Drop per-entry allocations in ListDir

archives.go

@@ -252,29 +252,23 @@ func normalizeDir(dirPath string) string {
 	return dirPath + "/"
 }
 
-// isInDir checks if filePath is directly in dirPath (not in subdirectories).
+// isInDir reports whether filePath is directly in dirPath (not in a
+// subdirectory). dirPath must already be normalized via normalizeDir;
+// callers do this once outside the per-entry loop.
 func isInDir(filePath, dirPath string) bool {
-	dirPath = normalizeDir(dirPath)
-
-	// Normalize file path by trimming trailing slash
 	filePath = strings.TrimSuffix(filePath, "/")
 
-	// Root directory
 	if dirPath == "" {
-		// File is in root if it has no slashes
-		parts := strings.Split(filePath, "/")
-		return len(parts) == 1
+		return strings.IndexByte(filePath, '/') < 0
 	}
 
-	// Check if file starts with directory path
-	if !strings.HasPrefix(filePath+"/", dirPath) {
+	// dirPath ends in "/"; an entry naming the directory itself counts as
+	// inside it so explicit dir entries in the archive are listed.
+	if filePath == dirPath[:len(dirPath)-1] {
+		return true
+	}
+	if !strings.HasPrefix(filePath, dirPath) {
 		return false
 	}
-
-	// Get relative path
-	rel := strings.TrimPrefix(filePath, strings.TrimSuffix(dirPath, "/"))
-	rel = strings.TrimPrefix(rel, "/")
-
-	// Should have no more slashes
-	return !strings.Contains(rel, "/")
+	return strings.IndexByte(filePath[len(dirPath):], '/') < 0
 }

archives_test.go

@@ -85,11 +85,13 @@ func TestIsInDir(t *testing.T) {
 	}{
 		{"file.txt", "", true},
 		{"dir/file.txt", "", false},
-		{"dir/file.txt", "dir", true},
-		{"dir/subdir/file.txt", "dir", false},
-		{"dir/subdir/file.txt", "dir/subdir", true},
-		{"other/file.txt", "dir", false},
-		{"dir/", "", true}, // dir entry is in root
+		{"dir/file.txt", "dir/", true},
+		{"dir/subdir/file.txt", "dir/", false},
+		{"dir/subdir/file.txt", "dir/subdir/", true},
+		{"other/file.txt", "dir/", false},
+		{"dir/", "", true},        // dir entry is in root
+		{"dir/", "dir/", true},    // explicit entry for the directory itself
+		{"dirX/file", "dir/", false},
 	}
 
 	for _, tt := range tests {
@@ -720,6 +722,34 @@ func TestTarListDirNoDuplicatesWithExplicitDirEntries(t *testing.T) {
 	assertNoDuplicates(t, "ListDir project-abc123/", files)
 }
 
+func TestListDirNoDuplicatesWithLateDirEntry(t *testing.T) {
+	// Some archives (hand-built or from tools that append a manifest
+	// pass) emit files before the explicit entry for their parent
+	// directory. ListDir synthesises a subdir entry on the first file
+	// and must then skip the explicit one.
+	buf := new(bytes.Buffer)
+	tw := tar.NewWriter(buf)
+	_ = tw.WriteHeader(&tar.Header{Name: "pkg/src/main.go", Mode: 0o644, Size: 1})
+	_, _ = tw.Write([]byte("x"))
+	_ = tw.WriteHeader(&tar.Header{Name: "pkg/src/", Typeflag: tar.TypeDir, Mode: 0o755})
+	_ = tw.Close()
+
+	reader, err := openTar(buf.Bytes(), "")
+	if err != nil {
+		t.Fatalf("openTar failed: %v", err)
+	}
+	defer func() { _ = reader.Close() }()
+
+	files, err := reader.ListDir("pkg/")
+	if err != nil {
+		t.Fatalf("ListDir failed: %v", err)
+	}
+	assertNoDuplicates(t, "ListDir pkg/", files)
+	if len(files) != 1 || files[0].Path != "pkg/src/" || !files[0].IsDir {
+		t.Errorf("ListDir pkg/ = %+v, want single pkg/src/ dir", files)
+	}
+}
+
 func TestGetStripPrefixNpm(t *testing.T) {
 	// Create npm-style archive
 	buf := new(bytes.Buffer)

tar.go

@@ -170,23 +170,26 @@ func (t *tarReader) ListDir(dirPath string) ([]FileInfo, error) {
 		// Check if this file/dir is directly in the requested directory
 		if isInDir(path, dirPath) {
 			if f.info.IsDir {
-				seenDirs[path] = true
+				name := strings.TrimSuffix(strings.TrimPrefix(path, dirPath), "/")
+				if seenDirs[name] {
+					continue
+				}
+				seenDirs[name] = true
 			}
 			files = append(files, f.info)
 			continue
 		}
 
