diff --git a/common/os.go b/common/os.go
index 9dc81c579..b1e778977 100644
--- a/common/os.go
+++ b/common/os.go
@@ -93,28 +93,30 @@ func MustWriteFile(filePath string, contents []byte, mode os.FileMode) {
 	}
 }
 
-// Writes to newBytes to filePath.
-// Guaranteed not to lose *both* oldBytes and newBytes,
-// (assuming that the OS is perfect)
+// WriteFileAtomic writes newBytes to temp and atomically moves to filePath
+// when everything else succeeds.
 func WriteFileAtomic(filePath string, newBytes []byte, mode os.FileMode) error {
-	// If a file already exists there, copy to filePath+".bak" (overwrite anything)
-	if _, err := os.Stat(filePath); !os.IsNotExist(err) {
-		fileBytes, err := ioutil.ReadFile(filePath)
-		if err != nil {
-			return fmt.Errorf("Could not read file %v. %v", filePath, err)
-		}
-		err = ioutil.WriteFile(filePath+".bak", fileBytes, mode)
-		if err != nil {
-			return fmt.Errorf("Could not write file %v. %v", filePath+".bak", err)
-		}
+	f, err := ioutil.TempFile("", "")
+	if err != nil {
+		return err
+	}
+	_, err = f.Write(newBytes)
+	if err == nil {
+		err = f.Sync()
+	}
+	if closeErr := f.Close(); err == nil {
+		err = closeErr
+	}
+	if permErr := os.Chmod(f.Name(), mode); err == nil {
+		err = permErr
+	}
+	if err == nil {
+		err = os.Rename(f.Name(), filePath)
 	}
-	// Write newBytes to filePath.new
-	err := ioutil.WriteFile(filePath+".new", newBytes, mode)
+	// any err should result in full cleanup
 	if err != nil {
-		return fmt.Errorf("Could not write file %v. %v", filePath+".new", err)
+		os.Remove(f.Name())
 	}
-	// Move filePath.new to filePath
-	err = os.Rename(filePath+".new", filePath)
 	return err
 }
 
diff --git a/common/os_test.go b/common/os_test.go
new file mode 100644
index 000000000..05359e36e
--- /dev/null
+++ b/common/os_test.go
@@ -0,0 +1,29 @@
+package common
+
+import (
+	"bytes"
+	"fmt"
+	"io/ioutil"
+	"os"
+	"testing"
+	"time"
+)
+
+func TestWriteFileAtomic(t *testing.T) {
+	data := []byte("Becatron")
+	fname := fmt.Sprintf("/tmp/write-file-atomic-test-%v.txt", time.Now().UnixNano())
+	err := WriteFileAtomic(fname, data, 0664)
+	if err != nil {
+		t.Fatal(err)
+	}
+	rData, err := ioutil.ReadFile(fname)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !bytes.Equal(data, rData) {
+		t.Fatalf("data mismatch: %v != %v", data, rData)
+	}
+	if err := os.Remove(fname); err != nil {
+		t.Fatal(err)
+	}
+}