Count hits, misses, fetches and transforms as Prometheus counters (closes #311)

The five counts the cache_stats table held are now Prometheus counters in
memory, as fields of the cache. NewCache makes them and registers them on
the registry it is given. pixad gives it Prometheus' default registry,
which /metrics serves; each test gives it a new one. The Increment methods
add to their cache's counters, so counting makes no database call.

cache_stats, Cache.Stats, Service.Stats, CacheStats and the pending counts
are gone, with their tests. The background goroutine now writes only access
times, and its names say so: StartPendingAccessTimeWrites,
StopPendingAccessTimeWrites and pendingaccesstimes.go. The two stop tests
whose write under way was a count now use access times.

Model: opus-5-5

internal/handlers/cache_hit_internal_test.go

@@ -17,6 +17,7 @@ import (
 	"time"
 
 	"github.com/go-chi/chi/v5"
+	"github.com/prometheus/client_golang/prometheus"
 
 	"sneak.berlin/go/pixa/internal/database"
 	"sneak.berlin/go/pixa/internal/encurl"
@@ -101,9 +102,8 @@ func newCountingDatabase(tb testing.TB) (*sql.DB, *atomic.Int64) {
 // newCountingDatabaseServer returns a router for both image routes whose
 // image cache uses newCountingDatabase, the count of the statements run on
 // that database, and a URL on each route for a different image of
-// photoPath on countingHost. Nothing writes the cache's pending counts and
-// access times in the background, so every statement counted comes from a
-// request.
+// photoPath on countingHost. Nothing writes the cache's access times in the
+// background, so every statement counted comes from a request.
 func newCountingDatabaseServer(
 	tb testing.TB,
 ) (http.Handler, *atomic.Int64, []string) {
@@ -121,7 +121,7 @@ func newCountingDatabaseServer(
 	cache, err := imgcache.NewCache(db, imgcache.CacheConfig{
 		StateDir:    tb.TempDir(),
 		NegativeTTL: 5 * time.Minute,
-	})
+	}, prometheus.NewRegistry())
 	if err != nil {
 		tb.Fatalf("imgcache.NewCache() error = %v", err)
 	}

internal/handlers/cache_max_bytes_internal_test.go

@@ -5,6 +5,7 @@ import (
 	"path/filepath"
 	"testing"
 
+	"github.com/prometheus/client_golang/prometheus"
 	"go.uber.org/fx/fxtest"
 
 	"sneak.berlin/go/pixa/internal/config"
@@ -127,7 +128,10 @@ func TestDiskCacheOffOnlyForExplicitZeroCacheMaxBytes(t *testing.T) {
 				t.Fatalf("database.New() error = %v", err)
 			}
 
-			_, err = New(lc, Params{Logger: log, Database: db, Config: cfg})
+			_, err = New(lc, Params{
+				Logger: log, Database: db, Config: cfg,
+				MetricsRegistry: prometheus.NewRegistry(),
+			})
 			if err != nil {
 				t.Fatalf("New() error = %v", err)
 			}

internal/handlers/fetcher_internal_test.go

@@ -8,6 +8,7 @@ import (
 	"time"
 
 	"github.com/go-chi/chi/v5"
+	"github.com/prometheus/client_golang/prometheus"
 	"go.uber.org/fx"
 	"go.uber.org/fx/fxtest"
 
@@ -47,7 +48,8 @@ func TestHandlersBuildTheirOwnFetcherWhenNoneIsProvided(t *testing.T) {
 
 	app := fxtest.New(t,
 		fx.Supply(cfg),
-		fx.Provide(globals.New, logger.New, database.New, healthcheck.New, New),
+		fx.Provide(globals.New, logger.New, database.New, healthcheck.New, New,
+			func() prometheus.Registerer { return prometheus.NewRegistry() }),
 		fx.Populate(&h),
 	)
 	app.RequireStart()

internal/handlers/first_bytes_internal_test.go

@@ -12,6 +12,7 @@ import (
 	"time"
 
