Don't inherit POSIX timers across fork

[emkey1]

tgroup_copy does *group = *old_group, which copies posix_timers[] wholesale — including each entry's struct timer *. The child therefore starts life believing it owns the parent's timers, and timer_delete on an inherited id frees a timer object the parent still owns and whose callback is still armed.

Linux does not inherit POSIX timers across fork. (A thread clone shares the tgroup, so that case is already correct.) This clears the array in the copy.

Testing

Repro: parent creates a SIGEV_SIGNAL timer, arms it, forks; the child calls timer_delete on the inherited id, and the parent then deletes its own.

	child's timer_delete(inherited)	outcome
Linux (x86 Ubuntu)	-1 EINVAL	parent's timer unaffected
master	0 (succeeds)	frees the parent's live timer, ish dies
this branch	-1 EINVAL	parent's own timer_delete still succeeds

One thing to note while you are looking at this: POSIX timers do not appear to deliver signals at all on master, fork or no fork — a SIGEV_SIGNAL timer armed with a 20ms interval never fired in my testing, with no fork involved. That is a separate gap and this PR does not touch it, but it means the "parent's timer keeps firing" half of the story cannot currently be demonstrated. The double-free is independently reproducible as above.

Also ran a shell smoke test over an Alpine rootfs (directory walks, /proc reads, fork/exec churn, redirects, pipes) with no change in behavior.

🤖 Generated with Claude Code

[saagarjha]

While we care less about commit messages this is clear enough that it doesn't really need a body

[saagarjha]

Suggested change

	// POSIX timers are not inherited across fork. The struct copy above
	// duplicated posix_timers[], including each entry's struct timer pointer,
	// so the child saw the parent's timers as its own and timer_delete() in
	// the child freed a timer the parent still owns and is still running.
	for (int i = 0; i < TIMERS_MAX; i++)
	group->posix_timers[i] = (struct posix_timer) {};
	// POSIX timers are not inherited across fork
	for (int i = 0; i < TIMERS_MAX; i++) {
	group->posix_timers[i] = (struct posix_timer) {};
	}