Synchronized And Coroutines

Compiler prevents some combined usage

Compiler Prevents Using Suspension in Synchronized Blocks

Good news: compiler prevents using synchronized with co-routine suspension.

GT-Sandbox-Snapshot

Code

package com.glassthought.sandbox
 
import kotlinx.coroutines.*
 
var counter = 0
val lock = Any()
 
suspend fun incrementCounter() {
  synchronized(lock) {
    val current = counter
 
    
    // Attempt to use a delay inside synchronized block
    // Compiler will not allow this with error:
    //
    // "The 'delay' suspension point is inside a critical section"
    delay(100)
 
    counter = current + 1
  }
}
 
fun main() = runBlocking {
  val jobs1 = List(1000) {
    launch(Dispatchers.Default) {
      incrementCounter()
    }
  }
  val jobs2 = List(1000) {
    launch(Dispatchers.IO) {
      incrementCounter()
    }
  }
  jobs1.forEach { it.join() }
  jobs2.forEach { it.join() }
  println("Final counter value: $counter")
}

Command to reproduce:

gt.sandbox.checkout.commit 457aa7534d22088c0603 \
&& cd "${GT_SANDBOX_REPO}" \
&& cmd.run.announce "_gt_no_op_for_snapshotting_with_printout"

Recorded output of command:

No command ran. Just snapshot.

Link to original