 	"github.com/go-chi/chi/v5"
+	"github.com/prometheus/client_golang/prometheus"
 	"sneak.berlin/go/pixa/internal/encurl"
 	"sneak.berlin/go/pixa/internal/httpfetcher"
 	"sneak.berlin/go/pixa/internal/imagerequest"
@@ -35,7 +36,7 @@ func newRoutesFetchingThrough(
 	cache, err := imgcache.NewCache(setupTestDB(t), imgcache.CacheConfig{
 		StateDir:    t.TempDir(),
 		NegativeTTL: 5 * time.Minute,
-	})
+	}, prometheus.NewRegistry())
 	if err != nil {
 		t.Fatalf("imgcache.NewCache() error = %v", err)
 	}

internal/handlers/handlers.go

@@ -9,6 +9,7 @@ import (
 	"net/http"
 	"time"
 
+	"github.com/prometheus/client_golang/prometheus"
 	"go.uber.org/fx"
 	"sneak.berlin/go/pixa/internal/allowlist"
 	"sneak.berlin/go/pixa/internal/config"
@@ -30,6 +31,11 @@ type Params struct {
 	Database    *database.Database
 	Config      *config.Config
 
+	// MetricsRegistry is the registry the image cache registers its
+	// counters on. pixad provides Prometheus' default registry, the one
+	// /metrics serves.
+	MetricsRegistry prometheus.Registerer
+
 	// Fetcher, when provided, fetches upstream images in place of the
 	// fetcher the handlers build from the config. Only tests provide one;
 	// pixad does not.
@@ -52,6 +58,10 @@ type Handlers struct {
 	// refererBlocklist matches the hosts of referer_blocklist; its IsAllowed
 	// reports whether a URL's host is on that list.
 	refererBlocklist *allowlist.HostAllowList
+
+	// metricsRegistry is the registry the image cache registers its
+	// counters on.
+	metricsRegistry prometheus.Registerer
 }
 
 // New creates a new Handlers instance.
@@ -69,6 +79,7 @@ func New(lc fx.Lifecycle, params Params) (*Handlers, error) {
 		fetcher:          params.Fetcher,
 		csrfProtect:      csrfProtect,
 		refererBlocklist: allowlist.New(params.Config.RefererBlocklist),
+		metricsRegistry:  params.MetricsRegistry,
 	}
 
 	lc.Append(fx.Hook{
@@ -76,8 +87,8 @@ func New(lc fx.Lifecycle, params Params) (*Handlers, error) {
 		OnStart: func(_ context.Context) error {
 			return s.initImageService()
 		},
-		// The pending counts and access times are written here, before the
-		// database's stop hook closes it.
+		// The pending access times are written here, before the database's
+		// stop hook closes it.
 		OnStop: func(ctx context.Context) error {
 			if s.imgCache == nil {
 				return nil
@@ -85,7 +96,7 @@ func New(lc fx.Lifecycle, params Params) (*Handlers, error) {
 
 			return errors.Join(
 				s.imgCache.StopEviction(ctx),
-				s.imgCache.StopPendingCountWrites(ctx),
+				s.imgCache.StopPendingAccessTimeWrites(ctx),
 			)
 		},
 	})
@@ -116,7 +127,8 @@ func newCacheConfig(cfg *config.Config, log *slog.Logger) imgcache.CacheConfig {
 
 // initImageService initializes the image cache and service.
 func (s *Handlers) initImageService() error {
-	cache, err := imgcache.NewCache(s.db.DB(), newCacheConfig(s.config, s.log))
+	cache, err := imgcache.NewCache(
+		s.db.DB(), newCacheConfig(s.config, s.log), s.metricsRegistry)
 	if err != nil {
 		return err
 	}
@@ -128,9 +140,9 @@ func (s *Handlers) initImageService() error {
 	// rows, which is all it does when the disk cache is disabled.
 	cache.StartEviction(imgcache.DefaultEvictionInterval)
 
