add WriteFile (#7)

writefile.go

@@ -0,0 +1,38 @@
+// Copyright 2018 Google Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package renameio
+
+import "os"
+
+// WriteFile mirrors ioutil.WriteFile, replacing an existing file with the same
+// name atomically.
+func WriteFile(filename string, data []byte, perm os.FileMode) error {
+	t, err := TempFile("", filename)
+	if err != nil {
+		return err
+	}
+	defer t.Cleanup()
+
+	// Set permissions before writing data, in case the data is sensitive.
+	if err := t.Chmod(perm); err != nil {
+		return err
+	}
+
+	if _, err := t.Write(data); err != nil {
+		return err
+	}
+
+	return t.CloseAtomicallyReplace()
+}

writefile_test.go

@@ -0,0 +1,57 @@
+// Copyright 2018 Google Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// +build darwin dragonfly freebsd linux nacl netbsd openbsd solaris windows
+
+package renameio
+
+import (
+	"bytes"
+	"io/ioutil"
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func TestWriteFile(t *testing.T) {
+	d, err := ioutil.TempDir("", "tempdirtest")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(d)
+
+	filename := filepath.Join(d, "hello.sh")
+
+	wantData := []byte("#!/bin/sh\necho \"Hello World\"\n")
+	wantPerm := os.FileMode(0755)
+	if err := WriteFile(filename, wantData, wantPerm); err != nil {
+		t.Fatal(err)
+	}
+
+	gotData, err := ioutil.ReadFile(filename)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !bytes.Equal(gotData, wantData) {
+		t.Errorf("got data %v, want data %v", gotData, wantData)
+	}
+
+	fi, err := os.Stat(filename)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if gotPerm := fi.Mode() & os.ModePerm; gotPerm != wantPerm {
+		t.Errorf("got permissions 0%o, want permissions 0%o", gotPerm, wantPerm)
+	}
+}