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package kotlinx.coroutines.sync
import kotlinx.atomicfu.*
import kotlinx.coroutines.*
import kotlinx.coroutines.internal.*
import kotlinx.coroutines.selects.*
import kotlin.contracts.*
import kotlin.coroutines.CoroutineContext
import kotlin.jvm.*
/**
* Mutual exclusion for coroutines.
*
* Mutex has two states: _locked_ and _unlocked_.
* It is **non-reentrant**, that is invoking [lock] even from the same thread/coroutine that currently holds
* the lock still suspends the invoker.
*
* JVM API note:
* Memory semantic of the [Mutex] is similar to `synchronized` block on JVM:
* An unlock operation on a [Mutex] happens-before every subsequent successful lock on that [Mutex].
* Unsuccessful call to [tryLock] do not have any memory effects.
*/
public interface Mutex {
/**
* Returns `true` if this mutex is locked.
*/
public val isLocked: Boolean
/**
* Tries to lock this mutex, returning `false` if this mutex is already locked.
*
* It is recommended to use [withLock] for safety reasons, so that the acquired lock is always
* released at the end of your critical section, and [unlock] is never invoked before a successful
* lock acquisition.
*
* @param owner Optional owner token for debugging. When `owner` is specified (non-null value) and this mutex
* is already locked with the same token (same identity), this function throws [IllegalStateException].
*/
public fun tryLock(owner: Any? = null): Boolean
/**
* Locks this mutex, suspending caller until the lock is acquired (in other words, while the lock is held elsewhere).
*
* This suspending function is cancellable: if the [Job] of the current coroutine is cancelled while this
* suspending function is waiting, this function immediately resumes with [CancellationException].
* There is a **prompt cancellation guarantee**: even if this function is ready to return the result, but was cancelled
* while suspended, [CancellationException] will be thrown. See [suspendCancellableCoroutine] for low-level details.
* This function releases the lock if it was already acquired by this function before the [CancellationException]
* was thrown.
*
* Note that this function does not check for cancellation when it is not suspended.
* Use [yield] or [CoroutineScope.isActive] to periodically check for cancellation in tight loops if needed.
*
* Use [tryLock] to try acquiring the lock without waiting.
*
* This function is fair; suspended callers are resumed in first-in-first-out order.
*
* It is recommended to use [withLock] for safety reasons, so that the acquired lock is always
* released at the end of the critical section, and [unlock] is never invoked before a successful
* lock acquisition.
*
* @param owner Optional owner token for debugging. When `owner` is specified (non-null value) and this mutex
* is already locked with the same token (same identity), this function throws [IllegalStateException].
*/
public suspend fun lock(owner: Any? = null)
/**
* Clause for [select] expression of [lock] suspending function that selects when the mutex is locked.
* Additional parameter for the clause in the `owner` (see [lock]) and when the clause is selected
* the reference to this mutex is passed into the corresponding block.
*/
@Deprecated(level = DeprecationLevel.WARNING, message = "Mutex.onLock deprecated without replacement. " +
"For additional details please refer to #2794") // WARNING since 1.6.0
public val onLock: SelectClause2
/**
* Checks whether this mutex is locked by the specified owner.
*
* @return `true` when this mutex is locked by the specified owner;
* `false` if the mutex is not locked or locked by another owner.
*/
public fun holdsLock(owner: Any): Boolean
/**
* Unlocks this mutex. Throws [IllegalStateException] if invoked on a mutex that is not locked or
* was locked with a different owner token (by identity).
*
* It is recommended to use [withLock] for safety reasons, so that the acquired lock is always
* released at the end of the critical section, and [unlock] is never invoked before a successful
* lock acquisition.
*
* @param owner Optional owner token for debugging. When `owner` is specified (non-null value) and this mutex
* was locked with the different token (by identity), this function throws [IllegalStateException].
*/
public fun unlock(owner: Any? = null)
}
/**
* Creates a [Mutex] instance.
* The mutex created is fair: lock is granted in first come, first served order.
