Serve a cached variant while its source URL is negative-cached (closes #245)

Get checked the negative cache before it looked for the variant on
disk, so after one failed fetch of a source every cached variant of it
answered 502 for 5 minutes. Get now serves a cached variant first and
checks the negative cache only on a miss, before processing. The 502
line under /v1/image/ in README.md and TODO.md say so.

Model: opus-5-5

TODO.md

@@ -45,15 +45,6 @@ P2: security: per-origin rate limiting
   commented entry under `example.com`. Getting Started and "Running under upaas"
   in `README.md` say that every image on an allowlisted host is served without a
   signature, so an allowlisted host must be one the operator controls.
-- 2026-10-15 a chunked upstream body of exactly `upstream_max_response_size`
-  bytes is accepted (closes #248): once the limit was used up, the fetcher's
-  next read refused the body without looking for its end. Go's HTTP client
-  returns the end of a body sent with a `Content-Length` together with its last
-  bytes, but the end of a chunked body can come in a later read, so such a body
-  was refused after it had been downloaded whole, and the image routes
-  answered 500. Once the limit is used up, the fetcher now reads one more byte:
-  if the body has ended it is accepted, and if there is more it is refused with
-  `ErrResponseTooLarge`, as before.
 - 2026-10-13 `fit=outside` no longer makes an output above 8192 on a side
   (closes #257): it scales the image to cover the requested size, and the side
   scaled past that size was held to no maximum, so `1x8192?fit=outside` of a

internal/httpfetcher/fetch_internal_test.go

@@ -9,7 +9,6 @@ import (
 	"net/http"
 	"net/http/httptest"
 	"slices"
-	"strconv"
 	"strings"
 	"sync"
 	"testing"
@@ -298,139 +297,6 @@ func TestFetchMaxResponseSizeEnforced(t *testing.T) {
 	}
 }
 
-// fetchWithMaxResponseSize serves handler as the fake upstream and fetches
-// its /image route with MaxResponseSize set to maxResponseSize.
-func fetchWithMaxResponseSize(
-	t *testing.T,
-	maxResponseSize int64,
-	handler http.HandlerFunc,
-) *FetchResult {
-	t.Helper()
-
-	srv := httptest.NewServer(handler)
-	t.Cleanup(srv.Close)
-
-	cfg := DefaultConfig()
-	cfg.MaxResponseSize = maxResponseSize
-
-	f, _ := newServerFetcher(t, srv, cfg)
-
-	res := fetchImage(t, f, "/image")
-	t.Cleanup(func() { _ = res.Content.Close() })
-
-	return res
-}
-
-// TestFetchContentLengthBodyAtMaxResponseSize checks that a body sent with a
-// Content-Length is read whole at exactly MaxResponseSize bytes and refused
-// at one byte more.
-func TestFetchContentLengthBodyAtMaxResponseSize(t *testing.T) {
-	t.Parallel()
-
-	const maxResponseSize = 8
-
-	for _, tc := range []struct {
-		size    int
-		wantErr error
-	}{
-		{maxResponseSize, nil},
-		{maxResponseSize + 1, ErrResponseTooLarge},
-	} {
-		t.Run(strconv.Itoa(tc.size)+" bytes", func(t *testing.T) {
-			t.Parallel()
-
-			body := strings.Repeat("x", tc.size)
-
-			res := fetchWithMaxResponseSize(t, maxResponseSize,
-				func(w http.ResponseWriter, _ *http.Request) {
-					w.Header().Set("Content-Type", contentTypeJPEG)
-					w.Header().Set("Content-Length", strconv.Itoa(tc.size))
-					_, _ = io.WriteString(w, body)
-				})
-
-			if res.ContentLength != int64(tc.size) {
-				t.Fatalf("ContentLength = %d, want %d", res.ContentLength, tc.size)
-			}
-
-			data, err := io.ReadAll(res.Content)
-			t.Logf("body of %d bytes: read %d, error %v", tc.size, len(data), err)
-
-			if !errors.Is(err, tc.wantErr) {
-				t.Fatalf("read error = %v, want %v", err, tc.wantErr)
-			}
-
-			if err == nil && string(data) != body {
-				t.Errorf("read %q, want %q", data, body)
-			}
-		})
-	}
-}
-
-// TestFetchChunkedBodyAtMaxResponseSize checks that a chunked body is read
-// whole at exactly MaxResponseSize bytes and refused at one byte more. The
-// upstream sends the end of the body only once the test has read
-// MaxResponseSize bytes, so the read after them comes before the end of the
-// body has arrived, as it can from a real upstream.
-func TestFetchChunkedBodyAtMaxResponseSize(t *testing.T) {
-	t.Parallel()
-
-	const maxResponseSize = 8
-
-	for _, tc := range []struct {
-		size    int
-		wantErr error
-	}{
-		{maxResponseSize, nil},
-		{maxResponseSize + 1, ErrResponseTooLarge},
-	} {
-		t.Run(strconv.Itoa(tc.size)+" bytes", func(t *testing.T) {
-			t.Parallel()
-
-			body := strings.Repeat("x", tc.size)
-			endBody := make(chan struct{})
-
-			res := fetchWithMaxResponseSize(t, maxResponseSize,
-				func(w http.ResponseWriter, r *http.Request) {
-					w.Header().Set("Content-Type", contentTypeJPEG)
-					_, _ = io.WriteString(w, body)
-					// With no Content-Length, flushing sends the body
-					// chunked; its end is sent when the handler returns.
-					_ = http.NewResponseController(w).Flush()
-
-					select {
-					case <-endBody:
-					case <-r.Context().Done():
-					}
-				})
-
-			if res.ContentLength != -1 {
-				t.Fatalf("ContentLength = %d, want -1 (chunked)", res.ContentLength)
-			}
-
-			head := make([]byte, maxResponseSize)
-
-			_, err := io.ReadFull(res.Content, head)
-			if err != nil {
-				t.Fatalf("read the first %d bytes: %v", maxResponseSize, err)
-			}
-
-			close(endBody)
-
-			rest, err := io.ReadAll(res.Content)
-			data := string(head) + string(rest)
-			t.Logf("body of %d bytes: read %d, error %v", tc.size, len(data), err)
-
-			if !errors.Is(err, tc.wantErr) {
-				t.Fatalf("read error = %v, want %v", err, tc.wantErr)
-			}
-
-			if err == nil && data != body {
-				t.Errorf("read %q, want %q", data, body)
-			}
-		})
-	}
-}
-
 func TestFetchSemaphoreReleasedOnError(t *testing.T) {
 	t.Parallel()
 

