chore(internal/aliasfix): properly truncate and backup files (#6637)

- Use os.Create to truncate existing file if needed
- Use named imports to capture error
- Create a bak incase anything goes wrong. We don't want to delete
  peoples code :)
- Bubble up error if any from WalkDir

internal/aliasfix/aliasfix.go

@@ -45,7 +45,7 @@ func ProcessPath(path string) error {
 		return err
 	}
 	if dir.IsDir() {
-		filepath.WalkDir(path, func(path string, d fs.DirEntry, err error) error {
+		err := filepath.WalkDir(path, func(path string, d fs.DirEntry, err error) error {
 			if err == nil && !d.IsDir() && strings.HasSuffix(d.Name(), ".go") {
 				err = processFile(path, nil)
 			}
@@ -54,6 +54,9 @@ func ProcessPath(path string) error {
 			}
 			return nil
 		})
+		if err != nil {
+			return err
+		}
 	} else {
 		if err := processFile(path, nil); err != nil {
 			return err
@@ -64,22 +67,16 @@ func ProcessPath(path string) error {
 
 // processFile checks to see if the given file needs any imports rewritten and
 // does so if needed. Note an io.Writer is injected here for testability.
-func processFile(name string, w io.Writer) error {
-	if w == nil {
-		file, err := os.Open(name)
-		if err != nil {
-			return err
-		}
-		defer file.Close()
-		w = file
-	}
-	f, err := parser.ParseFile(fset, name, nil, parser.ParseComments)
+func processFile(name string, w io.Writer) (err error) {
+	var f *ast.File
+	f, err = parser.ParseFile(fset, name, nil, parser.ParseComments)
 	if err != nil {
 		return err
 	}
 	var modified bool
 	for _, imp := range f.Imports {
-		importPath, err := strconv.Unquote(imp.Path.Value)
+		var importPath string
+		importPath, err = strconv.Unquote(imp.Path.Value)
 		if err != nil {
 			return err
 		}
@@ -98,18 +95,41 @@ func processFile(name string, w io.Writer) error {
 			modified = true
 		}
 	}
-	if modified {
-		var buf bytes.Buffer
-		if err := format.Node(&buf, fset, f); err != nil {
+	if !modified {
+		return nil
+	}
+
+	if w == nil {
+		backup := name + ".bak"
+		if err = os.Rename(name, backup); err != nil {
 			return err
 		}
-		b, err := imports.Process(name, buf.Bytes(), nil)
+		defer func() {
+			if err != nil {
+				os.Rename(backup, name)
+			} else {
+				os.Remove(backup)
+			}
+		}()
+		var file *os.File
+		file, err = os.Create(name)
 		if err != nil {
 			return err
 		}
-		if _, err := w.Write(b); err != nil {
-			return err
-		}
+		defer file.Close()
+		w = file
+	}
+	var buf bytes.Buffer
+	if err := format.Node(&buf, fset, f); err != nil {
+		return err
 	}
+	b, err := imports.Process(name, buf.Bytes(), nil)
+	if err != nil {
+		return err
+	}
+	if _, err := w.Write(b); err != nil {
+		return err
+	}
+
 	return nil
 }