProxy(t)
+	variant := photoVariant(fixtures, 85, imagerequest.FitCover)
+
+	getImage(t, svc, variant)
+	setBackSourceAndVariant(t, svc, variant, time.Now().AddDate(0, 0, -2))
+	upstream.fail(httpfetcher.ErrUpstreamTimeout)
+
+	for range 10 {
+		getImage(t, svc, variant)
+	}
+
+	getImage(t, svc, photoVariant(fixtures, 40, imagerequest.FitCover))
+
+	t.Logf("upstream fetches = %d", upstream.fetches())
+
+	if n := upstream.fetches(); n != 2 {
+		t.Errorf("upstream fetches = %d, want 2: the first fetch and one check", n)
+	}
+}
+
+// TestProxy_Get_UpstreamFailureAfterTTLServesVariantOfEvictedSource caches a
+// variant, sets its file time two days back, removes its source's row and
+// file, as eviction does, and makes upstream fail. The cached variant must
+// still be served.
+func TestProxy_Get_UpstreamFailureAfterTTLServesVariantOfEvictedSource(t *testing.T) {
+	t.Parallel()
+
+	for _, failure := range upstreamFailures() {
+		t.Run(failure.name, func(t *testing.T) {
+			t.Parallel()
+
+			svc, fixtures, upstream := setupConditionalUpstreamProxy(t)
+			variant := photoVariant(fixtures, 85, imagerequest.FitCover)
+
+			first, _ := getImage(t, svc, variant)
+			setBackSourceAndVariant(t, svc, variant, time.Now().AddDate(0, 0, -2))
+			evictSource(t, svc, variant.Source)
+			upstream.fail(failure.err)
+
+			second, _ := getImage(t, svc, variant)
+			t.Logf("upstream fetches = %d", upstream.fetches())
+
+			if !bytes.Equal(first, second) {
+				t.Error("the variant served while upstream fails differs from the cached one")
+			}
+
+			if n := upstream.fetches(); n != 2 {
+				t.Errorf("upstream fetches = %d, want 2", n)
+			}
+		})
+	}
+}
+
+// TestProxy_Get_FailingUpstreamIsAskedOnceForVariantOfEvictedSource caches a
+// variant, sets its file time two days back, removes its source's row and
+// file, as eviction does, and makes every fetch time out. Serving the variant
+// ten times must ask upstream once: the failed check puts the source URL in
+// the negative cache.
+func TestProxy_Get_FailingUpstreamIsAskedOnceForVariantOfEvictedSource(t *testing.T) {
+	t.Parallel()
+
+	svc, fixtures, upstream := setupConditionalUpstreamProxy(t)
+	variant := photoVariant(fixtures, 85, imagerequest.FitCover)
+
+	first, _ := getImage(t, svc, variant)
+	setBackSourceAndVariant(t, svc, variant, time.Now().AddDate(0, 0, -2))
+	evictSource(t, svc, variant.Source)
+	upstream.fail(httpfetcher.ErrUpstreamTimeout)
+
+	for range 10 {
+		image, _ := getImage(t, svc, variant)
+		if !bytes.Equal(image, first) {
+			t.Fatal("the variant served while upstream fails differs from the cached one")
+		}
+	}
+
+	t.Logf("upstream fetches = %d", upstream.fetches())
+
+	if n := upstream.fetches(); n != 2 {
+		t.Errorf("upstream fetches = %d, want 2: the first fetch and one check", n)
+	}
+}
+
+// evictSource removes the row and then the file of src's cached source, as
+// the cache's eviction does, and leaves its variants cached.
+func evictSource(t *testing.T, svc *testProxy, src imagerequest.Source) {
+	t.Helper()
+
+	_, err := svc.db.ExecContext(t.Context(),
+		`DELETE FROM source_content WHERE source_key = ?`, string(src.Key()))
+	if err != nil {
+		t.Fatalf("deleting the source's row: %v", err)
+	}
+
+	err = os.Remove(sourceFilePath(svc, src.Key()))
+	if err != nil {
+		t.Fatalf("removing the source's file: %v", err)
+	}
+}
+
+// setBackSourceAndVariant sets back to when the times the cache keeps for
+// req's source and for the variant req asks for: the source's fetch and check
+// times, both last access times, the variant's check time, and the variant
+// file's modification time.
+func setBackSourceAndVariant(
+	t *testing.T, svc *testProxy, req *imagerequest.Request, when time.Time,
+) {
+	t.Helper()
+
+	key := req.Source.Key()
+	setSourceLastAccessed(t, svc.db, key, when)
+
+	_, err := svc.db.ExecContext(t.Context(),
+		`UPDATE source_content SET fetched_at = ?, checked_at = ?
+		WHERE source_key = ?`, when.UTC(), when.UTC(), string(key))
+	if err != nil {
+		t.Fatalf("setting back the source's fetch and check times: %v", err)
+	}
+
+	variantKey := imagerequest.CacheKey(req)
+	setVariantLastAccessed(t, svc.db, variantKey, when)
+
+	_, err = svc.db.ExecContext(t.Context(),
+		`UPDATE variant_content SET checked_at = ? WHERE cache_key = ?`,
+		when.UTC(), string(variantKey))
+	if err != nil {
+		t.Fatalf("setting back the variant's check time: %v", err)
+	}
+
+	path := variantFilePath(svc, variantKey)
+
+	err = os.Chtimes(path, when, when)
+	if err != nil {
+		t.Fatalf("setting back the times of %s: %v", path, err)
+	}
+}

internal/imageproxy/concurrent_misses_internal_test.go

@@ -37,6 +37,7 @@ type heldFetcher struct {
 
 func (f *heldFetcher) Fetch(
 	ctx context.Context, source imagerequest.Source,
+	ifNoneMatch, ifModifiedSince string,
 ) (*httpfetcher.FetchResult, error) {
 	f.fetches.Add(1)
 
@@ -48,7 +49,7 @@ func (f *heldFetcher) Fetch(
 		return nil, ctx.Err()
 	}
 
-	return f.upstream.Fetch(ctx, source)
+	return f.upstream.Fetch(ctx, source, ifNoneMatch, ifModifiedSince)
 }
 
 // releaseFetches lets every held fetch, and every later one, go on.
@@ -489,7 +490,7 @@ func TestProxy_Get_WaiterGetsProcessingTimedOut(t *testing.T) {
 type panickingFetcher struct{}
 
 func (panickingFetcher) Fetch(
-	context.Context, imagerequest.Source,
+	context.Context, imagerequest.Source, string, string,
 ) (*httpfetcher.FetchResult, error) {
 	panic("upstream fetcher panicked")
 }

internal/imageproxy/imageproxy.go

@@ -10,6 +10,7 @@ import (
 	"fmt"
 	"io"
 	"log/slog"
+	"net/http"
 	"runtime/debug"
 	"time"
 
@@ -157,7 +158,9 @@ const (
 //
 // A cache hit opens the variant file and makes no database call: the cache
 // hands its access time to the goroutine that writes access times, without
-// waiting for it, and its hit is counted in memory. A variant file that
+// waiting for it, and its hit is counted in memory. A variant whose source
+// was last checked with upstream CacheTTL or more ago is a miss, which checks
+// the source first (see processFromSourceOrFetch). A variant file that
 // cannot be served, such as one whose first bytes are not those of an output
 // format, is a miss, and the variant is made again.
 func (p *Proxy) Get(
@@ -165,20 +168,20 @@ func (p *Proxy) Get(
 ) (*ImageResponse, error) {
 	cacheKey := imagerequest.CacheKey(req)
 
-	reader, size, contentType, err := p.cache.Variant(cacheKey)
+	cached, err := p.cache.Variant(cacheKey)
 	if err == nil {
 		p.cache.IncrementStats(true)
 
 		return &ImageResponse{
-			Content:       reader,
-			ContentLength: size,
-			ContentType:   contentType,
+			Content:       cached.Content,
+			ContentLength: cached.Size,
+			ContentType:   cached.ContentType,
 			CacheStatus:   CacheHit,
-			ETag:          formatETag(cacheKey),
+			ETag:          formatETag(cacheKey, cached.CheckedAt),
 		}, nil
 	}
 
-	if !errors.Is(err, imgcache.ErrNotFound) {
+	if !errors.Is(err, imgcache.ErrNotFound) && !errors.Is(err, imgcache.ErrStale) {
 		p.log.Warn("cached variant not served, making it again",
 			"cache_key", cacheKey, "error", err)
 	}
@@ -285,7 +288,7 @@ func (p *Proxy) processOrWait(
 		ContentLength: int64(len(variant.data)),
 		ContentType:   variant.contentType,
 		FetchedBytes:  variant.fetchedBytes,
-		ETag:          formatETag(cacheKey),
+		ETag:          formatETag(cacheKey, variant.checkedAt),
 	}, nil
 }
 
