mirror of https://github.com/hashicorp/consul
Daniel Nephin
4 years ago
parent
4e42b8f1d4
commit
28736e60fd
@ -0,0 +1,36 @@
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/*
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Package mutex implements the sync.Locker interface using x/sync/semaphore. It
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may be used as a replacement for sync.Mutex when one or more goroutines need to
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allow their calls to Lock to be cancelled by context cancellation.
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*/
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package mutex
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import (
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"context"
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"golang.org/x/sync/semaphore"
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)
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type Mutex semaphore.Weighted
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// New returns a Mutex that is ready for use.
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func New() *Mutex {
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return (*Mutex)(semaphore.NewWeighted(1))
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}
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func (m *Mutex) Lock() {
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_ = (*semaphore.Weighted)(m).Acquire(context.Background(), 1)
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}
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func (m *Mutex) Unlock() {
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(*semaphore.Weighted)(m).Release(1)
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}
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// TryLock acquires the mutex, blocking until resources are available or ctx is
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// done. On success, returns nil. On failure, returns ctx.Err() and leaves the
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// semaphore unchanged.
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//
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// If ctx is already done, Acquire may still succeed without blocking.
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func (m *Mutex) TryLock(ctx context.Context) error {
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return (*semaphore.Weighted)(m).Acquire(ctx, 1)
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}
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@ -0,0 +1,93 @@
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package mutex
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import (
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"context"
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"testing"
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"time"
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"github.com/stretchr/testify/require"
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)
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func TestMutex(t *testing.T) {
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t.Run("starts unlocked", func(t *testing.T) {
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m := New()
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canLock(t, m)
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})
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t.Run("Lock blocks when locked", func(t *testing.T) {
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m := New()
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m.Lock()
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lockIsBlocked(t, m)
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})
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t.Run("Unlock unblocks Lock", func(t *testing.T) {
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m := New()
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m.Lock()
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m.Unlock() // nolint:staticcheck // SA2001 is not relevant here
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canLock(t, m)
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})
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t.Run("TryLock acquires lock", func(t *testing.T) {
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m := New()
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ctx, cancel := context.WithTimeout(context.Background(), time.Second)
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t.Cleanup(cancel)
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require.NoError(t, m.TryLock(ctx))
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lockIsBlocked(t, m)
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})
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t.Run("TryLock blocks until timeout when locked", func(t *testing.T) {
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m := New()
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m.Lock()
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ctx, cancel := context.WithTimeout(context.Background(), 20*time.Millisecond)
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t.Cleanup(cancel)
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err := m.TryLock(ctx)
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require.Equal(t, err, context.DeadlineExceeded)
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})
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t.Run("TryLock acquires lock before timeout", func(t *testing.T) {
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m := New()
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m.Lock()
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go func() {
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time.Sleep(20 * time.Millisecond)
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m.Unlock()
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}()
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ctx, cancel := context.WithTimeout(context.Background(), time.Second)
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t.Cleanup(cancel)
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err := m.TryLock(ctx)
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require.NoError(t, err)
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})
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}
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func canLock(t *testing.T, m *Mutex) {
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t.Helper()
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chDone := make(chan struct{})
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go func() {
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m.Lock()
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close(chDone)
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}()
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select {
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case <-chDone:
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case <-time.After(20 * time.Millisecond):
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t.Fatal("failed to acquire lock before timeout")
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}
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}
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func lockIsBlocked(t *testing.T, m *Mutex) {
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t.Helper()
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chDone := make(chan struct{})
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go func() {
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m.Lock()
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close(chDone)
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}()
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select {
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case <-chDone:
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t.Fatal("expected Lock to block")
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case <-time.After(20 * time.Millisecond):
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}
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}
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@ -0,0 +1,136 @@
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// Copyright 2017 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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// Package semaphore provides a weighted semaphore implementation.
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package semaphore // import "golang.org/x/sync/semaphore"
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import (
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"container/list"
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"context"
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"sync"
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)
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type waiter struct {
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n int64
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ready chan<- struct{} // Closed when semaphore acquired.
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}
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// NewWeighted creates a new weighted semaphore with the given
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// maximum combined weight for concurrent access.
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func NewWeighted(n int64) *Weighted {
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w := &Weighted{size: n}
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return w
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}
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// Weighted provides a way to bound concurrent access to a resource.
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// The callers can request access with a given weight.
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type Weighted struct {
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size int64
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cur int64
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mu sync.Mutex
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waiters list.List
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}
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// Acquire acquires the semaphore with a weight of n, blocking until resources
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// are available or ctx is done. On success, returns nil. On failure, returns
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// ctx.Err() and leaves the semaphore unchanged.
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//
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// If ctx is already done, Acquire may still succeed without blocking.
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func (s *Weighted) Acquire(ctx context.Context, n int64) error {
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s.mu.Lock()
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if s.size-s.cur >= n && s.waiters.Len() == 0 {
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s.cur += n
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s.mu.Unlock()
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return nil
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}
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if n > s.size {
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// Don't make other Acquire calls block on one that's doomed to fail.
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s.mu.Unlock()
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<-ctx.Done()
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return ctx.Err()
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}
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ready := make(chan struct{})
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w := waiter{n: n, ready: ready}
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elem := s.waiters.PushBack(w)
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s.mu.Unlock()
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select {
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case <-ctx.Done():
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err := ctx.Err()
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s.mu.Lock()
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select {
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case <-ready:
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// Acquired the semaphore after we were canceled. Rather than trying to
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// fix up the queue, just pretend we didn't notice the cancelation.
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err = nil
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default:
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isFront := s.waiters.Front() == elem
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s.waiters.Remove(elem)
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// If we're at the front and there're extra tokens left, notify other waiters.
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if isFront && s.size > s.cur {
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s.notifyWaiters()
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}
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}
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s.mu.Unlock()
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return err
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case <-ready:
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return nil
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}
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}
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// TryAcquire acquires the semaphore with a weight of n without blocking.
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// On success, returns true. On failure, returns false and leaves the semaphore unchanged.
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func (s *Weighted) TryAcquire(n int64) bool {
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s.mu.Lock()
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success := s.size-s.cur >= n && s.waiters.Len() == 0
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if success {
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s.cur += n
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}
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s.mu.Unlock()
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return success
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}
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// Release releases the semaphore with a weight of n.
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func (s *Weighted) Release(n int64) {
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s.mu.Lock()
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s.cur -= n
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if s.cur < 0 {
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s.mu.Unlock()
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panic("semaphore: released more than held")
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}
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s.notifyWaiters()
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s.mu.Unlock()
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}
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func (s *Weighted) notifyWaiters() {
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for {
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next := s.waiters.Front()
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if next == nil {
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break // No more waiters blocked.
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}
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w := next.Value.(waiter)
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if s.size-s.cur < w.n {
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// Not enough tokens for the next waiter. We could keep going (to try to
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// find a waiter with a smaller request), but under load that could cause
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// starvation for large requests; instead, we leave all remaining waiters
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// blocked.
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//
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// Consider a semaphore used as a read-write lock, with N tokens, N
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// readers, and one writer. Each reader can Acquire(1) to obtain a read
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// lock. The writer can Acquire(N) to obtain a write lock, excluding all
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// of the readers. If we allow the readers to jump ahead in the queue,
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// the writer will starve — there is always one token available for every
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// reader.
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break
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}
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s.cur += w.n
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s.waiters.Remove(next)
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close(w.ready)
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}
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}
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