rewrite-timestamp: fix incompatibility with COPY --link

[AkihiroSuda]

Fix #4746

[AkihiroSuda]

git diff --ignore-all-spaces

diff --git a/frontend/dockerfile/dockerfile_test.go b/frontend/dockerfile/dockerfile_test.go
index d026fffc1..d8a04f991 100644
--- a/frontend/dockerfile/dockerfile_test.go
+++ b/frontend/dockerfile/dockerfile_test.go
@@ -6856,7 +6856,16 @@ func testReproSourceDateEpoch(t *testing.T, sb integration.Sandbox) {
        tm := time.Date(2023, time.January, 10, 12, 34, 56, 0, time.UTC) // 1673354096
        t.Logf("SOURCE_DATE_EPOCH=%d", tm.Unix())
 
-       dockerfile := []byte(`# The base image cannot be busybox, due to https://github.com/moby/buildkit/issues/3455
+       type testCase struct {
+               name           string
+               dockerfile     string
+               files          []fstest.Applier
+               expectedDigest string
+       }
+       testCases := []testCase{
+               {
+                       name: "Basic",
+                       dockerfile: `# The base image could not be busybox, due to https://github.com/moby/buildkit/issues/3455
 FROM amd64/debian:bullseye-20230109-slim
 RUN touch /foo
 RUN touch /foo.1
@@ -6867,7 +6876,20 @@ RUN touch -d '2030-01-01 12:34:56' /foo-2030.1
 RUN rm -f /foo.1
 RUN rm -f /foo-2010.1
 RUN rm -f /foo-2030.1
-`)
+`,
+                       expectedDigest: "sha256:04e5d0cbee3317c79f50494cfeb4d8a728402a970ef32582ee47c62050037e3f",
+               },
+               {
+                       // https://github.com/moby/buildkit/issues/4746
+                       name: "CopyLink",
+                       dockerfile: `FROM amd64/debian:bullseye-20230109-slim
+COPY --link foo foo
+`,
+                       files:          []fstest.Applier{fstest.CreateFile("foo", []byte("foo"), 0600)},
+                       expectedDigest: "sha256:9f75e4bdbf3d825acb36bb603ddef4a25742afb8ccb674763ffc611ae047d8a6",
+               },
+       }
+
        // https://explore.ggcr.dev/?image=amd64%2Fdebian%3Abullseye-20230109-slim
        baseImageLayers := []digest.Digest{
                "sha256:8740c948ffd4c816ea7ca963f99ca52f4788baa23f228da9581a9ea2edd3fcd7",
@@ -6877,19 +6899,19 @@ RUN rm -f /foo-2030.1
                timeMustParse(t, time.RFC3339Nano, "2023-01-11T02:34:44.829692296Z"),
        }
 
-       const expectedDigest = "sha256:04e5d0cbee3317c79f50494cfeb4d8a728402a970ef32582ee47c62050037e3f"
-
-       dir := integration.Tmpdir(
-               t,
-               fstest.CreateFile("Dockerfile", dockerfile, 0600),
-       )
-
        ctx := sb.Context()
        c, err := client.New(ctx, sb.Address())
        require.NoError(t, err)
        defer c.Close()
 
-       target := registry + "/buildkit/testreprosourcedateepoch:" + fmt.Sprintf("%d", tm.Unix())
+       for _, tc := range testCases {
+               t.Run(tc.name, func(t *testing.T) {
+                       dir := integration.Tmpdir(
+                               t,
+                               append([]fstest.Applier{fstest.CreateFile("Dockerfile", []byte(tc.dockerfile), 0600)}, tc.files...)...,
+                       )
+
+                       target := registry + "/buildkit/testreprosourcedateepoch-" + strings.ToLower(tc.name) + ":" + fmt.Sprintf("%d", tm.Unix())
                        solveOpt := client.SolveOpt{
                                FrontendAttrs: map[string]string{
                                        "build-arg:SOURCE_DATE_EPOCH": fmt.Sprintf("%d", tm.Unix()),
@@ -6927,7 +6949,7 @@ RUN rm -f /foo-2030.1
                        desc, manifest, img := readImage(t, ctx, target)
                        _, cacheManifest, _ := readImage(t, ctx, target+"-cache")
                        t.Log("The digest may change depending on the BuildKit version, the snapshotter configuration, etc.")
-       require.Equal(t, expectedDigest, desc.Digest.String())
+                       require.Equal(t, tc.expectedDigest, desc.Digest.String())
 
                        // Image history from the base config must remain immutable
                        for i, tm := range baseImageHistoryTimestamps {
@@ -6948,6 +6970,14 @@ RUN rm -f /foo-2030.1
                                require.Empty(t, l.Annotations["buildkit/rewritten-timestamp"])
                        }
 
+                       // Build again, after pruning the base image layer cache.
+                       // For testing https://github.com/moby/buildkit/issues/4746
+                       ensurePruneAll(t, c, sb)
+                       _, err = f.Solve(ctx, c, solveOpt, nil)
+                       require.NoError(t, err)
+                       descAfterPrune, _, _ := readImage(t, ctx, target)
+                       require.Equal(t, desc.Digest.String(), descAfterPrune.Digest.String())
+
                        // Build again, but without rewrite-timestamp
                        solveOpt2 := solveOpt
                        delete(solveOpt2.Exports[0].Attrs, "rewrite-timestamp")
@@ -6960,6 +6990,8 @@ RUN rm -f /foo-2030.1
                        for _, l := range manifest2.Layers {
                                require.Empty(t, l.Annotations["buildkit/rewritten-timestamp"])
                        }
+               })
+       }
 }
 
 func timeMustParse(t *testing.T, layout, value string) time.Time {
@@ -6978,6 +7010,14 @@ func readImage(t *testing.T, ctx context.Context, ref string) (ocispecs.Descript
        require.NoError(t, json.Unmarshal(dt, &manifest))
        imgDt, err := content.ReadBlob(ctx, provider, manifest.Config)
        require.NoError(t, err)
+       // Verify that all the layer blobs are present
+       for _, layer := range manifest.Layers {
+               layerRA, err := provider.ReaderAt(ctx, layer)
+               require.NoError(t, err)
+               layerDigest, err := layer.Digest.Algorithm().FromReader(content.NewReader(layerRA))
+               require.NoError(t, err)
+               require.Equal(t, layer.Digest, layerDigest)
+       }
        var img ocispecs.Image
        require.NoError(t, json.Unmarshal(imgDt, &img))
        return desc, manifest, img