Keep counts not written by the request's deadline in memory (closes #224)

TestService_Get_ReturnsByItsDeadline failed on a busy host because its
request, past its deadline, still waited to write its miss count, a write
with no deadline; in pixad that write waits for the one database
connection every request shares. Each count write now keeps the request's
deadline but not its cancellation. A count not written by then is kept in
memory, where Stats includes it, and one goroutine of the cache writes
those counts one UPDATE at a time, and once more at shutdown before the
database closes. 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/handlers/handlers.go

@@ -4,6 +4,7 @@ package handlers
 import (
 	"context"
 	"encoding/json"
+	"errors"
 	"log/slog"
 	"net/http"
 	"time"
@@ -71,16 +72,21 @@ func New(lc fx.Lifecycle, params Params) (*Handlers, error) {
 	}
 
 	lc.Append(fx.Hook{
-		//nolint:contextcheck // the eviction loop outlives OnStart; OnStop cancels it
+		//nolint:contextcheck // the cache's goroutines outlive OnStart; OnStop stops them
 		OnStart: func(_ context.Context) error {
 			return s.initImageService()
 		},
+		// The pending counts are written here, before the database's stop
+		// hook closes it.
 		OnStop: func(ctx context.Context) error {
 			if s.imgCache == nil {
 				return nil
 			}
 
-			return s.imgCache.StopEviction(ctx)
+			return errors.Join(
+				s.imgCache.StopEviction(ctx),
+				s.imgCache.StopPendingCountWrites(ctx),
+			)
 		},
 	})
 
@@ -122,6 +128,9 @@ func (s *Handlers) initImageService() error {
 	// write-pressure passes. No-op when the disk cache is disabled.
 	cache.StartEviction(imgcache.DefaultEvictionInterval)
 
+	// Writes the counts requests could not write by their deadline
+	cache.StartPendingCountWrites()
+
 	// Create the fetcher config
 	fetcherCfg := httpfetcher.DefaultConfig()
 	fetcherCfg.AllowHTTP = s.config.AllowHTTP