forkfd: implement vfork(2)-like support on Linux

fork() works by implementing Copy-On-Write for all pages that either the
parent or the child process write to. So if the parent process continues
running while the child is between fork(2) and execve(2), then it will
keep causing page faults and requiring the OS to duplicate those pages,
which may be expensive (page table updates, TLB flushes, etc.). This
problem is aggravated if the parent process is multithreaded, as the
simple act of running in the parent will cause those threads' stacks to
cause page faults.

The BSD solution for that was vfork(), which has two differences in
behavior: (1) it blocks the parent from running and (2) it shares memory
with it. But it's always been tricky, so POSIX.1-2001 deprecated it and
2008 removed its definition completely. Still, it is available somewhat
widely, and on Linux that can be achieved with clone(2) and the
CLONE_VFORK and CLONE_VM flags, for those two behaviors respectively.

Because of (2), we can't return from the forkfd() function in the child
(as that would trash the stack in the parent process), so to implement
this functionality vforkfd() adds a callback of the same signature as
glibc's clone(2) wrapper (something that hadn't occurred to me when we
attempted to use CLONE_VFORK last time).

On Linux, (1) is no problem, as clone(2) has native forkfd support. But
on other OSes, forkfd() requires the parent to run before the child
execve()s, in order to save the child PID in the list of children we're
going to handle SIGCHLD for in a non-racy way. Investigating if it is
possible to use vfork() anyway is left as an exercise for the reader.

Matching OpenDCDiag pull request:
https://github.com/opendcdiag/opendcdiag/pull/94

Pick-to: 6.4
Fixes: QTBUG-104493
Change-Id: Id0fb9ab0089845ee8843fffd16fa63c7c6f7dd1c
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@gmx.de>
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>
Reviewed-by: Milian Wolff <milian.wolff@kdab.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

0 files changed, 0 insertions, 0 deletions