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

README.md

@@ -233,8 +233,8 @@ These terms are the names the code uses:
 - **Variant**: the image pixa serves for an image request, made from its source
   with its transform.
 - **Variant key**: the SHA-256 hash of a variant's source, size, format, quality
-  and fit. It names the variant's file and its row in the database, and gives
-  its `ETag`.
+  and fit. It names the variant's file and its row in the database. The
+  variant's `ETag` is made from it and the file's modification time.
 - **Signed URL**: a `/v1/image/` URL whose `sig` parameter signs its image
   request and its expiry `exp` with `signing_key`.
 - **Encrypted URL**: a `/v1/e/` URL whose token holds its image request and any
@@ -249,8 +249,8 @@ These terms are the names the code uses:
     - a check of a cached image failed in any way (see Cache TTL).
 - **Image cache**: the part of pixa that keeps the sources, the variants and the
   negative cache (see Storage).
-- **Cache TTL**: how long pixa serves a cached source and its variants before it
-  checks the source with upstream again: 24 hours (see Cache TTL).
+- **Cache TTL**: how long pixa uses a cached source and serves its variants
+  before it checks the source with upstream again: 24 hours (see Cache TTL).
 - **Disk cache limit** (`cache_max_bytes`): the size in bytes that the cached
   sources and variants may reach together.
 - **Image proxy**: the part of pixa that serves a variant from the image cache
@@ -522,7 +522,9 @@ the image after pixa would refuse the URL. A URL with no expiry gets one year.
 `immutable` only stops a client revalidating while its copy is fresh. The
 response also carries `X-Pixa-Cache`: `HIT` for an image served from the cache,
 and `MISS` for one made for this request or older than 24 hours (see Cache TTL).
-The `ETag` is the first 16 characters of the variant key, in quotes.
+The `ETag` is made from the first 16 characters of the variant key and the
+variant file's modification time (see Cache TTL). So a variant made again from a
+changed source gets a new `ETag`.
 
 ### Signed URLs
 
@@ -894,16 +896,17 @@ also removes the expired rows of the negative cache every five minutes, at most
 
 ### Cache TTL
 
-pixa serves a cached source and its variants for 24 hours after upstream last
-sent the source or confirmed it unchanged. After that, the next request for an
-image of that source checks the source with upstream first:
+pixa uses a cached source for 24 hours after upstream last sent it or confirmed
+it unchanged. During that time it serves the source's variants from the cache.
+After that, the next request for an image of that source checks the source with
+upstream first:
 
 - pixa asks upstream to send the source only if it changed. It sends the
   source's `ETag` as `If-None-Match` and its `Last-Modified` as
   `If-Modified-Since`, when upstream gave them. With neither, upstream sends the
   whole image again.
-- When upstream answers 304 Not Modified, pixa serves the cached source and its
-  variants for another 24 hours. Nothing is downloaded or converted again.
+- When upstream answers 304 Not Modified, pixa uses the cached source and serves
+  its variants for another 24 hours. Nothing is downloaded or converted again.
 - When upstream sends the image, it replaces the cached source. Each variant
   made from the old image is converted again from the new one when it is next
   requested.
@@ -913,8 +916,8 @@ image of that source checks the source with upstream first:
   often it serves the image.
 
 pixa evicts a source and its variants 30 days after upstream last sent the
-source or confirmed it unchanged. So a source whose checks keep failing is
-served for at most 30 days. Eviction to stay under the disk cache limit can
+source or confirmed it unchanged. So pixa uses a source whose checks keep
+failing for at most 30 days. Eviction to stay under the disk cache limit can
 remove images sooner.
 
 That eviction can remove a source and keep its variants. Each variant's row

TODO.md

@@ -31,7 +31,7 @@ P2: security: per-origin rate limiting
 # Completed Steps
 
 - 2026-11-11 a cached source is checked with upstream once it is 24 hours old
-  (closes #69). Before, it and its variants were served until eviction, so a
+  (closes #69). Before, it was used and its variants served until eviction, so a
   changed image at the same URL was never picked up.
     - `source_content` keeps the source's `fetched_at`, `checked_at`, and the
       `ETag` and `Last-Modified` upstream sent.
@@ -40,6 +40,9 @@ P2: security: per-origin rate limiting
     - A variant file's modification time is its source's `checked_at` when the
       variant was made or last renewed, so a hit makes no database call. A
       variant made before the source's `fetched_at` is converted again.
+    - A variant's `ETag` is made from its variant key and its file's
+      modification time, so a variant made again from new bytes gets a new
+      `ETag`.
     - `variant_content` keeps the `ETag` and `Last-Modified` of the source a
       variant was made from. A variant whose source was evicted is checked with
       them, and a 304 renews the variant.

internal/handlers/handlers_internal_test.go

@@ -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/imageproxy/imageproxy.go