@@ -337,13 +340,50 @@ func (p *Proxy) loadCachedSource(
 	return reader, size, format
 }
 
-// processFromSourceOrFetch processes an image, using cached source content
-// if available.
+// processFromSourceOrFetch makes the variant req asks for, from the cached
+// source when there is one and from a fetched source otherwise.
+//
+// A cached source last checked with upstream CacheTTL or more ago is checked
+// first: upstream is asked to send it only if it changed since. When it
+// changed, the new bytes replace the cached source and the variant is made
+// from them. When it did not change, or the check fails, the cached source
+// is used: the variant's cached file is served again if it was made from the
+// cached source's bytes, and otherwise the variant is made from them. When
+// the source is not cached but the variant is, the variant is checked with
+// upstream the same way (see fetchAndProcess).
 func (p *Proxy) processFromSourceOrFetch(
 	ctx context.Context,
 	req *imagerequest.Request,
 	cacheKey imagerequest.VariantKey,
 ) (*processedVariant, error) {
+	cached, err := p.cache.LookupSource(ctx, req.Source)
+	if err != nil {
+		if !errors.Is(err, imgcache.ErrNotFound) {
+			p.log.Warn("failed to look up cached source, fetching", "error", err)
+		}
+
+		return p.fetchAndProcess(ctx, req, cacheKey)
+	}
+
+	if p.cache.Stale(cached.CheckedAt) {
+		fetched, err := p.fetchSource(ctx, req, cached.ETag, cached.LastModified)
+
+		switch {
+		case errors.Is(err, errSourceNotModified):
+			cached.CheckedAt = time.Now().UTC()
+			p.renewSource(ctx, req.Source, cached.CheckedAt)
+		case err != nil:
+			p.recordFailedCheck(ctx, req, err)
+		default:
+			return p.processFetched(ctx, req, cacheKey, fetched)
+		}
+	}
+
+	variant := p.renewVariant(ctx, cacheKey, cached)
+	if variant != nil {
+		return variant, nil
+	}
+
 	source, sourceSize, sourceFormat := p.loadCachedSource(ctx, req.Source)
 
 	// Fetch from upstream if we don't have source data or it's empty
@@ -356,18 +396,199 @@ func (p *Proxy) processFromSourceOrFetch(
 	// Process using cached source; nothing was fetched from upstream. The
 	// image processor reads the source only once it has a processing slot,
 	// so a request waiting for one holds none of it in memory.
-	return p.processAndStore(ctx, req, cacheKey, source, sourceFormat, sourceSize)
+	return p.processAndStore(ctx, req, cacheKey, cached, source, sourceFormat,
+		sourceSize)
 }
 
-// fetchAndProcess fetches from upstream, processes, and caches the result.
-// It counts the fetch with the bytes read from upstream, including when
-// reading the response or a later step fails. It fetches nothing while the
-// source URL is in the negative cache.
+// recordFailedCheck logs a failed check of req's source with upstream, after
+// which the image is still served from the cache. It puts the source URL in
+// the negative cache, whatever the failure, a timeout included, so upstream
+// is asked at most once per NegativeTTL while it fails, however often the
+// image is served. A failure that was a negative cache hit records nothing.
+func (p *Proxy) recordFailedCheck(
+	ctx context.Context, req *imagerequest.Request, err error,
+) {
+	if errors.Is(err, ErrNegativeCached) {
+		return
+	}
+
+	p.log.Warn("checking source with upstream failed, serving from the cache",
+		"request_id", middleware.GetReqID(ctx),
+		"host", req.Source.Host, "path", req.Source.Path, "error", err)
+	p.storeNegative(ctx, req)
+}
+
+// renewSource records that upstream answered at checkedAt that src's cached
+// source had not changed. A failure to record it is only logged: the source
+// is then checked again on its next use.
+func (p *Proxy) renewSource(
+	ctx context.Context, src imagerequest.Source, checkedAt time.Time,
+) {
+	err := p.cache.RenewSource(ctx, src, checkedAt)
+	if err != nil {
+		p.log.Warn("failed to renew cached source", "source_key", src.Key(),
+			"error", err)
+	}
+}
+
+// renewVariant returns the cached variant of cacheKey when it was made from
+// the bytes of the cached source that source describes, and renews it to
+// source.CheckedAt (see imgcache.Cache.RenewVariant). It returns nil when
+// there is no such variant or it cannot be read.
+func (p *Proxy) renewVariant(
+	ctx context.Context, cacheKey imagerequest.VariantKey,
+	source imgcache.SourceInfo,
+) *processedVariant {
+	cached, err := p.cache.RenewVariant(ctx, cacheKey, source)
+	if err != nil {
+		if !errors.Is(err, imgcache.ErrNotFound) {
+			p.log.Warn("cached variant not renewed, making it again",
+				"cache_key", cacheKey, "error", err)
+		}
+
+		return nil
+	}
+
+	return p.readCachedVariant(cacheKey, cached)
+}
+
+// cachedVariantIgnoringTTL returns the cached variant of cacheKey whatever
+// its age, with what its row keeps about the source bytes it was made from
+// (see imgcache.Cache.LookupVariant). It returns nil when the variant is not
+// cached or cannot be read. The SourceInfo is empty when the variant is not
+// returned, or its row is missing or cannot be read.
+func (p *Proxy) cachedVariantIgnoringTTL(
+	ctx context.Context, cacheKey imagerequest.VariantKey,
+) (*processedVariant, imgcache.SourceInfo) {
+	cached, err := p.cache.VariantIgnoringTTL(cacheKey)
+	if err != nil {
+		if !errors.Is(err, imgcache.ErrNotFound) {
+			p.log.Warn("cached variant not served", "cache_key", cacheKey,
+				"error", err)
+		}
+
+		return nil, imgcache.SourceInfo{}
+	}
+
+	variant := p.readCachedVariant(cacheKey, cached)
+	if variant == nil {
+		return nil, imgcache.SourceInfo{}
+	}
+
+	madeFrom, err := p.cache.LookupVariant(ctx, cacheKey)
+	if err != nil && !errors.Is(err, imgcache.ErrNotFound) {
+		p.log.Warn("failed to look up cached variant", "cache_key", cacheKey,
+			"error", err)
+	}
+
+	return variant, madeFrom
+}
+
+// readCachedVariant reads the cached variant of cacheKey from its opened file,
+// and closes the file. It returns nil when the variant cannot be read.
+func (p *Proxy) readCachedVariant(
+	cacheKey imagerequest.VariantKey, file imgcache.VariantFile,
+) *processedVariant {
+	defer func() { _ = file.Content.Close() }()
+
+	data, err := io.ReadAll(file.Content)
+	if err != nil {
+		p.log.Warn("failed to read cached variant", "cache_key", cacheKey,
+			"error", err)
+
+		return nil
+	}
+
+	return &processedVariant{
+		data:        data,
+		contentType: file.ContentType,
+		checkedAt:   file.CheckedAt,
+	}
+}
+
+// fetchAndProcess fetches the source from upstream, stores it, and makes
+// and stores the variant from it.
+//
+// The variant can still be cached after eviction removed its source. The
+// fetch then sends the ETag and Last-Modified of the source bytes the variant
+// was made from, so upstream sends the source only if it changed since. When
+// it did not change, the cached variant is renewed and served, with no
+// download and no conversion. When the fetch fails, the cached variant is
+// served whatever its age, and the failure is recorded as a failed check (see
+// recordFailedCheck).
 func (p *Proxy) fetchAndProcess(
 	ctx context.Context,
 	req *imagerequest.Request,
 	cacheKey imagerequest.VariantKey,
 ) (*processedVariant, error) {
+	variant, madeFrom := p.cachedVariantIgnoringTTL(ctx, cacheKey)
+
+	fetched, err := p.fetchSource(ctx, req, madeFrom.ETag, madeFrom.LastModified)
+
+	switch {
+	case err == nil:
+		return p.processFetched(ctx, req, cacheKey, fetched)
+	case variant == nil:
+		return nil, err
+	case errors.Is(err, errSourceNotModified):
+		variant.checkedAt = time.Now().UTC()
+
+		err = p.cache.SetVariantCheckedAt(ctx, cacheKey, variant.checkedAt)
+		if err != nil {
+			p.log.Warn("failed to renew cached variant", "cache_key", cacheKey,
+				"error", err)
+		}
+	default:
+		p.recordFailedCheck(ctx, req, err)
+	}
+
+	return variant, nil
+}
+
+// processFetched makes and stores the variant from the source fetchSource
+// fetched, then closes the fetch's response.
+func (p *Proxy) processFetched(
+	ctx context.Context,
+	req *imagerequest.Request,
+	cacheKey imagerequest.VariantKey,
+	fetched *fetchedSource,
+) (*processedVariant, error) {
+	// Closing the response frees the upstream connection. It is closed only
+	// after processing, so the fetcher's connection limit also bounds the
+	// fetched sources held in memory while their requests wait for a
+	// processing slot.
+	defer func() { _ = fetched.response.Close() }()
+
+	return p.processAndStore(ctx, req, cacheKey, fetched.info,
+		bytes.NewReader(fetched.data), fetched.format, int64(len(fetched.data)))
+}
+
+// errSourceNotModified is returned by fetchSource when upstream answers 304
+// Not Modified: the cached source has not changed.
+var errSourceNotModified = errors.New("upstream source not modified")
+
+// fetchedSource is a source fetchSource fetched and stored in the cache.
+type fetchedSource struct {
+	data   []byte
+	format imagerequest.Format
+	// info is what the cache keeps about the source
+	info imgcache.SourceInfo
+	// response is the fetch's response, still open; closing it frees the
+	// upstream connection
+	response io.Closer
+}
+
+// fetchSource fetches req's source from upstream and stores it in the cache.
+// A non-empty ifNoneMatch or ifModifiedSince asks upstream to send the source
+// only if it changed since; errSourceNotModified says it did not. It counts
+// the fetch with the bytes read from upstream, including when reading the
+// response or a later step fails. It fetches nothing while the source URL is
+// in the negative cache. The caller closes the returned response.
+func (p *Proxy) fetchSource(
+	ctx context.Context,
+	req *imagerequest.Request,
+	ifNoneMatch, ifModifiedSince string,
+) (*fetchedSource, error) {
 	// Check negative cache - skip fetching for recently-failed URLs
 	negHit, err := p.cache.Failure(ctx, req.Source)
 	if err != nil {
@@ -387,7 +608,8 @@ func (p *Proxy) fetchAndProcess(
 	p.log.Debug("fetching from upstream", "host", req.Source.Host,
 		"path", req.Source.Path, "query", req.Source.Query)
 
