gdb/mi: preserve user selected thread and frame when invoking MI commands

Fix for PR gdb/20684.  When invoking MI commands with --thread and/or
--frame, the user selected thread and frame was not preserved:

  (gdb)
  info thread
  &"info thread\n"
  ~"  Id   Target Id                                           Frame \n"
  ~"* 1    Thread 0x7ffff7c30740 (LWP 19302) \"user-selected-c\" main () at /home/uuu/gdb/gdb/testsuite/gdb.mi/user-selected-context-sync.c:60\n"
  ~"  2    Thread 0x7ffff7c2f700 (LWP 19306) \"user-selected-c\" child_sub_function () at /home/uuu/gdb/gdb/testsuite/gdb.mi/user-selected-context-sync.c:30\n"
  ~"  3    Thread 0x7ffff742e700 (LWP 19307) \"user-selected-c\" child_sub_function () at /home/uuu/gdb/gdb/testsuite/gdb.mi/user-selected-context-sync.c:30\n"
  ^done
  (gdb)
  info frame
  &"info frame\n"
  ~"Stack level 0, frame at 0x7fffffffdf90:\n"
  ~" rip = 0x555555555207 in main (/home/uuu/gdb/gdb/testsuite/gdb.mi/user-selected-context-sync.c:60); saved rip = 0x7ffff7c5709b\n"
  ~" source language c.\n"
  ~" Arglist at 0x7fffffffdf80, args: \n"
  ~" Locals at 0x7fffffffdf80, Previous frame's sp is 0x7fffffffdf90\n"
  ~" Saved registers:\n "
  ~" rbp at 0x7fffffffdf80, rip at 0x7fffffffdf88\n"
  ^done
  (gdb)
  -stack-info-depth --thread 3
  ^done,depth="4"
  (gdb)
  info thread
  &"info thread\n"
  ~"  Id   Target Id                                           Frame \n"
  ~"  1    Thread 0x7ffff7c30740 (LWP 19302) \"user-selected-c\" main () at /home/uuu/gdb/gdb/testsuite/gdb.mi/user-selected-context-sync.c:60\n"
  ~"  2    Thread 0x7ffff7c2f700 (LWP 19306) \"user-selected-c\" child_sub_function () at /home/uuu/gdb/gdb/testsuite/gdb.mi/user-selected-context-sync.c:30\n"
  ~"* 3    Thread 0x7ffff742e700 (LWP 19307) \"user-selected-c\" child_sub_function () at /home/uuu/gdb/gdb/testsuite/gdb.mi/user-selected-context-sync.c:30\n"
  ^done
  (gdb)
  info frame
  &"info frame\n"
  ~"Stack level 0, frame at 0x7ffff742dee0:\n"
  ~" rip = 0x555555555169 in child_sub_function (/home/uuu/gdb/gdb/testsuite/gdb.mi/user-selected-context-sync.c:30); saved rip = 0x555555555188\n"
  ~" called by frame at 0x7ffff742df00\n"
  ~" source language c.\n"
  ~" Arglist at 0x7ffff742ded0, args: \n"
  ~" Locals at 0x7ffff742ded0, Previous frame's sp is 0x7ffff742dee0\n"
  ~" Saved registers:\n "
  ~" rbp at 0x7ffff742ded0, rip at 0x7ffff742ded8\n"
  ^done
  (gdb)

This caused problems for frontends that provide access to CLI because UI
may silently change the context for CLI commands (as demonstrated above).

This commit fixes the problem by restoring thread and frame in
mi_cmd_execute (). With this change, there are only two GDB/MI commands
that can change user selected context: -thread-select and -stack-select-frame.
This allows us to remove all and rather complicated logic of notifying
about user selected context change from mi_execute_command (), leaving it
to these two commands themselves to notify.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=20684

3 files changed, 34 insertions, 63 deletions

diff --git a/gdb/mi/mi-cmd-stack.c b/gdb/mi/mi-cmd-stack.c
index e63f170..1be8aa8 100644
--- a/gdb/mi/mi-cmd-stack.c
+++ b/gdb/mi/mi-cmd-stack.c
@@ -36,6 +36,8 @@
 #include "mi-parse.h"
 #include "gdbsupport/gdb_optional.h"
 #include "safe-ctype.h"
+#include "inferior.h"
+#include "observable.h"
 
 enum what_to_list { locals, arguments, all };
 
@@ -756,7 +758,16 @@ mi_cmd_stack_select_frame (const char *command, char **argv, int argc)
   if (argc == 0 || argc > 1)
     error (_("-stack-select-frame: Usage: FRAME_SPEC"));
 
+  ptid_t previous_ptid = inferior_ptid;
+
   select_frame_for_mi (parse_frame_specification (argv[0]));
+
+  /* Notify if the thread has effectively changed.  */
+  if (inferior_ptid != previous_ptid)
+    {
+      gdb::observers::user_selected_context_changed.notify
+	(USER_SELECTED_THREAD | USER_SELECTED_FRAME);
+    }
 }
 
 void
diff --git a/gdb/mi/mi-cmds.h b/gdb/mi/mi-cmds.h
index 2a93a9f..785652e 100644
--- a/gdb/mi/mi-cmds.h
+++ b/gdb/mi/mi-cmds.h
@@ -23,6 +23,7 @@
 #define MI_MI_CMDS_H
 
