Fix OSC 51;E eval crashing process filter

`ghostel--osc51-eval` is dispatched synchronously from the native VT
parser inside `fnWriteInput`, so anything the callback does — or any
error it signals — unwinds back through `ghostel--write-input` into
whatever caller drove the flush: the process filter on the
immediate-redraw path, or `ghostel--delayed-redraw` on the bulk-timer
path.  Three distinct regressions from #82 all stem from that:

1. `find-file-other-window` / `dired-other-window` call `select-window`
   on the newly-popped window, which flips `current-buffer` as a side
   effect.  When control returns to `ghostel--delayed-redraw`, its
   `with-current-buffer` has already entered its body, and the
   remaining statements read `ghostel--term` from the *new* file
   buffer, where the buffer-local is nil.  The native module then
   receives nil and Emacs signals `Wrong type argument: user-ptrp,
   nil` — attributed to `ghostel--delayed-redraw` on the timer path
   and to the process filter on the immediate path.

   Fix: wrap the `ghostel--write-input` call in
   `ghostel--flush-pending-output` with `save-current-buffer`.  Done
   at the flush site so every caller — delayed-redraw, the filter's
   color-query fast path, `ghostel-clear`, `ghostel-clear-scrollback`,
   and the sentinel — is protected in one place.  The user-visible
   window selection still sticks because `save-current-buffer`
   restores only the elisp current buffer, not `selected-window`.

2. `dow` with no arguments called `dired-other-window` with zero
   arguments, which signals `Wrong number of arguments`.  That error
   propagates up the same path and again trips the filter.

   Fix: wrap the `apply` in `ghostel--osc51-eval` with
   `condition-case`, so any error a whitelisted function signals is
   reported via `message` instead of crashing the pipeline.  Also
   update the README's `dow()` example to default to `$PWD` so the
   bare invocation does the sensible thing.

3. After `find-file-other-window` deselected the ghostel window, the
   subsequent native redraw updated `buffer-point` but not
   `window-point` for any window showing the ghostel buffer (Emacs
   only keeps them in sync for the *selected* window).  When the
   user reselected the ghostel window the terminal cursor appeared
   at a stale position.

   Fix: after the native redraw in `ghostel--delayed-redraw`, push
   the new `(point)` into every window displaying the buffer via
   `set-window-point`.

Tests: `ghostel-test-flush-pending-output-preserves-buffer` stubs
`ghostel--write-input` to simulate the `select-window` side effect and
asserts the flush restores the ghostel buffer;
`ghostel-test-osc51-eval-catches-errors` dispatches a whitelisted
lambda that raises and asserts the error is messaged rather than
propagated.  Both run under `ghostel-test-run-elisp` — no native
module required.

Fixes #82

Author: dakra

README.md

@@ -358,8 +358,8 @@ if [[ "$INSIDE_EMACS" = 'ghostel' ]]; then
     # Open a file in Emacs from the terminal
     e()   { ghostel_cmd find-file-other-window "$@"; }
 
-    # Open dired in another window
-    dow() { ghostel_cmd dired-other-window "$@"; }
+    # Open dired in another window, defaulting to the current directory
+    dow() { ghostel_cmd dired-other-window "${1:-$PWD}"; }
 
     # Open magit for the current directory
     gst() { ghostel_cmd magit-status-setup-buffer "$(pwd)"; }

ghostel.el

@@ -1705,7 +1705,15 @@ Parses the command and arguments, looks up the command in
          (args (cdr parts))
          (entry (assoc command ghostel-eval-cmds)))
     (if entry
-        (apply (cadr entry) args)
+        ;; Catch errors from the dispatched function: this callback runs
+        ;; synchronously inside the native VT parser, so any unhandled
+        ;; error propagates back up through `ghostel--write-input' and
+        ;; crashes the process filter / redraw timer.
+        (condition-case err
+            (apply (cadr entry) args)
+          (error
+           (message "ghostel: error calling %s: %s"
+                    command (error-message-string err))))
       (message "ghostel: unknown eval command %S (add to `ghostel-eval-cmds' to allow)"
                command))))
 
