Check cached file sizes at startup, not per request (closes #252)

Lookup and LookupSource are as on next again: requests compare no
sizes and a hit reads no extra row. The reconciliation pass, at startup
and every five minutes, compares each cached source and variant file
with the size its row records and removes one that is empty or of
another size, with its rows and a warning naming the file; an empty
variant file without a row is removed rather than adopted. Source,
variant and .meta files all go through writeFileSynced (temp file,
sync, rename, directory sync), the .meta file before the variant, and
a failed directory sync removes the renamed file.

#252

Model: opus-5-5

README.md

@@ -160,11 +160,13 @@ name.
 Multiple source paths may reference the same content blob; the database tracks
 references rather than using filesystem refcounting.
 
-Source and transformed image files are written under a temporary name, synced to
+Source and transformed image files, and the `.meta` file beside each transformed
-disk, and only then renamed into place, so a crash leaves no file under its own
+image, are written under a temporary name, synced to disk, and only then renamed
-name with part of its data. A request that finds a cached source or transformed
+into place, so a crash leaves no file under its own name with part of its data.
-image whose file is empty, or not the size the database records for it, removes
+When pixa starts, and every five minutes after, it removes each cached source or
-it and fetches the source or converts the image again.
+transformed image whose file is empty, or not the size the database records for
+it, with a warning naming the file; the next request for it fetches the source
+or converts the image again.
 
 pixa's target is 1-5k r/s, which has not been measured at that rate (see Load
 Test). Toward it, pixa keeps in memory the content types of the 10,000

TODO.md

@@ -38,6 +38,18 @@ P2: security: per-origin rate limiting
   and the query now have each `%` written as `%25` and each `:` as `%3A`; a path
   and query holding neither are signed and keyed as before. Signature
   Specification in `README.md` says so.
+- 2026-10-15 a cached file left empty or partial is no longer served, or fetched
+  again on every request (closes #252). Source and transformed image files, and
+  each transformed image's `.meta` file, are synced to disk before they are
+  renamed into place, and their directory after; the `.meta` file is written
+  before the image, and a failed directory sync removes the renamed file. The
+  reconciliation pass, at startup and every five minutes, removes a cached
+  source or transformed image whose file is empty, or not the size the database
+  records for it, with its rows and a warning naming the file, so the next
+  request fetches the source or converts the image again; requests compare no
+  sizes. Storing a source over a file of another size replaces that file. A
+  source that fails to decode is not removed for that alone: a damaged upstream
+  image would then be fetched on every request.
 - 2026-10-15 the example config allowlists no host (closes #249): it allowlisted
   `github.com` and `user-images.githubusercontent.com`, where anyone can put an
   image, so a deployment set up as Getting Started says served those images to

internal/imgcache/cache.go

@@ -222,13 +222,13 @@ type LookupResult struct {
 	CacheStatus CacheStatus
 }
 
-// Lookup checks if a processed variant is on disk whole (see
+// Lookup checks if a processed variant exists on disk. Hits touch the
-// variantFileIsWhole). Hits touch the variant's LRU timestamp; a disabled
+// variant's LRU timestamp; a disabled cache always misses.
-// cache always misses.
 func (c *Cache) Lookup(ctx context.Context, req *ImageRequest) (*LookupResult, error) {
 	cacheKey := CacheKey(req)
 
-	if !c.disabled && c.variantFileIsWhole(ctx, cacheKey) {
+	// Check variant storage directly - no DB needed for cache hits
+	if !c.disabled && c.variants.Exists(cacheKey) {
 		c.touchVariant(ctx, cacheKey)
 
 		return &LookupResult{
@@ -393,11 +393,8 @@ func (c *Cache) StoreVariant(
 
 // LookupSource checks if we have cached source content for a request.
 // Returns the content hash and content type if found, or empty values
-// if not. A content file that is empty, or not the size its
+// if not. Hits touch the blob's LRU timestamp; a disabled cache always
-// source_content row records, is damaged: it is removed with its rows,
+// reports no cached source.
-// and no cached source is reported, so the source is fetched again. Hits
-// touch the blob's LRU timestamp; a disabled cache always reports no
-// cached source.
 func (c *Cache) LookupSource(
 	ctx context.Context, req *ImageRequest,
 ) (ContentHash, string, error) {
@@ -405,18 +402,12 @@ func (c *Cache) LookupSource(
 		return "", "", nil
 	}
 