 #include "gdbsupport/gdb_optional.h"
+#include "mi/mi-main.h"
 
 enum print_values {
    PRINT_NO_VALUES,
@@ -163,6 +164,17 @@ struct mi_command
      wrong.  */
   void invoke (struct mi_parse *parse) const;
 
+  /* Return whether this command preserves user selected context (thread
+     and frame).  */
+  bool preserve_user_selected_context () const
+  {
+    /* Here we exploit the fact that if MI command is supposed to change
+       user context, then it should not emit change notifications.  Therefore if
+       command does not suppress user context change notifications, then it should
+       preserve the context.  */
+    return m_suppress_notification != &mi_suppress_notification.user_selected_context;
+  }
+
 protected:
 
   /* The core of command invocation, this needs to be overridden in each
diff --git a/gdb/mi/mi-main.c b/gdb/mi/mi-main.c
index b359296..73380f5 100644
--- a/gdb/mi/mi-main.c
+++ b/gdb/mi/mi-main.c
@@ -1918,34 +1918,6 @@ mi_print_exception (const char *token, const struct gdb_exception &exception)
   fputs_unfiltered ("\n", mi->raw_stdout);
 }
 
-/* Determine whether the parsed command already notifies the
-   user_selected_context_changed observer.  */
-
-static int
-command_notifies_uscc_observer (struct mi_parse *command)
-{
-  if (command->op == CLI_COMMAND)
-    {
-      /* CLI commands "thread" and "inferior" already send it.  */
-      return (startswith (command->command, "thread ")
-	      || startswith (command->command, "inferior "));
-    }
-  else /* MI_COMMAND */
-    {
-      if (strcmp (command->command, "interpreter-exec") == 0
-	  && command->argc > 1)
-	{
-	  /* "thread" and "inferior" again, but through -interpreter-exec.  */
-	  return (startswith (command->argv[1], "thread ")
-		  || startswith (command->argv[1], "inferior "));
-	}
-
-      else
-	/* -thread-select already sends it.  */
-	return strcmp (command->command, "thread-select") == 0;
-    }
-}
-
 void
 mi_execute_command (const char *cmd, int from_tty)
 {
@@ -1971,8 +1943,6 @@ mi_execute_command (const char *cmd, int from_tty)
 
   if (command != NULL)
     {
-      ptid_t previous_ptid = inferior_ptid;
-
       command->token = token;
 
       if (do_timings)
@@ -2002,37 +1972,6 @@ mi_execute_command (const char *cmd, int from_tty)
 
       bpstat_do_actions ();
 
-      if (/* The notifications are only output when the top-level
-	     interpreter (specified on the command line) is MI.  */
-	  top_level_interpreter ()->interp_ui_out ()->is_mi_like_p ()
-	  /* Don't try report anything if there are no threads --
-	     the program is dead.  */
-	  && any_thread_p ()
-	  /* If the command already reports the thread change, no need to do it
-	     again.  */
-	  && !command_notifies_uscc_observer (command.get ()))
-	{
-	  int report_change = 0;
-
-	  if (command->thread == -1)
-	    {
-	      report_change = (previous_ptid != null_ptid
-			       && inferior_ptid != previous_ptid
-			       && inferior_ptid != null_ptid);
-	    }
-	  else if (inferior_ptid != null_ptid)
-	    {
-	      struct thread_info *ti = inferior_thread ();
-
-	      report_change = (ti->global_num != command->thread);
-	    }
-
-	  if (report_change)
-	    {
-	      gdb::observers::user_selected_context_changed.notify
-		(USER_SELECTED_THREAD | USER_SELECTED_FRAME);
-	    }
-	}
     }
 }
 
@@ -2076,6 +2015,7 @@ mi_cmd_execute (struct mi_parse *parse)
       set_current_program_space (inf->pspace);
     }
 
+  gdb::optional<scoped_restore_current_thread> thread_saver;
   if (parse->thread != -1)
     {
       thread_info *tp = find_thread_global_id (parse->thread);
@@ -2086,9 +2026,13 @@ mi_cmd_execute (struct mi_parse *parse)
       if (tp->state == THREAD_EXITED)
 	error (_("Thread id: %d has terminated"), parse->thread);
 
+      if (parse->cmd->preserve_user_selected_context ())
+	thread_saver.emplace ();
+
       switch_to_thread (tp);
     }
 
+  gdb::optional<scoped_restore_selected_frame> frame_saver;
   if (parse->frame != -1)
     {
       struct frame_info *fid;
@@ -2096,8 +2040,12 @@ mi_cmd_execute (struct mi_parse *parse)
 
       fid = find_relative_frame (get_current_frame (), &frame);
       if (frame == 0)
-	/* find_relative_frame was successful */
-	select_frame (fid);
+	{
+	  if (parse->cmd->preserve_user_selected_context ())
+	    frame_saver.emplace ();
+
+	  select_frame (fid);
+	}
       else
 	error (_("Invalid frame id: %d"), frame);
     }