bug#48408: BUGFIX: window-default-font-height: check for nil string

[Eli Zaretskii]

> Date: Sun, 16 May 2021 08:25:28 +0300
> From: Eli Zaretskii <eliz@gnu.org>
> Cc: 48408@debbugs.gnu.org
> 
> > on tty frame: (#<frame F152 0x5599a3b27720>)
> > on gui frame: (#<frame F152 0x5599a3b27720>)
> 
> Thanks.  So you have just one frame in that session, which is a TTY
> frame?  I thought it was a session with both GUI and TTY frames, is
> that not so?
> 
> Anyway, the above probably means we set up frame parameters
> incorrectly somewhere.  Hmm...

Turns out it's a feature, see this comment in server.el:

       ;; Note: TTY frames still get a `display' parameter set to the value of
       ;; $DISPLAY.  This is useful when running from that tty frame
       ;; sub-processes that want to connect to the X server, but that means we
       ;; have to be careful here not to be tricked into thinking those frames
       ;; are on `display'.

Boruch, can you try the patch below?

Martin, any comments on this change?  AFAIU we had this subtlety
waiting to bite us since about forever.

diff --git a/lisp/frame.el b/lisp/frame.el
index aff1d47..f6d6ddd 100644
--- a/lisp/frame.el
+++ b/lisp/frame.el
@@ -2025,11 +2025,23 @@ frame-size-changed-p
 
 (declare-function msdos-mouse-p "dosfns.c")
 
+(defun frame-type (&optional display)
+  "Return the type of frames on DISPLAY.
+DISPLAY can be a display name, a frame, or nil (meaning the selected
+frame's display).
+This function is almost like `framep-on-display', but it is safer
+when a session has both text-mode and GUI client frames, because
+text-mode frames can still have a non-nil `display' frame parameter."
+  (cond
+   ((null display) (framep (selected-frame)))
+   ((framep display) (framep display))
+   (t (framep-on-display display))))
+
 (defun display-mouse-p (&optional display)
   "Return non-nil if DISPLAY has a mouse available.
 DISPLAY can be a display name, a frame, or nil (meaning the selected
 frame's display)."
-  (let ((frame-type (framep-on-display display)))
+  (let ((frame-type (frame-type display)))
     (cond
      ((eq frame-type 'pc)
       (msdos-mouse-p))
@@ -2062,7 +2074,8 @@ display-graphic-p
 that use a window system such as X, and false for text-only terminals.
 DISPLAY can be a display name, a frame, or nil (meaning the selected
 frame's display)."
-  (not (null (memq (framep-on-display display) '(x w32 ns)))))
+  (let ((frame-type (frame-type display)))
+    (not (null (memq frame-type '(x w32 ns))))))
 