-	var (
+	var hashStr, contentType string
-		hashStr, contentType string
-		recordedSize         int64
-	)
 
 	err := c.db.QueryRowContext(ctx, `
-		SELECT source_metadata.content_hash, source_metadata.content_type,
+		SELECT content_hash, content_type FROM source_metadata
-		       source_content.size_bytes
-		FROM source_metadata JOIN source_content USING (content_hash)
 		WHERE source_host = ? AND source_path = ? AND source_query = ?
-	`, req.SourceHost, req.SourcePath, req.SourceQuery).
+	`, req.SourceHost, req.SourcePath, req.SourceQuery).Scan(&hashStr, &contentType)
-		Scan(&hashStr, &contentType, &recordedSize)
 
 	if errors.Is(err, sql.ErrNoRows) {
 		return "", "", nil
@@ -428,24 +419,8 @@ func (c *Cache) LookupSource(
 
 	contentHash := ContentHash(hashStr)
 
-	fileSize, err := c.srcContent.Size(contentHash)
+	// Verify the content file exists
-	if errors.Is(err, ErrNotFound) {
+	if !c.srcContent.Exists(contentHash) {
-		return "", "", nil
-	}
-
-	if err != nil {
-		return "", "", err
-	}
-
-	if fileSize == 0 || fileSize != recordedSize {
-		c.log.Warn("removing damaged cached source", "content_hash", contentHash,
-			"file_bytes", fileSize, "recorded_bytes", recordedSize)
-
-		err = c.evictSourceBlob(ctx, contentHash)
-		if err != nil {
-			return "", "", fmt.Errorf("failed to remove damaged source: %w", err)
-		}
-
 		return "", "", nil
 	}
 
@@ -668,10 +643,10 @@ func (c *Cache) readHitAndMissCounts(ctx context.Context, stats *CacheStats) err
 // hold: it reads the content type from the variant's .meta file and
 // keeps it in metaCache, unless a StoreVariant has put one there
 // meanwhile, as the store's is newer. A read that finds no .meta file,
-// as one can between a store's writing of the variant file and of its
+// as one can when the store's .meta write failed or an eviction removed
-// .meta file, serves application/octet-stream and keeps nothing, so
+// the file after the read opened the variant, serves
-// metaCache only ever holds a type read from a .meta file or passed to
+// application/octet-stream and keeps nothing, so metaCache only ever
-// StoreVariant.
+// holds a type read from a .meta file or passed to StoreVariant.
 func (c *Cache) loadVariantWithMeta(
 	cacheKey VariantKey,
 ) (io.ReadCloser, int64, string, error) {
@@ -719,43 +694,6 @@ func (c *Cache) writeMetadataSidecar(
 	}
 }
 
-// variantFileIsWhole reports whether the variant file of cacheKey is on
-// disk with the size its variant_content row records. A file that is
-// empty or of another size is damaged: it is removed with its row, so the
-// variant is converted again. A variant whose row cannot be read, as
-// between StoreVariant's writing of its file and of its row, is judged
-// by its file alone.
-func (c *Cache) variantFileIsWhole(ctx context.Context, cacheKey VariantKey) bool {
-	fileSize, err := c.variants.Size(cacheKey)
-	if err != nil {
-		return false
-	}
-
-	var recordedSize int64
-
-	err = c.db.QueryRowContext(ctx,
-		`SELECT size_bytes FROM variant_content WHERE cache_key = ?`,
-		string(cacheKey)).Scan(&recordedSize)
-	if err != nil {
-		recordedSize = fileSize
-	}
-
-	if fileSize > 0 && fileSize == recordedSize {
-		return true
-	}
-
-	c.log.Warn("removing damaged cached variant", "cache_key", cacheKey,
-		"file_bytes", fileSize, "recorded_bytes", recordedSize)
-
-	err = c.evictVariant(ctx, cacheKey)
-	if err != nil {
-		c.log.Warn("failed to remove damaged cached variant",
-			"cache_key", cacheKey, "error", err)
-	}
-
-	return false
-}
-
 // touchVariant updates the LRU timestamp of a variant, best-effort:
 // a failed touch only makes the entry look colder to eviction.
 func (c *Cache) touchVariant(ctx context.Context, cacheKey VariantKey) {

