Latch signal handlers

Now that our interactive signal handlers are a strict superset of
non-interactive ones, there is no reason to "reset" signals or take action
when becoming non-interactive. Clean up how signal handlers get installed.

src/fish_tests.cpp

@@ -1012,7 +1012,7 @@ static void test_cancellation() {
     // Enable fish's signal handling here. We need to make this interactive for fish to install its
     // signal handlers.
     proc_push_interactive(1);
-    signal_set_handlers();
+    signal_set_handlers(true);
 
     // This tests that we can correctly ctrl-C out of certain loop constructs, and that nothing gets
     // printed if we do.

src/proc.cpp

@@ -942,18 +942,15 @@ void proc_sanity_check(const parser_t &parser) {
 
 void proc_push_interactive(int value) {
     ASSERT_IS_MAIN_THREAD();
-    int old = is_interactive;
     interactive_stack.push_back(is_interactive);
     is_interactive = value;
-    if (old != value) signal_set_handlers();
+    signal_set_handlers_once(is_interactive);
 }
 
 void proc_pop_interactive() {
     ASSERT_IS_MAIN_THREAD();
-    int old = is_interactive;
     is_interactive = interactive_stack.back();
     interactive_stack.pop_back();
-    if (is_interactive != old) signal_set_handlers();
 }
 
 void proc_wait_any(parser_t &parser) {

src/reader.cpp

@@ -1738,7 +1738,7 @@ static void reader_interactive_init() {
             }
         }
 
-        signal_set_handlers();
+        signal_set_handlers(shell_is_interactive());
     }
 
     // It shouldn't be necessary to place fish in its own process group and force control

src/signal.cpp

@@ -325,7 +325,7 @@ static void set_interactive_handlers() {
 }
 
 /// Sets up appropriate signal handlers.
-void signal_set_handlers() {
+void signal_set_handlers(bool interactive) {
     struct sigaction act;
     act.sa_flags = 0;
     sigemptyset(&act.sa_mask);
@@ -354,11 +354,19 @@ void signal_set_handlers() {
         FATAL_EXIT();
     }
 
-    if (shell_is_interactive()) {
+    if (interactive) {
         set_interactive_handlers();
     }
 }
 
+void signal_set_handlers_once(bool interactive) {
+    static std::once_flag s_noninter_once;
+    std::call_once(s_noninter_once, signal_set_handlers, false);
+
+    static std::once_flag s_inter_once;
+    if (interactive) std::call_once(s_inter_once, set_interactive_handlers);
+}
+
 void signal_handle(int sig, int do_handle) {
     struct sigaction act;
 

src/signal.h

@@ -17,7 +17,13 @@ const wchar_t *signal_get_desc(int sig);
 void signal_reset_handlers();
 
 /// Set signal handlers to fish default handlers.
-void signal_set_handlers();
+/// If \p interactive is set, apply interactive handlers as well.
+/// Note interactive handlers, once applied, are not cleared; they are a strict superset of
+/// non-interactive handlers.
+void signal_set_handlers(bool interactive);
+
+/// Latch function. This sets signal handlers, but only the first time it is called.
+void signal_set_handlers_once(bool interactive);
 
 /// Tell fish what to do on the specified signal.
 ///