@@ -168,16 +168,16 @@ 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
 	}
 
@@ -288,7 +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
 }
 
@@ -439,7 +439,7 @@ func (p *Proxy) renewVariant(
 	ctx context.Context, cacheKey imagerequest.VariantKey,
 	source imgcache.SourceInfo,
 ) *processedVariant {
-	reader, _, contentType, err := p.cache.RenewVariant(ctx, cacheKey, source)
+	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",
@@ -449,7 +449,7 @@ func (p *Proxy) renewVariant(
 		return nil
 	}
 
-	return p.readCachedVariant(cacheKey, reader, contentType)
+	return p.readCachedVariant(cacheKey, cached)
 }
 
 // cachedVariantIgnoringTTL returns the cached variant of cacheKey whatever
@@ -460,7 +460,7 @@ func (p *Proxy) renewVariant(
 func (p *Proxy) cachedVariantIgnoringTTL(
 	ctx context.Context, cacheKey imagerequest.VariantKey,
 ) (*processedVariant, imgcache.SourceInfo) {
-	reader, _, contentType, err := p.cache.VariantIgnoringTTL(cacheKey)
+	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,
@@ -470,7 +470,7 @@ func (p *Proxy) cachedVariantIgnoringTTL(
 		return nil, imgcache.SourceInfo{}
 	}
 
-	variant := p.readCachedVariant(cacheKey, reader, contentType)
+	variant := p.readCachedVariant(cacheKey, cached)
 	if variant == nil {
 		return nil, imgcache.SourceInfo{}
 	}
@@ -484,15 +484,14 @@ func (p *Proxy) cachedVariantIgnoringTTL(
 	return variant, madeFrom
 }
 
-// readCachedVariant reads the cached variant of cacheKey that reader opened,
-// whose content type is contentType, and closes reader. It returns nil when
-// the variant cannot be read.
+// 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, reader io.ReadCloser, contentType string,
+	cacheKey imagerequest.VariantKey, file imgcache.VariantFile,
 ) *processedVariant {
-	defer func() { _ = reader.Close() }()
+	defer func() { _ = file.Content.Close() }()
 
-	data, err := io.ReadAll(reader)
+	data, err := io.ReadAll(file.Content)
 	if err != nil {
 		p.log.Warn("failed to read cached variant", "cache_key", cacheKey,
 			"error", err)
@@ -500,7 +499,11 @@ func (p *Proxy) readCachedVariant(
 		return nil
 	}
 
-	return &processedVariant{data: data, contentType: contentType}
+	return &processedVariant{
+		data:        data,
+		contentType: file.ContentType,
+		checkedAt:   file.CheckedAt,
+	}
 }
 
 // fetchAndProcess fetches the source from upstream, stores it, and makes
@@ -528,7 +531,9 @@ func (p *Proxy) fetchAndProcess(
 	case variant == nil:
 		return nil, err
 	case errors.Is(err, errSourceNotModified):
-		err = p.cache.SetVariantCheckedAt(ctx, cacheKey, time.Now().UTC())
+		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)
@@ -745,6 +750,9 @@ 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
@@ -826,6 +834,7 @@ func (p *Proxy) processAndStore(
 		data:         processedData,
 		contentType:  processResult.ContentType,
 		fetchedBytes: fetchBytes,
+		checkedAt:    sourceInfo.CheckedAt,
 	}, nil
 }
 
@@ -837,13 +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/imgcache/cache.go

@@ -3,8 +3,8 @@
 //   - 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. A source and its
-// variants are served until the cache TTL has passed since upstream last sent
+// 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
@@ -45,14 +45,14 @@ const cleanExpiredBatchSize = 1000
 const DefaultNegativeTTL = 5 * time.Minute
 
 // DefaultCacheTTL is how long after upstream last sent a cached source, or
-// answered that it had not changed, pixad serves the source and its
+// 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 served.
+// fail is used.
 const EvictionAfterLastCheck = 30 * 24 * time.Hour
 
 // ErrStale is returned by Variant for a variant whose file's modification
@@ -64,7 +64,7 @@ type Config struct {
 	StateDir    string
 	NegativeTTL time.Duration
 
-	// CacheTTL is how long a cached source and its variants are served
+	// 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
@@ -186,31 +186,43 @@ 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: 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
+// 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,
-) (io.ReadCloser, int64, string, error) {
+func (c *Cache) Variant(cacheKey imagerequest.VariantKey) (VariantFile, error) {
 	if c.config.MaxBytes == 0 {
-		return nil, 0, "", ErrNotFound
+		return VariantFile{}, ErrNotFound
 	}
 