internal/imgcache/eviction.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"database/sql"
 	"encoding/json"
+	"errors"
 	"fmt"
 	"io/fs"
 	"os"
@@ -542,10 +543,11 @@ func (c *Cache) runReconciliationPass(ctx context.Context) {
 // (or whose accounting insert failed, e.g. StoreVariant's best-effort
 // insert under transient DB contention), drops accounting rows whose
 // files are missing, removes source blob files the database does not
-// know (and rows whose files are gone), sweeps stale temp files left
+// know (and rows whose files are gone), removes source and variant files
-// behind by crashed writes, and last checks the total cache usage
+// that are empty or not the size their rows record, sweeps stale temp
-// against the tables. It reads the tables a page at a time. Running it
+// files left behind by crashed writes, and last checks the total cache
-// periodically, not just once, bounds how long such drift can
+// usage against the tables. It reads the tables a page at a time.
+// Running it periodically, not just once, bounds how long such drift can
 // accumulate unaccounted for on a long-running process to one eviction
 // interval. Once ctx is cancelled, it stops at the next file or row and
 // returns ctx's error.
@@ -577,8 +579,9 @@ func (c *Cache) reconcileAccounting(ctx context.Context) error {
 	return c.reconcileUsageTotal(ctx)
 }
 
-// reconcileVariantFiles walks the variant storage directory, adopting
+// reconcileVariantFiles walks the variant storage directory, checking
-// files without accounting rows and sweeping stale temp files.
+// each variant file against its accounting row (reconcileVariantFile)
+// and sweeping stale temp files.
 func (c *Cache) reconcileVariantFiles(ctx context.Context) error {
 	return filepath.WalkDir(
 		c.variants.baseDir,
@@ -603,38 +606,55 @@ func (c *Cache) reconcileVariantFiles(ctx context.Context) error {
 				return nil
 			}
 
-			return c.adoptVariantFile(ctx, path, entry, VariantKey(name))
+			return c.reconcileVariantFile(ctx, path, entry, VariantKey(name))
 		},
 	)
 }
 
-// adoptVariantFile inserts an accounting row for a variant file that
+// reconcileVariantFile checks one variant file against its accounting
-// has none, using the file's size and modification time.
+// row. A file without a row is adopted into the accounting, unless it is
-func (c *Cache) adoptVariantFile(
+// empty. A file that is empty, or not the size its row records, as a
+// crash or a restored snapshot can leave it, is removed with its row and
+// a warning naming it, so the variant is converted again.
+func (c *Cache) reconcileVariantFile(
 	ctx context.Context, path string, entry fs.DirEntry, cacheKey VariantKey,
 ) error {
-	var rowExists int
-
-	err := c.db.QueryRowContext(ctx,
-		`SELECT COUNT(*) FROM variant_content WHERE cache_key = ?`, string(cacheKey),
-	).Scan(&rowExists)
-	if err != nil {
-		return fmt.Errorf("failed to check variant accounting row: %w", err)
-	}
-
-	if rowExists > 0 {
-		return nil
-	}
-
 	info, err := entry.Info()
 	if err != nil {
 		return fmt.Errorf("failed to stat variant file: %w", err)
 	}
 
+	var recordedSize int64
+
+	err = c.db.QueryRowContext(ctx,
+		`SELECT size_bytes FROM variant_content WHERE cache_key = ?`, string(cacheKey),
+	).Scan(&recordedSize)
+
+	switch {
+	case errors.Is(err, sql.ErrNoRows) && info.Size() > 0:
+		return c.adoptVariantFile(ctx, path, info, cacheKey)
+	case err != nil && !errors.Is(err, sql.ErrNoRows):
+		return fmt.Errorf("failed to check variant accounting row: %w", err)
+	case info.Size() > 0 && info.Size() == recordedSize:
+		return nil
+	}
+
+	// The file is empty, or not the size its row records
+	c.log.Warn("removing damaged cached variant file", "path", path,
+		"file_bytes", info.Size(), "recorded_bytes", recordedSize)
+
+	return c.evictVariant(ctx, cacheKey)
+}
+
+// adoptVariantFile inserts an accounting row for a variant file that
+// has none, using the file's size and modification time.
+func (c *Cache) adoptVariantFile(
+	ctx context.Context, path string, info fs.FileInfo, cacheKey VariantKey,
+) error {
 	modTime := info.ModTime().UTC().Format(sqliteTimestampLayout)
 	contentType := c.variantContentTypeFromSidecar(path)
 
