Hi JDK Core Devs, I'm writing to you today with a question about the behavior of mapConcurrent() and its interaction with unchecked exceptions. I've been experimenting with the API and observed that mapConcurrent() blocks and joins all virtual threads upon an unchecked exception before propagating it.
Initially, I thought this design choice might provide a strong happens-before guarantee. My assumption was that an application catching a RuntimeException would be able to *observe all side effects* from the virtual threads, even though this practice is generally discouraged. This seemed like a potentially significant advantage, outweighing the risk of a virtual thread failing to respond to interruption or responding slowly. However, I've since realized that mapConcurrent() cannot fully guarantee a strong happens-before relationship when an unchecked exception occurs *somewhere* in the stream pipeline. While it can block and wait for exceptions thrown by the mapper function or downstream operations, it appears unable to intercept unchecked exceptions *thrown by an upstream* source. Consider a scenario with two input elements: if the first element starts a virtual thread, and then the second element causes an unchecked exception from the upstream *before* reaching the gather() call, the virtual thread initiated by the first element would not be interrupted. This makes the "happens-before" guarantee quite nuanced in practice. This brings me to my core questions: 1. Is providing a happens-before guarantee upon an unchecked exception a design goal for mapConcurrent()? 2. If not, would it be more desirable to *not* join on virtual threads when unchecked exceptions occur? This would allow the application code to catch the exception sooner and avoid the risk of being blocked indefinitely. Thank you for your time and insights. Best regards, Ben Yu
