package cn.businessworks.common.inject.internal;
import com.google.common.base.Preconditions;
import com.google.common.base.Supplier;
import com.google.common.collect.ImmutableListMultimap;
import com.google.common.collect.LinkedHashMultimap;
import com.google.common.collect.ListMultimap;
import com.google.common.collect.Lists;
import com.google.common.collect.Maps;
import com.google.common.collect.Multimap;
import com.google.common.collect.Multimaps;
import java.util.Collection;
import java.util.LinkedHashMap;
import java.util.List;
import java.util.Map;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;
/**
* Simplified version of {@link Lock} that is special due to how it handles deadlocks detection.
*
* <p>Is an inherent part of {@link SingletonScope}, moved into a upper level class due
* to its size and complexity.
*
* @param <ID> Lock identification provided by the client, is returned unmodified to the client
* when lock cycle is detected to identify it. Only toString() needs to be implemented.
* Lock references this object internally,
* for the purposes of Garbage Collection you should not use heavy IDs.
* Lock is referenced by a lock factory as long as it's owned by a thread.
*
* @see SingletonScope
* @see cn.businessworks.common.inject.internal.CycleDetectingLock.CycleDetectingLockFactory
*
* @author timofeyb (Timothy Basanov)
*/
interface CycleDetectingLock<ID> {
/**
* Takes a lock in a blocking fashion in case no potential deadlocks are detected.
* If the lock was successfully owned, returns an empty map indicating no detected potential
* deadlocks.
*
* Otherwise, a map indicating threads involved in a potential deadlock are returned.
* Map is ordered by dependency cycle and lists locks for each thread that are part of
* the loop in order. Returned map is created atomically.
*
* In case no cycle is detected performance is O(threads creating singletons),
* in case cycle is detected performance is O(singleton locks).
*/
ListMultimap<Long, ID> lockOrDetectPotentialLocksCycle();
/**
* Unlocks previously locked lock.
*/
void unlock();
/**
* Wraps locks so they would never cause a deadlock. On each
* {@link CycleDetectingLock#lockOrDetectPotentialLocksCycle} we check for dependency cycles
* within locks created by the same factory. Either we detect a cycle and return it
* or take it atomically.
*
* <p>Important to note that we do not prevent deadlocks in the client code. As an example:
* Thread A takes lock L and creates singleton class CA depending on the singleton class CB.
* Meanwhile thread B is creating class CB and is waiting on the lock L. Issue happens
* due to client code creating interdependent classes and using locks, where
* no guarantees on the creation order from Guice are provided.
*
* <p>Instances of these locks are not intended to be exposed outside of {@link SingletonScope}.
*/
class CycleDetectingLockFactory<ID> {
/**
* Specifies lock that thread is currently waiting on to own it.
* Used only for purposes of locks cycle detection.
*
* Key: thread id
* Value: lock that is being waited on
*
* Element is added inside {@link #lockOrDetectPotentialLocksCycle()} before {@link Lock#lock}
* is called. Element is removed inside {@link #lockOrDetectPotentialLocksCycle()} after
* {@link Lock#lock} and synchronously with adding it to {@link #locksOwnedByThread}.
*
* Same lock can be added for several threads in case all of them are trying to
* take it.
*
* Guarded by {@code CycleDetectingLockFactory.class}.
*/
private static Map<Long, ReentrantCycleDetectingLock<?>> lockThreadIsWaitingOn =
Maps.newHashMap();
/**
* Lists locks that thread owns.
* Used only to populate locks in a potential cycle when it is detected.
*
* Key: thread id
* Value: stack of locks that were owned.
*
* Element is added inside {@link #lockOrDetectPotentialLocksCycle()} after {@link Lock#lock}
* is called. Element is removed inside {@link #unlock()} synchronously with
* {@link Lock#unlock()} call.
*
* Same lock can only be present several times for the same thread as locks are
* reentrant. Lock can not be owned by several different threads as the same time.
*
* Guarded by {@code CycleDetectingLockFactory.class}.
