Rewrite README.md sentence by sentence (closes #327)

Rewrites README.md from scratch against the code on next, for
#327: what pixa is and how to
use it, deployment, design, the settings reference, then development
and testing. The Configuration list keeps one item per setting, as
TestReadmeNamesEverySetting reads it; the test is unchanged. The
statements where the old text disagreed with the code are listed in
the PR body.

Model: fable-5-1

TODO.md

@@ -30,6 +30,17 @@ P2: security: per-origin rate limiting
 
 # Completed Steps
 
+- 2026-10-27 the reconciliation pass skips a directory it cannot list and a file
+  whose info it cannot read, and goes on (closes #343). It stopped before its
+  later checks at a directory it could not list, or at a source or variant file
+  whose info it could not read.
+    - It logs the warning "skipped unreadable entry in cache directory", with
+      the path and the error.
+    - A file renamed or removed after its directory was listed gets no warning.
+    - `reconcileFiles` reads each file's info once, before the temp-file check,
+      and hands it to `sweepStaleTempFile`, `reconcileVariantFile` or
+      `reconcileSourceFile`.
+    - A cancelled context still stops the walk with its error.
 - 2026-10-26 the reconciliation pass drops a variant's accounting row only when
   its file is missing (closes #340). Any other error reading the file's size
   keeps the row. It dropped the row on any error and logged the file as missing.

internal/imgcache/eviction.go

@@ -515,12 +515,15 @@ func (c *Cache) reconcileAccounting(ctx context.Context) error {
 }
 
 // reconcileFiles walks baseDir, one of the cache's storage directories,
-// sweeping stale temp files and passing each other file, with its name, to
-// reconcileFile. Once ctx is cancelled it stops at the next file and
-// returns ctx's error.
+// sweeping stale temp files and passing each other file, with its info and
+// name, to reconcileFile. It skips a directory it cannot list and a file
+// whose info it cannot read, so one such entry does not stop the pass. It
+// logs a warning naming the path and the error. A file renamed or removed
+// after its directory was listed is skipped with no warning. Once ctx is
+// cancelled it stops at the next file and returns ctx's error.
 func (c *Cache) reconcileFiles(
 	ctx context.Context, baseDir string,
-	reconcileFile func(path string, entry fs.DirEntry, name string) error,
+	reconcileFile func(path string, info fs.FileInfo, name string) error,
 ) error {
 	return filepath.WalkDir(
 		baseDir,
@@ -529,19 +532,38 @@ func (c *Cache) reconcileFiles(
 				return ctx.Err()
 			}
 
-			if err != nil || entry.IsDir() {
-				return err
+			if err != nil {
+				c.log.Warn("skipped unreadable entry in cache directory",
+					"path", path, "error", err)
+
+				return nil
+			}
+
+			if entry.IsDir() {
+				return nil
+			}
+
+			info, err := entry.Info()
+			if errors.Is(err, fs.ErrNotExist) {
+				return nil
+			}
+
+			if err != nil {
+				c.log.Warn("skipped unreadable entry in cache directory",
+					"path", path, "error", err)
+
+				return nil
 			}
 
 			name := entry.Name()
 
 			if strings.HasPrefix(name, tempFilePrefix) {
-				c.sweepStaleTempFile(path, entry)
+				c.sweepStaleTempFile(path, info)
 
 				return nil
 			}
 
-			return reconcileFile(path, entry, name)
+			return reconcileFile(path, info, name)
 		},
 	)
 }
@@ -551,8 +573,8 @@ func (c *Cache) reconcileFiles(
 // and sweeping stale temp files.
 func (c *Cache) reconcileVariantFiles(ctx context.Context) error {
 	return c.reconcileFiles(ctx, c.variants.baseDir,
-		func(path string, entry fs.DirEntry, name string) error {
-			return c.reconcileVariantFile(ctx, path, entry, imagerequest.VariantKey(name))
+		func(path string, info fs.FileInfo, name string) error {
+			return c.reconcileVariantFile(ctx, path, info, imagerequest.VariantKey(name))
 		})
 }
 
@@ -562,16 +584,11 @@ func (c *Cache) reconcileVariantFiles(ctx context.Context) error {
 // 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 imagerequest.VariantKey,
+	ctx context.Context, path string, info fs.FileInfo, cacheKey imagerequest.VariantKey,
 ) error {
-	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,
+	err := c.db.QueryRowContext(ctx,
 		`SELECT size_bytes FROM variant_content WHERE cache_key = ?`, string(cacheKey),
 	).Scan(&recordedSize)
 
@@ -714,8 +731,8 @@ func queryStringColumn[T ~string](
 // and sweeping stale temp files.
 func (c *Cache) reconcileSourceFiles(ctx context.Context) error {
 	return c.reconcileFiles(ctx, c.srcContent.baseDir,
-		func(path string, entry fs.DirEntry, name string) error {
-			return c.reconcileSourceFile(ctx, path, entry, imagerequest.SourceKey(name))
+		func(path string, info fs.FileInfo, name string) error {
+			return c.reconcileSourceFile(ctx, path, info, imagerequest.SourceKey(name))
 		})
 }
 
