Treat unix sockets as regular files

This fix is similar to the fix in #1144; but was hit in a different code
path.

Signed-off-by: Alex Couture-Beil <alex@earthly.dev>

cache/contenthash/filehash.go

@@ -40,6 +40,9 @@ func NewFileHash(path string, fi os.FileInfo) (hash.Hash, error) {
 }
 
 func NewFromStat(stat *fstypes.Stat) (hash.Hash, error) {
+	// Clear the socket bit since archive/tar.FileInfoHeader does not handle it
+	stat.Mode &^= uint32(os.ModeSocket)
+
 	fi := &statInfo{stat}
 	hdr, err := tar.FileInfoHeader(fi, stat.Linkname)
 	if err != nil {

client/client_test.go

@@ -113,6 +113,7 @@ func TestIntegration(t *testing.T) {
 		testCacheMountNoCache,
 		testExporterTargetExists,
 		testTarExporterWithSocket,
+		testTarExporterWithSocketCopy,
 		testTarExporterSymlink,
 		testMultipleRegistryCacheImportExport,
 		testSourceMap,
@@ -1653,6 +1654,30 @@ func testTarExporterWithSocket(t *testing.T, sb integration.Sandbox) {
 	require.NoError(t, err)
 }
 
+func testTarExporterWithSocketCopy(t *testing.T, sb integration.Sandbox) {
+	if os.Getenv("TEST_DOCKERD") == "1" {
+		t.Skip("tar exporter is temporarily broken on dockerd")
+	}
+
+	requiresLinux(t)
+	c, err := New(context.TODO(), sb.Address())
+	require.NoError(t, err)
+	defer c.Close()
+
+	alpine := llb.Image("docker.io/library/alpine:latest")
+	state := alpine.Run(llb.Args([]string{"sh", "-c", "nc -l -s local:/root/socket.sock & usleep 100000; kill %1"})).Root()
+
+	fa := llb.Copy(state, "/root", "/roo2", &llb.CopyInfo{})
+
+	scratchCopy := llb.Scratch().File(fa)
+
+	def, err := scratchCopy.Marshal(context.TODO())
+	require.NoError(t, err)
+
+	_, err = c.Solve(context.TODO(), def, SolveOpt{}, nil)
+	require.NoError(t, err)
+}
+
 // moby/buildkit#1418
 func testTarExporterSymlink(t *testing.T, sb integration.Sandbox) {
 	requiresLinux(t)