1 files changed, 114 insertions, 0 deletions
diff --git a/pkg/amutex/amutex.go b/pkg/amutex/amutex.go
new file mode 100644
index 000000000..1cb73359a
--- /dev/null
+++ b/pkg/amutex/amutex.go
@@ -0,0 +1,114 @@
+// Copyright 2018 Google Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// Package amutex provides the implementation of an abortable mutex. It allows
+// the Lock() function to be canceled while it waits to acquire the mutex.
+package amutex
+
+import (
+	"sync/atomic"
+)
+
+// Sleeper must be implemented by users of the abortable mutex to allow for
+// cancelation of waits.
+type Sleeper interface {
+	// SleepStart is called by the AbortableMutex.Lock() function when the
+	// mutex is contended and the goroutine is about to sleep.
+	//
+	// A channel can be returned that causes the sleep to be canceled if
+	// it's readable. If no cancelation is desired, nil can be returned.
+	SleepStart() <-chan struct{}
+
+	// SleepFinish is called by AbortableMutex.Lock() once a contended mutex
+	// is acquired or the wait is aborted.
+	SleepFinish(success bool)
+}
+
+// NoopSleeper is a stateless no-op implementation of Sleeper for anonymous
+// embedding in other types that do not support cancelation.
+type NoopSleeper struct{}
+
+// SleepStart implements Sleeper.SleepStart.
+func (NoopSleeper) SleepStart() <-chan struct{} {
+	return nil
+}
+
+// SleepFinish implements Sleeper.SleepFinish.
+func (NoopSleeper) SleepFinish(success bool) {}
+
+// AbortableMutex is an abortable mutex. It allows Lock() to be aborted while it
+// waits to acquire the mutex.
+type AbortableMutex struct {
+	v  int32
+	ch chan struct{}
+}
+
+// Init initializes the abortable mutex.
+func (m *AbortableMutex) Init() {
+	m.v = 1
+	m.ch = make(chan struct{}, 1)
+}
+
+// Lock attempts to acquire the mutex, returning true on success. If something
+// is written to the "c" while Lock waits, the wait is aborted and false is
+// returned instead.
+func (m *AbortableMutex) Lock(s Sleeper) bool {
+	// Uncontended case.
+	if atomic.AddInt32(&m.v, -1) == 0 {
+		return true
+	}
+
+	var c <-chan struct{}
+	if s != nil {
+		c = s.SleepStart()
+	}
+
+	for {
+		// Try to acquire the mutex again, at the same time making sure
+		// that m.v is negative, which indicates to the owner of the
+		// lock that it is contended, which ill force it to try to wake
+		// someone up when it releases the mutex.
+		if v := atomic.LoadInt32(&m.v); v >= 0 && atomic.SwapInt32(&m.v, -1) == 1 {
+			if s != nil {
+				s.SleepFinish(true)
+			}
+			return true
+		}
+
+		// Wait for the owner to wake us up before trying again, or for
+		// the wait to be aborted by the provided channel.
+		select {
+		case <-m.ch:
+		case <-c:
+			// s must be non-nil, otherwise c would be nil and we'd
+			// never reach this path.
+			s.SleepFinish(false)
+			return false
+		}
+	}
+}
+
+// Unlock releases the mutex.
+func (m *AbortableMutex) Unlock() {
+	if atomic.SwapInt32(&m.v, 1) == 0 {
+		// There were no pending waiters.
+		return
+	}
+
+	// Wake some waiter up.
+	select {
+	case m.ch <- struct{}{}:
+	default:
+	}
+}