Added syncutil.Once

Change-Id: I83744a3646bac449934ab0dc6477b4dd1c351e5e

pkg/syncutil/once.go

@@ -0,0 +1,60 @@
+/*
+Copyright 2014 The Camlistore Authors
+
+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"
+	"sync/atomic"
+)
+
+// A Once will perform a successful action exactly once.
+//
+// Unlike a sync.Once, this Once's func returns an error
+// and is re-armed on failure.
+type Once struct {
+	m    sync.Mutex
+	done uint32
+}
+
+// Do calls the function f if and only if Do has not been invoked
+// without error for this instance of Once.  In other words, given
+// 	var once Once
+// if once.Do(f) is called multiple times, only the first call will
+// invoke f, even if f has a different value in each invocation unless
+// f returns an error.  A new instance of Once is required for each
+// function to execute.
+//
+// Do is intended for initialization that must be run exactly once.  Since f
+// is niladic, it may be necessary to use a function literal to capture the
+// arguments to a function to be invoked by Do:
+// 	err := config.once.Do(func() error { return config.init(filename) })
+func (o *Once) Do(f func() error) error {
+	if atomic.LoadUint32(&o.done) == 1 {
+		return nil
+	}
+	// Slow-path.
+	o.m.Lock()
+	defer o.m.Unlock()
+	var err error
+	if o.done == 0 {
+		err = f()
+		if err == nil {
+			atomic.StoreUint32(&o.done, 1)
+		}
+	}
+	return err
+}

pkg/syncutil/once_test.go

@@ -0,0 +1,57 @@
+package syncutil
+
+import (
+	"errors"
+	"testing"
+)
+
+func TestOnce(t *testing.T) {
+	timesRan := 0
+	f := func() error {
+		timesRan++
+		return nil
+	}
+
+	once := Once{}
+	grp := Group{}
+
+	for i := 0; i < 10; i++ {
+		grp.Go(func() error { return once.Do(f) })
+	}
+
+	if grp.Err() != nil {
+		t.Errorf("Expected no errors, got %v", grp.Err())
+	}
+
+	if timesRan != 1 {
+		t.Errorf("Expected to run one time, ran %d", timesRan)
+	}
+}
+
+// TestOnceErroring verifies we retry on every error, but stop after
+// the first success.
+func TestOnceErroring(t *testing.T) {
+	timesRan := 0
+	f := func() error {
+		timesRan++
+		if timesRan < 3 {
+			return errors.New("retry")
+		}
+		return nil
+	}
+
+	once := Once{}
+	grp := Group{}
+
+	for i := 0; i < 10; i++ {
+		grp.Go(func() error { return once.Do(f) })
+	}
+
+	if len(grp.Errs()) != 2 {
+		t.Errorf("Expected two errors, got %d", len(grp.Errs()))
+	}
+
+	if timesRan != 3 {
+		t.Errorf("Expected to run two times, ran %d", timesRan)
+	}
+}