--- /dev/null
+/* POSIX compatible signal blocking.
+ Copyright (C) 2008-2018 Free Software Foundation, Inc.
+ Written by Eric Blake <ebb9@byu.net>, 2008.
+
+ This program is free software: you can redistribute it and/or modify
+ it under the terms of the GNU General Public License as published by
+ the Free Software Foundation; either version 3 of the License, or
+ (at your option) any later version.
+
+ This program is distributed in the hope that it will be useful,
+ but WITHOUT ANY WARRANTY; without even the implied warranty of
+ MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ GNU General Public License for more details.
+
+ You should have received a copy of the GNU General Public License
+ along with this program. If not, see <https://www.gnu.org/licenses/>. */
+
+#include <config.h>
+
+/* Specification. */
+#include <signal.h>
+
+#include <errno.h>
+#include <stdint.h>
+#include <stdlib.h>
+
+/* This implementation of sigaction is tailored to native Windows behavior:
+ signal() has SysV semantics (ie. the handler is uninstalled before
+ it is invoked). This is an inherent data race if an asynchronous
+ signal is sent twice in a row before we can reinstall our handler,
+ but there's nothing we can do about it. Meanwhile, sigprocmask()
+ is not present, and while we can use the gnulib replacement to
+ provide critical sections, it too suffers from potential data races
+ in the face of an ill-timed asynchronous signal. And we compound
+ the situation by reading static storage in a signal handler, which
+ POSIX warns is not generically async-signal-safe. Oh well.
+
+ Additionally:
+ - We don't implement SA_NOCLDSTOP or SA_NOCLDWAIT, because SIGCHLD
+ is not defined.
+ - We don't implement SA_ONSTACK, because sigaltstack() is not present.
+ - We ignore SA_RESTART, because blocking native Windows API calls are
+ not interrupted anyway when an asynchronous signal occurs, and the
+ MSVCRT runtime never sets errno to EINTR.
+ - We don't implement SA_SIGINFO because it is impossible to do so
+ portably.
+
+ POSIX states that an application should not mix signal() and
+ sigaction(). We support the use of signal() within the gnulib
+ sigprocmask() substitute, but all other application code linked
+ with this module should stick with only sigaction(). */
+
+/* Check some of our assumptions. */
+#if defined SIGCHLD || defined HAVE_SIGALTSTACK || defined HAVE_SIGINTERRUPT
+# error "Revisit the assumptions made in the sigaction module"
+#endif
+
+/* Out-of-range substitutes make a good fallback for uncatchable
+ signals. */
+#ifndef SIGKILL
+# define SIGKILL (-1)
+#endif
+#ifndef SIGSTOP
+# define SIGSTOP (-1)
+#endif
+
+/* On native Windows, as of 2008, the signal SIGABRT_COMPAT is an alias
+ for the signal SIGABRT. Only one signal handler is stored for both
+ SIGABRT and SIGABRT_COMPAT. SIGABRT_COMPAT is not a signal of its own. */
+#if defined _WIN32 && ! defined __CYGWIN__
+# undef SIGABRT_COMPAT
+# define SIGABRT_COMPAT 6
+#endif
+
+/* A signal handler. */
+typedef void (*handler_t) (int signal);
+
+/* Set of current actions. If sa_handler for an entry is NULL, then
+ that signal is not currently handled by the sigaction handler. */
+static struct sigaction volatile action_array[NSIG] /* = 0 */;
+
+/* Signal handler that is installed for signals. */
+static void
+sigaction_handler (int sig)
+{
+ handler_t handler;
+ sigset_t mask;
+ sigset_t oldmask;
+ int saved_errno = errno;
+ if (sig < 0 || NSIG <= sig || !action_array[sig].sa_handler)
