Joining dead threads is allowed for two main reasons:
- Thread join behavior should not be racy when a thread is joined and
exiting at roughly the same time. This is common behavior when threads
are given a signal to end (meaning they are going to exit ASAP) and
then joined.
- POSIX requires that exited threads are joinable (at least, there is no
language in the specification forbidding it).
The behavior is still well-defined; e.g. it doesn't allow a dead
detached thread to be joined or a thread to be joined more than once.
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