@@ -726,16 +743,11 @@ func (c *Cache) reconcileSourceFiles(ctx context.Context) error {
 // is removed with its row and a warning naming it, so the source is
 // fetched again.
 func (c *Cache) reconcileSourceFile(
-	ctx context.Context, path string, entry fs.DirEntry, key imagerequest.SourceKey,
+	ctx context.Context, path string, info fs.FileInfo, key imagerequest.SourceKey,
 ) 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,
+	err := c.db.QueryRowContext(ctx,
 		`SELECT size_bytes FROM source_content WHERE source_key = ?`,
 		string(key),
 	).Scan(&recordedSize)
@@ -900,17 +912,12 @@ func (c *Cache) correctUsageTotal(
 
 // sweepStaleTempFile removes a temp file left behind by a crashed
 // write once it is old enough that no in-flight store can own it.
-func (c *Cache) sweepStaleTempFile(path string, entry fs.DirEntry) {
-	info, err := entry.Info()
-	if err != nil {
-		return
-	}
-
+func (c *Cache) sweepStaleTempFile(path string, info fs.FileInfo) {
 	if time.Since(info.ModTime()) < staleTempFileAge {
 		return
 	}
 
-	err = os.Remove(path)
+	err := os.Remove(path)
 	if err != nil && !os.IsNotExist(err) {
 		c.log.Warn("failed to remove stale temp file", "path", path, "error", err)
 

internal/imgcache/eviction_internal_test.go

@@ -9,8 +9,10 @@ import (
 	"log/slog"
 	"os"
 	"path/filepath"
+	"slices"
 	"strings"
 	"sync"
+	"syscall"
 	"testing"
 	"time"
 
@@ -785,6 +787,205 @@ func TestReconcileVariantRowsDropsOnlyRowsOfMissingFiles(t *testing.T) {
 	}
 }
 
