Add syncutil.Group: an easy way to do multiple things at once.

Change-Id: Ie28885a5f42019dd1c3fb14f92c7cc5d40e08742

pkg/syncutil/group.go

@@ -0,0 +1,64 @@
+/*
+Copyright 2013 Google Inc.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+     http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package syncutil
+
+import "sync"
+
+// A Group is like a sync.WaitGroup and coordinates doing
+// multiple things at once. Its zero value is ready to use.
+type Group struct {
+	wg   sync.WaitGroup
+	mu   sync.Mutex // guards errs
+	errs []error
+}
+
+// Go runs fn in its own goroutine, but does not wait for it to complete.
+// Call Err or Errs to wait for all the goroutines to complete.
+func (g *Group) Go(fn func() error) {
+	g.wg.Add(1)
+	go func() {
+		defer g.wg.Done()
+		err := fn()
+		if err != nil {
+			g.mu.Lock()
+			defer g.mu.Unlock()
+			g.errs = append(g.errs, err)
+		}
+	}()
+}
+
+// Wait waits for all the previous calls to Go to complete.
+func (g *Group) Wait() {
+	g.wg.Wait()
+}
+
+// Err waits for all previous calls to Go to complete and returns the
+// first non-nil error, or nil.
+func (g *Group) Err() error {
+	g.wg.Wait()
+	if len(g.errs) > 0 {
+		return g.errs[0]
+	}
+	return nil
+}
+
+// Err waits for all previous calls to Go to complete and returns the
+// first non-nil error, or nil.
+func (g *Group) Errs() []error {
+	g.wg.Wait()
+	return g.errs
+}