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until the mq notification event arrives, it is mandatory that signals
be blocked. otherwise, a signal can be received, and its handler
executed, in a thread which does not yet exist on the abstract
machine.
after the point of the event arriving, having signals blocked is not a
conformance requirement but a QoI requirement. while the application
can unblock any signals it wants unblocked in the event handler
thread, if they did not start out blocked, it could not block them
without a race window where they are momentarily unblocked, and this
would preclude controlled delivery or other forms of acceptance
(sigwait, etc.) anywhere in the application.
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this avoids leaving behind transient resource consumption whose
cleanup is subject to scheduling behavior.
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in the error path where the mq_notify syscall fails, the initiating
thread may have closed the socket before the worker thread calls recv
on it. even in the absence of such a race, if the recv call failed,
e.g. due to seccomp policy blocking it, the worker thread could
proceed to close, producing a double-close condition.
this can all be simplified by moving the mq_notify syscall into the
new thread, so that the error case does not require pthread_cancel.
now, the initiating thread only needs to read back the error status
after waiting for the worker thread to consume its arguments.
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semaphores are a much lighter primitive, and more idiomatic with
current usage in the code base.
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time64 syscall is used only if it's the only one defined for the arch,
or if the requested absolute timeout does not fit in 32 bits. on
current 32-bit archs where time_t is a 32-bit type, this makes it
statically unreachable.
on 64-bit archs, there is no change to the code after preprocessing.
on current 32-bit archs, the timeout is passed via an intermediate
copy to remove the assumption that time_t is a 32-bit type.
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to deal with the fact that the public headers may be used with pre-c99
compilers, __restrict is used in place of restrict, and defined
appropriately for any supported compiler. we also avoid the form
[restrict] since older versions of gcc rejected it due to a bug in the
original c99 standard, and instead use the form *restrict.
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why did gcc allow this invalid assignment to compile in the first place?
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