Stop waiting for count writes after a request's deadline (closes #224)

TestService_Get_ReturnsByItsDeadline failed on a busy host because its
request, past its deadline, still waited for its miss count to be
written, a database write with no deadline; in pixad that write waits
for the one database connection every request shares. The hit, miss,
upstream fetch and transform counts are now each written in a goroutine
of their own, which the request waits for only until its deadline; a
count not written by then is written after it returns. Shutdown waits
for those writes within ShutdownTimeout and reports any left unfinished.
The test phase also runs go test with -parallel 4: on a busy host, as
many tests at once as there are CPUs wait so long to be scheduled that
a timed request can fail.

Model: opus-5-5

internal/imgcache/count_writes_internal_test.go

@@ -86,8 +86,8 @@ func TestService_WaitForCountWrites(t *testing.T) {
 	ended, cancel := context.WithDeadline(t.Context(), time.Now())
 	defer cancel()
 
-	svc.writeCount(ended, func(ctx context.Context) {
-		svc.cache.IncrementStats(ctx, false, 0)
+	svc.writeCount(ended, func(writeCtx context.Context) {
+		svc.cache.IncrementStats(writeCtx, false, 0)
 	})
 
 	shortCtx, cancelShort := context.WithTimeout(t.Context(), 100*time.Millisecond)

internal/imgcache/service.go

@@ -165,8 +165,8 @@ func (s *Service) Get(ctx context.Context, req *ImageRequest) (*ImageResponse, e
 			s.log.Error("failed to get cached variant", "key", result.CacheKey, "error", err)
 			// Fall through to re-process
 		} else {
-			s.writeCount(ctx, func(ctx context.Context) {
-				s.cache.IncrementStats(ctx, true, 0)
+			s.writeCount(ctx, func(writeCtx context.Context) {
+				s.cache.IncrementStats(writeCtx, true, 0)
 			})
 
 			return &ImageResponse{
@@ -184,8 +184,8 @@ func (s *Service) Get(ctx context.Context, req *ImageRequest) (*ImageResponse, e
 	// failed or the request context has ended meanwhile
 	response, err := s.processOrWait(ctx, req)
 
-	s.writeCount(ctx, func(ctx context.Context) {
-		s.cache.IncrementStats(ctx, false, 0)
+	s.writeCount(ctx, func(writeCtx context.Context) {
+		s.cache.IncrementStats(writeCtx, false, 0)
 	})
 
 	if err != nil {
@@ -234,6 +234,34 @@ func (s *Service) WaitForCountWrites(ctx context.Context) bool {
 	}
 }
 
+// writeCount runs write, which adds to a count in the database, in a
+// goroutine of its own, and waits for it, though not past ctx's deadline.
+// The write gets ctx without its cancellation or deadline, so an ended
+// request is still counted. A request past its deadline thus does not wait
+// for the database connection, which every request shares; its count is
+// written after it returns.
+func (s *Service) writeCount(ctx context.Context, write func(context.Context)) {
+	written := make(chan struct{})
+
+	s.countWrites.Go(func() {
+		defer close(written)
+
+		write(context.WithoutCancel(ctx))
+	})
+
+	deadline, hasDeadline := ctx.Deadline()
+	if !hasDeadline {
+		<-written
+
+		return
+	}
+
+	select {
+	case <-written:
+	case <-time.After(time.Until(deadline)):
+	}
+}
+
 // ValidateRequest validates the request signature if required.
 func (s *Service) ValidateRequest(req *ImageRequest) error {
 	// Check if host is allowed (no signature required)
@@ -280,38 +308,6 @@ func (s *Service) GenerateSignedURL(
 		baseURL, path, sig, exp, req.Quality, req.FitMode), nil
 }
 
-// writeCount runs write, which adds to a count in the database, in a
-// goroutine of its own. The write gets ctx without its cancellation or
-// deadline, so an ended request is still counted. writeCount waits for it
-// until ctx's deadline, even if ctx is cancelled first, so a request
-// returns with its count written unless its deadline has passed; past
-// that, it does not wait for the database connection, which every request
-// shares.
-func (s *Service) writeCount(ctx context.Context, write func(context.Context)) {
-	written := make(chan struct{})
-
-	s.countWrites.Go(func() {
-		defer close(written)
-
-		write(context.WithoutCancel(ctx))
-	})
-
-	deadline, hasDeadline := ctx.Deadline()
-	if !hasDeadline {
-		<-written
-
-		return
-	}
-
-	deadlineTimer := time.NewTimer(time.Until(deadline))
-	defer deadlineTimer.Stop()
-
-	select {
-	case <-written:
-	case <-deadlineTimer.C:
-	}
-}
-
 // errPanicked is returned when processing a variant panicked.
 var errPanicked = errors.New("panic while processing image")
 
@@ -510,8 +506,8 @@ func (s *Service) fetchAndProcess(
 	sourceData, err := io.ReadAll(fetchResult.Content)
 	fetchBytes := int64(len(sourceData))
 
-	s.writeCount(ctx, func(ctx context.Context) {
-		s.cache.IncrementUpstreamFetch(ctx, fetchBytes)
+	s.writeCount(ctx, func(writeCtx context.Context) {
+		s.cache.IncrementUpstreamFetch(writeCtx, fetchBytes)
 	})
 
 	if err != nil {