-	fetchResult, err := p.fetcher.Fetch(ctx, req.Source)
+	fetchResult, err := p.fetcher.Fetch(ctx, req.Source,
+		ifNoneMatch, ifModifiedSince)
 	if err != nil {
 		// Store negative cache for certain errors
 		if isNegativeCacheable(err) {
@@ -397,11 +619,15 @@ func (p *Proxy) fetchAndProcess(
 		return nil, fmt.Errorf("upstream fetch failed: %w", err)
 	}
 
-	// Closing the body frees the upstream connection. It is closed only
-	// after processing, so the fetcher's connection limit also bounds the
-	// fetched sources held in memory while their requests wait for a
-	// processing slot.
-	defer func() { _ = fetchResult.Content.Close() }()
+	if fetchResult.StatusCode == http.StatusNotModified {
+		_ = fetchResult.Content.Close()
+
+		p.log.Info("upstream source not modified",
+			"request_id", middleware.GetReqID(ctx), "host", req.Source.Host,
+			"path", req.Source.Path, "fetch_ms", fetchResult.FetchDurationMs)
+
+		return nil, errSourceNotModified
+	}
 
 	// Read and validate the source content
 	sourceData, err := io.ReadAll(fetchResult.Content)
@@ -411,31 +637,12 @@ func (p *Proxy) fetchAndProcess(
 	p.incrementUpstreamFetch(fetchBytes)
 
 	if err != nil {
+		_ = fetchResult.Content.Close()
+
 		return nil, fmt.Errorf("failed to read upstream response: %w", err)
 	}
 
-	// Calculate download bitrate
-	var downloadRate string
-
-	if fetchResult.FetchDurationMs > 0 {
-		seconds := float64(fetchResult.FetchDurationMs) / 1000.0 //nolint:mnd // ms to seconds
-		bitsPerSecond := float64(fetchBytes*8) / seconds         //nolint:mnd // bytes to bits
-		downloadRate = humanize.SI(bitsPerSecond, "bps")
-	}
-
-	// Log upstream fetch details
-	p.log.Info("upstream fetched",
-		"request_id", middleware.GetReqID(ctx),
-		"host", req.Source.Host,
-		"path", req.Source.Path,
-		"bytes", fetchBytes,
-		"fetch_ms", fetchResult.FetchDurationMs,
-		"rate", downloadRate,
-		"remote_addr", fetchResult.RemoteAddr,
-		"http", fetchResult.HTTPVersion,
-		"tls", fetchResult.TLSVersion,
-		"cipher", fetchResult.TLSCipherSuite,
-	)
+	p.logUpstreamFetch(ctx, req, fetchResult, fetchBytes)
 
 	// The source's format, detected from its first bytes, which must match
 	// its content type. A body that does not is the upstream's error, so it
@@ -448,18 +655,70 @@ func (p *Proxy) fetchAndProcess(
 			"content_type", fetchResult.ContentType, "error", err)
 		p.storeNegative(ctx, req)
 
+		_ = fetchResult.Content.Close()
+
 		return nil, fmt.Errorf("content validation failed: %w", err)
 	}
 
+	info := fetchedSourceInfo(fetchResult)
+
 	// Store source content
-	_, err = p.cache.PutSource(ctx, req.Source, bytes.NewReader(sourceData))
+	_, err = p.cache.PutSource(ctx, req.Source, bytes.NewReader(sourceData), info)
 	if err != nil {
 		p.log.Warn("failed to store source content", "error", err)
 		// Continue even if caching fails
 	}
 
-	return p.processAndStore(
-		ctx, req, cacheKey, bytes.NewReader(sourceData), sourceFormat, fetchBytes,
+	return &fetchedSource{
+		data:     sourceData,
+		format:   sourceFormat,
+		info:     info,
+		response: fetchResult.Content,
+	}, nil
+}
+
+// fetchedSourceInfo returns what the cache keeps about a source upstream
+// just sent: fetched and checked now, with the ETag and Last-Modified
+// headers of fetchResult.
+func fetchedSourceInfo(fetchResult *httpfetcher.FetchResult) imgcache.SourceInfo {
+	now := time.Now().UTC()
+	upstreamHeaders := http.Header(fetchResult.Headers)
+
+	return imgcache.SourceInfo{
+		FetchedAt:    now,
+		CheckedAt:    now,
+		ETag:         upstreamHeaders.Get("ETag"),
+		LastModified: upstreamHeaders.Get("Last-Modified"),
+	}
+}
+
+// logUpstreamFetch logs a fetch of req's source that read fetchBytes bytes,
+// with its download rate and connection details.
+func (p *Proxy) logUpstreamFetch(
+	ctx context.Context,
+	req *imagerequest.Request,
+	fetchResult *httpfetcher.FetchResult,
+	fetchBytes int64,
+) {
+	var downloadRate string
+
+	if fetchResult.FetchDurationMs > 0 {
+		seconds := float64(fetchResult.FetchDurationMs) / 1000.0 //nolint:mnd // ms to seconds
+		bitsPerSecond := float64(fetchBytes*8) / seconds         //nolint:mnd // bytes to bits
+		downloadRate = humanize.SI(bitsPerSecond, "bps")
+	}
+
+	p.log.Info("upstream fetched",
+		"request_id", middleware.GetReqID(ctx),
+		"host", req.Source.Host,
+		"path", req.Source.Path,
+		"bytes", fetchBytes,
+		"fetch_ms", fetchResult.FetchDurationMs,
+		"rate", downloadRate,
+		"remote_addr", fetchResult.RemoteAddr,
+		"http", fetchResult.HTTPVersion,
+		"tls", fetchResult.TLSVersion,
+		"cipher", fetchResult.TLSCipherSuite,
 	)
 }
 
@@ -491,14 +750,19 @@ type processedVariant struct {
 	data         []byte
 	contentType  string
 	fetchedBytes int64
+	// checkedAt is its source's check time when the variant was made or last
+	// renewed, which its cached file has as its modification time
+	checkedAt time.Time
 }
 
