runtime: dieFromSignal should reinstall fwdSig[sig] instead of hard-coding _SIG_DFL

[bcmills]

runtime.dieFromSignal is called in a few places in the runtime where we've handled a fatal signal and intend to terminate the process.

At the moment, dieFromSignal unconditionally calls setsig(sig, _SIG_DFL) and re-raises the signal.
That's pretty much always the right thing to do in a pure-Go program.

However, in a mixed-language program there may be some existing signal handler, and that signal handler might want to do something useful — for example, save a core dump, mark an input record as a potential cause of the crash, or write an entry to a crash log.

It seems to me that we could address this use-case pretty easily by making dieFromSignal try to reinstall and raise to the previous handler before it resorts to SIG_DFL. Perhaps something like:

func dieFromSignal(sig uint32) {
	fwdFn := atomic.Loaduintptr(&fwdSig[sig])
	if fwdFn != _SIG_DFL {
		setsig(sig, fwdFn)
		unblocksig(sig)
		raise(sig)
	}

	// fwdSig[sig] either didn't exist or failed to terminate the process.
	// Try _SIG_DFL instead.
	setsig(sig, _SIG_DFL)
	unblocksig(sig)
	raise(sig)

	// That should have killed us. On some systems, though, raise
	// sends the signal to the whole process rather than to just
	// the current thread, which means that the signal may not yet
	// have been delivered. Give other threads a chance to run and
	// pick up the signal.
	osyield()
	osyield()
	osyield()

	// If we are still somehow running, just exit with the wrong status.
	exit(2)
}

In the typical case — when there are no handlers registered before ours — the behavior would be the same as it is today. On the off chance that some other language has registered a prior handler, we would instead invoke that handler, and if it isn't well behaved (i.e. doesn't terminate the process), then we fall back to today's behavior anyway.

[ianlancetaylor]

SGTM

It's only necessary to call unblockSig once.

[gopherbot]

CL https://golang.org/cl/49590 mentions this issue.

[gopherbot]

Change https://golang.org/cl/55032 mentions this issue: runtime: fix crashing with foreign signal handlers on Darwin