main: reset signal mask and signal handlers at startup

This ensures processes spawned by us (e.g. the shell, new terminal
instances etc) don’t inherit a flawed signal mask, or having signals
unknowingly ignored.

Closes #854

CHANGELOG.md

@@ -82,6 +82,9 @@
   (https://codeberg.org/dnkl/foot/issues/379).
 * Line-based selections now include a trailing newline when copied
   (https://codeberg.org/dnkl/foot/issues/869).
+* Foot now clears the signal mask and resets all signal handlers to
+  their default handlers at startup
+  (https://codeberg.org/dnkl/foot/issues/854).
 
 
 ### Deprecated

main.c

@@ -152,6 +152,20 @@ print_pid(const char *pid_file, bool *unlink_at_exit)
         return false;
 }
 
+static void
+sanitize_signals(void)
+{
+    sigset_t mask;
+    sigemptyset(&mask);
+    sigprocmask(SIG_SETMASK, &mask, NULL);
+
+    struct sigaction dfl = {.sa_handler = SIG_DFL};
+    sigemptyset(&dfl.sa_mask);
+
+    for (int i = 1; i < SIGRTMAX; i++)
+        sigaction(i, &dfl, NULL);
+}
+
 int
 main(int argc, char *const *argv)
 {
@@ -160,6 +174,8 @@ main(int argc, char *const *argv)
     static const int foot_exit_failure = -26;
     int ret = foot_exit_failure;
 
+    sanitize_signals();
+
     /* XDG startup notifications */
     const char *token = getenv("XDG_ACTIVATION_TOKEN");
     unsetenv("XDG_ACTIVATION_TOKEN");