pkg/util/walkers: truncate file when writing

To avoid leaving old content untouched when updating files, which may
happen if new content is smaller than the old one.

Closes #40

Signed-off-by: Mateusz Gozdek <mateusz@kinvolk.io>

pkg/util/walkers/copying.go

@@ -45,7 +45,7 @@ func CopyingWalker(path string, newDirPerms os.FileMode) assets.WalkFunc {
 // this is the only content stored in the file.
 func writeFile(p string, r io.Reader) error {
 	// TODO: If we start packing binaries, make sure they have executable bit set.
-	f, err := os.OpenFile(p, os.O_RDWR|os.O_CREATE, 0644)
+	f, err := os.OpenFile(p, os.O_RDWR|os.O_CREATE|os.O_TRUNC, 0644)
 	if err != nil {
 		return fmt.Errorf("failed to open target file %s: %w", p, err)
 	}

pkg/util/walkers/copying_test.go

@@ -48,3 +48,29 @@ func TestWriteFile(t *testing.T) {
 		t.Fatalf("Expected: '%s', got '%s'", testData, string(d))
 	}
 }
+
+func TestWriteFileTruncate(t *testing.T) {
+	f, err := ioutil.TempFile("", tmpPattern)
+	if err != nil {
+		t.Fatalf("Creating temp file should succeed, got: %v", err)
+	}
+
+	defer os.Remove(f.Name())
+
+	if err := writeFile(f.Name(), strings.NewReader("111111")); err != nil {
+		t.Fatalf("Writing to file should succeed, got: %v", err)
+	}
+
+	if err := writeFile(f.Name(), strings.NewReader(testData)); err != nil {
+		t.Fatalf("Updating file should succeed, got: %v", err)
+	}
+
+	d, err := ioutil.ReadFile(f.Name())
+	if err != nil {
+		t.Fatalf("Reading tmp file should succeed, got: %v", err)
+	}
+
+	if !reflect.DeepEqual(testData, string(d)) {
+		t.Fatalf("Expected: '%s', got '%s'", testData, string(d))
+	}
+}