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Is this an overkill? I don't know.
The thing is, correctly intercepting SIGTERM (also SIGINT, etc.) is
incredibly tricky. For example, before this commit, my I/O loops in
server.c and worker.c were inherently racy.
This was immediately obvious if you tried to run the tests. The tests
(especially the Valgrind flavour) would run a worker, wait until it
prints a "Waiting for a new command" line, and try to kill it using
SIGTERM. The problem is, the global_stop_flag check could have already
been executed by the worker, and it would hang forever in recv().
The solution seems to be to use signalfd and select()/poll(). I've never
used either before, but it seems to work well enough - at least the very
same tests pass and don't hang now.
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First, rename all API functions so that they start with net_.
Second, abstract the basic TCP server functionality into tcp_server.c.
This includes reworking net_accept so that it's a simple blocking
operation, and putting the callback stuff to tcp_server.c. Also, the
server now uses detached threads instead of fork(), since I want
connection handlers to share memory.
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