Serve a cache hit with no database read or write (closes #309)

A hit opens the variant file and nothing else: its content type comes
from the file's first bytes, and a file in no output format is a miss.
Its access time and hit count, and the access time of a cached source a
miss uses, go without waiting to the goroutine that wrote late counts,
which now writes them all once a second; each kind of access time waits
in a channel of 10,000 that drops the rest. checkNegativeCache leaves
expired rows to CleanExpired. The .meta files, metaCache, Lookup and
golang-lru go, with their tests. Dropping the .meta skip made the
variant and source reconciliation walks identical, so they share one.
make bench runs a parallel cache hit benchmark on the host, outside
make check.

Model: opus-5-5

internal/imgcache/cache_hit_internal_test.go

@@ -0,0 +1,299 @@
+package imgcache
+
+import (
+	"bytes"
+	"errors"
+	"fmt"
+	"image/color"
+	"io"
+	"os"
+	"testing"
+	"time"
+
+	"sneak.berlin/go/pixa/internal/imageprocessor"
+	"sneak.berlin/go/pixa/internal/imagerequest"
+	"sneak.berlin/go/pixa/internal/magic"
+)
+
+// TestGetVariant_ContentTypeFromFirstBytes stores an image in each output
+// format as a variant, recorded with the content type text/plain, and checks
+// that GetVariant serves it whole with the content type of its format, read
+// from the file.
+func TestGetVariant_ContentTypeFromFirstBytes(t *testing.T) {
+	t.Parallel()
+
+	cache, _ := newEvictionTestCache(t, 1<<30)
+	processor := imageprocessor.New(imageprocessor.Params{})
+	source := generateTestJPEG(t, 40, 30, color.RGBA{200, 40, 40, 255})
+
+	for _, format := range []imagerequest.Format{
+		imagerequest.FormatJPEG, imagerequest.FormatPNG, imagerequest.FormatGIF,
+		imagerequest.FormatWebP, imagerequest.FormatAVIF, imagerequest.FormatJXL,
+	} {
+		result, err := processor.Process(t.Context(), bytes.NewReader(source),
+			&imagerequest.Transform{
+				Size:    imagerequest.Size{Width: 20, Height: 15},
+				Format:  format,
+				Quality: 85,
+				FitMode: imagerequest.FitCover,
+			})
+		if err != nil {
+			t.Fatalf("making a %s image: %v", format, err)
+		}
+
+		image, err := io.ReadAll(result.Content)
+		if err != nil {
+			t.Fatalf("reading the %s image: %v", format, err)
+		}
+
+		key := imagerequest.VariantKey("aabbccdd10" + string(format))
+
+		err = cache.StoreVariant(t.Context(), key, bytes.NewReader(image), "text/plain")
+		if err != nil {
+			t.Fatalf("StoreVariant(%s) error = %v", format, err)
+		}
+
+		reader, size, contentType, err := cache.GetVariant(key)
+		if err != nil {
+			t.Fatalf("GetVariant(%s) error = %v", format, err)
+		}
+
+		served, err := io.ReadAll(reader)
+		_ = reader.Close()
+
+		if err != nil {
+			t.Fatalf("reading the %s variant: %v", format, err)
+		}
+
+		t.Logf("%s: %d bytes served as %s", format, size, contentType)
+
+		if want := imageprocessor.FormatToMIME(format); contentType != want {
+			t.Errorf("%s variant content type = %q, want %q", format, contentType, want)
+		}
+
+		if size != int64(len(image)) || !bytes.Equal(served, image) {
+			t.Errorf("%s variant served %d bytes, size %d, want the %d bytes stored",
+				format, len(served), size, len(image))
+		}
+	}
+}
+
+// TestGetVariant_RefusesFileOfNoOutputFormat checks that GetVariant refuses
+// a variant file whose first bytes are not those of an output format, with
+// an error other than ErrNotFound, and that a variant with no file, or any
+// variant of a disabled cache, is ErrNotFound.
