fs/fsutil: write files atomically

Writing files atomically is very important. Unfortunately, it seems
atomic rename is not available on Windows, so this feature is Unix-only.

fs/fsutil/os_driver.go

@@ -90,12 +90,6 @@ func (OSDriver) Symlink(oldname, newname string) error {
 	return os.Symlink(oldname, newname)
 }
 
-// WriteFile writes data to filename with permission perm.
-func (OSDriver) WriteFile(filename string, data []byte, perm os.FileMode) error {
-	// TODO(gbrlsnchs): use package "renameio"
-	return ioutil.WriteFile(filename, data, perm)
-}
-
 type fileInfo struct {
 	name     string
 	exists   bool

fs/fsutil/os_driver_unix.go

@@ -0,0 +1,14 @@
+// +build !windows
+
+package fsutil
+
+import (
+	"os"
+
+	"github.com/google/renameio"
+)
+
+// WriteFile writes data to filename atomically with permission perm.
+func (OSDriver) WriteFile(filename string, data []byte, perm os.FileMode) error {
+	return renameio.WriteFile(filename, data, perm)
+}

fs/fsutil/os_driver_windows.go

@@ -0,0 +1,13 @@
+// +build windows
+
+package fsutil
+
+import (
+	"io/ioutil"
+	"os"
+)
+
+// WriteFile writes data to filename with permission perm.
+func (OSDriver) WriteFile(filename string, data []byte, perm os.FileMode) error {
+	return ioutil.WriteFile(filename, data, perm)
+}

go.mod

@@ -7,6 +7,7 @@ require (
 	github.com/gbrlsnchs/cli v0.6.0
 	github.com/google/go-cmdtest v0.2.0
 	github.com/google/go-cmp v0.4.0
+	github.com/google/renameio v0.1.0
 	github.com/magefile/mage v1.9.0
 	github.com/sergi/go-diff v1.1.0 // indirect
 	golang.org/x/lint v0.0.0-20191125180803-fdd1cda4f05f