fs/diff: support symlink to abspath with second-precision mtime

Previously, `Changes()` for a symlink to absolute path, with second-precision
mtime of the link itself always resulted in ENOENT.

This is because `compareFileContent()` was called for the link target path, without changing the root path.

Fix moby/buildkit#172

Signed-off-by: Akihiro Suda <suda.akihiro@lab.ntt.co.jp>

fs/diff_test.go

@@ -150,27 +150,27 @@ func TestUpdateWithSameTime(t *testing.T) {
 	t2 := tt.Add(6 * time.Nanosecond)
 	l1 := fstest.Apply(
 		fstest.CreateFile("/file-modified-time", []byte("1"), 0644),
-		fstest.Chtime("/file-modified-time", t1),
+		fstest.Chtimes("/file-modified-time", t1, t1),
 		fstest.CreateFile("/file-no-change", []byte("1"), 0644),
-		fstest.Chtime("/file-no-change", t1),
+		fstest.Chtimes("/file-no-change", t1, t1),
 		fstest.CreateFile("/file-same-time", []byte("1"), 0644),
-		fstest.Chtime("/file-same-time", t1),
+		fstest.Chtimes("/file-same-time", t1, t1),
 		fstest.CreateFile("/file-truncated-time-1", []byte("1"), 0644),
-		fstest.Chtime("/file-truncated-time-1", t1),
+		fstest.Chtimes("/file-truncated-time-1", t1, t1),
 		fstest.CreateFile("/file-truncated-time-2", []byte("1"), 0644),
-		fstest.Chtime("/file-truncated-time-2", tt),
+		fstest.Chtimes("/file-truncated-time-2", tt, tt),
 	)
 	l2 := fstest.Apply(
 		fstest.CreateFile("/file-modified-time", []byte("2"), 0644),
-		fstest.Chtime("/file-modified-time", t2),
+		fstest.Chtimes("/file-modified-time", t2, t2),
 		fstest.CreateFile("/file-no-change", []byte("1"), 0644),
-		fstest.Chtime("/file-no-change", tt), // use truncated time, should be regarded as no change
+		fstest.Chtimes("/file-no-change", tt, tt), // use truncated time, should be regarded as no change
 		fstest.CreateFile("/file-same-time", []byte("2"), 0644),
-		fstest.Chtime("/file-same-time", t1),
+		fstest.Chtimes("/file-same-time", t1, t1),
 		fstest.CreateFile("/file-truncated-time-1", []byte("2"), 0644),
-		fstest.Chtime("/file-truncated-time-1", tt),
+		fstest.Chtimes("/file-truncated-time-1", tt, tt),
 		fstest.CreateFile("/file-truncated-time-2", []byte("2"), 0644),
-		fstest.Chtime("/file-truncated-time-2", tt),
+		fstest.Chtimes("/file-truncated-time-2", tt, tt),
 	)
 	diff := []TestChange{
 		// "/file-same-time" excluded because matching non-zero nanosecond values
@@ -184,6 +184,28 @@ func TestUpdateWithSameTime(t *testing.T) {
 	}
 }
 
+// buildkit#172
+func TestLchtimes(t *testing.T) {
+	skipDiffTestOnWindows(t)
+	mtimes := []time.Time{
+		time.Unix(0, 0),  // nsec is 0
+		time.Unix(0, 42), // nsec > 0
+	}
+	for _, mtime := range mtimes {
+		atime := time.Unix(424242, 42)
+		l1 := fstest.Apply(
+			fstest.CreateFile("/foo", []byte("foo"), 0644),
+			fstest.Symlink("/foo", "/lnk0"),
+			fstest.Lchtimes("/lnk0", atime, mtime),
+		)
+		l2 := fstest.Apply() // empty
+		diff := []TestChange{}
+		if err := testDiffWithBase(l1, l2, diff); err != nil {
+			t.Fatalf("Failed diff with base: %+v", err)
+		}
+	}
+}
+
 func testDiffWithBase(base, diff fstest.Applier, expected []TestChange) error {
 	t1, err := ioutil.TempDir("", "diff-with-base-lower-")
 	if err != nil {