*
* @param locked initial state of the mutex.
*/
@Suppress("FunctionName")
public fun Mutex(locked: Boolean = false): Mutex =
MutexImpl(locked)
/**
* Executes the given [action] under this mutex's lock.
*
* @param owner Optional owner token for debugging. When `owner` is specified (non-null value) and this mutex
* is already locked with the same token (same identity), this function throws [IllegalStateException].
*
* @return the return value of the action.
*/
@OptIn(ExperimentalContracts::class)
public suspend inline fun Mutex.withLock(owner: Any? = null, action: () -> T): T {
contract {
callsInPlace(action, InvocationKind.EXACTLY_ONCE)
}
lock(owner)
return try {
action()
} finally {
unlock(owner)
}
}
internal open class MutexImpl(locked: Boolean) : SemaphoreAndMutexImpl(1, if (locked) 1 else 0), Mutex {
/**
* After the lock is acquired, the corresponding owner is stored in this field.
* The [unlock] operation checks the owner and either re-sets it to [NO_OWNER],
* if there is no waiting request, or to the owner of the suspended [lock] operation
* to be resumed, otherwise.
*/
private val owner = atomic(if (locked) null else NO_OWNER)
private val onSelectCancellationUnlockConstructor: OnCancellationConstructor =
{ _: SelectInstance<*>, owner: Any?, _: Any? ->
{ _, _, _ -> unlock(owner) }
}
override val isLocked: Boolean get() =
availablePermits == 0
override fun holdsLock(owner: Any): Boolean = holdsLockImpl(owner) == HOLDS_LOCK_YES
/**
* [HOLDS_LOCK_UNLOCKED] if the mutex is unlocked
* [HOLDS_LOCK_YES] if the mutex is held with the specified [owner]
* [HOLDS_LOCK_ANOTHER_OWNER] if the mutex is held with a different owner
*/
private fun holdsLockImpl(owner: Any?): Int {
while (true) {
// Is this mutex locked?
if (!isLocked) return HOLDS_LOCK_UNLOCKED
val curOwner = this.owner.value
// Wait in a spin-loop until the owner is set
if (curOwner === NO_OWNER) continue // <-- ATTENTION, BLOCKING PART HERE
// Check the owner
return if (curOwner === owner) HOLDS_LOCK_YES else HOLDS_LOCK_ANOTHER_OWNER
}
}
override suspend fun lock(owner: Any?) {
if (tryLock(owner)) return
lockSuspend(owner)
}
private suspend fun lockSuspend(owner: Any?) = suspendCancellableCoroutineReusable { cont ->
val contWithOwner = CancellableContinuationWithOwner(cont, owner)
acquire(contWithOwner)
}
override fun tryLock(owner: Any?): Boolean = when (tryLockImpl(owner)) {
TRY_LOCK_SUCCESS -> true
TRY_LOCK_FAILED -> false
TRY_LOCK_ALREADY_LOCKED_BY_OWNER -> error("This mutex is already locked by the specified owner: $owner")
else -> error("unexpected")
}
private fun tryLockImpl(owner: Any?): Int {
while (true) {
if (tryAcquire()) {
assert { this.owner.value === NO_OWNER }
this.owner.value = owner
return TRY_LOCK_SUCCESS
} else {
// The semaphore permit acquisition has failed.
// However, we need to check that this mutex is not
// locked by our owner.
if (owner == null) return TRY_LOCK_FAILED
when (holdsLockImpl(owner)) {
// This mutex is already locked by our owner.
HOLDS_LOCK_YES -> return TRY_LOCK_ALREADY_LOCKED_BY_OWNER
// This mutex is locked by another owner, `trylock(..)` must return `false`.
HOLDS_LOCK_ANOTHER_OWNER -> return TRY_LOCK_FAILED
// This mutex is no longer locked, restart the operation.
