Show a message in locked frames in single-kboard mode

* src/keyboard.c (kbd_buffer_get_event): Pass the event's frame
up to the caller by means of a new 'struct frame **' argument.
(read_event_from_main_queue): Show a message in locked frames in
single-kboard mode (bug#79892).
* src/xdisp.c (log_message): Factor out of message3.
(message3): Call it.
(message3_nolog): Rename to ...
(message3_frame_nolog): ... this.  New 'struct frame *' argument
which causes temporarily switching to another frame when
displaying the message.
(message3_frame, message3_nolog): New functions.
* src/lisp.h: Declare message3_frame and message3_frame_nolog.
* admin/notes/multi-tty: Remove notes on showing a message.

4 files changed, 112 insertions, 48 deletions

diff --git a/admin/notes/multi-tty b/admin/notes/multi-tty
index 7783eb395e7..fff20a1769b 100644
--- a/admin/notes/multi-tty
+++ b/admin/notes/multi-tty
@@ -402,29 +402,15 @@ THINGS TO DO
 
 ** The single-keyboard mode of MULTI_KBOARD is extremely confusing
    sometimes; Emacs does not respond to stimuli from other keyboards.
-   At least a beep or a message would be important, if the single-mode
-   is still required to prevent interference.  (Reported by Dan
-   Nicolaescu.)
+   (Reported by Dan Nicolaescu.)
 
    Update: selecting a region with the mouse enables single_kboard
    under X.  This is very confusing.
 
    Update: After discussions with Richard Stallman, this will be
-   resolved by having locked displays warn the user to wait, and
-   introducing a complex protocol to remotely bail out of
+   resolved introducing a complex protocol to remotely bail out of
    single-kboard mode by pressing C-g.
 
-   Update: Warning the user is not trivial to implement, as Emacs has
-   only one echo area, shared by all frames.  Ideally the warning
-   should not be displayed on the display that is locking the others.
-   Perhaps the high probability of user confusion caused by
-   single_kboard mode deserves a special case in the display code.
-   Alternatively, it might be good enough to signal single_kboard mode
-   by changing the modelines or some other frame-local display element
-   on the locked out displays.
-
-   Update: In fact struct kboard does have an echo_string slot.
-
 ** The session management module is prone to crashes when the X
    connection is closed and then later I try to connect to a new X
    session:
diff --git a/src/keyboard.c b/src/keyboard.c
index ffe47460528..648dd81660a 100644
--- a/src/keyboard.c
+++ b/src/keyboard.c
@@ -2297,6 +2297,7 @@ show_help_echo (Lisp_Object help, Lisp_Object window, Lisp_Object object,
 /* Input of single characters from keyboard.  */
 
 static Lisp_Object kbd_buffer_get_event (KBOARD **kbp, bool *used_mouse_menu,
+					 struct frame **event_frame,
 					 struct timespec *end_time);
 static void record_char (Lisp_Object c);
 
@@ -2342,7 +2343,8 @@ read_event_from_main_queue (struct timespec *end_time,
   restore_getcjmp (local_getcjmp);
   if (!end_time)
     timer_start_idle ();
-  c = kbd_buffer_get_event (&kb, used_mouse_menu, end_time);
+  struct frame *frame;
+  c = kbd_buffer_get_event (&kb, used_mouse_menu, &frame, end_time);
   unbind_to (count, Qnil);
 
   if (! NILP (c) && (kb != current_kboard))
@@ -2360,9 +2362,26 @@ read_event_from_main_queue (struct timespec *end_time,
       else
         XSETCDR (last, list1 (c));
       kb->kbd_queue_has_data = true;
-      c = Qnil;
       if (single_kboard)
-        goto start;
+	{
+	  /* Typing and clicking in a locked frame is confusing because
+	     it seems like Emacs has completely locked up (bug#79892).
+	     Show a message about what's happening.  */
+	  /* FIXME: We also display the message in the unlocked frame.
+	     Can we avoid that?  */
+	  if (frame
+	      && (FIXNUMP (c) || (EVENT_HAS_PARAMETERS (c)
+				  && EQ (EVENT_HEAD_KIND (EVENT_HEAD (c)),
+					 Qmouse_click))))
+	    {
+	      AUTO_STRING (locked, "Frame is locked while another"
+			   " waits for input"
+			   " or is otherwise in a recursive edit");
+	      message3_frame (locked, frame);
+	    }
+	  c = Qnil;
+	  goto start;
+	}
       current_kboard = kb;
       return make_fixnum (-2);
     }
@@ -4005,6 +4024,7 @@ kbd_buffer_get_event_2 (Lisp_Object val)
 static Lisp_Object
 kbd_buffer_get_event (KBOARD **kbp,
                       bool *used_mouse_menu,
+                      struct frame **event_frame,
                       struct timespec *end_time)
 {
   Lisp_Object obj, str;
@@ -4019,6 +4039,8 @@ kbd_buffer_get_event (KBOARD **kbp,
   had_pending_conversion_events = false;
 #endif
 
