Re: [PATCH] gdb: notify of inferior switch when needed from 'thread' command

[Andrew Burgess <aburgess@redhat.com>]

Ping!

Hi Pedro, I wonder if you have any thoughts on my feedback here.  I
interpreted your initial reply as a NAK, which I'd usually just accept.
But in this case I do feel there was some misunderstanding about this
patch, which might change how you feel.

I rebased the patch to current HEAD, and added some additional text to
the commit message which might address your concerns.

Would be great to hear your thoughts.

Thanks,
Andrew


---

commit cf68b707614acbebd3b7525fd11784013adc8a92
Author: Andrew Burgess <aburgess@redhat.com>
Date:   Tue Sep 30 13:14:11 2025 +0100

    gdb: notify of inferior switch when needed from 'thread' command
    
    While working on the commit:
    
      commit 9959019545d8d6d71d927f20f088efba944b1e9c
      Date:   Sun Sep 28 16:16:53 2025 +0100
    
          gdb: fix for 'set suppress-cli-notifications on' missed case
    
    I spotted this message in the gdb.mi/user-selected-context-sync.exp
    test script:
    
      # Idea for the future: selecting a thread in a different inferior.  For now,
      # GDB doesn't show an inferior switch, but if it did, it would be a nice
      # place to test it.
    
    What this message is talking about is this behaviour:
    
      (gdb) info threads
        Id   Target Id                              Frame
        1.1  Thread 0xf7dbc700 (LWP 818430) "thr" 0xf7eb2888 in clone () from /lib/libc.so.6
        1.2  Thread 0xf7dbbb40 (LWP 818433) "thr" 0xf7fd0579 in __kernel_vsyscall ()
        1.3  Thread 0xf73ffb40 (LWP 818434) "thr" breakpt () at thr.c:19
        2.1  Thread 0xf7dbc700 (LWP 818456) "thr" 0xf7eb2888 in clone () from /lib/libc.so.6
        2.2  Thread 0xf7dbbb40 (LWP 818457) "thr" breakpt () at thr.c:19
      * 2.3  Thread 0xf73ffb40 (LWP 818458) "thr" breakpt () at thr.c:19
      (gdb) inferior 1
      [Switching to inferior 1 [process 818430] (/home/andrew/tmp/thr)]
      [Switching to thread 1.1 (Thread 0xf7dbc700 (LWP 818430))]
      #0  0xf7eb2888 in clone () from /lib/libc.so.6
      (gdb) thread 2.2
      [Switching to thread 2.2 (Thread 0xf7dbbb40 (LWP 818457))]
      #0  breakpt () at thr.c:19
      19      while (stop)
      (gdb)
    
    Notice that when we switch from thread 2.3 to 1.1 using the 'inferior
    1' command, GDB tells us that the inferior has changed, and that the
    thread has changed (and also that the frame has changed).
    
    But, when we switch from 1.1 to 2.2 using the 'thread 2.2' command, we
    are only told about the thread change.
    
    The 'Switching to inferior ...' line includes some useful information,
    the process PID and the executable name, and I think it is a shame
    that these are not presented when using the 'thread' command to switch
    inferior.
    
    So, this commit addresses this issue.
    
    A question that came up during review, and which I'm clarifying here:
    this change only effects the output of GDB when the thread command is
    also used to switch inferiors.  I am (in effect) arguing that the
    command 'thread 2.2' should be treated as a shorthand for 'inferior 2;
    thread 2', and should display all of the associated output.  If the
    user is only switching threads within a single inferior then it is not
    necessary to re-display the inferior information.
    
    I acknowledge that this does mean the output of the 'thread' command
    will now be different depending on whether the user changes inferior
    or not.  However, I think this is better than the alternative, having
    the 'thread' command always re-print the inferior information.  I
    think this would introduce excess noise that is not useful.
    
    There are changes in basically two areas.  The easy part is in
    thread_command (thread.c).  Here we spot when the inferior has changed
    as a result of the 'thread' command, and included
    USER_SELECTED_INFERIOR in the set of state passed to the
    notify_user_selected_context_changed function.
    
    The change in mi/mi-main.c is a little more involved.  In the
    mi_cmd_execute function we use an instance of user_selected_context to
    spot if any inferior state (frame, thread, or inferior) changes after
    an MI command, this is then used to decide if there should be a call
    to notify_user_selected_context_changed.
    
