renameio: initial commit (#39)

Based on https://go-review.googlesource.com/c/go/+/146377

Taken from Go repo as of f880efcc1666e2b99fbda644eeff258e4a15dd38

Closes #36.

Author: myitcv

renameio/renameio.go

@@ -0,0 +1,63 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package renameio writes files atomically by renaming temporary files.
+package renameio
+
+import (
+	"bytes"
+	"io"
+	"io/ioutil"
+	"os"
+	"path/filepath"
+)
+
+const patternSuffix = "*.tmp"
+
+// Pattern returns a glob pattern that matches the unrenamed temporary files
+// created when writing to filename.
+func Pattern(filename string) string {
+	return filepath.Join(filepath.Dir(filename), filepath.Base(filename)+patternSuffix)
+}
+
+// WriteFile is like ioutil.WriteFile, but first writes data to an arbitrary
+// file in the same directory as filename, then renames it atomically to the
+// final name.
+//
+// That ensures that the final location, if it exists, is always a complete file.
+func WriteFile(filename string, data []byte) (err error) {
+	return WriteToFile(filename, bytes.NewReader(data))
+}
+
+// WriteToFile is a variant of WriteFile that accepts the data as an io.Reader
+// instead of a slice.
+func WriteToFile(filename string, data io.Reader) (err error) {
+	f, err := ioutil.TempFile(filepath.Dir(filename), filepath.Base(filename)+patternSuffix)
+	if err != nil {
+		return err
+	}
+	defer func() {
+		// Only call os.Remove on f.Name() if we failed to rename it: otherwise,
+		// some other process may have created a new file with the same name after
+		// that.
+		if err != nil {
+			f.Close()
+			os.Remove(f.Name())
+		}
+	}()
+
+	if _, err := io.Copy(f, data); err != nil {
+		return err
+	}
+	// Sync the file before renaming it: otherwise, after a crash the reader may
+	// observe a 0-length file instead of the actual contents.
+	// See https://golang.org/issue/22397#issuecomment-380831736.
+	if err := f.Sync(); err != nil {
+		return err
+	}
+	if err := f.Close(); err != nil {
+		return err
+	}
+	return os.Rename(f.Name(), filename)
+}