[FAB-13347] introduce counting semaphore

The counting semaphore will be used as part of throttling
implementation.

Change-Id: Ie8741414a6b25373072a611a55620519051489cc
Signed-off-by: Matthew Sykes <sykesmat@us.ibm.com>
(cherry picked from commit f975549)

common/semaphore/semaphore.go

@@ -0,0 +1,35 @@
+/*
+Copyright IBM Corp. All Rights Reserved.
+
+SPDX-License-Identifier: Apache-2.0
+*/
+
+package semaphore
+
+import "context"
+
+type Semaphore chan struct{}
+
+func New(count int) Semaphore {
+	if count <= 0 {
+		panic("count must be greater than 0")
+	}
+	return make(chan struct{}, count)
+}
+
+func (s Semaphore) Acquire(ctx context.Context) error {
+	select {
+	case <-ctx.Done():
+		return ctx.Err()
+	case s <- struct{}{}:
+		return nil
+	}
+}
+
+func (s Semaphore) Release() {
+	select {
+	case <-s:
+	default:
+		panic("semaphore buffer is empty")
+	}
+}

common/semaphore/semaphore_test.go

@@ -0,0 +1,63 @@
+/*
+Copyright IBM Corp. All Rights Reserved.
+
+SPDX-License-Identifier: Apache-2.0
+*/
+
+package semaphore_test
+
+import (
+	"context"
+	"testing"
+
+	"github.com/hyperledger/fabric/common/semaphore"
+	. "github.com/onsi/gomega"
+	"github.com/stretchr/testify/assert"
+)
+
+func TestNewSemaphorePanic(t *testing.T) {
+	assert.PanicsWithValue(t, "count must be greater than 0", func() { semaphore.New(0) })
+}
+
+func TestSemaphoreBlocking(t *testing.T) {
+	gt := NewGomegaWithT(t)
+
+	sema := semaphore.New(5)
+	for i := 0; i < 5; i++ {
+		err := sema.Acquire(context.Background())
+		gt.Expect(err).NotTo(HaveOccurred())
+	}
+
+	done := make(chan struct{})
+	go func() {
+		err := sema.Acquire(context.Background())
+		gt.Expect(err).NotTo(HaveOccurred())
+
+		close(done)
+		sema.Release()
+	}()
+
+	gt.Consistently(done).ShouldNot(BeClosed())
+	sema.Release()
+	gt.Eventually(done).Should(BeClosed())
+}
+
+func TestSemaphoreContextError(t *testing.T) {
+	gt := NewGomegaWithT(t)
+
+	sema := semaphore.New(1)
+	err := sema.Acquire(context.Background())
+	gt.Expect(err).NotTo(HaveOccurred())
+
+	ctx, cancel := context.WithCancel(context.Background())
+	cancel()
+	errCh := make(chan error, 1)
+	go func() { errCh <- sema.Acquire(ctx) }()
+
+	gt.Eventually(errCh).Should(Receive(Equal(context.Canceled)))
+}
+
+func TestSemaphoreReleaseTooMany(t *testing.T) {
+	sema := semaphore.New(1)
+	assert.PanicsWithValue(t, "semaphore buffer is empty", func() { sema.Release() })
+}