postgres/src/port/pqsignal.c

/*-------------------------------------------------------------------------
 *
 * pqsignal.c
 *	  reliable BSD-style signal(2) routine stolen from RWW who stole it
 *	  from Stevens...
 *
 * Portions Copyright (c) 1996-2024, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *	  src/port/pqsignal.c
 *
 *	This is the signal() implementation from "Advanced Programming in the UNIX
 *	Environment", with minor changes.  It was originally a replacement needed
 *	for old SVR4 systems whose signal() behaved as if sa_flags = SA_RESETHAND |
 *	SA_NODEFER, also known as "unreliable" signals due to races when the
 *	handler was reset.
 *
 *	By now, all known modern Unix systems have a "reliable" signal() call.
 *	We still don't want to use it though, because it remains
 *	implementation-defined by both C99 and POSIX whether the handler is reset
 *	or signals are blocked when the handler runs, and default restart behavior
 *	is also unspecified.  Therefore we take POSIX's advice and call sigaction()
 *	so we can provide explicit sa_flags, but wrap it in this more convenient
 *	traditional interface style.  It also provides a place to set any extra
 *	flags we want everywhere, such as SA_NOCLDSTOP.
 *
 *	Windows, of course, is resolutely in a class by itself.  In the backend,
 *	this relies on pqsigaction() in src/backend/port/win32/signal.c, which
 *	provides limited emulation of reliable signals.
 *
 *	Frontend programs can use this version of pqsignal() to forward to the
 *	native Windows signal() call if they wish, but beware that Windows signals
 *	behave quite differently.  Only the 6 signals required by C are supported.
 *	SIGINT handlers run in another thread instead of interrupting an existing
 *	thread, and the others don't interrupt system calls either, so SA_RESTART
 *	is moot.  All except SIGFPE have SA_RESETHAND semantics, meaning the
 *	handler is reset to SIG_DFL each time it runs.  The set of things you are
 *	allowed to do in a handler is also much more restricted than on Unix,
 *	according to the documentation.
 *
 * ------------------------------------------------------------------------
 */

#include "c.h"

#include <signal.h>
#ifndef FRONTEND
#include <unistd.h>
#endif

#ifndef FRONTEND
#include "libpq/pqsignal.h"
#include "miscadmin.h"
#endif

#ifdef PG_SIGNAL_COUNT			/* Windows */
#define PG_NSIG (PG_SIGNAL_COUNT)
#elif defined(NSIG)
#define PG_NSIG (NSIG)
#else
#define PG_NSIG (64)			/* XXX: wild guess */
#endif

/* Check a couple of common signals to make sure PG_NSIG is accurate. */
StaticAssertDecl(SIGUSR2 < PG_NSIG, "SIGUSR2 >= PG_NSIG");
StaticAssertDecl(SIGHUP < PG_NSIG, "SIGHUP >= PG_NSIG");
StaticAssertDecl(SIGTERM < PG_NSIG, "SIGTERM >= PG_NSIG");
StaticAssertDecl(SIGALRM < PG_NSIG, "SIGALRM >= PG_NSIG");

static volatile pqsigfunc pqsignal_handlers[PG_NSIG];

/*
 * Except when called with SIG_IGN or SIG_DFL, pqsignal() sets up this function
 * as the handler for all signals.  This wrapper handler function checks that
 * it is called within a process that the server knows about (i.e., any process
 * that has called InitProcessGlobals(), such as a client backend), and not a
 * child process forked by system(3), etc.  This check ensures that such child
 * processes do not modify shared memory, which is often detrimental.  If the
 * check succeeds, the function originally provided to pqsignal() is called.
 * Otherwise, the default signal handler is installed and then called.
 *
 * This wrapper also handles restoring the value of errno.
 */
static void
wrapper_handler(SIGNAL_ARGS)
{
	int			save_errno = errno;

#ifndef FRONTEND

	/*
	 * We expect processes to set MyProcPid before calling pqsignal() or
	 * before accepting signals.
	 */
	Assert(MyProcPid);
	Assert(MyProcPid != PostmasterPid || !IsUnderPostmaster);

	if (unlikely(MyProcPid != (int) getpid()))
	{
		pqsignal(postgres_signal_arg, SIG_DFL);
		raise(postgres_signal_arg);
		return;
	}
#endif

	(*pqsignal_handlers[postgres_signal_arg]) (postgres_signal_arg);

	errno = save_errno;
}

/*
 * Set up a signal handler, with SA_RESTART, for signal "signo"
 *
 * Returns the previous handler.
 *
 * NB: If called within a signal handler, race conditions may lead to bogus
 * return values.  You should either avoid calling this within signal handlers
 * or ignore the return value.
 *
 * XXX: Since no in-tree callers use the return value, and there is little
 * reason to do so, it would be nice if we could convert this to a void
 * function instead of providing potentially-bogus return values.
 * Unfortunately, that requires modifying the pqsignal() in legacy-pqsignal.c,
 * which in turn requires an SONAME bump, which is probably not worth it.
 */
pqsigfunc
pqsignal(int signo, pqsigfunc func)
{
	pqsigfunc	orig_func = pqsignal_handlers[signo];	/* assumed atomic */
#if !(defined(WIN32) && defined(FRONTEND))
	struct sigaction act,
				oact;
#else
	pqsigfunc	ret;
#endif

	Assert(signo < PG_NSIG);

	if (func != SIG_IGN && func != SIG_DFL)
	{
		pqsignal_handlers[signo] = func;	/* assumed atomic */
		func = wrapper_handler;
	}

#if !(defined(WIN32) && defined(FRONTEND))
	act.sa_handler = func;
	sigemptyset(&act.sa_mask);
	act.sa_flags = SA_RESTART;
#ifdef SA_NOCLDSTOP
	if (signo == SIGCHLD)
		act.sa_flags |= SA_NOCLDSTOP;
#endif
	if (sigaction(signo, &act, &oact) < 0)
		return SIG_ERR;
	else if (oact.sa_handler == wrapper_handler)
		return orig_func;
	else
		return oact.sa_handler;
#else
	/* Forward to Windows native signal system. */
	if ((ret = signal(signo, func)) == wrapper_handler)
		return orig_func;
	else
		return ret;
#endif
}