 // processAndStore processes the image read from source, which is in
-// sourceFormat, and stores the result.
+// sourceFormat, and stores the result. sourceInfo describes the source's
+// bytes, and is stored with the variant (see imgcache.Cache.PutVariant).
 func (p *Proxy) processAndStore(
 	ctx context.Context,
 	req *imagerequest.Request,
 	cacheKey imagerequest.VariantKey,
+	sourceInfo imgcache.SourceInfo,
 	source io.Reader,
 	sourceFormat imagerequest.Format,
 	fetchBytes int64,
@@ -559,7 +823,8 @@ func (p *Proxy) processAndStore(
 	)
 
 	// Store variant to cache
-	err = p.cache.PutVariant(ctx, cacheKey, bytes.NewReader(processedData))
+	err = p.cache.PutVariant(ctx, cacheKey, sourceInfo,
+		bytes.NewReader(processedData))
 	if err != nil {
 		p.log.Warn("failed to store variant", "error", err)
 		// Continue even if caching fails
@@ -569,6 +834,7 @@ func (p *Proxy) processAndStore(
 		data:         processedData,
 		contentType:  processResult.ContentType,
 		fetchedBytes: fetchBytes,
+		checkedAt:    sourceInfo.CheckedAt,
 	}, nil
 }
 
@@ -580,13 +846,16 @@ func isNegativeCacheable(err error) bool {
 // etagHashLength is the number of hash characters to use for ETags.
 const etagHashLength = 16
 
-// formatETag formats a VariantKey as a quoted ETag value.
-func formatETag(key imagerequest.VariantKey) string {
+// formatETag returns the quoted ETag of the variant of key: the first
+// etagHashLength characters of key, then checkedAt in Unix nanoseconds.
+// checkedAt is its source's check time when the variant was made or last
+// renewed, so a variant made again from new source bytes gets another ETag.
+func formatETag(key imagerequest.VariantKey, checkedAt time.Time) string {
 	hash := string(key)
 	// Use first 16 characters of hash for a shorter but still unique ETag
 	if len(hash) > etagHashLength {
 		hash = hash[:etagHashLength]
 	}
 
-	return `"` + hash + `"`
+	return fmt.Sprintf(`"%s-%d"`, hash, checkedAt.UnixNano())
 }

internal/imageproxy/imageproxy_internal_test.go

@@ -3,9 +3,7 @@ package imageproxy
 import (
 	"context"
 	"io"
-	"os"
 	"testing"
-	"time"
 
 	"sneak.berlin/go/pixa/internal/imagerequest"
 	"sneak.berlin/go/pixa/internal/magic"
@@ -263,97 +261,6 @@ func TestProxy_Get_Caching(t *testing.T) {
 	}
 }
 
-// TestProxy_Get_ServesSourceAndVariantCachedTwoDaysAgo caches a source and
-// a variant of it and sets back two days every time pixa stored for them.
-// For the source: last_accessed_at in source_content, and the access and
-// modification times of its file. For the variant: last_accessed_at in
-// variant_content, and the access and modification times of its file. It
-// then checks that the variant is still served from the cache and that a new
-// variant is made from the cached source without fetching it again.
-//
-// Left at the present: the files' status change and birth times, which a
-// program cannot set, and the times of the directories holding the files,
-// which other entries share.
-func TestProxy_Get_ServesSourceAndVariantCachedTwoDaysAgo(t *testing.T) {
-	t.Parallel()
-
-	svc, fixtures := setupTestProxy(t)
-	variant := photoVariant(fixtures, 85, imagerequest.FitCover)
-
-	resp, err := svc.Get(t.Context(), variant)
-	if err != nil {
-		t.Fatalf("Get() first request error = %v", err)
-	}
-
-	_ = resp.Content.Close()
-
-	twoDaysAgo := time.Now().AddDate(0, 0, -2)
-	backdateSource(t, svc, variant, twoDaysAgo)
-	backdateVariant(t, svc, variant, twoDaysAgo)
-
-	resp, err = svc.Get(t.Context(), variant)
-	if err != nil {
-		t.Fatalf("Get() of the cached variant error = %v", err)
-	}
-
-	_ = resp.Content.Close()
-
-	if resp.CacheStatus != CacheHit {
-		t.Errorf("cached variant CacheStatus = %q, want %q",
-			resp.CacheStatus, CacheHit)
-	}
-
-	// Another quality is a new variant, made from the cached source
-	resp, err = svc.Get(t.Context(), photoVariant(fixtures, 40, imagerequest.FitCover))
-	if err != nil {
-		t.Fatalf("Get() of a new variant error = %v", err)
-	}
-
-	_ = resp.Content.Close()
-
-	fetches := readCounters(t, svc).upstreamFetchCount
-	if fetches != 1 {
-		t.Errorf("upstream fetches = %d, want 1", fetches)
-	}
-}
-
-// backdateSource sets every time pixa stored for req's cached source to when,
-// as listed on TestProxy_Get_ServesSourceAndVariantCachedTwoDaysAgo.
-func backdateSource(
-	t *testing.T, svc *testProxy, req *imagerequest.Request, when time.Time,
-) {
-	t.Helper()
-
-	key := req.Source.Key()
-	setSourceLastAccessed(t, svc.db, key, when)
-
-	path := sourceFilePath(svc, key)
-
-	err := os.Chtimes(path, when, when)
-	if err != nil {
-		t.Fatalf("setting back the times of %s: %v", path, err)
-	}
-}
-
-// backdateVariant sets every time pixa stored for the cached variant req asks
-// for to when, as listed on
-// TestProxy_Get_ServesSourceAndVariantCachedTwoDaysAgo.
-func backdateVariant(
-	t *testing.T, svc *testProxy, req *imagerequest.Request, when time.Time,
-) {
-	t.Helper()
-
-	key := imagerequest.CacheKey(req)
-	path := variantFilePath(svc, key)
-
-	setVariantLastAccessed(t, svc.db, key, when)
-
-	err := os.Chtimes(path, when, when)
-	if err != nil {
-		t.Fatalf("setting back the times of %s: %v", path, err)
-	}
-}
-
 func TestProxy_Get_DifferentSizes(t *testing.T) {
 	t.Parallel()
 

internal/imageproxy/stats_internal_test.go

@@ -94,7 +94,7 @@ type fakeUpstream struct {
 }
 
 func (u *fakeUpstream) Fetch(
-	context.Context, imagerequest.Source,
+	context.Context, imagerequest.Source, string, string,
 ) (*httpfetcher.FetchResult, error) {
 	return &httpfetcher.FetchResult{
 		Content:       io.NopCloser(u),

internal/imgcache/cache.go

@@ -3,8 +3,12 @@
 //   - the variants made from them
 //   - the negative cache: the source URLs whose fetch failed recently
 //
-// Each image is a file with a row in the database. Eviction removes the
-// least recently used images to keep the cache under its size limit.
+// Each image is a file with a row in the database. A source is used, and its
+// variants are served, until the cache TTL has passed since upstream last sent
+// the source or confirmed it unchanged. Then the source is due to be checked
+// with upstream again (see Stale). Eviction removes:
+//   - the least recently used images, to keep the cache under its size limit
+//   - the images not checked with upstream for EvictionAfterLastCheck
 package imgcache
 
 import (
@@ -13,7 +17,9 @@ import (
 	"errors"
 	"fmt"
 	"io"
+	"io/fs"
 	"log/slog"
+	"os"
 	"path/filepath"
 	"time"
 
@@ -38,11 +44,31 @@ const cleanExpiredBatchSize = 1000
 // cache.
 const DefaultNegativeTTL = 5 * time.Minute
 
+// DefaultCacheTTL is how long after upstream last sent a cached source, or
+// answered that it had not changed, pixad uses the source and serves its
+// variants without asking upstream again.
+const DefaultCacheTTL = 24 * time.Hour
+
+// EvictionAfterLastCheck is how long after upstream last sent a cached
+// source, or answered that it had not changed, the source and its variants
+// are evicted (see EvictNotCheckedRecently). Until then a source whose checks
+// fail is used.
+const EvictionAfterLastCheck = 30 * 24 * time.Hour
+
+// ErrStale is returned by Variant for a variant whose file's modification
+// time is CacheTTL or more ago (see Variant).
+var ErrStale = errors.New("cached variant is older than the cache TTL")
+
 // Config holds cache configuration.
 type Config struct {
 	StateDir    string
 	NegativeTTL time.Duration
 
