1f4394a0c9
For a while now, we've had intermittent reports about problems with os/exec on macOS, but no clear way to reproduce them. Recent changes in the os/exec package test seem to have aligned the stars just right, at least on my two x86 and ARM MacBook Pro laptops, to make the package test hang with roughly 50% probability. When it does hang, the stacks I see in the hung process match the ones reported for the Go-based hangs in #33565. (They do not match the ones reported in the so-called C reproducer in that issue, but I think that reproducer is actually reproducing a different race, between fork and exit.) The stacks obtained from the hung child processes are in libSystem_atfork_child, which is supposed to reinitialize various parts of the C library in the new process. One common stack dies in _notify_fork_child calling _notify_globals (inlined) calling _os_alloc_once, because _os_alloc_once detects that the once lock is held by the parent process and then calls _os_once_gate_corruption_abort. The allocation is setting up the globals for the notification subsystem. See the source code at [1]. To work around this, we can allocate the globals earlier in the Go program's lifetime, before any execs are involved, by calling any notify routine that is exported, calls _notify_globals, and doesn't do anything too expensive otherwise. notify_is_valid_token(0) fits the bill. The other common stack dies in xpc_atfork_child calling _objc_msgSend_uncached which ends up in WAITING_FOR_ANOTHER_THREAD_TO_FINISH_CALLING_+initialize. Of course, whatever thread the child is waiting for is in the parent process and is not going to finish anything in the child process. There is no public source code for these routines, so it is unclear exactly what the problem is. However, xpc_atfork_child turns out to be exported (for use by libSystem_atfork_child, which is in a different library, so xpc_atfork_child is unlikely to be unexported any time soon). It also stands to reason that since xpc_atfork_child is called at the start of any forked child process, it can't be too harmful to call at the start of an ordinary Go process. And whatever caches it needs for a non-deadlocking fast path during exec empirically do get initialized by calling it at startup. This CL introduces a function osinit_hack, called at osinit time, which calls notify_is_valid_token(0) and xpc_atfork_child(). Doing so makes the os/exec test pass reliably on both my laptops - I can run it successfully hundreds of times in a row when my previous record was twice in a row. Fixes #33565. Fixes #56784. [1] https://opensource.apple.com/source/Libnotify/Libnotify-241/notify_client.c.auto.html Change-Id: I16a14a800893c40244678203532a3e8d6214b6bd Reviewed-on: https://go-review.googlesource.com/c/go/+/451735 Run-TryBot: Russ Cox <rsc@golang.org> Auto-Submit: Russ Cox <rsc@golang.org> Reviewed-by: Cherry Mui <cherryyz@google.com> TryBot-Result: Gopher Robot <gobot@golang.org> |
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