fish_key_reader: stop looping on SIGHUP

Using the machinery in reader.cpp rather than going back to
intalling our own handlerss

(see 89644911a1883eea03ecc11ffbcef2df2a9c70fe)

Fixes #9309

src/fish_key_reader.cpp

@@ -227,7 +227,7 @@ static void process_input(bool continuous_mode, bool verbose) {
     std::vector<wchar_t> bind_chars;
 
     std::fwprintf(stderr, L"Press a key:\n");
-    for (;;) {
+    while (!check_exit_loop_maybe_warning(nullptr)) {
         maybe_t<char_event_t> evt{};
         if (reader_test_and_clear_interrupted()) {
             evt = char_event_t{shell_modes.c_cc[VINTR]};

src/reader.cpp

@@ -2909,12 +2909,12 @@ static bool try_warn_on_background_jobs(reader_data_t *data) {
 
 /// Check if we should exit the reader loop.
 /// \return true if we should exit.
-static bool check_exit_loop_maybe_warning(reader_data_t *data) {
+bool check_exit_loop_maybe_warning(reader_data_t *data) {
     // sighup always forces exit.
     if (s_sighup_received) return true;
 
     // Check if an exit is requested.
-    if (data->exit_loop_requested) {
+    if (data && data->exit_loop_requested) {
         if (try_warn_on_background_jobs(data)) {
             data->exit_loop_requested = false;
             return false;

src/reader.h

@@ -240,6 +240,9 @@ struct reader_config_t {
     int in{0};
 };
 
+class reader_data_t;
+bool check_exit_loop_maybe_warning(reader_data_t *data);
+
 /// Push a new reader environment controlled by \p conf, using the given history name.
 /// If \p history_name is empty, then save history in-memory only; do not write it to disk.
 void reader_push(parser_t &parser, const wcstring &history_name, reader_config_t &&conf);