    Currently, in mi_cmd_execute, notify_user_selected_context_changed is
    always passed 'USER_SELECTED_THREAD | USER_SELECTED_FRAME'.  This
    makes sense, the MI doesn't allow "switching inferiors" as a command,
    instead, an MI frontend must switch threads, and the inferior is
    switched as a consequence.  But this does mean that if a user has a
    CLI and MI interpreter running, and the MI switches threads, the CLI
    will only receive the thread switch style notifications, that is,
    there will be no "Switching to inferior ..." line.
    
    What I've done is rename user_selected_context::has_changed to
    user_selected_context::what_changed, this function is now responsible
    for returning the set of USER_SELECTED_* flags that indicate what
    changed.
    
    If anything has changed then we always return USER_SELECTED_THREAD |
    USER_SELECTED_FRAME as a minimum.  This retains the existing
    behaviour, but is possibly more aggressive that we need to be; the
    -stack-select-frame command can only change the frame, so maybe in
    this case we should only return USER_SELECTED_FRAME?  I've left that
    for the future though.
    
    However, the important change is that in ::what_changed, I now spot
    when the inferior has changed and include USER_SELECTED_INFERIOR in
    the set of flags that are returned.
    
    In mi_cmd_execute we now call the new what_changed function, and use
    the set of flags returned when calling
    notify_user_selected_context_changed.  This means that the CLI will
    now receive inferior changed notifications when appropriate.
    
    The gdb.mi/user-selected-context-sync.exp script has been updated,
    replacing the comment I quoted above with an actual test that the
    inferior change is announced correctly.

diff --git a/gdb/mi/mi-main.c b/gdb/mi/mi-main.c
index 77e56bc9fc8..a18209366ff 100644
--- a/gdb/mi/mi-main.c
+++ b/gdb/mi/mi-main.c
@@ -538,6 +538,11 @@ mi_cmd_thread_select (const char *command, const char *const *argv, int argc)
 
   thread_select (argv[0], thr);
 
+  /* We don't call print_selected_inferior here as this never prints
+     anything when the output is MI like (as it is now).  MI consumers are
+     expected to derive the inferior change from the global thread-id
+     included in the print_selected_thread_frame output.  */
+
   print_selected_thread_frame (current_uiout,
 			       USER_SELECTED_THREAD | USER_SELECTED_FRAME);
 }
@@ -1968,12 +1973,30 @@ struct user_selected_context
 
   /* Return true if the user selected context has changed since this object
      was created.  */
-  bool has_changed () const
+  user_selected_what what_changed () const
   {
+    /* If anything changed then we report both the thread and frame at a
+       minimum.  We optionally add the inferior if we know that it
+       changed.
+
+       This means that for pure frame changes, e.g. -stack-select-frame, we
+       still report both a thread and a frame, which isn't ideal, but
+       there's also the cases where -thread-select is used to re-select the
+       current thread, in this case we'd still like to see the thread
+       reported, at least, that's what we have historically done.  */
+    user_selected_what state
+      = USER_SELECTED_THREAD | USER_SELECTED_FRAME;
+
     /* Did the selected thread change?  */
     if (m_previous_ptid != null_ptid && inferior_ptid != null_ptid
 	&& m_previous_ptid != inferior_ptid)
-      return true;
+      {
+	/* Did the inferior change too?  */
+	if (m_previous_ptid.pid () != inferior_ptid.pid ())
+	  state |= USER_SELECTED_INFERIOR;
+
+	return state;
+      }
 
     /* Grab details of the currently selected frame, for comparison.  */
     frame_id current_frame_id;
@@ -1982,7 +2005,7 @@ struct user_selected_context
 
     /* Did the selected frame level change?  */
     if (current_frame_level != m_previous_frame_level)
-      return true;
+      return state;
 
     /* Did the selected frame id change?  If the innermost frame is
        selected then the level will be -1, and the frame-id will be
@@ -1991,10 +2014,10 @@ struct user_selected_context
        other than the innermost frame selected.  */
     if (current_frame_level != -1
 	&& current_frame_id != m_previous_frame_id)
-      return true;
+      return state;
 
     /* Nothing changed!  */
-    return false;
+    return 0;
   }
 private:
   /* The previously selected thread.  This might be null_ptid if there was
@@ -2098,10 +2121,13 @@ mi_cmd_execute (struct mi_parse *parse)
 
   parse->cmd->invoke (parse);
 
-  if (!parse->cmd->preserve_user_selected_context ()
-      && current_user_selected_context.has_changed ())
-    interps_notify_user_selected_context_changed
-      (USER_SELECTED_THREAD | USER_SELECTED_FRAME);
+  if (!parse->cmd->preserve_user_selected_context ())
+    {
+      user_selected_what what
+	= current_user_selected_context.what_changed ();
+      if (what != 0)
+	notify_user_selected_context_changed (what);
+    }
 }
 
