gdb: add debug prints in event loop

Add debug printouts about event loop-related events:

 - When a file descriptor handler gets invoked
 - When an async event/signal handler gets invoked

gdb/ChangeLog:

	* async-event.c (invoke_async_signal_handlers): Add debug
	print.
	(check_async_event_handlers): Likewise.
	* event-top.c (show_debug_event_loop): New function.
	(_initialize_event_top): Register "set debug event-loop"
	setting.

gdbserver/ChangeLog:

	* server.cc (handle_monitor_command): Handle "set
	debug-event-loop".
	(captured_main): Handle "--debug-event-loop".
	(monitor_show_help): Mention new setting.
	(gdbserver_usage): Mention new flag.

gdbsupport/ChangeLog:

	* event-loop.h (debug_event_loop): New variable declaration.
	(event_loop_debug_printf_1): New function declaration.
	(event_loop_debug_printf): New macro.
	* event-loop.cc (debug_event_loop): New variable.
	(handle_file_event): Add debug print.
	(event_loop_debug_printf_1): New function.

Change-Id: If78ed3a69179881368e7895b42940ce13b6a1a05

3 files changed, 86 insertions, 2 deletions

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index 9f74494..737091e 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,5 +1,14 @@
 2020-10-02  Simon Marchi  <simon.marchi@polymtl.ca>
 
+	* async-event.c (invoke_async_signal_handlers): Add debug
+	print.
+	(check_async_event_handlers): Likewise.
+	* event-top.c (show_debug_event_loop): New function.
+	(_initialize_event_top): Register "set debug event-loop"
+	setting.
+
+2020-10-02  Simon Marchi  <simon.marchi@polymtl.ca>
+
 	* debug.c (debug_prefixed_vprintf): Move to gdbsupport.
 	* debug.h: Remove.
 	* infrun.c: Include gdbsupport/common-debug.h.
diff --git a/gdb/async-event.c b/gdb/async-event.c
index 55be014..4228dfb 100644
--- a/gdb/async-event.c
+++ b/gdb/async-event.c
@@ -157,8 +157,23 @@ create_async_signal_handler (sig_handler_func * proc,
    for some event.  The caller of this function is the interrupt
    handler associated with a signal.  */
 void
-mark_async_signal_handler (async_signal_handler * async_handler_ptr)
+mark_async_signal_handler (async_signal_handler *async_handler_ptr)
 {
+  if (debug_event_loop != debug_event_loop_kind::OFF)
+    {
+      /* This is called by signal handlers, so we print it "by hand" using
+	 the async-signal-safe methods.  */
+      const char head[] = ("[event-loop] mark_async_signal_handler: marking"
+			   "async signal handler `");
+      gdb_stdlog->write_async_safe (head, strlen (head));
+
+      gdb_stdlog->write_async_safe (async_handler_ptr->name,
+				    strlen (async_handler_ptr->name));
+
+      const char tail[] = "`\n";
+      gdb_stdlog->write_async_safe (tail, strlen (tail));
+    }
+
   async_handler_ptr->ready = 1;
   serial_event_set (async_signal_handlers_serial_event);
 }
@@ -168,6 +183,8 @@ mark_async_signal_handler (async_signal_handler * async_handler_ptr)
 void
 clear_async_signal_handler (async_signal_handler *async_handler_ptr)
 {
+  event_loop_debug_printf ("clearing async signal handler `%s`",
+			   async_handler_ptr->name);
   async_handler_ptr->ready = 0;
 }
 
@@ -211,6 +228,8 @@ invoke_async_signal_handlers (void)
       /* Async signal handlers have no connection to whichever was the
 	 current UI, and thus always run on the main one.  */
       current_ui = main_ui;
+      event_loop_debug_printf ("invoking async signal handler `%s`",
+			       async_handler_ptr->name);
       (*async_handler_ptr->proc) (async_handler_ptr->client_data);
     }
 
@@ -274,6 +293,8 @@ create_async_event_handler (async_event_handler_func *proc,
 void
 mark_async_event_handler (async_event_handler *async_handler_ptr)
 {
+  event_loop_debug_printf ("marking async event handler `%s`",
+			   async_handler_ptr->name);
   async_handler_ptr->ready = 1;
 }
 
@@ -282,6 +303,8 @@ mark_async_event_handler (async_event_handler *async_handler_ptr)
 void
 clear_async_event_handler (async_event_handler *async_handler_ptr)
 {
+  event_loop_debug_printf ("clearing async event handler `%s`",
+			   async_handler_ptr->name);
   async_handler_ptr->ready = 0;
 }
 
@@ -300,6 +323,8 @@ check_async_event_handlers ()
       if (async_handler_ptr->ready)
 	{
 	  async_handler_ptr->ready = 0;
+	  event_loop_debug_printf ("invoking async event handler `%s`",
+				   async_handler_ptr->name);
 	  (*async_handler_ptr->proc) (async_handler_ptr->client_data);
 	  return 1;
 	}
diff --git a/gdb/event-top.c b/gdb/event-top.c
index fdce5de..63d2295 100644
--- a/gdb/event-top.c
+++ b/gdb/event-top.c
@@ -525,7 +525,7 @@ void
 ui_register_input_event_handler (struct ui *ui)
 {
   add_file_handler (ui->input_fd, stdin_event_handler, ui,
-		    string_printf ("ui-%d", ui->num));
+		    string_printf ("ui-%d", ui->num), true);
 }
 
 /* See top.h.  */
@@ -1287,3 +1287,53 @@ gdb_disable_readline (void)
     gdb_rl_callback_handler_remove ();
   delete_file_handler (ui->input_fd);
 }
+
+static const char debug_event_loop_off[] = "off";
+static const char debug_event_loop_all_except_ui[] = "all-except-ui";
+static const char debug_event_loop_all[] = "all";
+
+static const char *debug_event_loop_enum[] = {
+  debug_event_loop_off,
+  debug_event_loop_all_except_ui,
+  debug_event_loop_all,
+  nullptr
+};
+
+static const char *debug_event_loop_value = debug_event_loop_off;
+
+static void
+set_debug_event_loop_command (const char *args, int from_tty,
+			      cmd_list_element *c)
+{
+  if (debug_event_loop_value == debug_event_loop_off)
+    debug_event_loop = debug_event_loop_kind::OFF;
+  else if (debug_event_loop_value == debug_event_loop_all_except_ui)
+    debug_event_loop = debug_event_loop_kind::ALL_EXCEPT_UI;
+  else if (debug_event_loop_value == debug_event_loop_all)
+    debug_event_loop = debug_event_loop_kind::ALL;
+  else
+    gdb_assert_not_reached ("Invalid debug event look kind value.");
+}
+
+static void
+show_debug_event_loop_command (struct ui_file *file, int from_tty,
+			       struct cmd_list_element *cmd, const char *value)
+{
+  fprintf_filtered (file, _("Event loop debugging is %s.\n"), value);
+}
+
+void _initialize_event_top ();
+void
+_initialize_event_top ()
+{
+  add_setshow_enum_cmd ("event-loop", class_maintenance,
+			debug_event_loop_enum,
+			&debug_event_loop_value,
+			_("Set event-loop debugging."),
+			_("Show event-loop debugging."),
+			_("\
+Control whether to show event loop-related debug messages."),
+			set_debug_event_loop_command,
+			show_debug_event_loop_command,
+			&setdebuglist, &showdebuglist);
+}