Count interrupted misses and the upstream bytes they read (closes #56)

The miss and transform counters are written with context.WithoutCancel,
so a client disconnect or the request timeout during or after the work
no longer loses them. A failed read of the upstream body now returns
the bytes read before the error, so an over-size or cut-off body still
moves the upstream fetch counters. processFromSourceOrFetch passes the
cached source's length directly instead of through a local named
fetchBytes.

Model: opus-5-5

internal/imgcache/service.go

@@ -156,12 +156,13 @@ func (s *Service) Get(ctx context.Context, req *ImageRequest) (*ImageResponse, e
 	}
 
 	// Cache miss - process the cached source or fetch it, then count the
-	// miss with the bytes it fetched from upstream, also when it failed
+	// miss with the bytes it fetched from upstream, also when it failed or
+	// the request context has ended meanwhile
 	cacheKey := CacheKey(req)
 
 	response, fetchedBytes, err := s.processFromSourceOrFetch(ctx, req, cacheKey)
 
-	s.cache.IncrementStats(ctx, false, fetchedBytes)
+	s.cache.IncrementStats(context.WithoutCancel(ctx), false, fetchedBytes)
 
 	if err != nil {
 		return nil, err
@@ -283,10 +284,7 @@ func (s *Service) processFromSourceOrFetch(
 		s.log.Warn("source lookup failed", "error", err)
 	}
 
-	var (
-		sourceData []byte
-		fetchBytes int64
-	)
+	var sourceData []byte
 
 	if contentHash != "" {
 		s.log.Debug("using cached source", "hash", contentHash)
@@ -299,16 +297,16 @@ func (s *Service) processFromSourceOrFetch(
 	}
 
 	// Process using cached source; nothing was fetched from upstream
-	fetchBytes = int64(len(sourceData))
-
-	resp, err := s.processAndStore(ctx, req, cacheKey, sourceData, fetchBytes)
+	resp, err := s.processAndStore(
+		ctx, req, cacheKey, sourceData, int64(len(sourceData)),
+	)
 
 	return resp, 0, err
 }
 
 // fetchAndProcess fetches from upstream, processes, and caches the result.
-// It also returns the number of bytes fetched from upstream once the
-// response has been read, including when a later step fails.
+// It also returns the number of bytes read from upstream, including when
+// reading the response or a later step fails.
 func (s *Service) fetchAndProcess(
 	ctx context.Context,
 	req *ImageRequest,
@@ -339,13 +337,13 @@ func (s *Service) fetchAndProcess(
 
 	// Read and validate the source content
 	sourceData, err := io.ReadAll(fetchResult.Content)
+	fetchBytes := int64(len(sourceData))
+
 	if err != nil {
-		return nil, 0, fmt.Errorf("failed to read upstream response: %w", err)
+		return nil, fetchBytes, fmt.Errorf("failed to read upstream response: %w", err)
 	}
 
 	// Calculate download bitrate
-	fetchBytes := int64(len(sourceData))
-
 	var downloadRate string
 
 	if fetchResult.FetchDurationMs > 0 {
@@ -410,7 +408,8 @@ func (s *Service) processAndStore(
 
 	processDuration := time.Since(processStart)
 
-	s.cache.IncrementTransformCount(ctx)
+	// Counted also when the request context has ended meanwhile
+	s.cache.IncrementTransformCount(context.WithoutCancel(ctx))
 
 	// Read processed content
 	processedData, err := io.ReadAll(processResult.Content)