Fix interactive --on-signal INT handlers

f8ba0ac5bf introduced a bug where INT handlers would themselves be
cancelled, due to the signal. Defer processing handlers until the
parser is ready to execute more fish script.

Fixes the interactive case of #6649.

src/event.cpp

@@ -288,6 +288,8 @@ void event_fire_delayed(parser_t &parser) {
     auto &ld = parser.libdata();
     // Do not invoke new event handlers from within event handlers.
     if (ld.is_event) return;
+    // Do not invoke new event handlers if we are unwinding (#6649).
+    if (parser.get_cancel_signal()) return;
 
     std::vector<shared_ptr<event_t>> to_send;
     to_send.swap(ld.blocked_events);

src/parser.h

@@ -376,6 +376,9 @@ class parser_t : public std::enable_shared_from_this<parser_t> {
     void get_backtrace(const wcstring &src, const parse_error_list_t &errors,
                        wcstring &output) const;
 
+    /// \return the signal triggering cancellation, or 0 if none.
+    int get_cancel_signal() const { return cancellation_signal; }
+
     /// Output profiling data to the given filename.
     void emit_profiling(const char *path) const;