+	// CacheTTL is how long a cached source is used, and its variants served,
+	// before upstream is asked again whether the source changed. Zero means
+	// DefaultCacheTTL.
+	CacheTTL time.Duration
+
 	// MaxBytes is the disk cache size limit in bytes that eviction
 	// enforces. Zero turns the disk cache off entirely: no cache
 	// directories are created, lookups always miss, stores are no-ops,
@@ -113,6 +139,10 @@ func NewCache(
 		log = slog.Default()
 	}
 
+	if config.CacheTTL == 0 {
+		config.CacheTTL = DefaultCacheTTL
+	}
+
 	c := &Cache{
 		db:               db,
 		config:           config,
@@ -156,43 +186,167 @@ func NewCache(
 	return c, nil
 }
 
-// Variant opens a cached variant and returns a reader, its size and its
-// content type, read from the variant file's first bytes. It opens that one
-// file and makes no database call. A variant it opens has its access time
-// handed to the background writer without waiting (see touchVariant). A
-// variant not in the cache, or any variant of a disabled cache, is
-// ErrNotFound; a variant file whose first bytes are not those of an output
-// format is another error.
-func (c *Cache) Variant(
-	cacheKey imagerequest.VariantKey,
-) (io.ReadCloser, int64, string, error) {
+// VariantFile is the open file of a cached variant, as Variant,
+// VariantIgnoringTTL and RenewVariant return it.
+type VariantFile struct {
+	// Content reads the file. The caller closes it.
+	Content io.ReadCloser
+	// Size is the file's size in bytes.
+	Size int64
+	// ContentType is the MIME type of the variant's format, read from the
+	// file's first bytes.
+	ContentType string
+	// CheckedAt is the file's modification time: its source's check time
+	// when the variant was made or last renewed.
+	CheckedAt time.Time
+}
+
+// Variant opens a cached variant and returns its file (see VariantFile). It
+// opens that one file and makes no database call: the file's modification
+// time is its source's check time when the variant was made or last renewed
+// (see PutVariant), and a variant whose time is CacheTTL or more ago is
+// ErrStale. A variant it opens has its access time handed to the background
+// writer without waiting (see touchVariant). A variant not in the cache, or
+// any variant of a disabled cache, is ErrNotFound; a variant file whose first
+// bytes are not those of an output format is another error.
+func (c *Cache) Variant(cacheKey imagerequest.VariantKey) (VariantFile, error) {
 	if c.config.MaxBytes == 0 {
-		return nil, 0, "", ErrNotFound
+		return VariantFile{}, ErrNotFound
 	}
 
-	file, size, err := c.variants.Open(string(cacheKey))
+	file, info, err := c.variants.Open(string(cacheKey))
 	if err != nil {
-		return nil, 0, "", err
+		return VariantFile{}, err
+	}
+
+	if c.Stale(info.ModTime()) {
+		_ = file.Close()
+
+		return VariantFile{}, ErrStale
+	}
+
+	return c.finishOpeningVariant(cacheKey, file, info)
+}
+
+// VariantIgnoringTTL opens a cached variant whatever its age, and returns it
+// as Variant does. The image proxy reads it for a variant whose source is not
+// cached, before it checks the source with upstream (see LookupVariant).
+func (c *Cache) VariantIgnoringTTL(
+	cacheKey imagerequest.VariantKey,
+) (VariantFile, error) {
+	if c.config.MaxBytes == 0 {
+		return VariantFile{}, ErrNotFound
+	}
+
+	file, info, err := c.variants.Open(string(cacheKey))
+	if err != nil {
+		return VariantFile{}, err
+	}
+
+	return c.finishOpeningVariant(cacheKey, file, info)
+}
+
+// RenewVariant opens a cached variant made from the source bytes that source
+// describes, whatever its age, and returns it as Variant does. It moves the
+// variant's check time forward to source.CheckedAt (see SetVariantCheckedAt).
+// A variant made before source.FetchedAt was made from older bytes, and is
+// ErrNotFound, as is a variant not in the cache.
+func (c *Cache) RenewVariant(
+	ctx context.Context, cacheKey imagerequest.VariantKey, source SourceInfo,
+) (VariantFile, error) {
+	if c.config.MaxBytes == 0 {
+		return VariantFile{}, ErrNotFound
+	}
+
+	file, info, err := c.variants.Open(string(cacheKey))
+	if err != nil {
+		return VariantFile{}, err
+	}
+
+	if info.ModTime().Before(source.FetchedAt) {
+		_ = file.Close()
+
+		return VariantFile{}, ErrNotFound
 	}
 
 	format, err := readFormat(file)
 	if err != nil {
 		_ = file.Close()
 
-		return nil, 0, "", err
+		return VariantFile{}, err
+	}
+
+	checkedAt := info.ModTime()
+
+	if checkedAt.Before(source.CheckedAt) {
+		err = c.SetVariantCheckedAt(ctx, cacheKey, source.CheckedAt)
+		if err != nil {
+			_ = file.Close()
+
+			return VariantFile{}, err
+		}
+
+		checkedAt = source.CheckedAt
 	}
 
 	c.touchVariant(cacheKey)
 
-	return file, size, format.MIMEType(), nil
+	return VariantFile{
+		Content:     file,
+		Size:        info.Size(),
+		ContentType: format.MIMEType(),
+		CheckedAt:   checkedAt,
+	}, nil
 }
 
-// PutVariant stores a processed variant by its cache key and records
-// it in the size accounting. On a disabled cache it is a no-op. The
-// accounting insert is best-effort (the startup reconciliation pass
-// adopts any variant file that misses its accounting row).
+// SetVariantCheckedAt records checkedAt as when upstream last sent the source
+// bytes the cached variant of cacheKey was made from, or answered that they
+// had not changed. It sets the variant file's modification time to checkedAt,
+// so Variant serves the file until CacheTTL after it. It sets the checked_at
+// of the variant's row too, so eviction keeps the variant until
+// EvictionAfterLastCheck after it. A failure to write the row is only logged.
+// On a disabled cache it is a no-op.
+func (c *Cache) SetVariantCheckedAt(
+	ctx context.Context, cacheKey imagerequest.VariantKey, checkedAt time.Time,
+) error {
+	if c.config.MaxBytes == 0 {
+		return nil
+	}
+
+	err := c.variants.SetModTime(string(cacheKey), checkedAt)
+	if err != nil {
+		return err
+	}
+
+	_, err = c.db.ExecContext(ctx,
+		`UPDATE variant_content SET checked_at = ? WHERE cache_key = ?`,
+		checkedAt.UTC(), string(cacheKey))
+	if err != nil {
+		c.log.Warn("failed to renew variant's checked_at",
+			"cache_key", cacheKey, "error", err)
+	}
+
+	return nil
+}
+
+// Stale reports whether a source last checked with upstream at checkedAt,
+// and its variants, are due to be checked again.
+func (c *Cache) Stale(checkedAt time.Time) bool {
+	return time.Since(checkedAt) >= c.config.CacheTTL
+}
+
+// PutVariant stores a processed variant by its cache key and records it in
+// the size accounting. source describes the source bytes the variant was made
+// from. Its CheckedAt becomes the modification time of the variant's file,
+// which Variant and RenewVariant read. The variant's row keeps its CheckedAt,
+// ETag and LastModified (see LookupVariant). On a disabled cache it is a
+// no-op. The accounting insert is best-effort (the startup reconciliation
+// pass adopts any variant file that misses its accounting row).
 func (c *Cache) PutVariant(
-	ctx context.Context, cacheKey imagerequest.VariantKey, content io.Reader,
+	ctx context.Context,
+	cacheKey imagerequest.VariantKey,
+	source SourceInfo,
+	content io.Reader,
 ) error {
 	if c.config.MaxBytes == 0 {
 		return nil
@@ -208,15 +362,33 @@ func (c *Cache) PutVariant(
 		return err
 	}
 
+	// A file left with the time of its writing could be served as made from
+	// bytes fetched after the ones it was made from
+	err = c.variants.SetModTime(string(cacheKey), source.CheckedAt)
+	if err != nil {
+		deleteErr := c.variants.Delete(string(cacheKey))
+		if deleteErr != nil {
+			c.log.Warn("failed to remove variant file whose time was not set",
+				"cache_key", cacheKey, "error", deleteErr)
+		}
+
+		return err
+	}
+
 	size := int64(len(data))
 