 (defun display-images-p (&optional display)
   "Return non-nil if DISPLAY can display images.
@@ -2083,7 +2096,7 @@ display-selections-p
 via special system buffers called `selection' or `clipboard'.
 DISPLAY can be a display name, a frame, or nil (meaning the selected
 frame's display)."
-  (let ((frame-type (framep-on-display display)))
+  (let ((frame-type (frame-type display)))
     (cond
      ((eq frame-type 'pc)
       ;; MS-DOS frames support selections when Emacs runs inside
@@ -2099,7 +2112,7 @@ display-symbol-keys-p
   "Return non-nil if DISPLAY supports symbol names as keys.
 This means that, for example, DISPLAY can differentiate between
 the keybinding RET and [return]."
-  (let ((frame-type (framep-on-display display)))
+  (let ((frame-type (frame-type display)))
     (or (memq frame-type '(x w32 ns pc))
         ;; MS-DOS and MS-Windows terminals have built-in support for
         ;; function (symbol) keys
@@ -2111,7 +2124,7 @@ display-screens
   "Return the number of screens associated with DISPLAY.
 DISPLAY should be either a frame or a display name (a string).
 If DISPLAY is omitted or nil, it defaults to the selected frame's display."
-  (let ((frame-type (framep-on-display display)))
+  (let ((frame-type (frame-type display)))
     (cond
      ((memq frame-type '(x w32 ns))
       (x-display-screens display))
@@ -2131,7 +2144,7 @@ display-pixel-height
 refers to the pixel height for all physical monitors associated
 with DISPLAY.  To get information for each physical monitor, use
 `display-monitor-attributes-list'."
-  (let ((frame-type (framep-on-display display)))
+  (let ((frame-type (frame-type display)))
     (cond
      ((memq frame-type '(x w32 ns))
       (x-display-pixel-height display))
@@ -2151,7 +2164,7 @@ display-pixel-width
 refers to the pixel width for all physical monitors associated
 with DISPLAY.  To get information for each physical monitor, use
 `display-monitor-attributes-list'."
-  (let ((frame-type (framep-on-display display)))
+  (let ((frame-type (frame-type display)))
     (cond
      ((memq frame-type '(x w32 ns))
       (x-display-pixel-width display))
@@ -2191,7 +2204,7 @@ display-mm-height
 refers to the height in millimeters for all physical monitors
 associated with DISPLAY.  To get information for each physical
 monitor, use `display-monitor-attributes-list'."
-  (and (memq (framep-on-display display) '(x w32 ns))
+  (and (memq (frame-type display) '(x w32 ns))
        (or (cddr (assoc (or display (frame-parameter nil 'display))
                        display-mm-dimensions-alist))
           (cddr (assoc t display-mm-dimensions-alist))
@@ -2212,7 +2225,7 @@ display-mm-width
 refers to the width in millimeters for all physical monitors
 associated with DISPLAY.  To get information for each physical
 monitor, use `display-monitor-attributes-list'."
-  (and (memq (framep-on-display display) '(x w32 ns))
+  (and (memq (frame-type display) '(x w32 ns))
        (or (cadr (assoc (or display (frame-parameter nil 'display))
                        display-mm-dimensions-alist))
           (cadr (assoc t display-mm-dimensions-alist))
@@ -2228,7 +2241,7 @@ display-backing-store
 the question is inapplicable to a certain kind of display.
 DISPLAY can be a display name or a frame.
 If DISPLAY is omitted or nil, it defaults to the selected frame's display."
-  (let ((frame-type (framep-on-display display)))
+  (let ((frame-type (frame-type display)))
     (cond
      ((memq frame-type '(x w32 ns))
       (x-display-backing-store display))
@@ -2241,7 +2254,7 @@ display-save-under
   "Return non-nil if DISPLAY's screen supports the SaveUnder feature.
 DISPLAY can be a display name or a frame.
 If DISPLAY is omitted or nil, it defaults to the selected frame's display."
-  (let ((frame-type (framep-on-display display)))
+  (let ((frame-type (frame-type display)))
     (cond
      ((memq frame-type '(x w32 ns))
       (x-display-save-under display))
@@ -2254,7 +2267,7 @@ display-planes
   "Return the number of planes supported by DISPLAY.
 DISPLAY can be a display name or a frame.
 If DISPLAY is omitted or nil, it defaults to the selected frame's display."
-  (let ((frame-type (framep-on-display display)))
+  (let ((frame-type (frame-type display)))
     (cond
      ((memq frame-type '(x w32 ns))
       (x-display-planes display))
@@ -2269,7 +2282,7 @@ display-color-cells
   "Return the number of color cells supported by DISPLAY.
 DISPLAY can be a display name or a frame.
 If DISPLAY is omitted or nil, it defaults to the selected frame's display."
-  (let ((frame-type (framep-on-display display)))
+  (let ((frame-type (frame-type display)))
     (cond
      ((memq frame-type '(x w32 ns))
       (x-display-color-cells display))
@@ -2286,7 +2299,7 @@ display-visual-class
 `static-color', `pseudo-color', `true-color', or `direct-color'.
 DISPLAY can be a display name or a frame.
 If DISPLAY is omitted or nil, it defaults to the selected frame's display."
-  (let ((frame-type (framep-on-display display)))
+  (let ((frame-type (frame-type display)))
     (cond
      ((memq frame-type '(x w32 ns))
       (x-display-visual-class display))
@@ -2342,7 +2355,7 @@ display-monitor-attributes-list
 in a graphical display is dominated by exactly one physical
 monitor at a time, though it can span multiple (or no) physical
 monitors."
-  (let ((frame-type (framep-on-display display)))
+  (let ((frame-type (frame-type display)))
     (cond
      ((eq frame-type 'x)
       (x-display-monitor-attributes-list display))