-	// Writes the counts and the access times of cache hits that requests
-	// hand over, so that no request waits for the database to record them
-	cache.StartPendingCountWrites()
+	// Writes the access times that requests hand over, so that no request
+	// waits for the database to record them
+	cache.StartPendingAccessTimeWrites()
 
 	// Create the fetcher config
 	fetcherCfg := httpfetcher.DefaultConfig()

internal/handlers/handlers_internal_test.go

@@ -18,6 +18,7 @@ import (
 	"time"
 
 	"github.com/go-chi/chi/v5"
+	"github.com/prometheus/client_golang/prometheus"
 	"sneak.berlin/go/pixa/internal/database"
 	"sneak.berlin/go/pixa/internal/httpfetcher"
 	"sneak.berlin/go/pixa/internal/imgcache"
@@ -48,7 +49,7 @@ func setupTestHandler(t *testing.T) *testFixtures {
 	cache, err := imgcache.NewCache(db, imgcache.CacheConfig{
 		StateDir:    tmpDir,
 		NegativeTTL: 5 * time.Minute,
-	})
+	}, prometheus.NewRegistry())
 	if err != nil {
 		t.Fatalf("failed to create cache: %v", err)
 	}

internal/handlers/image_cache_control_internal_test.go

@@ -12,6 +12,7 @@ import (
 	"time"
 
 	"github.com/go-chi/chi/v5"
+	"github.com/prometheus/client_golang/prometheus"
 
 	"sneak.berlin/go/pixa/internal/encurl"
 	"sneak.berlin/go/pixa/internal/imagerequest"
@@ -33,7 +34,7 @@ func newSignedHostServer(
 	cache, err := imgcache.NewCache(setupTestDB(t), imgcache.CacheConfig{
 		StateDir:    t.TempDir(),
 		NegativeTTL: 5 * time.Minute,
-	})
+	}, prometheus.NewRegistry())
 	if err != nil {
 		t.Fatalf("imgcache.NewCache() error = %v", err)
 	}

internal/handlers/image_errors_internal_test.go

@@ -13,6 +13,7 @@ import (
 	"time"
 
 	"github.com/go-chi/chi/v5"
+	"github.com/prometheus/client_golang/prometheus"
 	"sneak.berlin/go/pixa/internal/httpfetcher"
 	"sneak.berlin/go/pixa/internal/imagerequest"
 	"sneak.berlin/go/pixa/internal/imgcache"
@@ -31,7 +32,7 @@ func newImageRoute(t *testing.T, fetcher httpfetcher.Fetcher) http.Handler {
 	cache, err := imgcache.NewCache(setupTestDB(t), imgcache.CacheConfig{
 		StateDir:    t.TempDir(),
 		NegativeTTL: 5 * time.Minute,
-	})
+	}, prometheus.NewRegistry())
 	if err != nil {
 		t.Fatalf("failed to create cache: %v", err)
 	}

internal/handlers/referer_blocklist_internal_test.go

@@ -10,6 +10,7 @@ import (
 	"time"
 
 	"github.com/go-chi/chi/v5"
+	"github.com/prometheus/client_golang/prometheus"
 	"sneak.berlin/go/pixa/internal/allowlist"
 	"sneak.berlin/go/pixa/internal/encurl"
 	"sneak.berlin/go/pixa/internal/httpfetcher"
@@ -50,7 +51,7 @@ func newRefererRoutes(t *testing.T) (http.Handler, *Handlers, *countingFetcher)
 	cache, err := imgcache.NewCache(setupTestDB(t), imgcache.CacheConfig{
 		StateDir:    t.TempDir(),
 		NegativeTTL: 5 * time.Minute,
-	})
+	}, prometheus.NewRegistry())
 	if err != nil {
 		t.Fatalf("imgcache.NewCache() error = %v", err)
 	}