+  *event_frame = NULL;
+
 #ifdef subprocesses
   if (kbd_on_hold_p () && kbd_buffer_nr_stored () < KBD_BUFFER_SIZE / 4)
     {
@@ -4179,6 +4201,9 @@ kbd_buffer_get_event (KBOARD **kbp,
       if (*kbp == 0)
 	*kbp = current_kboard;  /* Better than returning null ptr?  */
 
+      if (FRAMEP (event->ie.frame_or_window))
+	*event_frame = XFRAME (event->ie.frame_or_window);
+
       obj = Qnil;
 
       /* These two kinds of events get special handling
@@ -4492,13 +4517,14 @@ kbd_buffer_get_event (KBOARD **kbp,
   /* Try generating a mouse motion event.  */
   else if (some_mouse_moved ())
     {
-      struct frame *f, *movement_frame = some_mouse_moved ();
+      struct frame *f;
       Lisp_Object bar_window;
       enum scroll_bar_part part;
       Lisp_Object x, y;
       Time t;
 
-      f = movement_frame;
+      *event_frame = some_mouse_moved ();
+      f = *event_frame;
       *kbp = current_kboard;
       /* Note that this uses F to determine which terminal to look at.
 	 If there is no valid info, it does not store anything
@@ -4535,8 +4561,8 @@ kbd_buffer_get_event (KBOARD **kbp,
 	obj = make_lispy_movement (f, bar_window, part, x, y, t);
 
       if (!NILP (obj))
-	Vlast_event_device = (STRINGP (movement_frame->last_mouse_device)
-			      ? movement_frame->last_mouse_device
+	Vlast_event_device = (STRINGP ((*event_frame)->last_mouse_device)
+			      ? (*event_frame)->last_mouse_device
 			      : virtual_core_pointer_name);
     }
 #ifdef HAVE_X_WINDOWS
diff --git a/src/lisp.h b/src/lisp.h
index c7afb07576b..cefca3d6fbb 100644
--- a/src/lisp.h
+++ b/src/lisp.h
@@ -4390,6 +4390,8 @@ extern void message1 (const char *);
 extern void message1_nolog (const char *);
 extern void message3 (Lisp_Object);
 extern void message3_nolog (Lisp_Object);
+extern void message3_frame (Lisp_Object, struct frame *);
+extern void message3_frame_nolog (Lisp_Object, struct frame *);
 extern void message_dolog (const char *, ptrdiff_t, bool, bool);
 extern void message_with_string (const char *, Lisp_Object, bool);
 extern void message_log_maybe_newline (void);
diff --git a/src/xdisp.c b/src/xdisp.c
index f5e2b6e2d24..c8e4bf0cf10 100644
--- a/src/xdisp.c
+++ b/src/xdisp.c
@@ -510,6 +510,10 @@ along with GNU Emacs.  If not, see <https://www.gnu.org/licenses/>.  */
 /* Holds the list (error).  */
 static Lisp_Object list_of_error;
 
+/* Forward declarations.  */
+static void restore_selected_window (Lisp_Object);
+static void restore_frame_selected_window (Lisp_Object);
+
 #ifdef HAVE_WINDOW_SYSTEM
 
 /* Test if overflow newline into fringe.  Called with iterator IT
@@ -12481,15 +12485,8 @@ message_log_check_duplicate (ptrdiff_t prev_bol_byte, ptrdiff_t this_bol_byte)
 }
 
 
-/* Display an echo area message M with a specified length of NBYTES
-   bytes.  The string may include null characters.  If M is not a
-   string, clear out any existing message, and let the mini-buffer
-   text show through.
-
-   This function cancels echoing.  */
-
-void
-message3 (Lisp_Object m)
+static void
+log_message (Lisp_Object m)
 {
   clear_message (true, true);
   cancel_echoing ();
@@ -12506,10 +12503,34 @@ message3 (Lisp_Object m)
       message_dolog (buffer, nbytes, true, multibyte);
       SAFE_FREE ();
     }
+}
+
+/* Display an echo area message M with a specified length of NBYTES
+   bytes.  The string may include null characters.  If M is not a
+   string, clear out any existing message, and let the mini-buffer
+   text show through.
+
+   This function cancels echoing.  */
+
+void
+message3 (Lisp_Object m)
+{
+  log_message (m);
   if (! inhibit_message)
     message3_nolog (m);
 }
 