HOLDS_LOCK_UNLOCKED -> continue
}
}
}
}
override fun unlock(owner: Any?) {
while (true) {
// Is this mutex locked?
check(isLocked) { "This mutex is not locked" }
// Read the owner, waiting until it is set in a spin-loop if required.
val curOwner = this.owner.value
if (curOwner === NO_OWNER) continue // <-- ATTENTION, BLOCKING PART HERE
// Check the owner.
check(curOwner === owner || owner == null) { "This mutex is locked by $curOwner, but $owner is expected" }
// Try to clean the owner first. We need to use CAS here to synchronize with concurrent `unlock(..)`-s.
if (!this.owner.compareAndSet(curOwner, NO_OWNER)) continue
// Release the semaphore permit at the end.
release()
return
}
}
@Suppress("UNCHECKED_CAST", "OverridingDeprecatedMember", "OVERRIDE_DEPRECATION")
override val onLock: SelectClause2 get() = SelectClause2Impl(
clauseObject = this,
regFunc = MutexImpl::onLockRegFunction as RegistrationFunction,
processResFunc = MutexImpl::onLockProcessResult as ProcessResultFunction,
onCancellationConstructor = onSelectCancellationUnlockConstructor
)
protected open fun onLockRegFunction(select: SelectInstance<*>, owner: Any?) {
if (owner != null && holdsLock(owner)) {
select.selectInRegistrationPhase(ON_LOCK_ALREADY_LOCKED_BY_OWNER)
} else {
onAcquireRegFunction(SelectInstanceWithOwner(select as SelectInstanceInternal<*>, owner), owner)
}
}
protected open fun onLockProcessResult(owner: Any?, result: Any?): Any? {
if (result == ON_LOCK_ALREADY_LOCKED_BY_OWNER) {
error("This mutex is already locked by the specified owner: $owner")
}
return this
}
@OptIn(InternalForInheritanceCoroutinesApi::class)
private inner class CancellableContinuationWithOwner(
@JvmField
val cont: CancellableContinuationImpl,
@JvmField
val owner: Any?
) : CancellableContinuation by cont, Waiter by cont {
override fun tryResume(
value: R,
idempotent: Any?,
onCancellation: ((cause: Throwable, value: R, context: CoroutineContext) -> Unit)?
): Any? {
assert { [email protected] === NO_OWNER }
val token = cont.tryResume(value, idempotent) { _, _, _ ->
assert { [email protected] { it === NO_OWNER || it === owner } }
[email protected] = owner
unlock(owner)
}
if (token != null) {
assert { [email protected] === NO_OWNER }
[email protected] = owner
}
return token
}
override fun resume(
value: R,
onCancellation: ((cause: Throwable, value: R, context: CoroutineContext) -> Unit)?
) {
assert { [email protected] === NO_OWNER }
[email protected] = owner
cont.resume(value) { unlock(owner) }
}
}
private inner class SelectInstanceWithOwner(
@JvmField
val select: SelectInstanceInternal,
@JvmField
val owner: Any?
) : SelectInstanceInternal by select {
override fun trySelect(clauseObject: Any, result: Any?): Boolean {
assert { [email protected] === NO_OWNER }
return select.trySelect(clauseObject, result).also { success ->
if (success) [email protected] = owner
}
}
override fun selectInRegistrationPhase(internalResult: Any?) {
assert { [email protected] === NO_OWNER }
[email protected] = owner
select.selectInRegistrationPhase(internalResult)
}
}
override fun toString() = "Mutex@${hexAddress}[isLocked=$isLocked,owner=${owner.value}]"
}
private val NO_OWNER = Symbol("NO_OWNER")
private val ON_LOCK_ALREADY_LOCKED_BY_OWNER = Symbol("ALREADY_LOCKED_BY_OWNER")
private const val TRY_LOCK_SUCCESS = 0
private const val TRY_LOCK_FAILED = 1
private const val TRY_LOCK_ALREADY_LOCKED_BY_OWNER = 2
private const val HOLDS_LOCK_UNLOCKED = 0
private const val HOLDS_LOCK_YES = 1
private const val HOLDS_LOCK_ANOTHER_OWNER = 2
