Comment 21 for bug 1868486

IIUC the fix to the TX-side issue should look something like the attached patch. We don't have any tests that express the fault that this should fix, but the result does (still) pass epicsMessageQueueTest.

However I spent yesterday afternoon looking at osi/os/default/osdMessageQueue.cpp properly and I have a more extensive solution that I think fixes the remaining issues too. I wrote some tests to check the TX-side problems but unfortunately they don't seem to trigger any bugs other than possibly in the test code itself. I will put together a PR with what I have so far in case anyone else wants to take a look.