Hi I recently encountered a strange behavior I have a `HashM

Question

Hi I recently encountered a strange behavior I have a `HashMap` which is only accessed and modified in a single coroutine context ``` val pushNotificationGeneratorThread = newSingleThreadContext name = Push Notification Generator Thread suspend fun clear = run pushNotificationDispatcherThread lastNotificationDates clear suspend fun put key String value Any = run pushNotificationDispatcherThread lastNotificationDates put key value ``` However sometimes I still get a `ConcurrentModificationException` In my understanding this should be threadsafe

functionaldude · Answer

Here is the stacktrace ``` java util ConcurrentModificationException at java util HashMap$EntryIterator next HashMap java 1471 at java util HashMap$HashIterator nextNode HashMap java 1437 at xxx$startCleanupThread$1 doResume PushModule kt 114 at java util HashMap$EntryIterator next HashMap java 1469 at kotlin coroutines experimental jvm internal CoroutineImpl resume CoroutineImpl kt 54 at kotlinx coroutines experimental CancellableContinuationImpl afterCompletion CancellableContinuation kt 318 at kotlinx coroutines experimental JobSupport completeUpdateState Job kt 426 at kotlinx coroutines experimental JobSupport updateState Job kt 382 at kotlinx coroutines experimental AbstractCoroutine resume CoroutineScope kt 80 at kotlinx coroutines experimental CancellableContinuationImpl resumeUndispatched CancellableContinuation kt 287 at java util concurrent Executors$RunnableAdapter call Executors java 511 at java util concurrent ScheduledThreadPoolExecutor$ScheduledFutureTask access$201 ScheduledThreadPoolExecutor java 180 at java util concurrent ThreadPoolExecutor runWorker ThreadPoolExecutor java 1149 at java lang Thread run Thread java 748 at kotlinx coroutines experimental ResumeUndispatchedRunnable run Executors kt 79 at java util concurrent FutureTask run FutureTask java 266 at java util concurrent ScheduledThreadPoolExecutor$ScheduledFutureTask run ScheduledThreadPoolExecutor java 293 at java util concurrent ThreadPoolExecutor$Worker run ThreadPoolExecutor java 624 ```

bj0 · Answer

are you iterating over it somewhere else

functionaldude · Answer

yes I have a cleanup function which removes some entries

functionaldude · Answer

``` suspend fun cleanup = run pushNotificationGeneratorThread while true val tenSecondsAgo = Date delay time = 10 unit = TimeUnit SECONDS lastNotificationDates forEach userId dateMap gt dateMap forEach payloadId lastNotificationDate gt remove if dateMap isEmpty remove ```

functionaldude · Answer

it looks somewhat like this

bj0 · Answer

I mean where is it read used

functionaldude · Answer

yes but that function also runs on `pushNotificationGeneratorThread`

bj0 · Answer

what does the `remove ` function do does it modify the list

functionaldude · Answer

yes it removes one element from the map

bj0 · Answer

isn t that the problem modifying the map while iterating over it

functionaldude · Answer

yeah so that was exactly the problem grinning it turns out this had nothing to do with coroutines grinning I tried way too hard to understand the coroutine magic so I didn t saw the obvious Thanks for the help

functionaldude · Answer

solved slightly smiling face