Build the image pipeline in pixad's fx graph (closes #313)

internal/app now provides the cache, the fetcher, the processor, the
image service, the session manager and the encrypted URL generator, and
handlers.New receives them built. database.New opens the database, so
these parts can read it while fx builds them; the cache's stop hook
still runs before the database's.

Each setting's default is declared in internal/config, the default
cache_max_bytes included, which internal/app works out before it builds
the cache. The cache's MaxBytes is the one value for the disk cache
limit, and 0 turns the disk cache off. The fetcher and the processor
take explicit values. The server tests build the pipeline themselves,
as they cannot import internal/app.

Model: opus-5-5

internal/imgcache/cache.go

@@ -32,23 +32,12 @@ type CacheConfig struct {
 	NegativeTTL time.Duration
 
 	// MaxBytes is the disk cache size limit in bytes that eviction
-	// enforces. Zero means no limit is enforced (no eviction).
+	// enforces. Zero turns the disk cache off entirely: no cache
+	// directories are created, lookups always miss, stores are no-ops,
+	// and the eviction goroutine only removes the expired negative cache
+	// rows.
 	MaxBytes int64
 
-	// UseDefaultMaxBytes makes NewCache replace MaxBytes with the
-	// default limit: 75% of the sum of the space free on the filesystem
-	// holding the cache and the bytes the cache already holds, at least
-	// DefaultCacheMaxBytesFloor. The config layer sets this when the
-	// operator omits cache_max_bytes.
-	UseDefaultMaxBytes bool
-
-	// DisableDiskCache turns the disk cache off entirely: no cache
-	// directories are created, lookups always miss, stores are
-	// no-ops, and the eviction goroutine only removes the expired
-	// negative cache rows. The config layer sets this when the
-	// operator configures cache_max_bytes: 0.
-	DisableDiskCache bool
-
 	// Logger receives accounting and eviction log output. A nil
 	// Logger means slog.Default().
 	Logger *slog.Logger
@@ -62,10 +51,6 @@ type Cache struct {
 	config     CacheConfig
 	log        *slog.Logger
 
-	// disabled means the disk cache is turned off entirely: lookups
-	// always miss, stores are no-ops, and no eviction runs.
-	disabled bool
-
 	// Eviction machinery. The channels are created in NewCache so
 	// stores can signal write pressure without racing StartEviction.
 	// evictionCancel, set by StartEviction, cancels the eviction
@@ -110,20 +95,9 @@ type Cache struct {
 
 // NewCache creates a new cache instance and registers its counters on
 // metricsRegistry. pixad passes Prometheus' default registry, the one
-// /metrics serves.
+// /metrics serves. It makes no database call.
 func NewCache(
 	db *sql.DB, config CacheConfig, metricsRegistry prometheus.Registerer,
-) (*Cache, error) {
-	return newCache(db, config, metricsRegistry, defaultFreeSpaceProbe)
-}
-
-// newCache is NewCache with the free-space probe passed in, so tests
-// can fake the free space the default limit is worked out from.
-func newCache(
-	db *sql.DB,
-	config CacheConfig,
-	metricsRegistry prometheus.Registerer,
-	probe FreeSpaceProbeFunc,
 ) (*Cache, error) {
 	log := config.Logger
 	if log == nil {
@@ -134,7 +108,6 @@ func newCache(
 		db:               db,
 		config:           config,
 		log:              log,
-		disabled:         config.DisableDiskCache,
 		evictionPressure: make(chan struct{}, 1),
 		evictionDone:     make(chan struct{}),
 
@@ -150,7 +123,7 @@ func newCache(
 		return nil, err
 	}
 
-	if c.disabled {
+	if config.MaxBytes == 0 {
 		return c, nil
 	}
 
@@ -171,20 +144,6 @@ func newCache(
 	c.srcContent = srcContent
 	c.variants = variants
 
-	if config.UseDefaultMaxBytes {
-		limit, err := c.computeDefaultMaxBytes(context.Background(), probe)
-		if err != nil {
-			return nil, err
-		}
-
-		c.config.MaxBytes = limit
-
-		log.Info("computed default cache size limit from free space and cache contents",
-			"cache_max_bytes", limit,
-			"cache_dir", filepath.Join(config.StateDir, "cache"),
-		)
-	}
-
 	return c, nil
 }
 
@@ -196,7 +155,7 @@ func newCache(
 func (c *Cache) GetVariant(
 	cacheKey imagerequest.VariantKey,
 ) (io.ReadCloser, int64, string, error) {
-	if c.disabled {
+	if c.config.MaxBytes == 0 {
 		return nil, 0, "", ErrNotFound
 	}
 
@@ -214,7 +173,7 @@ func (c *Cache) StoreSource(
 	req *imagerequest.Request,
 	content io.Reader,
 ) (imagerequest.SourceKey, error) {
-	if c.disabled {
+	if c.config.MaxBytes == 0 {
 		return "", nil
 	}
 
@@ -259,7 +218,7 @@ func (c *Cache) StoreSource(
 func (c *Cache) StoreVariant(
 	ctx context.Context, cacheKey imagerequest.VariantKey, content io.Reader,
 ) error {
-	if c.disabled {
+	if c.config.MaxBytes == 0 {
 		return nil
 	}
 
@@ -293,7 +252,7 @@ func (c *Cache) StoreVariant(
 func (c *Cache) LookupSource(
 	ctx context.Context, req *imagerequest.Request,
 ) (imagerequest.SourceKey, error) {
-	if c.disabled {
+	if c.config.MaxBytes == 0 {
 		return "", nil
 	}
 
@@ -345,7 +304,7 @@ func (c *Cache) StoreNegative(ctx context.Context, req *imagerequest.Request) er
 func (c *Cache) GetSourceContent(
 	key imagerequest.SourceKey,
 ) (io.ReadCloser, int64, error) {
-	if c.disabled {
+	if c.config.MaxBytes == 0 {
 		return nil, 0, ErrNotFound
 	}