-	_, err = c.db.ExecContext(ctx, `
+	_, err := c.db.ExecContext(ctx, `
 		INSERT INTO variant_content
 			(cache_key, size_bytes, content_type, created_at, last_accessed_at)
 		VALUES (?, ?, ?, ?, ?)
@@ -753,10 +773,9 @@ func queryStringColumn[T ~string](
 	return values, nil
 }
 
-// reconcileSourceFiles walks the source content directory, removing
+// reconcileSourceFiles walks the source content directory, checking
-// blob files the database does not track (they are unreachable: source
+// each blob file against its source_content row (reconcileSourceFile)
-// lookups always go through source_metadata) and sweeping stale temp
+// and sweeping stale temp files.
-// files.
 func (c *Cache) reconcileSourceFiles(ctx context.Context) error {
 	return filepath.WalkDir(
 		c.srcContent.baseDir,
@@ -777,31 +796,55 @@ func (c *Cache) reconcileSourceFiles(ctx context.Context) error {
 				return nil
 			}
 
-			return c.removeUntrackedSourceFile(ctx, path, ContentHash(name))
+			return c.reconcileSourceFile(ctx, path, entry, ContentHash(name))
 		},
 	)
 }
 
+// reconcileSourceFile checks one source blob file against its
+// source_content row. A file without a row is removed: it is
+// unreachable, as source lookups always go through source_metadata. A
+// file that is empty, or not the size its row records, as a crash or a
+// restored snapshot can leave it, is removed with its rows and a warning
+// naming it, so the source is fetched again.
+func (c *Cache) reconcileSourceFile(
+	ctx context.Context, path string, entry fs.DirEntry, contentHash ContentHash,
+) error {
+	info, err := entry.Info()
+	if err != nil {
+		return fmt.Errorf("failed to stat source content file: %w", err)
+	}
+
+	var recordedSize int64
+
+	err = c.db.QueryRowContext(ctx,
+		`SELECT size_bytes FROM source_content WHERE content_hash = ?`,
+		string(contentHash),
+	).Scan(&recordedSize)
+
+	switch {
+	case errors.Is(err, sql.ErrNoRows):
+		return c.removeUntrackedSourceFile(ctx, path, contentHash)
+	case err != nil:
+		return fmt.Errorf("failed to check source content row: %w", err)
+	case info.Size() > 0 && info.Size() == recordedSize:
+		return nil
+	}
+
+	// The file is empty, or not the size its row records
+	c.log.Warn("removing damaged cached source file", "path", path,
+		"file_bytes", info.Size(), "recorded_bytes", recordedSize)
+
+	return c.evictSourceBlob(ctx, contentHash)
+}
+
 // removeUntrackedSourceFile deletes a source blob file that has no
 // source_content row. Any source_metadata rows referencing the hash
 // are removed first so no row ever points at a deleted file.
 func (c *Cache) removeUntrackedSourceFile(
 	ctx context.Context, path string, contentHash ContentHash,
 ) error {
-	var rowExists int
+	_, err := c.db.ExecContext(ctx,
-
-	err := c.db.QueryRowContext(ctx,
-		`SELECT COUNT(*) FROM source_content WHERE content_hash = ?`, string(contentHash),
-	).Scan(&rowExists)
-	if err != nil {
-		return fmt.Errorf("failed to check source content row: %w", err)
-	}
-
-	if rowExists > 0 {
-		return nil
-	}
-
-	_, err = c.db.ExecContext(ctx,
 		`DELETE FROM source_metadata WHERE content_hash = ?`, string(contentHash))
 	if err != nil {
 		return fmt.Errorf("failed to delete metadata rows for untracked blob: %w", err)

internal/imgcache/service.go

@@ -148,7 +148,7 @@ func (s *Service) Get(ctx context.Context, req *ImageRequest) (*ImageResponse, e
 		return nil, fmt.Errorf("%w: %w", httpfetcher.ErrUpstreamError, ErrNegativeCached)
 	}
 