 	_, err = c.db.ExecContext(ctx, `
-		INSERT INTO variant_content (cache_key, size_bytes)
-		VALUES (?, ?)
+		INSERT INTO variant_content (cache_key, size_bytes, checked_at,
+			upstream_etag, upstream_last_modified)
+		VALUES (?, ?, ?, ?, ?)
 		ON CONFLICT(cache_key) DO UPDATE SET
 			size_bytes = excluded.size_bytes,
-			last_accessed_at = CURRENT_TIMESTAMP
-	`, string(cacheKey), size)
+			last_accessed_at = CURRENT_TIMESTAMP,
+			checked_at = excluded.checked_at,
+			upstream_etag = excluded.upstream_etag,
+			upstream_last_modified = excluded.upstream_last_modified
+	`, string(cacheKey), size, source.CheckedAt.UTC(), source.ETag,
+		source.LastModified)
 	if err != nil {
 		c.log.Warn("failed to record variant in size accounting",
 			"cache_key", cacheKey, "error", err)
@@ -255,7 +427,7 @@ func (c *Cache) Source(
 		return nil, 0, "", ErrNotFound
 	}
 
-	file, size, err := c.srcContent.Open(string(key))
+	file, info, err := c.srcContent.Open(string(key))
 	if err != nil {
 		return nil, 0, "", err
 	}
@@ -269,19 +441,107 @@ func (c *Cache) Source(
 
 	c.touchSourceContent(key)
 
-	return file, size, format, nil
+	return file, info.Size(), format, nil
 }
 
-// PutSource stores fetched source content: it writes the file named by
-// the source's key, then the source's row. When the row cannot be written
-// the file is removed and the store fails, as a file with no row is never
-// served or evicted; if that removal fails too, the next reconciliation
-// pass removes the file. On a disabled cache it is a no-op returning an
-// empty key.
+// SourceInfo is what the cache keeps in a cached source's row besides its
+// size and access time.
+type SourceInfo struct {
+	// FetchedAt is when the cached bytes were fetched.
+	FetchedAt time.Time
+	// CheckedAt is when upstream last sent the bytes or answered that they
+	// had not changed.
+	CheckedAt time.Time
+	// ETag is the ETag header upstream sent with the bytes, or "".
+	ETag string
+	// LastModified is the Last-Modified header upstream sent with the
+	// bytes, or "".
+	LastModified string
+}
+
+// LookupSource returns what the cache keeps about the cached source of src.
+// A source with no row is ErrNotFound, as is any source of a disabled cache.
+func (c *Cache) LookupSource(
+	ctx context.Context, src imagerequest.Source,
+) (SourceInfo, error) {
+	if c.config.MaxBytes == 0 {
+		return SourceInfo{}, ErrNotFound
+	}
+
+	var info SourceInfo
+
+	err := c.db.QueryRowContext(ctx, `
+		SELECT fetched_at, checked_at, upstream_etag, upstream_last_modified
+		FROM source_content WHERE source_key = ?
+	`, string(src.Key())).Scan(
+		&info.FetchedAt, &info.CheckedAt, &info.ETag, &info.LastModified)
+	if errors.Is(err, sql.ErrNoRows) {
+		return SourceInfo{}, ErrNotFound
+	}
+
+	if err != nil {
+		return SourceInfo{}, fmt.Errorf("failed to look up source: %w", err)
+	}
+
+	return info, nil
+}
+
+// LookupVariant returns what the row of the cached variant of cacheKey keeps
+// about the source bytes the variant was made from: the CheckedAt, ETag and
+// LastModified they had when the variant was made or last renewed. The row
+// keeps no FetchedAt, so it is zero. With the ETag and LastModified the image
+// proxy checks the variant with upstream after its source is evicted. A
+// variant with no row is ErrNotFound, as is any variant of a disabled cache.
+func (c *Cache) LookupVariant(
+	ctx context.Context, cacheKey imagerequest.VariantKey,
+) (SourceInfo, error) {
+	if c.config.MaxBytes == 0 {
+		return SourceInfo{}, ErrNotFound
+	}
+
+	var info SourceInfo
+
+	err := c.db.QueryRowContext(ctx, `
+		SELECT checked_at, upstream_etag, upstream_last_modified
+		FROM variant_content WHERE cache_key = ?
+	`, string(cacheKey)).Scan(&info.CheckedAt, &info.ETag, &info.LastModified)
+	if errors.Is(err, sql.ErrNoRows) {
+		return SourceInfo{}, ErrNotFound
+	}
+
+	if err != nil {
+		return SourceInfo{}, fmt.Errorf("failed to look up variant: %w", err)
+	}
+
+	return info, nil
+}
+
+// RenewSource records that upstream answered at checkedAt that the cached
+// source of src had not changed.
+func (c *Cache) RenewSource(
+	ctx context.Context, src imagerequest.Source, checkedAt time.Time,
+) error {
+	_, err := c.db.ExecContext(ctx,
+		`UPDATE source_content SET checked_at = ? WHERE source_key = ?`,
+		checkedAt.UTC(), string(src.Key()))
+	if err != nil {
+		return fmt.Errorf("failed to renew source: %w", err)
+	}
+
+	return nil
+}
+
+// PutSource stores fetched source content with what info says about it: it
+// writes the file named by the source's key, then the source's row. When the
+// row cannot be written the file is removed and the store fails, as a file
+// with no row is never served or evicted; if that removal fails too, the
+// next reconciliation pass removes the file. On a disabled cache it is a
+// no-op returning an empty key.
 func (c *Cache) PutSource(
 	ctx context.Context,
 	src imagerequest.Source,
 	content io.Reader,
+	info SourceInfo,
 ) (imagerequest.SourceKey, error) {
 	if c.config.MaxBytes == 0 {
 		return "", nil
@@ -302,12 +562,18 @@ func (c *Cache) PutSource(
 	size := int64(len(data))
 
 	_, err = c.db.ExecContext(ctx, `
-		INSERT INTO source_content (source_key, size_bytes)
-		VALUES (?, ?)
+		INSERT INTO source_content (source_key, size_bytes, fetched_at,
+			checked_at, upstream_etag, upstream_last_modified)
+		VALUES (?, ?, ?, ?, ?, ?)
 		ON CONFLICT(source_key) DO UPDATE SET
 			size_bytes = excluded.size_bytes,
-			last_accessed_at = CURRENT_TIMESTAMP
-	`, string(key), size)
+			last_accessed_at = CURRENT_TIMESTAMP,
+			fetched_at = excluded.fetched_at,
+			checked_at = excluded.checked_at,
+			upstream_etag = excluded.upstream_etag,
+			upstream_last_modified = excluded.upstream_last_modified
+	`, string(key), size, info.FetchedAt.UTC(), info.CheckedAt.UTC(), info.ETag,
+		info.LastModified)
 	if err != nil {
 		deleteErr := c.srcContent.Delete(string(key))
 		if deleteErr != nil {
@@ -449,6 +715,31 @@ func (c *Cache) cleanExpiredBatch(ctx context.Context) (int64, error) {
 	return deleted, nil
 }
 
+// finishOpeningVariant returns a variant file that FileStore.Open opened, with
+// its information info, as Variant does. It reads the content type from the
+// file's first bytes and hands the variant's access time to the background
+// writer (see touchVariant). It closes the file when its first bytes are not
+// those of an output format.
+func (c *Cache) finishOpeningVariant(
+	cacheKey imagerequest.VariantKey, file *os.File, info fs.FileInfo,
+) (VariantFile, error) {
+	format, err := readFormat(file)
+	if err != nil {
+		_ = file.Close()
+
+		return VariantFile{}, err
+	}
+
+	c.touchVariant(cacheKey)
+
+	return VariantFile{
+		Content:     file,
+		Size:        info.Size(),
+		ContentType: format.MIMEType(),
+		CheckedAt:   info.ModTime(),
+	}, nil
+}
+
 // readFormat reads the first bytes of a source or variant file, without
 // moving its offset, and returns the format they show. Sources and variants
 // are in the same six formats.

internal/imgcache/cache_hit_internal_test.go

@@ -49,16 +49,19 @@ func TestVariant_ContentTypeFromFirstBytes(t *testing.T) {
 
 		key := imagerequest.VariantKey("aabbccdd10" + string(format))
 
-		err = cache.PutVariant(t.Context(), key, bytes.NewReader(image))
+		err = cache.PutVariant(t.Context(), key, SourceInfo{CheckedAt: time.Now()},
+			bytes.NewReader(image))
 		if err != nil {
 			t.Fatalf("PutVariant(%s) error = %v", format, err)
 		}
 
-		reader, size, contentType, err := cache.Variant(key)
+		variant, err := cache.Variant(key)
 		if err != nil {
 			t.Fatalf("Variant(%s) error = %v", format, err)
 		}
 
