semaphore: add a weighted semaphore implementation

This package provides a weighted semaphore that is context-aware.

The code is derived from a similar package inside Google.

Change-Id: Id1dad96d79e8ccfd289e4299e8265aa5bdad3a5b
Reviewed-on: https://go-review.googlesource.com/38298
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Damien Neil <dneil@google.com>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

semaphore/semaphore_test.go

@@ -0,0 +1,168 @@
+// Copyright 2017 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 semaphore
+
+import (
+	"math/rand"
+	"runtime"
+	"sync"
+	"testing"
+	"time"
+
+	"golang.org/x/net/context"
+	"golang.org/x/sync/errgroup"
+)
+
+const maxSleep = 1 * time.Millisecond
+
+func HammerWeighted(sem *Weighted, n int64, loops int) {
+	for i := 0; i < loops; i++ {
+		sem.Acquire(context.Background(), n)
+		time.Sleep(time.Duration(rand.Int63n(int64(maxSleep/time.Nanosecond))) * time.Nanosecond)
+		sem.Release(n)
+	}
+}
+
+func TestWeighted(t *testing.T) {
+	t.Parallel()
+
+	n := runtime.GOMAXPROCS(0)
+	sem := NewWeighted(int64(n))
+	var wg sync.WaitGroup
+	wg.Add(n)
+	for i := 0; i < n; i++ {
+		go func() {
+			defer wg.Done()
+			HammerWeighted(sem, int64(i), 1000)
+		}()
+	}
+	wg.Wait()
+}
+
+func TestWeightedPanic(t *testing.T) {
+	t.Parallel()
+
+	defer func() {
+		if recover() == nil {
+			t.Fatal("release of an unacquired weighted semaphore did not panic")
+		}
+	}()
+	w := NewWeighted(1)
+	w.Release(1)
+}
+
+func TestWeightedTryAcquire(t *testing.T) {
+	t.Parallel()
+
+	ctx := context.Background()
+	sem := NewWeighted(2)
+	tries := []bool{}
+	sem.Acquire(ctx, 1)
+	tries = append(tries, sem.TryAcquire(1))
+	tries = append(tries, sem.TryAcquire(1))
+
+	sem.Release(2)
+
+	tries = append(tries, sem.TryAcquire(1))
+	sem.Acquire(ctx, 1)
+	tries = append(tries, sem.TryAcquire(1))
+
+	want := []bool{true, false, true, false}
+	for i := range tries {
+		if tries[i] != want[i] {
+			t.Errorf("tries[%d]: got %t, want %t", i, tries[i], want[i])
+		}
+	}
+}
+
+func TestWeightedAcquire(t *testing.T) {
+	t.Parallel()
+
+	ctx := context.Background()
+	sem := NewWeighted(2)
+	tryAcquire := func(n int64) bool {
+		ctx, cancel := context.WithTimeout(ctx, 10*time.Millisecond)
+		defer cancel()
+		return sem.Acquire(ctx, n) == nil
+	}
+
+	tries := []bool{}
+	sem.Acquire(ctx, 1)
+	tries = append(tries, tryAcquire(1))
+	tries = append(tries, tryAcquire(1))
+
+	sem.Release(2)
+
+	tries = append(tries, tryAcquire(1))
+	sem.Acquire(ctx, 1)
+	tries = append(tries, tryAcquire(1))
+
+	want := []bool{true, false, true, false}
+	for i := range tries {
+		if tries[i] != want[i] {
+			t.Errorf("tries[%d]: got %t, want %t", i, tries[i], want[i])
+		}
+	}
+}
+
+func TestWeightedDoesntBlockIfTooBig(t *testing.T) {
+	t.Parallel()
+
+	const n = 2
+	sem := NewWeighted(n)
+	{
+		ctx, cancel := context.WithCancel(context.Background())
+		defer cancel()
+		go sem.Acquire(ctx, n+1)
+	}
+
+	g, ctx := errgroup.WithContext(context.Background())
+	for i := n * 3; i > 0; i-- {
+		g.Go(func() error {
+			err := sem.Acquire(ctx, 1)
+			if err == nil {
+				time.Sleep(1 * time.Millisecond)
+				sem.Release(1)
+			}
+			return err
+		})
+	}
+	if err := g.Wait(); err != nil {
+		t.Errorf("NewWeighted(%v) failed to AcquireCtx(_, 1) with AcquireCtx(_, %v) pending", n, n+1)
+	}
+}
+
+// TestLargeAcquireDoesntStarve times out if a large call to Acquire starves.
+// Merely returning from the test function indicates success.
+func TestLargeAcquireDoesntStarve(t *testing.T) {
+	t.Parallel()
+
+	ctx := context.Background()
+	n := int64(runtime.GOMAXPROCS(0))
+	sem := NewWeighted(n)
+	running := true
+
+	var wg sync.WaitGroup
+	wg.Add(int(n))
+	for i := n; i > 0; i-- {
+		sem.Acquire(ctx, 1)
+		go func() {
+			defer func() {
+				sem.Release(1)
+				wg.Done()
+			}()
+			for running {
+				time.Sleep(1 * time.Millisecond)
+				sem.Release(1)
+				sem.Acquire(ctx, 1)
+			}
+		}()
+	}
+
+	sem.Acquire(ctx, n)
+	running = false
+	sem.Release(n)
+	wg.Wait()
+}