Only allow QUIT on the main thread

Pedro pointed out that gdb worker threads should not react to quits.
While I don't think that the new DWARF reader can call QUIT from a
worker thread (and I don't think the existing minsym threading code
can either), it seems safest to address this before checking in the
new code.  This patch arranges for the QUIT macro to only work on the
main thread.

gdb/run-on-main-thread.c

@@ -20,6 +20,7 @@
 #include "run-on-main-thread.h"
 #include "ser-event.h"
 #if CXX_STD_THREAD
+#include <thread>
 #include <mutex>
 #endif
 #include "gdbsupport/event-loop.h"
@@ -38,6 +39,10 @@ static std::vector<std::function<void ()>> runnables;
 
 static std::mutex runnable_mutex;
 
+/* The main thread.  */
+
+static std::thread::id main_thread;
+
 #endif
 
 /* Run all the queued runnables.  */
@@ -89,10 +94,25 @@ run_on_main_thread (std::function<void ()> &&func)
   serial_event_set (runnable_event);
 }
 
+/* See run-on-main-thread.h.  */
+
+bool
+is_main_thread ()
+{
+#if CXX_STD_THREAD
+  return std::this_thread::get_id () == main_thread;
+#else
+  return true;
+#endif
+}
+
 void _initialize_run_on_main_thread ();
 void
 _initialize_run_on_main_thread ()
 {
+#if CXX_STD_THREAD
+  main_thread = std::this_thread::get_id ();
+#endif
   runnable_event = make_serial_event ();
   add_file_handler (serial_event_fd (runnable_event), run_events, nullptr,
 		    "run-on-main-thread");

gdb/run-on-main-thread.h

@@ -25,4 +25,8 @@
 
 extern void run_on_main_thread (std::function<void ()> &&);
 
+/* Return true on the main thread.  */
+
+extern bool is_main_thread ();
+
 #endif /* GDB_RUN_ON_MAIN_THREAD_H */

gdb/utils.c

@@ -78,6 +78,7 @@
 #include "bt-utils.h"
 #include "gdbsupport/buildargv.h"
 #include "pager.h"
+#include "run-on-main-thread.h"
 
 void (*deprecated_error_begin_hook) (void);
 
@@ -681,6 +682,9 @@ quit (void)
 void
 maybe_quit (void)
 {
+  if (!is_main_thread ())
+    return;
+
   if (sync_quit_force_run)
     quit ();