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

@@ -0,0 +1,116 @@
+package imgcache
+
+import (
+	"context"
+	"errors"
+	"testing"
+	"time"
+)
+
+// holdDatabase takes the one connection of the test service's database, so
+// that every other query waits for it, and returns the func that frees it.
+func holdDatabase(t *testing.T, svc *Service) func() {
+	t.Helper()
+
+	conn, err := svc.cache.db.Conn(t.Context())
+	if err != nil {
+		t.Fatalf("failed to take the database connection: %v", err)
+	}
+
+	release := func() { _ = conn.Close() }
+	t.Cleanup(release)
+
+	return release
+}
+
+// TestService_Get_ReturnsByItsDeadlineWhileTheDatabaseIsBusy holds the
+// database's one connection while a request whose fetch is held reaches its
+// deadline. The request must still return by its deadline with the
+// deadline's error, and its miss must be counted once the connection is free.
+func TestService_Get_ReturnsByItsDeadlineWhileTheDatabaseIsBusy(t *testing.T) {
+	t.Parallel()
+
+	svc, fixtures, fetcher := setupHeldFetchService(t)
+
+	const timeout = 200 * time.Millisecond
+
+	ctx, cancel := context.WithTimeout(t.Context(), timeout)
+	defer cancel()
+
+	results := startGet(ctx, svc, photoVariant(fixtures, 85, FitCover))
+
+	// The request has made its database reads by the time it fetches.
+	<-fetcher.started
+
+	releaseDatabase := holdDatabase(t, svc)
+
+	select {
+	case got := <-results:
+		deadline, _ := ctx.Deadline()
+		t.Logf("Get() returned %v after its deadline, error = %v",
+			time.Since(deadline), got.err)
+
+		if !errors.Is(got.err, context.DeadlineExceeded) {
+			t.Errorf("Get() error = %v, want %v", got.err, context.DeadlineExceeded)
+		}
+	case <-time.After(timeout + time.Second):
+		t.Fatal("request did not return by its deadline while the database was busy")
+	}
+
+	releaseDatabase()
+
+	waitCtx, cancelWait := context.WithTimeout(t.Context(), 5*time.Second)
+	defer cancelWait()
+
+	if !svc.WaitForCountWrites(waitCtx) {
+		t.Fatal("the miss was not counted once the database was free")
+	}
+
+	want := cacheStatsCounters{missCount: 1}
+	if got := readCacheStatsCounters(t, svc.cache); got != want {
+		t.Errorf("counters = %+v, want %+v", got, want)
+	}
+}
+
+// TestService_WaitForCountWrites holds the database's one connection while a
+// request past its deadline counts a miss. WaitForCountWrites must report the
+// count unwritten when its context ends, and written once the connection is
+// free.
+func TestService_WaitForCountWrites(t *testing.T) {
+	t.Parallel()
+
+	svc, _, _ := setupHeldFetchService(t)
+
+	releaseDatabase := holdDatabase(t, svc)
+
+	ended, cancel := context.WithDeadline(t.Context(), time.Now())
+	defer cancel()
+
+	svc.writeCount(ended, func(ctx context.Context) {
+		svc.cache.IncrementStats(ctx, false, 0)
+	})
+
+	shortCtx, cancelShort := context.WithTimeout(t.Context(), 100*time.Millisecond)
+	defer cancelShort()
+
+	written := svc.WaitForCountWrites(shortCtx)
+	t.Logf("WaitForCountWrites() while the database was busy = %t", written)
+
+	if written {
+		t.Fatal("WaitForCountWrites() = true while the database was busy")
+	}
+
+	releaseDatabase()
+
+	waitCtx, cancelWait := context.WithTimeout(t.Context(), 5*time.Second)
+	defer cancelWait()
+
+	if !svc.WaitForCountWrites(waitCtx) {
+		t.Fatal("WaitForCountWrites() = false once the database was free")
+	}
+
+	want := cacheStatsCounters{missCount: 1}
+	if got := readCacheStatsCounters(t, svc.cache); got != want {
+		t.Errorf("counters = %+v, want %+v", got, want)
+	}
+}