executor: stubs cleaner should remove empty directory mounts

On Linux, an empty directory is usually 4096 bytes, not 0, so we need an
additional explicit check here.

Signed-off-by: Justin Chadwell <me@jedevc.com>

client/client_test.go

@@ -185,6 +185,7 @@ func TestIntegration(t *testing.T) {
 		testSBOMScan,
 		testSBOMScanSingleRef,
 		testMultipleCacheExports,
+		testMountStubsDirectory,
 		testMountStubsTimestamp,
 	)
 }
@@ -8106,6 +8107,52 @@ func testMultipleCacheExports(t *testing.T, sb integration.Sandbox) {
 	ensureFileContents(t, filepath.Join(destDir, "unique"), string(uniqueFile))
 }
 
+func testMountStubsDirectory(t *testing.T, sb integration.Sandbox) {
+	c, err := New(sb.Context(), sb.Address())
+	require.NoError(t, err)
+	defer c.Close()
+
+	st := llb.Image("busybox:latest").Run(
+		llb.Args([]string{"/bin/echo", "dummy"}),
+		llb.AddMount("/foo/bar", llb.Scratch(), llb.Tmpfs()),
+	)
+	def, err := st.Marshal(sb.Context())
+	require.NoError(t, err)
+
+	tmpDir := t.TempDir()
+	tarFile := filepath.Join(tmpDir, "out.tar")
+	tarFileW, err := os.Create(tarFile)
+	require.NoError(t, err)
+	defer tarFileW.Close()
+
+	_, err = c.Solve(sb.Context(), def, SolveOpt{
+		Exports: []ExportEntry{
+			{
+				Type:   ExporterTar,
+				Output: fixedWriteCloser(tarFileW),
+			},
+		},
+	}, nil)
+	require.NoError(t, err)
+	tarFileW.Close()
+
+	tarFileR, err := os.Open(tarFile)
+	require.NoError(t, err)
+	defer tarFileR.Close()
+	tarR := tar.NewReader(tarFileR)
+
+	for {
+		hd, err := tarR.Next()
+		if errors.Is(err, io.EOF) {
+			break
+		}
+		require.NoError(t, err)
+		if hd.Name == "foo/bar/" {
+			require.Fail(t, "foo/bar/ should not be in the tar")
+		}
+	}
+}
+
 // https://github.com/moby/buildkit/issues/3148
 func testMountStubsTimestamp(t *testing.T, sb integration.Sandbox) {
 	c, err := New(sb.Context(), sb.Address())

executor/stubs.go

@@ -42,9 +42,18 @@ func MountStubsCleaner(dir string, mounts []Mount) func() {
 			if err != nil {
 				continue
 			}
-			if st.Size() != 0 {
+			if st.IsDir() {
+				entries, err := os.ReadDir(p)
+				if err != nil {
+					continue
+				}
+				if len(entries) != 0 {
+					continue
+				}
+			} else if st.Size() != 0 {
 				continue
 			}
+
 			// Back up the timestamps of the dir for reproducible builds
 			// https://github.com/moby/buildkit/issues/3148
 			dir := filepath.Dir(p)