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I'm not entirely happy with the amount of ugliness here, but since
F_DUPFD_CLOEXEC is used elsewhere in code that's expected to work on
old kernels (popen), it seems necessary. reportedly even some modern
kernels went back and broke F_DUPFD_CLOEXEC (making it behave like
plain F_DUPFD), so it might be necessary to add some additional fixup
code later to deal with that issue too.
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on old kernels, there's no way to detect errors; we must assume
negative syscall return values are pgrp ids. but if the F_GETOWN_EX
fcntl works, we can get a reliable answer.
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these will NOT be used when compiling with -D_LARGEFILE64_SOURCE on
musl; instead, they exist in the hopes of eventually being able to run
some glibc-linked apps with musl sitting in place of glibc.
also remove the (apparently incorrect) fcntl alias.
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the fcntl syscall can return a negative value when the command is
F_GETOWN, and this is not an error code but an actual value. thus we
must special-case it and avoid calling __syscall_ret to set errno.
this fix is better than the glibc fix (using F_GETOWN_EX) which only
works on newer kernels and is more complex.
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this patch improves the correctness, simplicity, and size of
cancellation-related code. modulo any small errors, it should now be
completely conformant, safe, and resource-leak free.
the notion of entering and exiting cancellation-point context has been
completely eliminated and replaced with alternative syscall assembly
code for cancellable syscalls. the assembly is responsible for setting
up execution context information (stack pointer and address of the
syscall instruction) which the cancellation signal handler can use to
determine whether the interrupted code was in a cancellable state.
these changes eliminate race conditions in the previous generation of
cancellation handling code (whereby a cancellation request received
just prior to the syscall would not be processed, leaving the syscall
to block, potentially indefinitely), and remedy an issue where
non-cancellable syscalls made from signal handlers became cancellable
if the signal handler interrupted a cancellation point.
x86_64 asm is untested and may need a second try to get it right.
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