Make LazyMultiError only constructable via function

common/errors/lazymultierror.go

Index: common/errors/lazymultierror.go
diff --git a/common/errors/lazymultierror.go b/common/errors/lazymultierror.go
new file mode 100644
index 0000000000000000000000000000000000000000..bfdafb417e8d3622b31efeaa058d85b7876db8d3
--- /dev/null
+++ b/common/errors/lazymultierror.go
@@ -0,0 +1,80 @@
+// 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
+}