+func TestGetVariant_RefusesFileOfNoOutputFormat(t *testing.T) {
+	t.Parallel()
+
+	cache, _ := newEvictionTestCache(t, 1<<30)
+	jpegImage := generateTestJPEG(t, 10, 10, color.RGBA{0, 0, 255, 255})
+
+	tests := []struct {
+		name    string
+		content []byte
+		wantErr error
+	}{
+		{"text", []byte("this is text, not an image"), magic.ErrUnknownFormat},
+		{"SVG", []byte(`<svg xmlns="http://www.w3.org/2000/svg"></svg>`),
+			errNotAnOutputFormat},
+		{"JPEG cut to 10 bytes", jpegImage[:10], io.EOF},
+		{"empty", nil, io.EOF},
+	}
+
+	for i, tt := range tests {
+		key := imagerequest.VariantKey(fmt.Sprintf("aabbccdd20%02d", i))
+
+		_, err := cache.variants.Store(key, bytes.NewReader(tt.content))
+		if err != nil {
+			t.Fatalf("%s: storing the variant file: %v", tt.name, err)
+		}
+
+		reader, _, contentType, err := cache.GetVariant(key)
+		t.Logf("%s: GetVariant() error = %v", tt.name, err)
+
+		if reader != nil || contentType != "" || !errors.Is(err, tt.wantErr) {
+			t.Errorf("%s: GetVariant() = reader %v, content type %q, error %v; "+
+				"want no reader and %v", tt.name, reader, contentType, err, tt.wantErr)
+		}
+	}
+
+	reader, _, _, err := cache.GetVariant(testVariantKeyFour)
+	if reader != nil || !errors.Is(err, ErrNotFound) {
+		t.Errorf("GetVariant() of a variant with no file = reader %v, error %v; "+
+			"want no reader and %v", reader, err, ErrNotFound)
+	}
+
+	disabled, _ := newEvictionTestCache(t, 0)
+
+	reader, _, _, err = disabled.GetVariant(testVariantKeyOne)
+	if reader != nil || !errors.Is(err, ErrNotFound) {
+		t.Errorf("GetVariant() on a disabled cache = reader %v, error %v; "+
+			"want no reader and %v", reader, err, ErrNotFound)
+	}
+}
+
+// TestService_Get_MakesAgainVariantOfNoOutputFormat replaces a cached
+// variant's file with text, and checks that the next request is a miss
+// answered with the variant made again, and the one after a hit.
+func TestService_Get_MakesAgainVariantOfNoOutputFormat(t *testing.T) {
+	t.Parallel()
+
+	svc, fixtures := SetupTestService(t)
+	req := photoVariant(fixtures, 85, imagerequest.FitCover)
+
+	want, _ := getImage(t, svc, req)
+
+	path := svc.cache.variants.keyToPath(imagerequest.CacheKey(req))
+
+	err := os.WriteFile(path, []byte("this is text, not an image"), StorageFilePerm)
+	if err != nil {
+		t.Fatalf("replacing the variant file: %v", err)
+	}
+
+	got, status := getImage(t, svc, req)
+	t.Logf("after the replacement: %d bytes, %s", len(got), status)
+
+	if status != CacheMiss || !bytes.Equal(got, want) {
+		t.Errorf("after the replacement: Get() = %d bytes, %s; want the %d bytes "+
+			"made again, %s", len(got), status, len(want), CacheMiss)
+	}
+
+	got, status = getImage(t, svc, req)
+
+	if status != CacheHit || !bytes.Equal(got, want) {
+		t.Errorf("next request: Get() = %d bytes, %s; want the %d bytes, %s",
+			len(got), status, len(want), CacheHit)
+	}
+}
+
+// TestService_Get_HitWaitsForNoDatabaseWrite holds the database's one
+// connection while a cached variant is requested. The hit must be answered
+// while the connection is still held, and its access time and hit count
+// must reach the database once it is free.
