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I have a crazy idea, but I'm not 100% sure how to implement it: If Python was able to detect and report this error in a friendly way, it would allow people to easily understand what is happening and to work around it. How can we do it?
First idea: In the implementation of os.fork(), detect if libdispatch has been used. If it has, throw an exception. I think this is probably possible using the libdispatch public APIs, but I'll need to figure out the details. In general, this could apply on Linux as well: throw an exception if the process has more than one thread running?
Second idea: On Mac OS X only, libdispatch is intentionally crashing the process. We could install a signal handler that attempts to detect *this specific crash* in order to throw a friendlier exception, or at worst crash with a useful message.
Third idea: Add documentation to the multiprocessing module and os.fork that they are very unsafe on Mac OS X?
Maybe there is a better way of making this crash "friendlier"?