+		reader, size, contentType := variant.Content, variant.Size, variant.ContentType
+
 		served, err := io.ReadAll(reader)
 		_ = reader.Close()
 
@@ -109,16 +112,19 @@ func TestVariant_RefusesFileOfNoOutputFormat(t *testing.T) {
 			t.Fatalf("%s: storing the variant file: %v", tt.name, err)
 		}
 
-		reader, _, contentType, err := cache.Variant(key)
+		variant, err := cache.Variant(key)
 		t.Logf("%s: Variant() error = %v", tt.name, err)
 
+		reader, contentType := variant.Content, variant.ContentType
 		if reader != nil || contentType != "" || !errors.Is(err, tt.wantErr) {
 			t.Errorf("%s: Variant() = reader %v, content type %q, error %v; "+
 				"want no reader and %v", tt.name, reader, contentType, err, tt.wantErr)
 		}
 	}
 
-	reader, _, _, err := cache.Variant(testVariantKeyFour)
+	variant, err := cache.Variant(testVariantKeyFour)
+
+	reader := variant.Content
 	if reader != nil || !errors.Is(err, ErrNotFound) {
 		t.Errorf("Variant() of a variant with no file = reader %v, error %v; "+
 			"want no reader and %v", reader, err, ErrNotFound)
@@ -126,7 +132,9 @@ func TestVariant_RefusesFileOfNoOutputFormat(t *testing.T) {
 
 	disabled, _ := newEvictionTestCache(t, 0)
 
-	reader, _, _, err = disabled.Variant(testVariantKeyOne)
+	variant, err = disabled.Variant(testVariantKeyOne)
+
+	reader = variant.Content
 	if reader != nil || !errors.Is(err, ErrNotFound) {
 		t.Errorf("Variant() on a disabled cache = reader %v, error %v; "+
 			"want no reader and %v", reader, err, ErrNotFound)

internal/imgcache/evict_not_checked_recently_internal_test.go

@@ -0,0 +1,88 @@
+package imgcache
+
+import (
+	"bytes"
+	"os"
+	"slices"
+	"testing"
+	"time"
+
+	"sneak.berlin/go/pixa/internal/imagerequest"
+)
+
+// TestEvictNotCheckedRecently stores a source and a variant last checked with
+// upstream 31 days ago and a source and a variant checked 29 days ago.
+// EvictNotCheckedRecently must remove the rows and files of the first two and
+// keep the other two.
+func TestEvictNotCheckedRecently(t *testing.T) {
+	t.Parallel()
+
+	cache, _ := newEvictionTestCache(t, 1<<30)
+	old := time.Now().Add(-31 * 24 * time.Hour)
+	recent := time.Now().Add(-29 * 24 * time.Hour)
+
+	oldSource := imagerequest.Source{Host: testHostCDN, Path: "/old.jpg"}
+	recentSource := imagerequest.Source{Host: testHostCDN, Path: "/recent.jpg"}
+
+	for src, checkedAt := range map[imagerequest.Source]time.Time{
+		oldSource: old, recentSource: recent,
+	} {
+		_, err := cache.PutSource(t.Context(), src, bytes.NewReader([]byte("source")),
+			SourceInfo{FetchedAt: checkedAt, CheckedAt: checkedAt})
+		if err != nil {
+			t.Fatalf("PutSource(%s) error = %v", src.Path, err)
+		}
+	}
+
+	oldVariant := imagerequest.VariantKey("aabbccdd0001")
+	recentVariant := imagerequest.VariantKey("aabbccdd0002")
+
+	for key, checkedAt := range map[imagerequest.VariantKey]time.Time{
+		oldVariant: old, recentVariant: recent,
+	} {
+		err := cache.PutVariant(t.Context(), key, SourceInfo{CheckedAt: checkedAt},
+			bytes.NewReader([]byte("variant")))
+		if err != nil {
+			t.Fatalf("PutVariant(%s) error = %v", key, err)
+		}
+	}
+
+	err := cache.EvictNotCheckedRecently(t.Context())
+	if err != nil {
+		t.Fatalf("EvictNotCheckedRecently() error = %v", err)
+	}
+
+	sourceRows, err := queryStringColumn[imagerequest.SourceKey](t.Context(),
+		cache.db, `SELECT source_key FROM source_content`, "sources", "source")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	variantRows, err := queryStringColumn[imagerequest.VariantKey](t.Context(),
+		cache.db, `SELECT cache_key FROM variant_content`, "variants", "variant")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	t.Logf("source rows left = %v, variant rows left = %v", sourceRows, variantRows)
+
+	if !slices.Equal(sourceRows, []imagerequest.SourceKey{recentSource.Key()}) {
+		t.Errorf("source rows left = %v, want only %s", sourceRows, recentSource.Key())
+	}
+
+	if !slices.Equal(variantRows, []imagerequest.VariantKey{recentVariant}) {
+		t.Errorf("variant rows left = %v, want only %s", variantRows, recentVariant)
+	}
+
+	for path, wantKept := range map[string]bool{
+		cache.srcContent.keyToPath(string(oldSource.Key())):    false,
+		cache.srcContent.keyToPath(string(recentSource.Key())): true,
+		cache.variants.keyToPath(string(oldVariant)):           false,
+		cache.variants.keyToPath(string(recentVariant)):        true,
+	} {
+		_, statErr := os.Stat(path)
+		if kept := statErr == nil; kept != wantKept {
+			t.Errorf("file %s kept = %t, want %t (%v)", path, kept, wantKept, statErr)
+		}
+	}
+}

internal/imgcache/eviction.go

@@ -131,6 +131,56 @@ func (c *Cache) EvictToLimit(ctx context.Context) error {
 	}
 }
 
+// EvictNotCheckedRecently evicts each source and variant whose checked_at is
+// EvictionAfterLastCheck or more ago, evictionBatchSize of each at a time,
+// until none is left. It is a no-op when the cache is disabled.
+func (c *Cache) EvictNotCheckedRecently(ctx context.Context) error {
+	if c.config.MaxBytes == 0 {
+		return nil
+	}
+
+	cutoff := time.Now().UTC().Add(-EvictionAfterLastCheck)
+
+	for {
+		sources, err := queryStringColumn[imagerequest.SourceKey](ctx, c.db, `
+			SELECT source_key FROM source_content WHERE checked_at < ? LIMIT ?
+		`, "sources not checked recently", "source key", cutoff, evictionBatchSize)
+		if err != nil {
+			return err
+		}
+
+		for _, key := range sources {
+			err = c.evictSourceBlob(ctx, key)
+			if err != nil {
+				return err
+			}
+		}
+
+		variants, err := queryStringColumn[imagerequest.VariantKey](ctx, c.db, `
+			SELECT cache_key FROM variant_content WHERE checked_at < ? LIMIT ?
+		`, "variants not checked recently", "variant key", cutoff, evictionBatchSize)
+		if err != nil {
+			return err
+		}
+
+		for _, key := range variants {
+			err = c.evictVariant(ctx, key)
+			if err != nil {
+				return err
+			}
+		}
+
+		if len(sources) > 0 || len(variants) > 0 {
+			c.log.Info("evicted cache content not checked with upstream recently",
+				"sources", len(sources), "variants", len(variants))
+		}
+
+		if len(sources) < evictionBatchSize && len(variants) < evictionBatchSize {
+			return nil
+		}
+	}
+}
+
 // evictBatch fetches one batch of LRU candidates across variants and
 // source blobs and evicts them oldest-first until excessBytes are
 // freed or the batch is exhausted. It returns the bytes freed. A
@@ -344,8 +394,9 @@ func (c *Cache) notifyWritePressure() {
 }
 
 // StartEviction launches the background eviction goroutine, which
-// removes the expired negative cache rows at startup and again on every
-// periodic tick thereafter, reconciles the database accounting with the
+// removes the expired negative cache rows, and the sources and variants not
+// checked for EvictionAfterLastCheck, at startup and again on every periodic
+// tick thereafter, reconciles the database accounting with the
 // cache directories at startup and again on the first periodic tick
 // that comes at least ten times as long after a reconciliation pass
 // ended as that pass took, and evicts to the configured limit on the given
@@ -392,6 +443,7 @@ func (c *Cache) evictionLoop(ctx context.Context, interval time.Duration) {
 
 	nextReconciliation := c.runReconciliationPass(ctx)
 	c.runCleanExpired(ctx)
+	c.runEvictNotCheckedRecently(ctx)
 	c.runEvictionPass(ctx)
 
 	ticker := time.NewTicker(interval)
@@ -417,6 +469,7 @@ func (c *Cache) evictionLoop(ctx context.Context, interval time.Duration) {
 			}
 
 			c.runCleanExpired(ctx)
+			c.runEvictNotCheckedRecently(ctx)
 		case <-c.evictionPressure:
 		}
 
