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/server/server.go

@@ -29,6 +29,10 @@ const (
 // still being processed once ShutdownTimeout has passed.
 var errStillProcessing = errors.New("images still being processed at shutdown")
 
+// errStillWritingCounts is returned by the server's stop hook when counts
+// are still being written to the database once ShutdownTimeout has passed.
+var errStillWritingCounts = errors.New("counts still being written at shutdown")
+
 // Params defines dependencies for Server.
 type Params struct {
 	fx.In
@@ -117,9 +121,11 @@ func (s *Server) enableSentry() error {
 }
 
 // cleanShutdown stops the HTTP server, waits for the images still being
-// processed, then flushes Sentry. The first two share ShutdownTimeout. It
-// returns errStillProcessing when images are still being processed after
-// that, as their work is abandoned.
+// processed and then for the counts still being written to the database,
+// which closes after this hook, then flushes Sentry. The first three share
+// ShutdownTimeout. It returns errStillProcessing and errStillWritingCounts
+// for the images and counts still unfinished after that, as their work is
+// abandoned.
 func (s *Server) cleanShutdown(ctx context.Context) error {
 	s.log.Info("shutting down")
 
@@ -132,17 +138,26 @@ func (s *Server) cleanShutdown(ctx context.Context) error {
 	}
 
 	stillProcessing := s.h.WaitForProcessing(ctxShutdown)
+	countsWritten := s.h.WaitForCountWrites(ctxShutdown)
 
 	if s.sentryEnabled {
 		sentry.Flush(SentryFlushTimeout)
 	}
 
+	var unfinished []error
+
 	if stillProcessing > 0 {
 		s.log.Error("images still being processed at shutdown",
 			"count", stillProcessing)
 
-		return errStillProcessing
+		unfinished = append(unfinished, errStillProcessing)
 	}
 
-	return nil
+	if !countsWritten {
+		s.log.Error("counts still being written at shutdown")
+
+		unfinished = append(unfinished, errStillWritingCounts)
+	}
+
+	return errors.Join(unfinished...)
 }