withContext(IO) blocks thread

Currently we are using Vert.x web for HTTP server, and uses JedisPool for Redis client. Since that Redis client has a blocking API, we used withContext(IO) to not block the Vert.x thread.

suspend fun <R> JedisPool.useAsync(block: (Jedis) -> R) = withContext(IO) {
    this.resource.use { block(it) }
}

But the Vert.x thread is blocked by withContext. Coroutines version is “0.25.0”, Vertx version is “3.5.3”.

Aug 31, 2018 3:24:53 AM io.vertx.core.impl.BlockedThreadChecker
WARNING: Thread Thread[vert.x-eventloop-thread-0,5,main] has been blocked for 39234941 ms, time limit is 2000
io.vertx.core.VertxException: Thread blocked
	at kotlinx.coroutines.experimental.scheduling.CoroutineScheduler.parkedWorkersStackPop(CoroutineScheduler.kt:982)
	at kotlinx.coroutines.experimental.scheduling.CoroutineScheduler.tryUnpark(CoroutineScheduler.kt:395)
	at kotlinx.coroutines.experimental.scheduling.CoroutineScheduler.requestCpuWorker(CoroutineScheduler.kt:366)
	at kotlinx.coroutines.experimental.scheduling.CoroutineScheduler.dispatch(CoroutineScheduler.kt:345)
	at kotlinx.coroutines.experimental.scheduling.ExperimentalCoroutineDispatcher.dispatchWithContext$kotlinx_coroutines_core(ExperimentalCoroutineDispatcher.kt:83)
	at kotlinx.coroutines.experimental.scheduling.LimitingDispatcher.dispatch(ExperimentalCoroutineDispatcher.kt:123)
	at kotlinx.coroutines.experimental.scheduling.LimitingDispatcher.dispatch(ExperimentalCoroutineDispatcher.kt:113)
	at kotlinx.coroutines.experimental.DispatchedKt.resumeCancellable(Dispatched.kt:189)
	at kotlinx.coroutines.experimental.intrinsics.CancellableKt.startCoroutineCancellable(Cancellable.kt:16)
	at kotlinx.coroutines.experimental.CoroutineStart.invoke(CoroutineStart.kt:83)
	at kotlinx.coroutines.experimental.BuildersKt__Builders_commonKt.withContext(Builders.common.kt:136)
	at kotlinx.coroutines.experimental.BuildersKt.withContext(Unknown Source)
	at kotlinx.coroutines.experimental.BuildersKt__Builders_commonKt.withContext$default(Builders.common.kt:107)
	at kotlinx.coroutines.experimental.BuildersKt.withContext$default(Unknown Source)
...
	at kotlin.coroutines.experimental.jvm.internal.CoroutineImpl.resume(CoroutineImpl.kt:42)
	at kotlinx.coroutines.experimental.DispatchedTask$DefaultImpls.run(Dispatched.kt:149)
	at kotlinx.coroutines.experimental.DispatchedContinuation.run(Dispatched.kt:13)
	at io.vertx.kotlin.coroutines.VertxCoroutineExecutor$execute$1.handle(VertxCoroutine.kt:408)
	at io.vertx.kotlin.coroutines.VertxCoroutineExecutor$execute$1.handle(VertxCoroutine.kt:404)
	at io.vertx.core.impl.ContextImpl.lambda$wrapTask$2(ContextImpl.java:339)
	at io.vertx.core.impl.ContextImpl$$Lambda$33/1483526283.run(Unknown Source)
	at io.netty.util.concurrent.AbstractEventExecutor.safeExecute(AbstractEventExecutor.java:163)
	at io.netty.util.concurrent.SingleThreadEventExecutor.runAllTasks(SingleThreadEventExecutor.java:404)
	at io.netty.channel.nio.NioEventLoop.run(NioEventLoop.java:463)
	at io.netty.util.concurrent.SingleThreadEventExecutor$5.run(SingleThreadEventExecutor.java:886)
	at io.netty.util.concurrent.FastThreadLocalRunnable.run(FastThreadLocalRunnable.java:30)
	at java.lang.Thread.run(Thread.java:745)

Where the ... is 3 lines of our code.

I would love to understand things under the hood but currently we will just use the non-blocking Redis client to avoid the problem first.

[elizarov]

@objcode We are cleaning up and finalizing API for 1.0 release of this library, so I’d suggest to also watch for issues with “for 1.0 release” label: https://github.com/Kotlin/kotlinx.coroutines/issues?q=is%3Aissue+is%3Aopen+label%3A"for+1.0+release"

[LouisCAD]

Shouldn’t this issue have its labels updated? (like having the bug label in place of the current one)