 		// Check if we should add a subdirectory entry
 		if dirPath == "" || strings.HasPrefix(path, dirPath) {
-			rel := strings.TrimPrefix(path, dirPath)
-			parts := strings.Split(strings.TrimSuffix(rel, "/"), "/")
-			if len(parts) > 1 {
-				subdir := dirPath + parts[0] + "/"
-				if !seenDirs[subdir] {
-					seenDirs[subdir] = true
+			rel := strings.TrimSuffix(strings.TrimPrefix(path, dirPath), "/")
+			if i := strings.IndexByte(rel, '/'); i >= 0 {
+				name := rel[:i]
+				if !seenDirs[name] {
+					seenDirs[name] = true
 					files = append(files, FileInfo{
-						Path:  subdir,
-						Name:  parts[0],
+						Path:  dirPath + name + "/",
+						Name:  name,
 						IsDir: true,
 					})
 				}

tar_bench_test.go

@@ -1,6 +1,8 @@
 package archives
 
 import (
+	"archive/tar"
+	"bytes"
 	"fmt"
 	"io"
 	"testing"
@@ -20,6 +22,39 @@ func BenchmarkTarBrowse(b *testing.B) {
 	}
 }
 
+func BenchmarkListDir(b *testing.B) {
+	const (
+		files    = 2000
+		subdirs  = 20
+		filePerm = 0o644
+	)
+	var buf bytes.Buffer
+	tw := tar.NewWriter(&buf)
+	for i := range files {
+		_ = tw.WriteHeader(&tar.Header{
+			Name: fmt.Sprintf("package/lib/sub%02d/file%04d.dat", i%subdirs, i),
+			Mode: filePerm,
+		})
+	}
+	_ = tw.Close()
+	r, err := OpenBytesWithPrefix("test.tar", buf.Bytes(), "package/")
+	if err != nil {
+		b.Fatal(err)
+	}
+	b.Cleanup(func() { _ = r.Close() })
+
+	for _, dir := range []string{"", "lib", "lib/sub00"} {
+		b.Run(dir, func(b *testing.B) {
+			b.ReportAllocs()
+			for b.Loop() {
+				if _, err := r.ListDir(dir); err != nil {
+					b.Fatal(err)
+				}
+			}
+		})
+	}
+}
+
 func benchmarkTarBrowse(b *testing.B, filename string, payload []byte) {
 	b.Helper()
 	raw := compressTar(b, filename, tarWithPayloads(b, payload, payload, payload, payload))

zip.go

@@ -171,24 +171,26 @@ func (z *zipReader) ListDir(dirPath string) ([]FileInfo, error) {
 		// Check if this file/dir is directly in the requested directory
 		if isInDir(path, dirPath) {
 			if f.FileInfo().IsDir() {
-				seenDirs[path] = true
+				name := strings.TrimSuffix(strings.TrimPrefix(path, dirPath), "/")
+				if seenDirs[name] {
+					continue
+				}
+				seenDirs[name] = true
 			}
 			files = append(files, fileInfoFromZip(f))
 			continue
 		}
 
 		// Check if we should add a subdirectory entry
 		if dirPath == "" || strings.HasPrefix(path, dirPath) {
-			rel := strings.TrimPrefix(path, dirPath)
-			parts := strings.Split(strings.TrimSuffix(rel, "/"), "/")
-			if len(parts) > 1 {
-				// This file is in a subdirectory
-				subdir := dirPath + parts[0] + "/"
-				if !seenDirs[subdir] {
-					seenDirs[subdir] = true
+			rel := strings.TrimSuffix(strings.TrimPrefix(path, dirPath), "/")
+			if i := strings.IndexByte(rel, '/'); i >= 0 {
+				name := rel[:i]
+				if !seenDirs[name] {
+					seenDirs[name] = true
 					files = append(files, FileInfo{
-						Path:  subdir,
-						Name:  parts[0],
+						Path:  dirPath + name + "/",
+						Name:  name,
 						IsDir: true,
 					})
 				}