This flag asks forkfd() to use vfork semantics wherever available. That
is, suspend the calling process execution until the child either does an
execve(2) or _exit(2). The advantage of that is that it puts lower
pressure on the OS VMM system, as the number of pages that need to be
copy-on-write duplicated is much smaller (still not zero, as at least
the stack in the child will be written to).
However, the only implementation that supports using this flag for now
is Linux's pidfd. It would be possible to add to FreeBSD, but pdfork(2)
does not have a flag for this behavior -- if it gets one, we can add
support for it later. Everywhere else, we need to force the child to not
exit until we store the child process's PID in the ProcessInfo structure
we allocated, which means the parent process must run before we even
return from forkfd().
Change-Id: I1bee3bc466a04f19bd6efffd15f447f28c201aa9
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
defd49f7bf