Prevent FD leakage when calling fork(2)+exec(2) #557
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In order to prevent file descriptor leakage when calling fork(2)+exec(2), the FD_CLOEXEC flag needs to be set. Just setting the flag using fcntl(2) after socket creation is, however, not enough, since, in a multi-threaded application, another thread might fork after we created the socket but before we had the chance of setting the flag. To remedy this, Linux introduced the SOCK_CLOEXEC flag in 2.6.27, to be set on socket creation. At the time of this commit, it has been ported to other systems, such as FreeBSD. Should the flag not be available, as is the case in macOS or older versions of the Linux kernel, we do the next best thing, which is trying to call fcntl(2) right after socket creation.
AppVeyor fails due to an implicit declaration of strerror_r.
I had renamed the definition but forgot to rename the call. My bad.
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I couldn't think of a better way to test this behavior, but I'm open to suggestions.
This should resolve #555.