1 files changed, 127 insertions, 0 deletions
diff --git a/src/sync/waitgroup.go b/src/sync/waitgroup.go
new file mode 100644
index 0000000..be21417
--- /dev/null
+++ b/src/sync/waitgroup.go
@@ -0,0 +1,127 @@
+// 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 (
+	"internal/race"
+	"sync/atomic"
+	"unsafe"
+)
+
+// 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.
+//
+// A WaitGroup must not be copied after first use.
+//
+// In the terminology of the Go memory model, a call to Done
+// “synchronizes before” the return of any Wait call that it unblocks.
+type WaitGroup struct {
+	noCopy noCopy
+
+	state atomic.Uint64 // high 32 bits are counter, low 32 bits are waiter count.
+	sema  uint32
+}
+
+// Add adds delta, which may be negative, to the WaitGroup counter.
+// If the counter becomes zero, all goroutines blocked on Wait are released.
+// If the counter goes negative, Add panics.
+//
+// Note that calls with a positive delta that occur when the counter is zero
+// must happen before a Wait. Calls with a negative delta, or calls with a
+// positive delta that start when the counter is greater than zero, may happen
+// at any time.
+// Typically this means the calls to Add should execute before the statement
+// creating the goroutine or other event to be waited for.
+// If a WaitGroup is reused to wait for several independent sets of events,
+// new Add calls must happen after all previous Wait calls have returned.
+// See the WaitGroup example.
+func (wg *WaitGroup) Add(delta int) {
+	if race.Enabled {
+		if delta < 0 {
+			// Synchronize decrements with Wait.
+			race.ReleaseMerge(unsafe.Pointer(wg))
+		}
+		race.Disable()
+		defer race.Enable()
+	}
+	state := wg.state.Add(uint64(delta) << 32)
+	v := int32(state >> 32)
+	w := uint32(state)
+	if race.Enabled && delta > 0 && v == int32(delta) {
+		// The first increment must be synchronized with Wait.
+		// Need to model this as a read, because there can be
+		// several concurrent wg.counter transitions from 0.
+		race.Read(unsafe.Pointer(&wg.sema))
+	}
+	if v < 0 {
+		panic("sync: negative WaitGroup counter")
+	}
+	if w != 0 && delta > 0 && v == int32(delta) {
+		panic("sync: WaitGroup misuse: Add called concurrently with Wait")
+	}
+	if v > 0 || w == 0 {
+		return
+	}
+	// This goroutine has set counter to 0 when waiters > 0.
+	// Now there can't be concurrent mutations of state:
+	// - Adds must not happen concurrently with Wait,
+	// - Wait does not increment waiters if it sees counter == 0.
+	// Still do a cheap sanity check to detect WaitGroup misuse.
+	if wg.state.Load() != state {
+		panic("sync: WaitGroup misuse: Add called concurrently with Wait")
+	}
+	// Reset waiters count to 0.
+	wg.state.Store(0)
+	for ; w != 0; w-- {
+		runtime_Semrelease(&wg.sema, false, 0)
+	}
+}
+
+// Done decrements the WaitGroup counter by one.
+func (wg *WaitGroup) Done() {
+	wg.Add(-1)
+}
+
+// Wait blocks until the WaitGroup counter is zero.
+func (wg *WaitGroup) Wait() {
+	if race.Enabled {
+		race.Disable()
+	}
+	for {
+		state := wg.state.Load()
+		v := int32(state >> 32)
+		w := uint32(state)
+		if v == 0 {
+			// Counter is 0, no need to wait.
+			if race.Enabled {
+				race.Enable()
+				race.Acquire(unsafe.Pointer(wg))
+			}
+			return
+		}
+		// Increment waiters count.
+		if wg.state.CompareAndSwap(state, state+1) {
+			if race.Enabled && w == 0 {
+				// Wait must be synchronized with the first Add.
+				// Need to model this is as a write to race with the read in Add.
+				// As a consequence, can do the write only for the first waiter,
+				// otherwise concurrent Waits will race with each other.
+				race.Write(unsafe.Pointer(&wg.sema))
+			}
+			runtime_Semacquire(&wg.sema)
+			if wg.state.Load() != 0 {
+				panic("sync: WaitGroup is reused before previous Wait has returned")
+			}
+			if race.Enabled {
+				race.Enable()
+				race.Acquire(unsafe.Pointer(wg))
+			}
+			return
+		}
+	}
+}