 /* See mi-main.h.  */
diff --git a/gdb/testsuite/gdb.mi/user-selected-context-sync.exp b/gdb/testsuite/gdb.mi/user-selected-context-sync.exp
index 3434ffa2a46..4f02b950f6e 100644
--- a/gdb/testsuite/gdb.mi/user-selected-context-sync.exp
+++ b/gdb/testsuite/gdb.mi/user-selected-context-sync.exp
@@ -698,9 +698,21 @@ proc_with_prefix test_cli_thread { mode } {
 	}
     }
 
-    # Idea for the future: selecting a thread in a different inferior.  For now,
-    # GDB doesn't show an inferior switch, but if it did, it would be a nice
-    # place to test it.
+    with_test_prefix "thread 2.2" {
+	# Select a thread in a different inferior.  This should trigger an
+	# 'inferior changed' and 'thread changed' notification on the CLI,
+	# and a single MI async notification.
+	set mi_re [make_mi_re $mode 5 0 event]
+	set cli_re [make_cli_re $mode 2 2.2 0]
+
+	with_spawn_id $gdb_main_spawn_id {
+	    gdb_test "thread 2.2" $cli_re "select thread"
+	}
+
+	with_spawn_id $mi_spawn_id {
+	    match_re_or_ensure_no_output $mi_re "select thread, event on MI"
+	}
+    }
 }
 
 # Test frame selection from CLI.
@@ -995,9 +1007,22 @@ proc_with_prefix test_mi_thread_select { mode } {
 	}
     }
 
-    # Idea for the future: selecting a thread in a different inferior.  For now,
-    # GDB doesn't show an inferior switch, but if it did, it would be a nice
-    # place to test it.
+    with_test_prefix "thread 2.2" {
+	# Select a thread in a different inferior.  This should trigger an
+	# 'inferior changed' and 'thread changed' notification on the CLI,
+	# and a single MI async notification.
+	set mi_re [make_mi_re $mode 5 0 response]
+	set cli_re [make_cli_re $mode 2 2.2 0]
+
+	with_spawn_id $mi_spawn_id {
+	    mi_gdb_test "-thread-select 5" $mi_re "-thread-select"
+	}
+
+	with_spawn_id $gdb_main_spawn_id {
+	    match_re_or_ensure_no_output "$cli_re\r\n" "-thread-select, event on CLI"
+	}
+
+    }
 }
 
 proc_with_prefix test_mi_stack_select_frame { mode } {
@@ -1290,9 +1315,22 @@ proc_with_prefix test_cli_in_mi_thread { mode cli_in_mi_mode } {
 	}
     }
 
-    # Idea for the future: selecting a thread in a different inferior.  For now,
-    # GDB doesn't show an inferior switch, but if it did, it would be a nice
-    # place to test it.
+    with_test_prefix "thread 2.2" {
+	# Select a thread in a different inferior.  This should trigger an
+	# 'inferior changed' and 'thread changed' notification on the CLI,
+	# and a single MI async notification.
+	set command [make_cli_in_mi_command $cli_in_mi_mode "thread 2.2"]
+	set mi_re [make_cli_in_mi_re $command $cli_in_mi_mode $mode 1 2 2.2 5 0]
+	set cli_re [make_cli_re $mode 2 2.2 0]
+
+	with_spawn_id $mi_spawn_id {
+	    mi_gdb_test $command $mi_re "select thread"
+	}
+
+	with_spawn_id $gdb_main_spawn_id {
+	    match_re_or_ensure_no_output "$cli_re\r\n" "select thread, event on CLI"
+	}
+    }
 }
 
 # Test selecting the frame using a CLI command in the MI channel.
diff --git a/gdb/thread.c b/gdb/thread.c
index 2c5292d24b7..0f44eacd1f4 100644
--- a/gdb/thread.c
+++ b/gdb/thread.c
@@ -1977,10 +1977,19 @@ thread_command (const char *tidstr, int from_tty)
     }
   else
     {
+      inferior *previous_inferior = current_inferior ();
+
       thread_select (tidstr, parse_thread_id (tidstr, NULL));
 
-      notify_user_selected_context_changed
-	(USER_SELECTED_THREAD | USER_SELECTED_FRAME);
+      user_selected_what selection = (USER_SELECTED_THREAD
+				      | USER_SELECTED_FRAME);
+
+      /* If the inferior changed as a consequence of the thread change,
+	 then let the user know.  */
+      if (previous_inferior != current_inferior ())
+	selection |= USER_SELECTED_INFERIOR;
+
+      notify_user_selected_context_changed (selection);
     }
 }