No backpressure (unlimited buffers) when using ExecutionModel.SynchronousExecution

[ghik]

Using ExecutionModel.SynchronousExecution has some very surprising and dangerous consequences that are not mentioned in its documentation.

Namely, the value of ExecutionModel.recommendedBatchSize (which is close to Int.MaxValue for SynchronousExecution) is used as default buffer size when converting a Reactive Streams Publisher to Observable (Observable.fromReactivePublisher). This basically means that the buffer becomes unbounded and effectively turns off backpressure and quickly exhausts available memory, leading to catastrophic failures.

My understanding of ExecutionModel.recommendedBatchSize (as explained by its documentation) is that it only affects the internal trampoline of Scheduler, controlling the frequency of asynchronous boundaries. In my opinion it should not affect other places like the culprit buffer size.

[alexandru]

This might be the case indeed. Thanks for the report.

[ghik]

Note: the original reason why we switched to ExecutionModel.SynchronousExecution is #1672 - which is interesting on its own.