+func TestService_Get_HitWaitsForNoDatabaseWrite(t *testing.T) {
+	t.Parallel()
+
+	svc, fixtures := SetupTestService(t)
+	req := photoVariant(fixtures, 85, imagerequest.FitCover)
+	key := imagerequest.CacheKey(req)
+
+	getImage(t, svc, req)
+	waitForPendingCountsWritten(t, svc.cache)
+
+	anHourAgo := time.Now().Add(-time.Hour)
+	setVariantLastAccessed(t, svc.cache, key, anHourAgo)
+
+	want := readCacheStatsCounters(t, svc.cache)
+	want.hitCount++
+
+	releaseDatabase := holdDatabase(t, svc.cache)
+	defer releaseDatabase()
+
+	results := startGet(t.Context(), svc, req)
+
+	select {
+	case got := <-results:
+		if got.err != nil {
+			t.Fatalf("Get() of the cached variant error = %v", got.err)
+		}
+	case <-time.After(pendingCountsWait):
+		t.Fatal("the cache hit waited for the database")
+	}
+
+	releaseDatabase()
+
+	waitForCounters(t, svc.cache, want)
+
+	deadline := time.Now().Add(pendingCountsWait)
+
+	for countRows(t, svc.cache, `
+		SELECT COUNT(*) FROM variant_content
+		WHERE cache_key = ? AND last_accessed_at > ?
+	`, string(key), anHourAgo.UTC().Format(sqliteTimestampFormat)) == 0 {
+		if time.Now().After(deadline) {
+			t.Fatal("the hit's access time was not written")
+		}
+
+		time.Sleep(10 * time.Millisecond)
+	}
+}
+
+// TestTouchVariantAndTouchSourceContent_DropAccessTimesPastTheLimit hands one
+// more access time of a variant, and one more of a source, than may wait to
+// be written. touchVariant and touchSourceContent must return each time
+// without waiting, and the last of each must be dropped.
+func TestTouchVariantAndTouchSourceContent_DropAccessTimesPastTheLimit(t *testing.T) {
+	t.Parallel()
+
+	cache, _ := newEvictionTestCache(t, 1<<30)
+
+	done := make(chan struct{})
+
+	go func() {
+		defer close(done)
+
+		for range maxPendingAccessTimes + 1 {
+			cache.touchVariant(testVariantKeyOne)
+			cache.touchSourceContent("aabbccdd3001")
+		}
+	}()
+
+	select {
+	case <-done:
+	case <-time.After(pendingCountsWait):
+		t.Fatal("touchVariant or touchSourceContent waited once the access " +
+			"times reached the limit")
+	}
+
+	if waiting := len(cache.pendingVariantAccessTimes); waiting != maxPendingAccessTimes {
+		t.Errorf("%d access times of variants wait to be written, want %d",
+			waiting, maxPendingAccessTimes)
+	}
+
+	if waiting := len(cache.pendingSourceAccessTimes); waiting != maxPendingAccessTimes {
+		t.Errorf("%d access times of sources wait to be written, want %d",
+			waiting, maxPendingAccessTimes)
+	}
+}
+
+// TestStopPendingCountWrites_WritesPendingAccessTimes hands the access time
+// of one of two variants, three times, then stops the goroutine that writes
+// them, which never started. The stop must write that access time and
+// leave the other variant's.
+func TestStopPendingCountWrites_WritesPendingAccessTimes(t *testing.T) {
+	t.Parallel()
+
+	cache, _ := newEvictionTestCache(t, 1<<30)
+	anHourAgo := time.Now().Add(-time.Hour)
+
+	for _, key := range []imagerequest.VariantKey{testVariantKeyOne, testVariantKeyTwo} {
+		storeEvictionTestVariant(t, cache, key, []byte("variant bytes"))
+		setVariantLastAccessed(t, cache, key, anHourAgo)
+	}
+
+	for range 3 {
+		cache.touchVariant(testVariantKeyOne)
+	}
+
+	err := cache.StopPendingCountWrites(t.Context())
+	if err != nil {
+		t.Fatalf("StopPendingCountWrites() error = %v", err)
+	}
+
+	for key, wantTouched := range map[imagerequest.VariantKey]int{
+		testVariantKeyOne: 1, testVariantKeyTwo: 0,
+	} {
+		touched := countRows(t, cache, `
+			SELECT COUNT(*) FROM variant_content
+			WHERE cache_key = ? AND last_accessed_at > ?
+		`, string(key), anHourAgo.UTC().Format(sqliteTimestampFormat))
+		if touched != wantTouched {
+			t.Errorf("variant %s: %d rows with a new access time, want %d",
+				key, touched, wantTouched)
+		}
+	}
+
+	if waiting := len(cache.pendingVariantAccessTimes); waiting != 0 {
+		t.Errorf("%d access times still wait to be written, want 0", waiting)
+	}
+}