TODO.md

@@ -34,10 +34,11 @@ P2: security: referer blacklist
   limits the images decoded and encoded at once, and `upstream_connections`
   (default 64) the connections to all upstream hosts together, on top of
   `upstream_connections_per_host`; a fetch holds its connection until its image
-  has been processed; a request that finds either limit reached waits up to 10
-  seconds for a free one, then gets 503 `server busy, try again later`; libvips
-  runs one worker thread per image with its operation cache off; documented in
-  `README.md` and `config.example.yml`.
+  has been processed, and a request whose source is cached reads it only once it
+  has a processing slot; a request that finds either limit reached waits up to
+  10 seconds for a free one, then gets 503 `server busy, try again later`;
+  libvips runs one worker thread per image with its operation cache off;
+  documented in `README.md` and `config.example.yml`.
 - 2026-09-29 Dockerfiles install through `script/bootstrap` (closes #95): the
   `Dockerfile` lint and build stages and `Dockerfile.lint` copy `script/`,
   `go.mod` and `go.sum`, then run `script/bootstrap` in place of their own

internal/imgcache/cache.go

@@ -383,13 +383,16 @@ func (c *Cache) GetSourceMetadataID(
 	return id, nil
 }
 
-// GetSourceContent returns a reader for cached source content by its hash.
-func (c *Cache) GetSourceContent(contentHash ContentHash) (io.ReadCloser, error) {
+// GetSourceContent returns a reader for cached source content by its hash,
+// and the content's size in bytes.
+func (c *Cache) GetSourceContent(
+	contentHash ContentHash,
+) (io.ReadCloser, int64, error) {
 	if c.disabled {
-		return nil, ErrNotFound
+		return nil, 0, ErrNotFound
 	}
 
-	return c.srcContent.Load(contentHash)
+	return c.srcContent.LoadWithSize(contentHash)
 }
 
 // CleanExpired removes expired entries from the cache.

internal/imgcache/service.go

@@ -241,38 +241,37 @@ func (s *Service) GenerateSignedURL(
 		baseURL, path, sig, exp, req.Quality, req.FitMode), nil
 }
 
-// loadCachedSource attempts to load source content from cache, returning nil
-// if the cached data is unavailable or exceeds maxResponseSize.
-func (s *Service) loadCachedSource(contentHash ContentHash) []byte {
-	reader, err := s.cache.GetSourceContent(contentHash)
+// loadCachedSource opens source content from cache, without reading it, and
+// returns it with its size; nil if the cached data is unavailable, empty or
+// exceeds maxResponseSize.
+func (s *Service) loadCachedSource(
+	contentHash ContentHash,
+) (io.ReadCloser, int64) {
+	reader, size, err := s.cache.GetSourceContent(contentHash)
 	if err != nil {
 		s.log.Warn("failed to load cached source, fetching", "error", err)
 
-		return nil
+		return nil, 0
 	}
 
-	// Bound the read to maxResponseSize to prevent unbounded memory use
-	// from unexpectedly large cached files.
-	limited := io.LimitReader(reader, s.maxResponseSize+1)
-	data, err := io.ReadAll(limited)
+	if size > s.maxResponseSize {
 		_ = reader.Close()
 
-	if err != nil {
-		s.log.Warn("failed to read cached source, fetching", "error", err)
-
-		return nil
-	}
-
-	if int64(len(data)) > s.maxResponseSize {
 		s.log.Warn("cached source exceeds max response size, discarding",
 			"hash", contentHash,
 			"max_bytes", s.maxResponseSize,
 		)
 
-		return nil
+		return nil, 0
 	}
 
-	return data
+	if size == 0 {
+		_ = reader.Close()
+
+		return nil, 0
+	}
+
+	return reader, size
 }
 
 // processFromSourceOrFetch processes an image, using cached source content
@@ -289,22 +288,27 @@ func (s *Service) processFromSourceOrFetch(
 		s.log.Warn("source lookup failed", "error", err)
 	}
 
-	var sourceData []byte
+	var (
+		source     io.ReadCloser
+		sourceSize int64
+	)
 
 	if contentHash != "" {
 		s.log.Debug("using cached source", "hash", contentHash)
-		sourceData = s.loadCachedSource(contentHash)
+		source, sourceSize = s.loadCachedSource(contentHash)
 	}
 
 	// Fetch from upstream if we don't have source data or it's empty
-	if len(sourceData) == 0 {
+	if source == nil {
 		return s.fetchAndProcess(ctx, req, cacheKey)
 	}
 
-	// Process using cached source; nothing was fetched from upstream
-	resp, err := s.processAndStore(
-		ctx, req, cacheKey, sourceData, int64(len(sourceData)),
-	)
+	defer func() { _ = source.Close() }()
+
+	// 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.
+	resp, err := s.processAndStore(ctx, req, cacheKey, source, sourceSize)
 
 	return resp, 0, err
 }
@@ -338,6 +342,10 @@ func (s *Service) fetchAndProcess(
 		return nil, 0, 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() }()
 
 	// Read and validate the source content
@@ -383,17 +391,20 @@ func (s *Service) fetchAndProcess(
 		// Continue even if caching fails
 	}
 
-	resp, err := s.processAndStore(ctx, req, cacheKey, sourceData, fetchBytes)
+	resp, err := s.processAndStore(
+		ctx, req, cacheKey, bytes.NewReader(sourceData), fetchBytes,
+	)
 
 	return resp, fetchBytes, err
 }
 
-// processAndStore processes an image and stores the result.
+// processAndStore processes the image read from source and stores the
+// result.
 func (s *Service) processAndStore(
 	ctx context.Context,
 	req *ImageRequest,
 	cacheKey VariantKey,
-	sourceData []byte,
+	source io.Reader,
 	fetchBytes int64,
 ) (*ImageResponse, error) {
 	// Process the image
@@ -406,7 +417,7 @@ func (s *Service) processAndStore(
 		FitMode: imageprocessor.FitMode(req.FitMode),
 	}
 
-	processResult, err := s.processor.Process(ctx, bytes.NewReader(sourceData), processReq)
+	processResult, err := s.processor.Process(ctx, source, processReq)
 	if err != nil {
 		return nil, fmt.Errorf("image processing failed: %w", err)
 	}