when copying, skip files with the same name

When using the COPY command, if the source and destination have the same
the file should be skipped rather than copied. This is to prevent the
file from being overwritten and therefore producing an empty file.

fixes #904

pkg/util/fs_util.go

@@ -551,6 +551,12 @@ func CopyFile(src, dest, buildcontext string) (bool, error) {
 		logrus.Debugf("%s found in .dockerignore, ignoring", src)
 		return true, nil
 	}
+	if src == dest {
+		// This is a no-op. Move on, but don't list it as ignored.
+		// We have to make sure we do this so we don't overwrite our own file.
+		// See iusse #904 for an example.
+		return false, nil
+	}
 	fi, err := os.Stat(src)
 	if err != nil {
 		return false, err

pkg/util/fs_util_test.go

@@ -825,3 +825,41 @@ func Test_correctDockerignoreFileIsUsed(t *testing.T) {
 		}
 	}
 }
+
+func Test_CopyFile_skips_self(t *testing.T) {
+	t.Parallel()
+	tempDir, err := ioutil.TempDir("", "kaniko_test")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	tempFile := filepath.Join(tempDir, "foo")
+	expected := "bar"
+
+	if err := ioutil.WriteFile(
+		tempFile,
+		[]byte(expected),
+		0755,
+	); err != nil {
+		t.Fatal(err)
+	}
+
+	ignored, err := CopyFile(tempFile, tempFile, "")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if ignored {
+		t.Fatal("expected file to NOT be ignored")
+	}
+
+	// Ensure file has expected contents
+	actualData, err := ioutil.ReadFile(tempFile)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if actual := string(actualData); actual != expected {
+		t.Fatalf("expected file contents to be %q, but got %q", expected, actual)
+	}
+}