*/
private static final Multimap<Long, ReentrantCycleDetectingLock<?>> locksOwnedByThread =
LinkedHashMultimap.create();
/**
* Creates new lock within this factory context. We can guarantee that locks created by
* the same factory would not deadlock.
*
* @param userLockId lock id that would be used to report lock cycles if detected
*/
CycleDetectingLock<ID> create(ID userLockId) {
return new ReentrantCycleDetectingLock<ID>(this, userLockId, new ReentrantLock());
}
/** The implementation for {@link CycleDetectingLock}. */
static class ReentrantCycleDetectingLock<ID> implements CycleDetectingLock<ID> {
/** Underlying lock used for actual waiting when no potential deadlocks are detected. */
private final Lock lockImplementation;
/** User id for this lock. */
private final ID userLockId;
/** Factory that was used to create this lock. */
private final CycleDetectingLockFactory<ID> lockFactory;
/**
* Thread id for the thread that owned this lock. Nullable.
* Guarded by {@code CycleDetectingLockFactory.this}.
*/
private Long lockOwnerThreadId = null;
/**
* Number of times that thread owned this lock.
* Guarded by {@code CycleDetectingLockFactory.this}.
*/
private int lockReentranceCount = 0;
ReentrantCycleDetectingLock(CycleDetectingLockFactory<ID> lockFactory,
ID userLockId, Lock lockImplementation) {
this.lockFactory = lockFactory;
this.userLockId = Preconditions.checkNotNull(userLockId, "userLockId");
this.lockImplementation = Preconditions.checkNotNull(
lockImplementation, "lockImplementation");
}
@Override public ListMultimap<Long, ID> lockOrDetectPotentialLocksCycle() {
final long currentThreadId = Thread.currentThread().getId();
synchronized (CycleDetectingLockFactory.class) {
checkState();
ListMultimap<Long, ID> locksInCycle = detectPotentialLocksCycle();
if (!locksInCycle.isEmpty()) {
// potential deadlock is found, we don't try to take this lock
return locksInCycle;
}
lockThreadIsWaitingOn.put(currentThreadId, this);
}
// this may be blocking, but we don't expect it to cause a deadlock
lockImplementation.lock();
synchronized (CycleDetectingLockFactory.class) {
// current thread is no longer waiting on this lock
lockThreadIsWaitingOn.remove(currentThreadId);
checkState();
// mark it as owned by us
lockOwnerThreadId = currentThreadId;
lockReentranceCount++;
// add this lock to the list of locks owned by a current thread
locksOwnedByThread.put(currentThreadId, this);
}
// no deadlock is found, locking successful
return ImmutableListMultimap.of();
}
@Override public void unlock() {
final long currentThreadId = Thread.currentThread().getId();
synchronized (CycleDetectingLockFactory.class) {
checkState();
Preconditions.checkState(lockOwnerThreadId != null,
"Thread is trying to unlock a lock that is not locked");
Preconditions.checkState(lockOwnerThreadId == currentThreadId,
"Thread is trying to unlock a lock owned by another thread");
// releasing underlying lock
lockImplementation.unlock();
// be sure to release the lock synchronously with updating internal state
lockReentranceCount--;
if (lockReentranceCount == 0) {
// we no longer own this lock
lockOwnerThreadId = null;
Preconditions.checkState(locksOwnedByThread.remove(currentThreadId, this),
"Internal error: Can not find this lock in locks owned by a current thread");
if (locksOwnedByThread.get(currentThreadId).isEmpty()) {
// clearing memory
locksOwnedByThread.removeAll(currentThreadId);
}
}
}
}
/** Check consistency of an internal state. */
void checkState() throws IllegalStateException {
final long currentThreadId = Thread.currentThread().getId();
Preconditions.checkState(!lockThreadIsWaitingOn.containsKey(currentThreadId),
"Internal error: Thread should not be in a waiting thread on a lock now");
if (lockOwnerThreadId != null) {
// check state of a locked lock
Preconditions.checkState(lockReentranceCount >= 0,
"Internal error: Lock ownership and reentrance count internal states do not match");
Preconditions.checkState(locksOwnedByThread.get(lockOwnerThreadId).contains(this),
"Internal error: Set of locks owned by a current thread and lock "
+ "ownership status do not match");
} else {
// check state of a non locked lock
Preconditions.checkState(lockReentranceCount == 0,
"Internal error: Reentrance count of a non locked lock is expect to be zero");
Preconditions.checkState(!locksOwnedByThread.values().contains(this),
"Internal error: Non locked lock should not be owned by any thread");
}
}
/**
* Algorithm to detect a potential lock cycle.
