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// Copyright 2015 Google Inc. All rights reserved.
//
// 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 com.google.devtools.build.lib.concurrent;
import com.google.devtools.build.lib.concurrent.ThreadSafety.ThreadSafe;
/** A keyed store of locks. */
@ThreadSafe
public interface KeyedLocker<K> {
/** Used to yield access to the implicit lock granted by {@link #lock}. */
@ThreadSafe
interface AutoUnlocker extends AutoCloseable {
/** Exception used to indicate illegal use of {@link AutoUnlocker#close}. */
class IllegalUnlockException extends RuntimeException {
public IllegalUnlockException(String msg) {
super(msg);
}
}
/**
* Closes the {@link AutoUnlocker} instance. If this instance was the last unclosed one
* returned by {@code lock(k)} owned by then the current thread, then exclusive access to
* {@code k} is yielded.
*
* <p>This method may only be called at most once per {@link AutoUnlocker} instance and must
* be called by the same thread that acquired the {@link AutoUnlocker} via {@link #lock}.
* Otherwise, an {@link IllegalUnlockException} is thrown.
*/
@Override
void close();
}
/**
* Blocks the current thread until it has exclusive access to do things with {@code k} and
* returns a {@link AutoUnlocker} instance for yielding the implicit lock.
*
* <p>Notably, this means that a thread is allowed to call {@code lock(k)} again before calling
* {@link AutoUnlocker#close} for the first call to {@code lock(k)}. Each call to {@link #lock}
* will return a different {@link AutoUnlocker} instance.
*
* <p>The intended usage is:
*
* <pre>
* {@code
* try (AutoUnlocker unlocker = locker.lock(k)) {
* // Your code here.
* }
* }
* </pre>
*
* <p>Note that the usual caveats about mutexes apply here, e.g. the following may deadlock:
*
* <pre>
* {@code
* // Thread A
* try (AutoUnlocker unlocker = locker.lock(k1)) {
* // This will deadlock if Thread A already acquired a lock for k2.
* try (AutoUnlocker unlocker = locker.lock(k2)) {
* }
* }
* // end Thread A
*
* // Thread B
* try (AutoUnlocker unlocker = locker.lock(k2)) {
* // This will deadlock if Thread A already acquired a lock for k1.
* try (AutoUnlocker unlocker = locker.lock(k1)) {
* }
* }
* // end Thread B
* }
* </pre>
*/
AutoUnlocker lock(K key);
}
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