1 files changed, 97 insertions, 0 deletions
diff --git a/src/pkg/sync/waitgroup.go b/src/pkg/sync/waitgroup.go
new file mode 100644
index 000000000..a4c9b7e43
--- /dev/null
+++ b/src/pkg/sync/waitgroup.go
@@ -0,0 +1,97 @@
+// Copyright 2011 The Go 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 sync
+
+import (
+	"runtime"
+	"sync/atomic"
+)
+
+// A WaitGroup waits for a collection of goroutines to finish.
+// The main goroutine calls Add to set the number of
+// goroutines to wait for.  Then each of the goroutines
+// runs and calls Done when finished.  At the same time,
+// Wait can be used to block until all goroutines have finished.
+//
+// For example:
+//
+//   for i := 0; i < n; i++ {
+//       if !condition(i) {
+//           continue
+//       }
+//       wg.Add(1)
+//       go func() {
+//           // Do something.
+//           wg.Done()
+//       }()
+//   }
+//   wg.Wait()
+// 
+type WaitGroup struct {
+	m       Mutex
+	counter int32
+	waiters int32
+	sema    *uint32
+}
+
+// WaitGroup creates a new semaphore each time the old semaphore
+// is released. This is to avoid the following race:
+//
+// G1: Add(1)
+// G1: go G2()
+// G1: Wait() // Context switch after Unlock() and before Semacquire().
+// G2: Done() // Release semaphore: sema == 1, waiters == 0. G1 doesn't run yet.
+// G3: Wait() // Finds counter == 0, waiters == 0, doesn't block.
+// G3: Add(1) // Makes counter == 1, waiters == 0.
+// G3: go G4()
+// G3: Wait() // G1 still hasn't run, G3 finds sema == 1, unblocked! Bug.
+
+// Add adds delta, which may be negative, to the WaitGroup counter.
+// If the counter becomes zero, all goroutines blocked on Wait() are released.
+func (wg *WaitGroup) Add(delta int) {
+	v := atomic.AddInt32(&wg.counter, int32(delta))
+	if v < 0 {
+		panic("sync: negative WaitGroup count")
+	}
+	if v > 0 || atomic.LoadInt32(&wg.waiters) == 0 {
+		return
+	}
+	wg.m.Lock()
+	for i := int32(0); i < wg.waiters; i++ {
+		runtime.Semrelease(wg.sema)
+	}
+	wg.waiters = 0
+	wg.sema = nil
+	wg.m.Unlock()
+}
+
+// Done decrements the WaitGroup counter.
+func (wg *WaitGroup) Done() {
+	wg.Add(-1)
+}
+
+// Wait blocks until the WaitGroup counter is zero.
+func (wg *WaitGroup) Wait() {
+	if atomic.LoadInt32(&wg.counter) == 0 {
+		return
+	}
+	wg.m.Lock()
+	atomic.AddInt32(&wg.waiters, 1)
+	// This code is racing with the unlocked path in Add above.
+	// The code above modifies counter and then reads waiters.
+	// We must modify waiters and then read counter (the opposite order)
+	// to avoid missing an Add.
+	if atomic.LoadInt32(&wg.counter) == 0 {
+		atomic.AddInt32(&wg.waiters, -1)
+		wg.m.Unlock()
+		return
+	}
+	if wg.sema == nil {
+		wg.sema = new(uint32)
+	}
+	s := wg.sema
+	wg.m.Unlock()
+	runtime.Semacquire(s)
+}