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Palantir open source project
/*
* (c) Copyright 2018 Palantir Technologies 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.palantir.lock.impl;
import com.google.common.base.MoreObjects;
import com.palantir.lock.LockClient;
import com.palantir.logsafe.Preconditions;
import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
import java.util.concurrent.locks.AbstractQueuedSynchronizer;
import javax.annotation.Nullable;
import javax.annotation.concurrent.GuardedBy;
import org.eclipse.collections.api.iterator.IntIterator;
import org.eclipse.collections.api.map.primitive.MutableIntIntMap;
import org.eclipse.collections.impl.factory.primitive.IntIntMaps;
class LockServerSync extends AbstractQueuedSynchronizer {
private static final long serialVersionUID = 1L;
private final LockClientIndices clients;
private @GuardedBy("this") boolean frozen;
private @GuardedBy("this") int writeLockHolder = 0;
private @GuardedBy("this") MutableIntIntMap readLockHolders;
public LockServerSync(LockClientIndices clients) {
this.clients = Preconditions.checkNotNull(clients);
}
private boolean isAnonymous(int clientIndex) {
return clientIndex < 0;
}
synchronized boolean safeHoldsWriteLock(int clientIndex) {
return getState() > 0 && clientIndex == writeLockHolder && !isAnonymous(clientIndex);
}
private synchronized boolean holdsWriteLock(int clientIndex) {
Preconditions.checkState(getState() > 0);
return clientIndex == writeLockHolder && !isAnonymous(clientIndex);
}
@Override
protected synchronized boolean tryAcquire(int clientIndex) {
if (frozen) {
return false;
}
int writeCount = getState();
if (writeCount > 0 && holdsWriteLock(clientIndex)) {
setState(writeCount + 1);
return true;
}
if (hasBlockedPredecessors()) {
return false;
}
if (writeCount == 0 && !isReadLockHeld()) {
setState(1);
writeLockHolder = clientIndex;
return true;
}
if (holdsReadLock(clientIndex)) {
throw LockServerLock.throwIllegalMonitorStateException(
clients.fromIndex(clientIndex) + " currently holds the read lock");
}
return false;
}
@Override
protected synchronized boolean tryRelease(int clientIndex) {
int newWriteCount = getState() - 1;
if (writeLockHolder != clientIndex || newWriteCount < 0) {
throw LockServerLock.throwIllegalMonitorStateException(
clients.fromIndex(clientIndex) + " does not hold the write lock");
}
setState(newWriteCount);
if (newWriteCount == 0 && !isReadLockHeld()) {
frozen = false;
}
return newWriteCount == 0;
}
@Override
protected synchronized int tryAcquireShared(int clientIndex) {
if (frozen) {
return -1;
}
int writeCount = getState();
if (writeCount == 0 && !holdsReadLock(clientIndex) && hasBlockedPredecessors()) {
return -1;
}
if (writeCount > 0 && !holdsWriteLock(clientIndex)) {
return -1;
}
incrementReadCount(clientIndex);
return 1;
}
@Override
protected synchronized boolean tryReleaseShared(int clientIndex) {
decrementReadCount(clientIndex);
if (!isReadLockHeld() && getState() == 0) {
frozen = false;
return true;
}
return false;
}
synchronized void unlockAndFreeze(int clientIndex) {
if (isAnonymous(clientIndex)) {
throw LockServerLock.throwIllegalMonitorStateException("anonymous clients cannot call unlockAndFreeze()");
}
if (writeLockHolder != clientIndex || getState() == 0) {
throw LockServerLock.throwIllegalMonitorStateException(
clients.fromIndex(clientIndex) + " does not hold the write lock");
}
if (!release(clientIndex) || isReadLockHeld()) {
frozen = true;
}
}
synchronized void changeOwnerShared(int oldClient, LockClient newClient) {
int newIndex = clients.toIndex(newClient);
if (oldClient == newIndex) {
throw LockServerLock.throwIllegalMonitorStateException(
"new owner must be different from old owner, owner=" + clients.fromIndex(oldClient));
}
if (frozen) {
throw LockServerLock.throwIllegalMonitorStateException("cannot change owner because the lock is frozen");
}
if (!holdsReadLock(oldClient)) {
throw LockServerLock.throwIllegalMonitorStateException(
clients.fromIndex(oldClient) + " does not hold the read lock");
}
if (getState() > 0) {
throw LockServerLock.throwIllegalMonitorStateException(
clients.fromIndex(oldClient) + " currently holds both the read and write locks");
}
decrementReadCount(oldClient);
incrementReadCount(newIndex);
}
synchronized void changeOwner(int oldClient, LockClient newClient) {
int newIndex = clients.toIndex(newClient);
if (oldClient == newIndex) {
throw LockServerLock.throwIllegalMonitorStateException(
"new owner must be different from old owner, owner=" + clients.fromIndex(oldClient));
}
if (frozen) {
throw LockServerLock.throwIllegalMonitorStateException("Cannot change owner because the lock is frozen");
}
int writeCount = getState();
if (writeCount == 0 || writeLockHolder != oldClient) {
throw LockServerLock.throwIllegalMonitorStateException(
clients.fromIndex(oldClient) + " does not hold the write lock");
}
if (holdsReadLock(oldClient)) {
throw LockServerLock.throwIllegalMonitorStateException(
clients.fromIndex(oldClient) + " currently holds both the read and write locks");
}
if (writeCount > 1) {
throw LockServerLock.throwIllegalMonitorStateException(
clients.fromIndex(oldClient) + " is attempting to create a lock grant"
+ " while being supported by multiple clients."