fs/fstest/file.go

@@ -71,10 +71,11 @@ func Chown(name string, uid, gid int) Applier {
 	})
 }
 
-// Chtime changes access and mod time of file
-func Chtime(name string, t time.Time) Applier {
+// Chtimes changes access and mod time of file.
+// Use Lchtimes for symbolic links.
+func Chtimes(name string, atime, mtime time.Time) Applier {
 	return applyFn(func(root string) error {
-		return os.Chtimes(filepath.Join(root, name), t, t)
+		return os.Chtimes(filepath.Join(root, name), atime, mtime)
 	})
 }
 

fs/fstest/file_unix.go

@@ -2,7 +2,13 @@
 
 package fstest
 
-import "github.com/containerd/continuity/sysx"
+import (
+	"path/filepath"
+	"time"
+
+	"github.com/containerd/continuity/sysx"
+	"golang.org/x/sys/unix"
+)
 
 // SetXAttr sets the xatter for the file
 func SetXAttr(name, key, value string) Applier {
@@ -10,3 +16,14 @@ func SetXAttr(name, key, value string) Applier {
 		return sysx.LSetxattr(name, key, []byte(value), 0)
 	})
 }
+
+// Lchtimes changes access and mod time of file without following symlink
+func Lchtimes(name string, atime, mtime time.Time) Applier {
+	return applyFn(func(root string) error {
+		path := filepath.Join(root, name)
+		at := unix.NsecToTimespec(atime.UnixNano())
+		mt := unix.NsecToTimespec(mtime.UnixNano())
+		utimes := [2]unix.Timespec{at, mt}
+		return unix.UtimesNanoAt(unix.AT_FDCWD, path, utimes[0:], unix.AT_SYMLINK_NOFOLLOW)
+	})
+}

fs/fstest/file_windows.go

@@ -0,0 +1,14 @@
+package fstest
+
+import (
+	"time"
+
+	"github.com/containerd/containerd/errdefs"
+)
+
+// Lchtimes changes access and mod time of file without following symlink
+func Lchtimes(name string, atime, mtime time.Time) Applier {
+	return applyFn(func(root string) error {
+		return errdefs.ErrNotImplemented
+	})
+}

fs/path.go

@@ -74,11 +74,14 @@ func sameFile(f1, f2 *currentPath) (bool, error) {
 		// If the timestamp may have been truncated in one of the
 		// files, check content of file to determine difference
 		if t1.Nanosecond() == 0 || t2.Nanosecond() == 0 {
-			if f1.f.Size() > 0 {
-				eq, err := compareFileContent(f1.fullPath, f2.fullPath)
-				if err != nil || !eq {
-					return eq, err
-				}
+			var eq bool
+			if (f1.f.Mode() & os.ModeSymlink) == os.ModeSymlink {
+				eq, err = compareSymlinkTarget(f1.fullPath, f2.fullPath)
+			} else if f1.f.Size() > 0 {
+				eq, err = compareFileContent(f1.fullPath, f2.fullPath)
+			}
+			if err != nil || !eq {
+				return eq, err
 			}
 		} else if t1.Nanosecond() != t2.Nanosecond() {
 			return false, nil
@@ -88,6 +91,18 @@ func sameFile(f1, f2 *currentPath) (bool, error) {
 	return true, nil
 }
 
+func compareSymlinkTarget(p1, p2 string) (bool, error) {
+	t1, err := os.Readlink(p1)
+	if err != nil {
+		return false, err
+	}
+	t2, err := os.Readlink(p2)
+	if err != nil {
+		return false, err
+	}
+	return t1 == t2, nil
+}
+
 const compareChuckSize = 32 * 1024
 
 // compareFileContent compares the content of 2 same sized files