drmgr: unsafe behavior in signal handler

[nathanlynch]

Consider drmgr's signal handler:

void
sighandler(int signo)
{
	say(ERROR, "Received signal %d, attempting to cleanup and exit\n",
	    signo);

	if (log_fd) {
		void *callstack[128];
		int sz;

		sz = backtrace(callstack, 128);
		backtrace_symbols_fd(callstack, sz, log_fd);
	}

	dr_fini();
	exit(-1);
}

There are several actions that are not safe[1] and could result in process hangs/crashes:

1. say(), which uses a couple of functions from the printf family: AS-Unsafe[2]
2. backtrace() and backtrace_symbols_fd(): AS-Unsafe[3]
3. dr_fini() which performs too many AS-Unsafe things to list individually

I think that we would be OK to simply remove sighandler and sig_setup. Perhaps in the past there was legitimate reason for drmgr to handle normally fatal signals such as SIGSEGV and SIGBUS, but I don't think that's true any more. The only reason for caution I see is that this handler releases the dr_lock, but since that's an advisory record lock it will be released on process termination anyway.

A benefit of cleaning this up will be that drmgr will produce proper core dumps reliably when it crashes.

[1] https://www.gnu.org/software/libc/manual/html_node/POSIX-Safety-Concepts.html and https://man7.org/linux/man-pages/man7/signal-safety.7.html
[2] https://www.gnu.org/software/libc/manual/html_node/Formatted-Output-Functions.html
[3] https://www.gnu.org/software/libc/manual/html_node/Backtraces.html

[nathanlynch]

proposed change from Scott: https://groups.google.com/g/powerpc-utils-devel/c/yNb4q339fM8