ReentrantMutex implementation for Kotlin Coroutines

ReentrantMutex.kt

/*
This is free and unencumbered software released into the public domain.

Anyone is free to copy, modify, publish, use, compile, sell, or
distribute this software, either in source code form or as a compiled
binary, for any purpose, commercial or non-commercial, and by any
means.

In jurisdictions that recognize copyright laws, the author or authors
of this software dedicate any and all copyright interest in the
software to the public domain. We make this dedication for the benefit
of the public at large and to the detriment of our heirs and
successors. We intend this dedication to be an overt act of
relinquishment in perpetuity of all present and future rights to this
software under copyright law.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
IN NO EVENT SHALL THE AUTHORS BE LIABLE FOR ANY CLAIM, DAMAGES OR
OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE,
ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
OTHER DEALINGS IN THE SOFTWARE.

For more information, please refer to <https://unlicense.org>
*/

import kotlinx.coroutines.*
import kotlinx.coroutines.sync.*
import kotlin.coroutines.*

val mutex = Mutex()

suspend fun foo() {
    mutex.withReentrantLock {
        doSomeWork()
    }
}

suspend fun doSomeWork() {
    mutex.withReentrantLock {
        println("Working!")
    }
}

suspend fun main() {
    foo()
}

suspend fun <T> Mutex.withReentrantLock(block: suspend () -> T): T {
    val key = ReentrantMutexContextKey(this)
    // call block directly when this mutex is already locked in the context
    if (coroutineContext[key] != null) return block()
    // otherwise add it to the context and lock the mutex
    return withContext(ReentrantMutexContextElement(key)) {
        withLock { block() }
    }
}

class ReentrantMutexContextElement(
    override val key: ReentrantMutexContextKey
) : CoroutineContext.Element

data class ReentrantMutexContextKey(
    val mutex: Mutex
) : CoroutineContext.Key<ReentrantMutexContextElement>

In this implementation it's needed to create a key object each time just to check if we are already inside the corresponding reentrant context:

val key = ReentrantMutexContextKey(this)
if (coroutineContext[key] != null) return block()

Possibly we can move mutex out from ReentrantMutexContextKey, make this ReentrantMutexContextKey a Kotlin object having no properties inside and keep track of acquired mutexes inside ReentrantMutexContextElement instead. So the check is going to require no allocations (if we are already inside a reentrant context):

suspend fun <T> Mutex.withReentrantLock(block: suspend () -> T): T {
    if (coroutineContext[ReentrantMutexContextKey]?.acquiredMutexes?.contains(this) == true) return block()
    // ...

Of course this little code block suggested by me above is just an illustration and requires at least some renaming applied.