Add a new internal/safeio package for safer I/O functions

This patch adds a new internal/safeio package, which is meant to implement
safer version of some I/O related functions.

For now, only an atomic version of ioutil.WriteFile is implemented. More may
be added later if there's a need for them.

internal/safeio/safeio.go

@@ -0,0 +1,43 @@
+// Package safeio implements convenient I/O routines that provide additional
+// levels of safety in the presence of unexpected failures.
+package safeio
+
+import (
+	"io/ioutil"
+	"os"
+	"path"
+)
+
+// WriteFile writes data to a file named by filename, atomically.
+// It's a wrapper to ioutil.WriteFile, but provides atomicity (and increased
+// safety) by writing to a temporary file and renaming it at the end.
+//
+// Note this relies on same-directory Rename being atomic, which holds in most
+// reasonably modern filesystems.
+func WriteFile(filename string, data []byte, perm os.FileMode) error {
+	// Note we create the temporary file in the same directory, otherwise we
+	// would have no expectation of Rename being atomic.
+	tmpf, err := ioutil.TempFile(path.Dir(filename), path.Base(filename))
+	if err != nil {
+		return err
+	}
+
+	if err = os.Chmod(tmpf.Name(), perm); err != nil {
+		tmpf.Close()
+		os.Remove(tmpf.Name())
+		return err
+	}
+
+	if _, err = tmpf.Write(data); err != nil {
+		tmpf.Close()
+		os.Remove(tmpf.Name())
+		return err
+	}
+
+	if err = tmpf.Close(); err != nil {
+		os.Remove(tmpf.Name())
+		return err
+	}
+
+	return os.Rename(tmpf.Name(), filename)
+}