-	file, info, err := c.openVariant(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 nil, 0, "", ErrStale
+		return VariantFile{}, ErrStale
 	}
 
 	return c.finishOpeningVariant(cacheKey, file, info)
@@ -221,14 +233,14 @@ func (c *Cache) Variant(
 // cached, before it checks the source with upstream (see LookupVariant).
 func (c *Cache) VariantIgnoringTTL(
 	cacheKey imagerequest.VariantKey,
-) (io.ReadCloser, int64, string, error) {
+) (VariantFile, error) {
 	if c.config.MaxBytes == 0 {
-		return nil, 0, "", ErrNotFound
+		return VariantFile{}, ErrNotFound
 	}
 
-	file, info, err := c.openVariant(cacheKey)
+	file, info, err := c.variants.Open(string(cacheKey))
 	if err != nil {
-		return nil, 0, "", err
+		return VariantFile{}, err
 	}
 
 	return c.finishOpeningVariant(cacheKey, file, info)
@@ -241,41 +253,50 @@ func (c *Cache) VariantIgnoringTTL(
 // ErrNotFound, as is a variant not in the cache.
 func (c *Cache) RenewVariant(
 	ctx context.Context, cacheKey imagerequest.VariantKey, source SourceInfo,
-) (io.ReadCloser, int64, string, error) {
+) (VariantFile, error) {
 	if c.config.MaxBytes == 0 {
-		return nil, 0, "", ErrNotFound
+		return VariantFile{}, ErrNotFound
 	}
 
-	file, info, err := c.openVariant(cacheKey)
+	file, info, err := c.variants.Open(string(cacheKey))
 	if err != nil {
-		return nil, 0, "", err
+		return VariantFile{}, err
 	}
 
 	if info.ModTime().Before(source.FetchedAt) {
 		_ = file.Close()
 
-		return nil, 0, "", ErrNotFound
+		return VariantFile{}, ErrNotFound
 	}
 
 	format, err := readFormat(file)
 	if err != nil {
 		_ = file.Close()
 
-		return nil, 0, "", err
+		return VariantFile{}, err
 	}
 
-	if info.ModTime().Before(source.CheckedAt) {
+	checkedAt := info.ModTime()
+
+	if checkedAt.Before(source.CheckedAt) {
 		err = c.SetVariantCheckedAt(ctx, cacheKey, source.CheckedAt)
 		if err != nil {
 			_ = file.Close()
 
-			return nil, 0, "", err
+			return VariantFile{}, err
 		}
+
+		checkedAt = source.CheckedAt
 	}
 
 	c.touchVariant(cacheKey)
 
-	return file, info.Size(), format.MIMEType(), nil
+	return VariantFile{
+		Content:     file,
+		Size:        info.Size(),
+		ContentType: format.MIMEType(),
+		CheckedAt:   checkedAt,
+	}, nil
 }
 
 // SetVariantCheckedAt records checkedAt as when upstream last sent the source
@@ -406,7 +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
 	}
@@ -420,7 +441,7 @@ func (c *Cache) Source(
 
 	c.touchSourceContent(key)
 
-	return file, size, format, nil
+	return file, info.Size(), format, nil
 }
 
 // SourceInfo is what the cache keeps in a cached source's row besides its
@@ -694,43 +715,29 @@ func (c *Cache) cleanExpiredBatch(ctx context.Context) (int64, error) {
 	return deleted, nil
 }
 
-// openVariant opens the file of a cached variant and returns it with its
-// information, which gives its size and its modification time.
-func (c *Cache) openVariant(
-	cacheKey imagerequest.VariantKey,
-) (*os.File, fs.FileInfo, error) {
-	file, _, err := c.variants.Open(string(cacheKey))
-	if err != nil {
-		return nil, nil, err
-	}
-
-	info, err := file.Stat()
-	if err != nil {
-		_ = file.Close()
-
-		return nil, nil, fmt.Errorf("failed to stat variant: %w", err)
-	}
-
-	return file, info, nil
-}
-
-// finishOpeningVariant reads the content type of a variant file openVariant
-// opened from its first bytes, hands the variant's access time to the
-// background writer (see touchVariant), and returns the file as Variant does.
-// It closes the file when its first bytes are not those of an output format.
+// 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,
-) (io.ReadCloser, int64, string, error) {
+) (VariantFile, error) {
 	format, err := readFormat(file)
 	if err != nil {
 		_ = file.Close()
 
-		return nil, 0, "", err
+		return VariantFile{}, err
 	}
 
 	c.touchVariant(cacheKey)
 
-	return file, info.Size(), format.MIMEType(), nil
+	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

internal/imgcache/cache_hit_internal_test.go

@@ -55,11 +55,13 @@ func TestVariant_ContentTypeFromFirstBytes(t *testing.T) {
 			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()
 
@@ -110,16 +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)
@@ -127,7 +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/filestore.go

@@ -3,6 +3,7 @@ package imgcache
 import (
 	"errors"
 	"fmt"
+	"io/fs"
 	"os"
 	"path/filepath"
 	"time"
@@ -55,26 +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

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)