internal/httpfetcher/httpfetcher.go

@@ -660,33 +660,14 @@ func dialSSRFSafe(
 	return nil, fmt.Errorf("%w to %s", errConnectFailed, host)
 }
 
-// limitedReader wraps a reader and refuses a body longer than remaining
-// bytes with ErrResponseTooLarge. remaining goes below 0 once the body is
-// found to be longer.
+// limitedReader wraps a reader and limits the number of bytes read.
 type limitedReader struct {
 	reader    io.Reader
 	remaining int64
 }
 
-// Read reads at most the remaining bytes. Once they are read, it reads one
-// more byte to tell the end of the body from more data: at the end it
-// returns the wrapped reader's error, io.EOF; with more data it returns
-// ErrResponseTooLarge, as does every later Read.
 func (r *limitedReader) Read(p []byte) (int, error) {
-	if r.remaining < 0 {
-		return 0, ErrResponseTooLarge
-	}
-
-	if r.remaining == 0 {
-		var extra [1]byte
-
-		n, err := r.reader.Read(extra[:])
-		r.remaining -= int64(n)
-
-		if n == 0 {
-			return 0, err
-		}
-
+	if r.remaining <= 0 {
 		return 0, ErrResponseTooLarge
 	}
 

internal/imgcache/negative_cache_internal_test.go

@@ -97,9 +97,8 @@ func TestNegativeCache_Expired(t *testing.T) {
 // TestService_Get_ServesCachedVariantOfNegativeCachedURL caches a variant of
 // the test photo, then keeps a failed fetch of the photo's URL in the negative
 // cache, as a failed fetch for another variant does once the source has been
-// evicted. The cached variant is still served, and a variant not in the cache
-// is made from the cached source. Once the source is evicted, a variant not in
-// the cache needs a fetch and is refused.
+// evicted. The cached variant is still served; a variant not in the cache is
+// still refused.
 func TestService_Get_ServesCachedVariantOfNegativeCachedURL(t *testing.T) {
 	t.Parallel()
 
@@ -130,31 +129,11 @@ func TestService_Get_ServesCachedVariantOfNegativeCachedURL(t *testing.T) {
 			resp.CacheStatus, CacheHit)
 	}
 
-	resp, err = svc.Get(t.Context(), photoVariant(fixtures, 40, FitCover))
-	t.Logf("Get() of a variant made from the cached source: error = %v", err)
-
-	if err != nil {
-		t.Fatalf("Get() of a variant made from the cached source error = %v", err)
-	}
-
-	_ = resp.Content.Close()
-
-	contentHash, _, err := svc.cache.LookupSource(t.Context(),
-		photoVariant(fixtures, 85, FitCover))
-	if err != nil || contentHash == "" {
-		t.Fatalf("LookupSource() = %q, %v, want the cached source", contentHash, err)
-	}
-
-	err = svc.cache.evictSourceBlob(t.Context(), contentHash)
-	if err != nil {
-		t.Fatalf("evictSourceBlob() error = %v", err)
-	}
-
-	_, err = svc.Get(t.Context(), photoVariant(fixtures, 60, FitCover))
-	t.Logf("Get() of a variant whose source was evicted: error = %v", err)
+	_, err = svc.Get(t.Context(), photoVariant(fixtures, 40, FitCover))
+	t.Logf("Get() of a variant not in the cache: error = %v", err)
 
 	if !errors.Is(err, ErrNegativeCached) {
-		t.Errorf("Get() of a variant whose source was evicted error = %v, want %v",
+		t.Errorf("Get() of a variant not in the cache error = %v, want %v",
 			err, ErrNegativeCached)
 	}
 }