signal handling: support SA_RESETHAND

Add support for SA_RESETHAND signal handler flag, which means that the signal
handler is reset to the default one after handling the signal once.

I admit it's not a very useful feature (our default handler is powering off
the system...) but no reason not to support it.

Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>

libc/signal.cc

@@ -223,12 +223,20 @@ int kill(pid_t pid, int sig)
         // very Unix-like behavior, but if we assume that the program doesn't
         // care which of its threads handle the signal - why not just create
         // a completely new thread and run it there...
-        const auto& sa = signal_actions[sig];
+        const auto sa = signal_actions[sig];
         auto t = new sched::thread([=] {
+            if (sa.sa_flags & SA_RESETHAND) {
+                signal_actions[sig].sa_flags = 0;
+                signal_actions[sig].sa_handler = SIG_DFL;
+            }
             if (sa.sa_flags & SA_SIGINFO) {
                 // FIXME: proper second (siginfo) and third (context) arguments (See example in call_signal_handler)
                 sa.sa_sigaction(sig, nullptr, nullptr);
             } else {
+                if (sa.sa_flags & SA_RESETHAND) {
+                    signal_actions[sig].sa_flags = 0;
+                    signal_actions[sig].sa_handler = SIG_DFL;
+                }
                 sa.sa_handler(sig);
             }
         }, sched::thread::attr().detached().stack(65536).name("signal_handler"));

tests/tst-kill.cc

@@ -113,6 +113,36 @@ int main(int ac, char** av)
     report(recv_result == -1,   "timed out syscall returns -1");
     report(recv_errno == EWOULDBLOCK, "timed out syscall doesn't return EINTR");
 
+    // Test with and without SA_RESETHAND (for __sysv_signal support)
+    struct sigaction act = {};
+    act.sa_handler = handler1;
+    global = 0;
+    r = sigaction(SIGUSR1, &act, nullptr);
+    report(r == 0, "set SIGUSR1 handler");
+    r = kill(0, SIGUSR1);
+    report(r == 0, "kill SIGUSR1 succeeds");
+    for (int i=0; i<100; i++) {
+        if (global == 1) break;
+        usleep(10000);
+    }
+    report(global == 1, "'global' is now 1");
+    struct sigaction oldact;
+    r = sigaction(SIGUSR1, nullptr, &oldact);
+    report(r == 0 && oldact.sa_handler == handler1, "without SA_RESETHAND, signal handler is not reset");
+    act.sa_flags = SA_RESETHAND;
+    r = sigaction(SIGUSR1, &act, nullptr);
+    report(r == 0, "set SIGUSR1 handler with SA_RESETHAND");
+    r = kill(0, SIGUSR1);
+    report(r == 0, "kill SIGUSR1 succeeds");
+    for (int i=0; i<100; i++) {
+        if (global == 1) break;
+        usleep(10000);
+    }
+    report(global == 1, "'global' is now 1");
+    r = sigaction(SIGUSR1, nullptr, &oldact);
+    report(r == 0 && oldact.sa_handler == SIG_DFL, "with SA_RESETHAND, signal handler is reset");
+
+
     debug("SUMMARY: %d tests, %d failures\n", tests, fails);
 }