*
* For lock's thread owner check which lock is it trying to take.
* Repeat recursively. When current thread is found a potential cycle is detected.
*
* @see CycleDetectingLock#lockOrDetectPotentialLocksCycle()
*/
private ListMultimap<Long, ID> detectPotentialLocksCycle() {
final long currentThreadId = Thread.currentThread().getId();
if (lockOwnerThreadId == null || lockOwnerThreadId == currentThreadId) {
// if nobody owns this lock, lock cycle is impossible
// if a current thread owns this lock, we let Guice to handle it
return ImmutableListMultimap.of();
}
ListMultimap<Long, ID> potentialLocksCycle = Multimaps.newListMultimap(
new LinkedHashMap<Long, Collection<ID>>(),
new Supplier<List<ID>>() {
@Override
public List<ID> get() {
return Lists.newArrayList();
}
});
// lock that is a part of a potential locks cycle, starts with current lock
ReentrantCycleDetectingLock lockOwnerWaitingOn = this;
// try to find a dependency path between lock's owner thread and a current thread
while (lockOwnerWaitingOn != null && lockOwnerWaitingOn.lockOwnerThreadId != null) {
Long threadOwnerThreadWaits = lockOwnerWaitingOn.lockOwnerThreadId;
// in case locks cycle exists lock we're waiting for is part of it
potentialLocksCycle.putAll(threadOwnerThreadWaits,
getAllLockIdsAfter(threadOwnerThreadWaits, lockOwnerWaitingOn));
if (threadOwnerThreadWaits == currentThreadId) {
// owner thread depends on current thread, cycle detected
return potentialLocksCycle;
}
// going for the next thread we wait on indirectly
lockOwnerWaitingOn = lockThreadIsWaitingOn.get(threadOwnerThreadWaits);
}
// no dependency path from an owner thread to a current thread
return ImmutableListMultimap.of();
}
/** Return locks owned by a thread after a lock specified, inclusive. */
private List<ID> getAllLockIdsAfter(long threadId, ReentrantCycleDetectingLock lock) {
List<ID> ids = Lists.newArrayList();
boolean found = false;
Collection<ReentrantCycleDetectingLock<?>> ownedLocks = locksOwnedByThread.get(threadId);
Preconditions.checkNotNull(ownedLocks,
"Internal error: No locks were found taken by a thread");
for (ReentrantCycleDetectingLock ownedLock : ownedLocks) {
if (ownedLock == lock) {
found = true;
}
if (found && ownedLock.lockFactory == this.lockFactory) {
// All locks are stored in a shared map therefore there is no way to
// enforce type safety. We know that our cast is valid as we check for a lock's
// factory. If the lock was generated by the
// same factory it has to have same type as the current lock.
@SuppressWarnings("unchecked") ID userLockId = (ID) ownedLock.userLockId;
ids.add(userLockId);
}
}
Preconditions.checkState(found, "Internal error: We can not find locks that "
+ "created a cycle that we detected");
return ids;
}
@Override public String toString() {
// copy is made to prevent a data race
// no synchronization is used, potentially stale data, should be good enough
Long threadId = this.lockOwnerThreadId;
if (threadId != null) {
return String.format("%s[%s][locked by Id=%d]", super.toString(), userLockId, threadId);
} else {
return String.format("%s[%s][unlocked]", super.toString(), userLockId);
}
}
}
}
}
