Why is it not recommended to use synchronization blocks and similar constructs when working with coroutines?

Using blocking constructs such as synchronized or lock from the standard library when working with coroutines leads to inefficient resource utilization and can cause issues.

Main reasons:

• Thread blocking: Synchronization blocks stop the execution of the thread, not just coroutines. If a Suspending function is executed inside a synchronized block, the thread will be blocked until the Suspending function completes and the lock is released. This can lead to "starvation" of other coroutines using the same thread.
• Incompatibility with Suspending functions: The coroutine mechanism (context switching, continuations) does not account for thread-level locks. Executing a Suspending function inside a synchronized block breaks the non-blocking nature of coroutines.
• Violation of structured concurrency principles: Coroutines have their own cancellation system and hierarchy. Thread-level locks ignore this system, complicating proper cancellation handling and lifecycle management.
• Risk of deadlocks: Combining thread locks and coroutine concurrency mechanisms increases the likelihood of complex deadlocks, which are difficult to debug.

Instead of thread-level locks for synchronizing access to shared resources in coroutines, specialized concurrency primitives from the kotlinx.coroutines library should be used:

• Mutex: Ensures mutual exclusion. A coroutine attempting to acquire a busy mutex suspends (without blocking the thread) until the mutex is released.
• Semaphore: Limits the number of coroutines that can access a resource simultaneously.
• Channel: Used for data exchange between coroutines.
• Atomic variables: For simple atomic operations.

Example of using Mutex:

import kotlinx.coroutines.sync.Mutex
import kotlinx.coroutines.sync.withLock
import kotlinx.coroutines.*

val mutex = Mutex()
var counter = 0

suspend fun incrementCounter() {
    mutex.withLock {
        // Critical section
        counter++
    }
}

fun main() = runBlocking {
    // Create multiple coroutines for concurrent access
    repeat(1000) {
        launch {
            incrementCounter()
        }
    }
    delay(1000) // Give time for coroutines to complete
    println("Counter: $counter") // Expect 1000
}

mutex.withLock is a suspending function. If the mutex is busy, the coroutine that called withLock suspends and releases the thread for other tasks. When the mutex becomes available, the coroutine resumes. This does not block the thread, unlike synchronized.