+/* Display an echo area message M on frame F, which may not be the
+   selected frame.  */
+
+void
+message3_frame (Lisp_Object m, struct frame *f)
+{
+  log_message (m);
+  if (! inhibit_message)
+    message3_frame_nolog (m, f);
+}
+
 /* Log the message M to stderr.  Log an empty line if M is not a string.  */
 
 static void
@@ -12538,7 +12559,7 @@ message_to_stderr (Lisp_Object m)
     errputc ('\n');
 }
 
-/* The non-logging version of message3.
+/* The non-logging versions of message3 & message3_frame.
    This does not cancel echoing, because it is used for echoing.
    Perhaps we need to make a separate function for echoing
    and make this cancel echoing.  */
@@ -12546,28 +12567,55 @@ message_to_stderr (Lisp_Object m)
 void
 message3_nolog (Lisp_Object m)
 {
+  message3_frame_nolog (m, NULL);
+}
+
+void
+message3_frame_nolog (Lisp_Object m, struct frame *f)
+{
   struct frame *sf = SELECTED_FRAME ();
+  if (!f) f = sf;
 
-  if (FRAME_INITIAL_P (sf))
+  if (FRAME_INITIAL_P (f))
     message_to_stderr (m);
-  /* Error messages get reported properly by cmd_error, so this must be just an
-     informative message; if the frame hasn't really been initialized yet, just
-     toss it.  */
-  else if (INTERACTIVE && sf->glyphs_initialized_p)
+  /* Error messages get reported properly by cmd_error, so this must be
+     just an informative message; therefore if the frame hasn't really
+     been initialized yet, just toss it.  */
+  else if (INTERACTIVE && f->glyphs_initialized_p)
     {
-      /* Get the frame containing the mini-buffer
-	 that the selected frame is using.  */
-      Lisp_Object mini_window = FRAME_MINIBUF_WINDOW (sf);
-      Lisp_Object frame = XWINDOW (mini_window)->frame;
-      struct frame *f = XFRAME (frame);
+      Lisp_Object frame = Qnil;
+      struct frame *mbf = NULL;
+      specpdl_ref count = SPECPDL_INDEX ();
+
+      if (f == sf)
+	{
+	  /* Get the frame containing the mini-buffer that the selected
+	     frame is using.  */
+	  Lisp_Object mini_window = FRAME_MINIBUF_WINDOW (f);
+	  frame = XWINDOW (mini_window)->frame;
+	  mbf = XFRAME (frame);
 
-      if (FRAME_VISIBLE_P (sf) && !FRAME_VISIBLE_P (f))
-	Fmake_frame_visible (frame);
+	  if (FRAME_VISIBLE_P (f) && !FRAME_VISIBLE_P (mbf))
+	    Fmake_frame_visible (frame);
+	}
+      else
+	{
+	  /* We temporarily switch frame, show the message, and then
+	     when we unwind the message will normally still be visible
+	     in the other frame, at least for a few seconds.  */
+	  record_unwind_protect
+	    (restore_selected_window, selected_window);
+	  record_unwind_protect
+	    (restore_frame_selected_window, f->selected_window);
+	  XSETFRAME (frame, f);
+	  selected_frame = frame;
+	  selected_window = FRAME_SELECTED_WINDOW (f);
+	}
 
       if (STRINGP (m) && SCHARS (m) > 0)
 	{
 	  set_message (m);
-	  if (minibuffer_auto_raise)
+	  if (minibuffer_auto_raise && !NILP (frame))
 	    Fraise_frame (frame);
 	  /* Assume we are not echoing.
 	     (If we are, echo_now will override this.)  */
@@ -12579,8 +12627,10 @@ message3_nolog (Lisp_Object m)
       do_pending_window_change (false);
       echo_area_display (true);
       do_pending_window_change (false);
-      if (FRAME_TERMINAL (f)->frame_up_to_date_hook)
-	(*FRAME_TERMINAL (f)->frame_up_to_date_hook) (f);
+      if (mbf && FRAME_TERMINAL (mbf)->frame_up_to_date_hook)
+	(*FRAME_TERMINAL (mbf)->frame_up_to_date_hook) (mbf);
+
+      unbind_to (count, Qnil);
     }
 }