+// openDirectoryNearPathLimit makes a chain of directories inside dir, each
+// named with 255 zeros, the longest name Linux allows. Linux refuses a path
+// of 4096 bytes or more, and the chain goes as deep as it can below that.
+// It returns the chain's deepest directory as an os.Root, closed when the
+// test ends. An entry made through it with a 255-byte name has a path even
+// the root user cannot read. The os.Root can make that entry because it
+// names the entry only by its name, not by the whole path.
+func openDirectoryNearPathLimit(t *testing.T, dir string) *os.Root {
+	t.Helper()
+
+	name := strings.Repeat("0", 255)
+	deepest := dir
+
+	for len(filepath.Join(deepest, name)) < 4096 {
+		deepest = filepath.Join(deepest, name)
+	}
+
+	err := os.MkdirAll(deepest, StorageDirPerm)
+	if err != nil {
+		t.Fatalf("failed to make a chain of directories: %v", err)
+	}
+
+	root, err := os.OpenRoot(deepest)
+	if err != nil {
+		t.Fatalf("os.OpenRoot(%s) error = %v", deepest, err)
+	}
+
+	t.Cleanup(func() { _ = root.Close() })
+
+	return root
+}
+
+// makeEntriesTooLongToRead makes a new directory and a new file in the
+// deepest directory of the chain that openDirectoryNearPathLimit makes
+// inside dir. Even the root user can then neither list the new directory
+// nor read the new file's info. It returns the paths of the new directory
+// and the new file.
+func makeEntriesTooLongToRead(t *testing.T, dir string) []string {
+	t.Helper()
+
+	deepest := openDirectoryNearPathLimit(t, dir)
+	dirName := strings.Repeat("0", 255)
+
+	err := deepest.Mkdir(dirName, StorageDirPerm)
+	if err != nil {
+		t.Fatalf("failed to make a directory too deep to list: %v", err)
+	}
+
+	fileName := strings.Repeat("1", 255)
+
+	err = deepest.WriteFile(fileName, []byte("unreadable"), StorageFilePerm)
+	if err != nil {
+		t.Fatalf("failed to make a file too deep to read: %v", err)
+	}
+
+	return []string{
+		filepath.Join(deepest.Name(), dirName),
+		filepath.Join(deepest.Name(), fileName),
+	}
+}
+
+// makeTempFileTooLongToRead makes a temp file in the deepest directory of
+// the chain that openDirectoryNearPathLimit makes inside dir. Even the
+// root user then cannot read the temp file's info. It returns the temp
+// file's path.
+func makeTempFileTooLongToRead(t *testing.T, dir string) string {
+	t.Helper()
+
+	deepest := openDirectoryNearPathLimit(t, dir)
+	name := tempFilePrefix + strings.Repeat("1", 255-len(tempFilePrefix))
+
+	err := deepest.WriteFile(name, []byte("partial"), StorageFilePerm)
+	if err != nil {
+		t.Fatalf("failed to make a temp file too deep to read: %v", err)
+	}
+
+	return filepath.Join(deepest.Name(), name)
+}
+
+// assertSkippedEntryWarning checks that logs hold the warning "skipped
+// unreadable entry in cache directory" naming path, with the error
+// ENAMETOOLONG for that path.
+func assertSkippedEntryWarning(t *testing.T, logs, path string) {
+	t.Helper()
+
+	for line := range strings.Lines(logs) {
+		if strings.Contains(line, `"level":"WARN"`) &&
+			strings.Contains(line, `"msg":"skipped unreadable entry in cache directory"`) &&
+			strings.Contains(line, `"path":"`+path+`"`) &&
+			strings.Contains(line, path+": "+syscall.ENAMETOOLONG.Error()) {
+			return
+		}
+	}
+
+	t.Errorf("no warning %q names %s with the error %q",
+		"skipped unreadable entry in cache directory", path, syscall.ENAMETOOLONG)
+}
+
+// TestReconciliationGoesOnPastEntriesItCannotRead puts, in each storage
+// directory, a directory the pass cannot list and a file whose info it
+// cannot read. The pass walks each directory in name order, so it reaches
+// them before the cache's files, whose keys start with a letter. It must
+// log a warning naming each one's path and the error, and still do the
+// rest of its work:
+//   - adopt a variant file that has no row
+//   - drop the row of a variant whose file is missing
+//   - remove a source file that has no row
+//   - correct a wrong total cache usage
+func TestReconciliationGoesOnPastEntriesItCannotRead(t *testing.T) {
+	t.Parallel()
+
+	cache, _ := newEvictionTestCache(t, 1<<30)
+	ctx := t.Context()
+
+	untrackedVariant := []byte("variant with no row")
+
+	err := cache.variants.Write(string(testVariantKeyOne), untrackedVariant)
+	if err != nil {
+		t.Fatalf("failed to write a variant file with no row: %v", err)
+	}
+
+	storeEvictionTestVariant(t, cache, testVariantKeyTwo, []byte("variant two"))
+
+	err = os.Remove(cache.variants.keyToPath(string(testVariantKeyTwo)))
+	if err != nil {
+		t.Fatalf("failed to remove the second variant's file: %v", err)
+	}
+
+	untrackedSourceKey := "ffeeddcc0001"
+
+	err = cache.srcContent.Write(untrackedSourceKey, []byte("source with no row"))
+	if err != nil {
+		t.Fatalf("failed to write a source file with no row: %v", err)
+	}
+
+	_, err = cache.db.ExecContext(ctx,
+		`UPDATE cache_usage SET total_size_bytes = 1 WHERE id = 1`)
+	if err != nil {
+		t.Fatalf("failed to set a wrong total: %v", err)
+	}
+
+	unreadable := slices.Concat(
+		makeEntriesTooLongToRead(t, cache.variants.baseDir),
+		makeEntriesTooLongToRead(t, cache.srcContent.baseDir),
+	)
+
+	logs := reconcileAsAtStartup(t, cache)
+
+	for _, path := range unreadable {
+		assertSkippedEntryWarning(t, logs, path)
+	}
+
+	rowQuery := `SELECT COUNT(*) FROM variant_content WHERE cache_key = ?`
+
+	if n := countRows(t, cache, rowQuery, string(testVariantKeyOne)); n != 1 {
+		t.Errorf("the variant file with no row got %d rows, want 1", n)
+	}
+
+	if n := countRows(t, cache, rowQuery, string(testVariantKeyTwo)); n != 0 {
+		t.Errorf("the variant with no file kept %d rows, want 0", n)
+	}
+
+	_, err = os.Stat(cache.srcContent.keyToPath(untrackedSourceKey))
+	if err == nil {
+		t.Error("the source file with no row is still on disk")
+	}
+
+	usage, err := cache.UsageBytes(ctx)
+	if err != nil {
+		t.Fatalf("UsageBytes() error = %v", err)
+	}
+
+	if usage != int64(len(untrackedVariant)) {
+		t.Errorf("UsageBytes() = %d, want %d, the size of the adopted variant",
+			usage, len(untrackedVariant))
+	}
+}
+
+// TestReconciliationWarnsOfTempFilesWhoseInfoItCannotRead puts, in each
+// storage directory, a temp file whose info the pass cannot read. The
+// pass must log a warning naming each one's path and the error, as it
+// does for any other file whose info it cannot read.
+func TestReconciliationWarnsOfTempFilesWhoseInfoItCannotRead(t *testing.T) {
+	t.Parallel()
+
+	cache, _ := newEvictionTestCache(t, 1<<30)
+
+	unreadable := []string{
+		makeTempFileTooLongToRead(t, cache.variants.baseDir),
+		makeTempFileTooLongToRead(t, cache.srcContent.baseDir),
+	}
+
+	logs := reconcileAsAtStartup(t, cache)
+
+	for _, path := range unreadable {
+		assertSkippedEntryWarning(t, logs, path)
+	}
+}
+
 // TestPeriodicReconciliationAdoptsFileThatAppearsAfterStartup proves
 // reconciliation is not a one-shot startup-only pass: it must also run
 // on the periodic ticker, so a variant file that lands on disk with no