1 files changed, 51 insertions, 0 deletions
diff --git a/internal/atomicfile/atomicfile.go b/internal/atomicfile/atomicfile.go
new file mode 100644
index 0000000..542f58a
--- /dev/null
+++ b/internal/atomicfile/atomicfile.go
@@ -0,0 +1,51 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Package atomicfile contains code related to writing to filesystems
+// atomically.
+//
+// This package should be considered internal; its API is not stable.
+package atomicfile // import "tailscale.com/atomicfile"
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+	"runtime"
+)
+
+// WriteFile writes data to filename+some suffix, then renames it into filename.
+// The perm argument is ignored on Windows. If the target filename already
+// exists but is not a regular file, WriteFile returns an error.
+func WriteFile(filename string, data []byte, perm os.FileMode) (err error) {
+	fi, err := os.Stat(filename)
+	if err == nil && !fi.Mode().IsRegular() {
+		return fmt.Errorf("%s already exists and is not a regular file", filename)
+	}
+	f, err := os.CreateTemp(filepath.Dir(filename), filepath.Base(filename)+".tmp")
+	if err != nil {
+		return err
+	}
+	tmpName := f.Name()
+	defer func() {
+		if err != nil {
+			f.Close()          //nolint:errcheck
+			os.Remove(tmpName) //nolint:errcheck
+		}
+	}()
+	if _, err := f.Write(data); err != nil {
+		return err
+	}
+	if runtime.GOOS != "windows" {
+		if err := f.Chmod(perm); err != nil {
+			return err
+		}
+	}
+	if err := f.Sync(); err != nil {
+		return err
+	}
+	if err := f.Close(); err != nil {
+		return err
+	}
+	return os.Rename(tmpName, filename)
+}