-	// Check variant cache first
+	// Check variant cache first (disk only, no DB)
 	result, err := s.cache.Lookup(ctx, req)
 	if err != nil {
 		s.log.Warn("cache lookup failed", "error", err)

internal/imgcache/storage.go

@@ -152,24 +152,8 @@ func (s *ContentStorage) Exists(hash ContentHash) bool {
 	return err == nil
 }
 
-// Size returns the size in bytes of the content with the given hash.
+// writeIfAbsent writes data with writeFileSynced to the path derived
-func (s *ContentStorage) Size(hash ContentHash) (int64, error) {
+// from hash, unless a file of the data's size is already there.
-	info, err := os.Stat(s.hashToPath(hash))
-	if err != nil {
-		if os.IsNotExist(err) {
-			return 0, ErrNotFound
-		}
-
-		return 0, fmt.Errorf("failed to stat content: %w", err)
-	}
-
-	return info.Size(), nil
-}
-
-// writeIfAbsent writes data to the path derived from hash, unless a
-// file of the data's size is already there. It writes a temp file,
-// syncs it to disk and renames it into place, so neither a concurrent
-// reader nor a crash finds a partial file under that path.
 func (s *ContentStorage) writeIfAbsent(hash ContentHash, data []byte) error {
 	// Build path: <basedir>/<ab>/<cd>/<hash>
 	path := s.hashToPath(hash)
@@ -180,15 +164,23 @@ func (s *ContentStorage) writeIfAbsent(hash ContentHash, data []byte) error {
 		return nil
 	}
 
-	// Create directory structure
+	err := os.MkdirAll(filepath.Dir(path), StorageDirPerm)
-	dir := filepath.Dir(path)
-
-	err := os.MkdirAll(dir, StorageDirPerm)
 	if err != nil {
 		return fmt.Errorf("failed to create directory: %w", err)
 	}
 
-	// Write to temp file first, then rename for atomicity
+	return writeFileSynced(path, data)
+}
+
+// writeFileSynced writes data to path through a temp file in the same
+// directory: the temp file is synced to disk, renamed to path, and the
+// directory synced after, so that neither a concurrent reader nor a
+// crash finds a partial file under path. When the directory sync fails,
+// the file is removed from path, so that a failed write leaves no file
+// there.
+func writeFileSynced(path string, data []byte) error {
+	dir := filepath.Dir(path)
+
 	tmpFile, err := os.CreateTemp(dir, ".tmp-*")
 	if err != nil {
 		return fmt.Errorf("failed to create temp file: %w", err)
@@ -196,18 +188,12 @@ func (s *ContentStorage) writeIfAbsent(hash ContentHash, data []byte) error {
 
 	tmpPath := tmpFile.Name()
 
-	// Each failure path below unlinks the temp file explicitly. This
-	// replaces a deferred cleanup that read a named result, which the
-	// canonical config does not permit; the set of paths that remove
-	// tmpPath, and the order relative to Close, is unchanged. This
-	// mirrors how MetadataStorage.Store and VariantStorage.Store below
-	// already express the same cleanup.
 	_, err = tmpFile.Write(data)
 	if err != nil {
 		_ = tmpFile.Close()
 		_ = os.Remove(tmpPath)
 
-		return fmt.Errorf("failed to write content: %w", err)
+		return fmt.Errorf("failed to write temp file: %w", err)
 	}
 
 	err = tmpFile.Sync()
@@ -225,7 +211,6 @@ func (s *ContentStorage) writeIfAbsent(hash ContentHash, data []byte) error {
 		return fmt.Errorf("failed to close temp file: %w", err)
 	}
 
-	// Atomic rename
 	err = os.Rename(filepath.Clean(tmpPath), filepath.Clean(path))
 	if err != nil {
 		_ = os.Remove(tmpPath)
@@ -233,7 +218,14 @@ func (s *ContentStorage) writeIfAbsent(hash ContentHash, data []byte) error {
 		return fmt.Errorf("failed to rename temp file: %w", err)
 	}
 