@@ -474,6 +527,21 @@ func (c *Cache) runCleanExpired(ctx context.Context) {
 	}
 }
 
+// runEvictNotCheckedRecently runs EvictNotCheckedRecently, logging a failure
+// instead of propagating it (the loop must keep running). It does nothing
+// once ctx is cancelled.
+func (c *Cache) runEvictNotCheckedRecently(ctx context.Context) {
+	if ctx.Err() != nil {
+		return
+	}
+
+	err := c.EvictNotCheckedRecently(ctx)
+	if err != nil {
+		c.log.Warn("evicting cache content not checked recently failed",
+			"error", err)
+	}
+}
+
 // reconcileAccounting synchronizes the database size accounting with
 // the actual contents of the cache directories. It runs at startup and
 // then on the first periodic eviction tick that comes at least ten times
@@ -609,16 +677,18 @@ func (c *Cache) reconcileVariantFile(
 }
 
 // adoptVariantFile inserts an accounting row for a variant file that
-// has none, using the file's size and modification time.
+// has none, using the file's size and modification time, which is also
+// the variant's checked_at.
 func (c *Cache) adoptVariantFile(
 	ctx context.Context, info fs.FileInfo, cacheKey imagerequest.VariantKey,
 ) error {
 	modTime := info.ModTime().UTC().Format(sqliteTimestampLayout)
 
 	_, err := c.db.ExecContext(ctx, `
-		INSERT INTO variant_content (cache_key, size_bytes, last_accessed_at)
-		VALUES (?, ?, ?)
-	`, string(cacheKey), info.Size(), modTime)
+		INSERT INTO variant_content (cache_key, size_bytes, last_accessed_at,
+			checked_at)
+		VALUES (?, ?, ?, ?)
+	`, string(cacheKey), info.Size(), modTime, info.ModTime().UTC())
 	if err != nil {
 		return fmt.Errorf("failed to adopt variant file into accounting: %w", err)
 	}

internal/imgcache/eviction_internal_test.go

@@ -101,7 +101,7 @@ func storeEvictionTestSource(
 	}
 
 	key, err := cache.PutSource(
-		context.Background(), req.Source, bytes.NewReader(content),
+		context.Background(), req.Source, bytes.NewReader(content), SourceInfo{},
 	)
 	if err != nil {
 		t.Fatalf("PutSource(%s%s) failed: %v", host, path, err)
@@ -117,7 +117,8 @@ func storeEvictionTestVariant(
 ) {
 	t.Helper()
 
-	err := cache.PutVariant(t.Context(), key, bytes.NewReader(content))
+	err := cache.PutVariant(t.Context(), key, SourceInfo{CheckedAt: time.Now()},
+		bytes.NewReader(content))
 	if err != nil {
 		t.Fatalf("PutVariant(%s) failed: %v", key, err)
 	}
@@ -432,13 +433,14 @@ func assertDisabledCacheWritesAreNoOps(
 	ctx := t.Context()
 
 	err := cache.PutVariant(
-		ctx, imagerequest.CacheKey(req), bytes.NewReader([]byte("data")),
+		ctx, imagerequest.CacheKey(req), SourceInfo{}, bytes.NewReader([]byte("data")),
 	)
 	if err != nil {
 		t.Fatalf("PutVariant on disabled cache must be a no-op, got error: %v", err)
 	}
 
-	key, err := cache.PutSource(ctx, req.Source, bytes.NewReader([]byte("data")))
+	key, err := cache.PutSource(ctx, req.Source, bytes.NewReader([]byte("data")),
+		SourceInfo{})
 	if err != nil {
 		t.Fatalf("PutSource on disabled cache must be a no-op, got error: %v", err)
 	}

internal/imgcache/filestore.go

@@ -3,8 +3,10 @@ package imgcache
 import (
 	"errors"
 	"fmt"
+	"io/fs"
 	"os"
 	"path/filepath"
+	"time"
 )
 
 // Storage constants.
@@ -54,26 +56,26 @@ func (s *FileStore) Write(key string, data []byte) error {
 	return writeFileSynced(path, data)
 }
 
-// Open opens the file for key and returns it with its size in bytes. A
-// missing file is ErrNotFound.
-func (s *FileStore) Open(key string) (*os.File, int64, error) {
+// Open opens the file for key and returns it with its information, which
+// gives its size and its modification time. A missing file is ErrNotFound.
+func (s *FileStore) Open(key string) (*os.File, fs.FileInfo, error) {
 	f, err := os.Open(s.keyToPath(key))
 	if err != nil {
 		if os.IsNotExist(err) {
-			return nil, 0, ErrNotFound
+			return nil, nil, ErrNotFound
 		}
 
-		return nil, 0, fmt.Errorf("failed to open content: %w", err)
+		return nil, nil, fmt.Errorf("failed to open content: %w", err)
 	}
 
-	stat, err := f.Stat()
+	info, err := f.Stat()
 	if err != nil {
 		_ = f.Close()
 
-		return nil, 0, fmt.Errorf("failed to stat content: %w", err)
+		return nil, nil, fmt.Errorf("failed to stat content: %w", err)
 	}
 
-	return f, stat.Size(), nil
+	return f, info, nil
 }
 
 // Size returns the size in bytes of the file for key. A missing file is
@@ -91,6 +93,17 @@ func (s *FileStore) Size(key string) (int64, error) {
 	return stat.Size(), nil
 }
 
+// SetModTime sets the modification time of the file for key to modTime,
+// leaving its access time as it is.
+func (s *FileStore) SetModTime(key string, modTime time.Time) error {
+	err := os.Chtimes(s.keyToPath(key), time.Time{}, modTime)
+	if err != nil {
+		return fmt.Errorf("failed to set modification time: %w", err)
+	}
+
+	return nil
+}
+
 // Delete removes the file for key. A missing file is not an error.
 func (s *FileStore) Delete(key string) error {
 	err := os.Remove(s.keyToPath(key))

internal/imgcache/filestore_internal_test.go

@@ -38,15 +38,15 @@ func TestFileStore_WriteAndOpen(t *testing.T) {
 	}
 
 	// Open and verify size and content
-	r, size, err := storage.Open(key)
+	r, info, err := storage.Open(key)
 	if err != nil {
 		t.Fatalf("Open() error = %v", err)
 	}
 
 	defer func() { _ = r.Close() }()
 
-	if size != int64(len(content)) {
-		t.Errorf("Open() size = %d, want %d", size, len(content))
+	if info.Size() != int64(len(content)) {
+		t.Errorf("Open() size = %d, want %d", info.Size(), len(content))
 	}
 
 	loaded, err := io.ReadAll(r)

internal/imgcache/source_content_internal_test.go

@@ -128,7 +128,7 @@ func TestPutSource_FailedRowWriteLeavesNoFile(t *testing.T) {
 		Source: imagerequest.Source{Host: testHostCDN, Path: testPathCat},
 	}
 	key, err := cache.PutSource(t.Context(), req.Source,
-		bytes.NewReader([]byte("source bytes")))
+		bytes.NewReader([]byte("source bytes")), SourceInfo{})
 	t.Logf("PutSource() = %q, error = %v", key, err)
 
 	if err == nil {
@@ -170,7 +170,7 @@ func TestSource_FormatFromFirstBytes(t *testing.T) {
 			t.Fatalf("reading the %s image: %v", format, err)
 		}
 
-		_, err = cache.PutSource(t.Context(), src, bytes.NewReader(image))
+		_, err = cache.PutSource(t.Context(), src, bytes.NewReader(image), SourceInfo{})
 		if err != nil {
 			t.Fatalf("PutSource(%s) error = %v", format, err)
 		}

internal/providers/providers.go

@@ -83,6 +83,7 @@ func newCache(lc fx.Lifecycle, params cacheParams) (*imgcache.Cache, error) {
 	cache, err := imgcache.NewCache(db, imgcache.Config{
 		StateDir:    cfg.StateDir,
 		NegativeTTL: imgcache.DefaultNegativeTTL,
+		CacheTTL:    imgcache.DefaultCacheTTL,
 		MaxBytes:    maxBytes,
 		Logger:      params.Logger.Get(),
 	}, params.MetricsRegistry)