Make LazyMultiError only constructable via function

common/errors/lazymultierror.go

+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+package errors
+
+import (
+        "sync"
+)
+
+// LazyMultiError is a lazily-constructed MultiError.
+//
+// LazyMultiError is like MultiError, except that you know the ultimate size up
+// front, and then you call Assign for each error encountered, and it's
+// potential index. The underlying MultiError will only be allocated if one of
+// the Assign'd errors is non-nil. Similarly, Get will retrieve either the
+// allocated MultiError, or nil if no error was encountered.
+// Build one with NewLazyMultiError.
+type LazyMultiError interface {
+        // Assign semantically assigns the error to the given index in the MultiError.
+        // If the error is nil, no action is taken. Otherwise the MultiError is
+        // allocated to its full size (if not already), and the error assigned into
+        // it.
+        //
+        // Returns true iff err != nil (i.e. "was it assigned?"), so you can use this
+        // like:
+        //   if !lme.Assign(i, err) {
+        //     // stuff requiring err == nil
+        //   }
+        Assign(int, error) bool
+
+        // GetOne returns the error at the given index (which may be nil)
+        GetOne(int) error
+
+        // Get returns the MultiError, or nil, if no non-nil error was Assign'd.
+        Get() error
+}
+
+type lazyMultiError struct {
+        sync.Mutex
+
+        size int
+        me   MultiError
+}
+
+// NewLazyMultiError makes a new LazyMultiError of the provided size.
+func NewLazyMultiError(size int) *lazyMultiError {
+        return &lazyMultiError{size: size}
+}
+
+func (e *lazyMultiError) Assign(i int, err error) bool {
+        if err == nil {
+                return false
+        }
+        e.Lock()
+        defer e.Unlock()
+        if e.me == nil {
+                e.me = make(MultiError, e.size)
+        }
+        e.me[i] = err
+        return true
+}
+
+func (e *lazyMultiError) GetOne(i int) error {
+        e.Lock()
+        defer e.Unlock()
+        if e.me == nil {
+                return nil
+        }
+        return e.me[i]
+}
+
+func (e *lazyMultiError) Get() error {
+        e.Lock()
+        defer e.Unlock()
+        if e.me == nil {
+                return nil
+        }
+        return e.me
+}