-	return syncDirectory(dir)
+	err = syncDirectory(dir)
+	if err != nil {
+		_ = os.Remove(path)
+
+		return err
+	}
+
+	return nil
 }
 
 // syncDirectory syncs the directory dir to disk, so that a file just
@@ -468,7 +460,9 @@ func NewVariantStorage(baseDir string, log *slog.Logger) (*VariantStorage, error
 	return &VariantStorage{baseDir: baseDir, log: log}, nil
 }
 
-// Store writes content and metadata to storage at the given key.
+// Store writes content and metadata to storage at the given key, each
+// with writeFileSynced. The .meta file is written first, so that a crash
+// cannot leave the content in place without it.
 func (s *VariantStorage) Store(
 	key VariantKey, r io.Reader, contentType string,
 ) (int64, error) {
@@ -481,61 +475,11 @@ func (s *VariantStorage) Store(
 	path := s.keyToPath(key)
 	metaPath := path + ".meta"
 
-	// Create directory structure
+	err = os.MkdirAll(filepath.Dir(path), StorageDirPerm)
-	dir := filepath.Dir(path)
-
-	err = os.MkdirAll(dir, StorageDirPerm)
 	if err != nil {
 		return 0, fmt.Errorf("failed to create directory: %w", err)
 	}
 
-	// Write content to temp file first, sync it to disk, then rename for
-	// atomicity, so neither a concurrent reader nor a crash finds a
-	// partial file under the key's path
-	tmpFile, err := os.CreateTemp(dir, ".tmp-*")
-	if err != nil {
-		return 0, fmt.Errorf("failed to create temp file: %w", err)
-	}
-
-	tmpPath := tmpFile.Name()
-
-	_, err = tmpFile.Write(data)
-	if err != nil {
-		_ = tmpFile.Close()
-		_ = os.Remove(tmpPath)
-
-		return 0, fmt.Errorf("failed to write content: %w", err)
-	}
-
-	err = tmpFile.Sync()
-	if err != nil {
-		_ = tmpFile.Close()
-		_ = os.Remove(tmpPath)
-
-		return 0, fmt.Errorf("failed to sync temp file: %w", err)
-	}
-
-	err = tmpFile.Close()
-	if err != nil {
-		_ = os.Remove(tmpPath)
-
-		return 0, fmt.Errorf("failed to close temp file: %w", err)
-	}
-
-	// Atomic rename content
-	err = os.Rename(filepath.Clean(tmpPath), filepath.Clean(path))
-	if err != nil {
-		_ = os.Remove(tmpPath)
-
-		return 0, fmt.Errorf("failed to rename temp file: %w", err)
-	}
-
-	err = syncDirectory(dir)
-	if err != nil {
-		return 0, err
-	}
-
-	// Write metadata
 	meta := VariantMeta{
 		ContentType: contentType,
 		Size:        size,
@@ -547,13 +491,19 @@ func (s *VariantStorage) Store(
 		return 0, fmt.Errorf("failed to marshal metadata: %w", err)
 	}
 
-	// Metadata write failure is non-fatal; content is already stored.
+	// Metadata write failure is non-fatal: the content is then served as
-	err = os.WriteFile(metaPath, metaData, StorageFilePerm)
+	// application/octet-stream.
+	err = writeFileSynced(metaPath, metaData)
 	if err != nil {
 		s.log.Warn("failed to write variant metadata sidecar",
 			"path", metaPath, "error", err)
 	}
 
+	err = writeFileSynced(path, data)
+	if err != nil {
+		return 0, err
+	}
+
 	return size, nil
 }
 
@@ -631,20 +581,6 @@ func (s *VariantStorage) Exists(key VariantKey) bool {
 	return err == nil
 }
 
-// Size returns the size in bytes of the content at the given key.
-func (s *VariantStorage) Size(key VariantKey) (int64, error) {
-	info, err := os.Stat(s.keyToPath(key))
-	if err != nil {
-		if os.IsNotExist(err) {
-			return 0, ErrNotFound
-		}
-
-		return 0, fmt.Errorf("failed to stat content: %w", err)
-	}
-
-	return info.Size(), nil
-}
-
 // Delete removes the content at the given key together with its .meta
 // sidecar file. A missing file is not an error.
 func (s *VariantStorage) Delete(key VariantKey) error {