(I'm fairly new to resteasy and async jax-rs so apologies if I've gotten this all wrong)

There appears to be a bug in the implementation of AsyncResponse.setTimeout for values of 0 or less - the javadoc says

Value lower or equal to 0 causes the context to suspend indefinitely

I believe this is caused by the implementation in SynchronousExecutionContext@125 which uses syncLatch.await(long,TimeUnit) and passes in the exact values given in the AsyncResponse.setTimeout. However the intended semantics of these two methods w.r.t. <=0 values is the opposite, from the CountDownLatch docs:

If the time is less than or equal to zero, the method will not wait at all

A work around is to specify a timeout when configuring the AsyncResponse that is some distant time in the future.