fix(system): write all files atomically

system/file.go

@@ -31,33 +31,55 @@ func (f *File) Permissions() (os.FileMode, error) {
 	return os.FileMode(perm), nil
 }
 
-func WriteFile(f *File) error {
+func WriteFile(f *File, root string) (string, error) {
 	if f.Encoding != "" {
-		return fmt.Errorf("Unable to write file with encoding %s", f.Encoding)
+		return "", fmt.Errorf("Unable to write file with encoding %s", f.Encoding)
 	}
 
-	if err := os.MkdirAll(path.Dir(f.Path), os.FileMode(0755)); err != nil {
-		return err
+	fullpath := path.Join(root, f.Path)
+	dir := path.Dir(fullpath)
+
+	if err := EnsureDirectoryExists(dir); err != nil {
+		return "", err
 	}
 
 	perm, err := f.Permissions()
 	if err != nil {
-		return err
+		return "", err
 	}
 
-	if err := ioutil.WriteFile(f.Path, []byte(f.Content), perm); err != nil {
-		return err
+	var tmp *os.File
+	// Create a temporary file in the same directory to ensure it's on the same filesystem
+	if tmp, err = ioutil.TempFile(dir, "cloudinit-temp"); err != nil {
+		return "", err
+	}
+
+	if err := ioutil.WriteFile(tmp.Name(), []byte(f.Content), perm); err != nil {
+		return "", err
+	}
+
+	if err := tmp.Close(); err != nil {
+		return "", err
+	}
+
+	// Ensure the permissions are as requested (since WriteFile can be affected by sticky bit)
+	if err := os.Chmod(tmp.Name(), perm); err != nil {
+		return "", err
 	}
 
 	if f.Owner != "" {
 		// We shell out since we don't have a way to look up unix groups natively
-		cmd := exec.Command("chown", f.Owner, f.Path)
+		cmd := exec.Command("chown", f.Owner, tmp.Name())
 		if err := cmd.Run(); err != nil {
-			return err
+			return "", err
 		}
 	}
 
-	return nil
+	if err := os.Rename(tmp.Name(), fullpath); err != nil {
+		return "", err
+	}
+
+	return fullpath, nil
 }
 
 func EnsureDirectoryExists(dir string) error {

system/file_test.go

@@ -4,7 +4,6 @@ import (
 	"io/ioutil"
 	"os"
 	"path"
-	"syscall"
 	"testing"
 )
 
@@ -13,18 +12,22 @@ func TestWriteFileUnencodedContent(t *testing.T) {
 	if err != nil {
 		t.Fatalf("Unable to create tempdir: %v", err)
 	}
-	defer syscall.Rmdir(dir)
+	defer os.RemoveAll(dir)
 
-	fullPath := path.Join(dir, "tmp", "foo")
+	fn := "foo"
+	fullPath := path.Join(dir, fn)
 
 	wf := File{
-		Path:        fullPath,
-		Content:     "bar",
+		Path:               fn,
+		Content:            "bar",
 		RawFilePermissions: "0644",
 	}
 
-	if err := WriteFile(&wf); err != nil {
+	path, err := WriteFile(&wf, dir)
+	if err != nil {
 		t.Fatalf("Processing of WriteFile failed: %v", err)
+	} else if path != fullPath {
+		t.Fatalf("WriteFile returned bad path: want %s, got %s", fullPath, path)
 	}
 
 	fi, err := os.Stat(fullPath)
@@ -51,15 +54,15 @@ func TestWriteFileInvalidPermission(t *testing.T) {
 	if err != nil {
 		t.Fatalf("Unable to create tempdir: %v", err)
 	}
-	defer syscall.Rmdir(dir)
+	defer os.RemoveAll(dir)
 
 	wf := File{
-		Path:        path.Join(dir, "tmp", "foo"),
-		Content:     "bar",
+		Path:               path.Join(dir, "tmp", "foo"),
+		Content:            "bar",
 		RawFilePermissions: "pants",
 	}
 
-	if err := WriteFile(&wf); err == nil {
+	if _, err := WriteFile(&wf, dir); err == nil {
 		t.Fatalf("Expected error to be raised when writing file with invalid permission")
 	}
 }
@@ -69,17 +72,21 @@ func TestWriteFilePermissions(t *testing.T) {
 	if err != nil {
 		t.Fatalf("Unable to create tempdir: %v", err)
 	}
-	defer syscall.Rmdir(dir)
+	defer os.RemoveAll(dir)
 
-	fullPath := path.Join(dir, "tmp", "foo")
+	fn := "foo"
+	fullPath := path.Join(dir, fn)
 
 	wf := File{
-		Path:               fullPath,
+		Path:               fn,
 		RawFilePermissions: "0755",
 	}
 
-	if err := WriteFile(&wf); err != nil {
+	path, err := WriteFile(&wf, dir)
+	if err != nil {
 		t.Fatalf("Processing of WriteFile failed: %v", err)
+	} else if path != fullPath {
+		t.Fatalf("WriteFile returned bad path: want %s, got %s", fullPath, path)
 	}
 
 	fi, err := os.Stat(fullPath)
@@ -97,15 +104,15 @@ func TestWriteFileEncodedContent(t *testing.T) {
 	if err != nil {
 		t.Fatalf("Unable to create tempdir: %v", err)
 	}
-	defer syscall.Rmdir(dir)
+	defer os.RemoveAll(dir)
 
 	wf := File{
-		Path: path.Join(dir, "tmp", "foo"),
-		Content: "",
+		Path:     path.Join(dir, "tmp", "foo"),
+		Content:  "",
 		Encoding: "base64",
 	}
 
-	if err := WriteFile(&wf); err == nil {
+	if _, err := WriteFile(&wf, dir); err == nil {
 		t.Fatalf("Expected error to be raised when writing file with encoding")
 	}
 }

system/systemd.go

@@ -78,12 +78,12 @@ func PlaceUnit(u *Unit, dst string) error {
 	}
 
 	file := File{
-		Path:               dst,
+		Path:               filepath.Base(dst),
 		Content:            u.Content,
 		RawFilePermissions: "0644",
 	}
 
-	err := WriteFile(&file)
+	_, err := WriteFile(&file, dir)
 	if err != nil {
 		return err
 	}