syncutil: add RWMutexTracker debugging type

Change-Id: Ifac22bbea3ef116685baa617d42a2cd7f16a1dd3

pkg/syncutil/lock.go

@@ -0,0 +1,65 @@
+/*
+Copyright 2013 The Camlistore Authors
+
+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 syncutil
+
+import (
+	"log"
+	"runtime"
+	"sync"
+)
+
+// RWMutexTracker is a sync.RWMutex that tracks who owns the current
+// exclusive lock.  It's used for debugging deadlocks.
+type RWMutexTracker struct {
+	mu sync.RWMutex
+
+	hmu    sync.Mutex
+	holder []byte
+}
+
+const stackBufSize = 16 << 20
+
+func (m *RWMutexTracker) Lock() {
+	m.mu.Lock()
+
+	m.hmu.Lock()
+	if len(m.holder) == 0 {
+		m.holder = make([]byte, stackBufSize)
+	}
+	m.holder = m.holder[:runtime.Stack(m.holder[:stackBufSize], false)]
+	log.Printf("Lock at %s", string(m.holder))
+	m.hmu.Unlock()
+}
+
+func (m *RWMutexTracker) Unlock() {
+	m.hmu.Lock()
+	m.holder = m.holder[:0]
+	m.hmu.Unlock()
+	m.mu.Unlock()
+}
+
+func (m *RWMutexTracker) RLock()   { m.mu.RLock() }
+func (m *RWMutexTracker) RUnlock() { m.mu.RUnlock() }
+
+// Holder returns the stack trace of the current exclusive lock holder's stack
+// when it acquired the lock (with Lock). It returns the empty string if the lock
+// is not currently held.
+func (m *RWMutexTracker) Holder() string {
+	m.hmu.Lock()
+	defer m.hmu.Unlock()
+	return string(m.holder)
+}