+ " This is not currently supported.");
}
writeLockHolder = newIndex;
}
// Returns null if there is no lock holder. Note: There
// may be no lock holder even if tryAcquire or tryAcquireShared
// failed because those methods may fail in order to prevent
// barging.
@GuardedBy("this")
@Nullable
LockClient getLockHolder() {
if (getState() > 0) {
return clients.fromIndex(writeLockHolder);
}
if (!isReadLockHeld()) {
return null;
}
IntIterator iter = readLockHolders.keysView().intIterator();
Preconditions.checkState(iter.hasNext());
return clients.fromIndex(iter.next());
}
LockClient getClient(int clientIndex) {
return clients.fromIndex(clientIndex);
}
public int getClientIndex(LockClient client) {
return clients.toIndex(client);
}
// See https://bugs.openjdk.java.net/browse/JDK-8191483
// AbstractQueuedSynchronizer has a bug where simultaneous cancelAcquire() calls can cause
// future invocations of hasQueuedThreads() and hasQueuedPredecessors() to return true, when in fact there are no
// queued threads. This causes us to spin indefinitely in LockServerLock#tryLock.
// To get around this, we use getFirstQueuedThread(), which is not vulnerable to this bug.
private boolean hasBlockedPredecessors() {
Thread queuedThread = getFirstQueuedThread();
return queuedThread != null && queuedThread != Thread.currentThread();
}
synchronized boolean isFrozen() {
return frozen;
}
@Override
public synchronized String toString() {
return MoreObjects.toStringHelper(getClass().getSimpleName())
.add("hashCode", hashCode())
.add("writeLockCount", getState())
.add("writeClient", writeLockHolder == 0 ? null : clients.fromIndex(writeLockHolder))
.add("readClients", getReadClients())
.add("queuedThreads", getQueueLength())
.add("isFrozen", frozen)
.toString();
}
private synchronized boolean isReadLockHeld() {
return readLockHolders != null && !readLockHolders.isEmpty();
}
synchronized boolean holdsReadLock(int clientIndex) {
return !isAnonymous(clientIndex) && readLockHolders != null && readLockHolders.get(clientIndex) > 0;
}
private synchronized void incrementReadCount(int clientIndex) {
if (readLockHolders == null) {
readLockHolders = IntIntMaps.mutable.ofInitialCapacity(1);
}
readLockHolders.addToValue(clientIndex, 1);
}
private synchronized void decrementReadCount(int clientIndex) {
if (readLockHolders == null) {
throw LockServerLock.throwIllegalMonitorStateException(
clients.fromIndex(clientIndex) + " does not hold the read lock");
}
int readCount = readLockHolders.removeKeyIfAbsent(clientIndex, 0);
if (readCount > 1) {
readLockHolders.put(clientIndex, readCount - 1);
} else if (readCount == 0) {
throw LockServerLock.throwIllegalMonitorStateException(
clients.fromIndex(clientIndex) + " does not hold the read lock");
}
}
synchronized List getReadClients() {
if (readLockHolders == null) {
return List.of();
}
return Collections.unmodifiableList(
readLockHolders.keysView().collect(clients::fromIndex, new ArrayList<>(readLockHolders.size())));
}
}