+ {
+ /* Unexpected situation; be careful to avoid recursive abort. */
+ if (sig == SIGABRT)
+ signal (SIGABRT, SIG_DFL);
+ abort ();
+ }
+
+ /* Reinstall the signal handler when required; otherwise update the
+ bookkeeping so that the user's handler may call sigaction and get
+ accurate results. We know the signal isn't currently blocked, or
+ we wouldn't be in its handler, therefore we know that we are not
+ interrupting a sigaction() call. There is a race where any
+ asynchronous instance of the same signal occurring before we
+ reinstall the handler will trigger the default handler; oh
+ well. */
+ handler = action_array[sig].sa_handler;
+ if ((action_array[sig].sa_flags & SA_RESETHAND) == 0)
+ signal (sig, sigaction_handler);
+ else
+ action_array[sig].sa_handler = NULL;
+
+ /* Block appropriate signals. */
+ mask = action_array[sig].sa_mask;
+ if ((action_array[sig].sa_flags & SA_NODEFER) == 0)
+ sigaddset (&mask, sig);
+ sigprocmask (SIG_BLOCK, &mask, &oldmask);
+
+ /* Invoke the user's handler, then restore prior mask. */
+ errno = saved_errno;
+ handler (sig);
+ saved_errno = errno;
+ sigprocmask (SIG_SETMASK, &oldmask, NULL);
+ errno = saved_errno;
+}
+
+/* Change and/or query the action that will be taken on delivery of
+ signal SIG. If not NULL, ACT describes the new behavior. If not
+ NULL, OACT is set to the prior behavior. Return 0 on success, or
+ set errno and return -1 on failure. */
+int
+sigaction (int sig, const struct sigaction *restrict act,
+ struct sigaction *restrict oact)
+{
+ sigset_t mask;
+ sigset_t oldmask;
+ int saved_errno;
+
+ if (sig < 0 || NSIG <= sig || sig == SIGKILL || sig == SIGSTOP
+ || (act && act->sa_handler == SIG_ERR))
+ {
+ errno = EINVAL;
+ return -1;
+ }
+
+#ifdef SIGABRT_COMPAT
+ if (sig == SIGABRT_COMPAT)
+ sig = SIGABRT;
+#endif
+
+ /* POSIX requires sigaction() to be async-signal-safe. In other
+ words, if an asynchronous signal can occur while we are anywhere
+ inside this function, the user's handler could then call
+ sigaction() recursively and expect consistent results. We meet
+ this rule by using sigprocmask to block all signals before
+ modifying any data structure that could be read from a signal
+ handler; this works since we know that the gnulib sigprocmask
+ replacement does not try to use sigaction() from its handler. */
+ if (!act && !oact)
+ return 0;
+ sigfillset (&mask);
+ sigprocmask (SIG_BLOCK, &mask, &oldmask);
+ if (oact)
+ {
+ if (action_array[sig].sa_handler)
+ *oact = action_array[sig];
+ else
+ {
+ /* Safe to change the handler at will here, since all
+ signals are currently blocked. */
+ oact->sa_handler = signal (sig, SIG_DFL);
+ if (oact->sa_handler == SIG_ERR)
+ goto failure;
+ signal (sig, oact->sa_handler);
+ oact->sa_flags = SA_RESETHAND | SA_NODEFER;
+ sigemptyset (&oact->sa_mask);
+ }
+ }
+
+ if (act)
+ {
+ /* Safe to install the handler before updating action_array,
+ since all signals are currently blocked. */
+ if (act->sa_handler == SIG_DFL || act->sa_handler == SIG_IGN)
+ {
+ if (signal (sig, act->sa_handler) == SIG_ERR)
+ goto failure;
+ action_array[sig].sa_handler = NULL;
+ }
+ else
+ {
+ if (signal (sig, sigaction_handler) == SIG_ERR)
+ goto failure;
+ action_array[sig] = *act;
+ }
+ }
+ sigprocmask (SIG_SETMASK, &oldmask, NULL);
+ return 0;
+
+ failure:
+ saved_errno = errno;
+ sigprocmask (SIG_SETMASK, &oldmask, NULL);
+ errno = saved_errno;
+ return -1;
+}
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