@@ -2269,7 +2277,13 @@ frame after idle to improve interactive responsiveness."
   (when ghostel--pending-output
     (let ((combined (apply #'concat (nreverse ghostel--pending-output))))
       (setq ghostel--pending-output nil)
-      (ghostel--write-input ghostel--term combined))))
+      ;; An OSC 51;E callback dispatched synchronously from the native
+      ;; parser (e.g. `find-file-other-window') can change the current
+      ;; buffer via `select-window'.  Isolate that so callers keep
+      ;; reading buffer-locals — notably `ghostel--term' — from the
+      ;; ghostel buffer after this returns.
+      (save-current-buffer
+        (ghostel--write-input ghostel--term combined)))))
 
 (defun ghostel--delayed-redraw (buffer)
   "Perform the actual redraw in BUFFER."
@@ -2289,7 +2303,16 @@ frame after idle to improve interactive responsiveness."
                 (inhibit-modification-hooks t))
             (ghostel--redraw ghostel--term ghostel-full-redraw))
           (when ghostel--has-wide-chars
-            (ghostel--compensate-wide-chars)))))))
+            (ghostel--compensate-wide-chars))
+          ;; Native redraw updates buffer-point via `goto-char', which
+          ;; only propagates to `window-point' for the selected window.
+          ;; If an OSC 51;E callback moved selection elsewhere (e.g.
+          ;; `find-file-other-window'), the ghostel window's
+          ;; window-point is stale and the terminal cursor will display
+          ;; at the wrong place when the user reselects it.  Sync it.
+          (let ((pt (point)))
+            (dolist (win (get-buffer-window-list buffer nil t))
+              (set-window-point win pt))))))))
 
 (defun ghostel-force-redraw ()
   "Force a full terminal redraw (for debugging)."

test/ghostel-test.el

@@ -1454,6 +1454,48 @@ rendered by `ghostel--delayed-redraw'.  This is the exact real-world path."
       (should (car messages))
       (should (string-match-p "unknown eval command" (car messages))))))
 
+(ert-deftest ghostel-test-osc51-eval-catches-errors ()
+  "Errors signaled by a dispatched OSC 51;E function must not
+propagate out of `ghostel--osc51-eval' — otherwise they crash the
+process filter / redraw timer that invoked the native parser.
+Regression for a follow-up to #82 where `dow' with no args called
+`dired-other-window' with 0 arguments and signaled up through the
+filter."
+  (let* ((ghostel-eval-cmds
+          `(("boom" ,(lambda (&rest _) (error "kaboom")))))
+         (messages nil))
+    (cl-letf (((symbol-function 'message)
+               (lambda (fmt &rest args) (push (apply #'format fmt args) messages))))
+      ;; Must not raise.
+      (ghostel--osc51-eval "\"boom\"")
+      (should (car messages))
+      (should (string-match-p "error calling boom" (car messages)))
+      (should (string-match-p "kaboom" (car messages))))))
+
+(ert-deftest ghostel-test-flush-pending-output-preserves-buffer ()
+  "Regression for #82: a buffer switch performed by a synchronous
+native callback (as OSC 51;E dispatch does when it calls
+`find-file-other-window') must not leak out of
+`ghostel--flush-pending-output'.  Otherwise callers such as
+`ghostel--delayed-redraw' read `ghostel--term' from the wrong
+buffer and hand nil to the native module."
+  (let ((ghostel-buf (generate-new-buffer " *ghostel-test-flush-buf*"))
+        (other-buf (generate-new-buffer " *ghostel-test-flush-other*")))
+    (unwind-protect
+        (with-current-buffer ghostel-buf
+          (setq-local ghostel--term 'fake-handle)
+          (setq-local ghostel--pending-output (list "payload"))
+          (cl-letf (((symbol-function 'ghostel--write-input)
+                     (lambda (_term _data)
+                       ;; Simulate `find-file-other-window' flipping
+                       ;; the current buffer via `select-window'.
+                       (set-buffer other-buf))))
+            (ghostel--flush-pending-output))
+          (should (eq (current-buffer) ghostel-buf))
+          (should (null ghostel--pending-output)))
+      (kill-buffer ghostel-buf)
+      (kill-buffer other-buf))))
+
 ;; -----------------------------------------------------------------------
 ;; Test: copy-mode cursor visibility
 ;; -----------------------------------------------------------------------
@@ -2496,6 +2538,8 @@ while :; do sleep 0.1; done'\n")
     ghostel-test-apply-palette-default-colors
     ghostel-test-osc51-eval
     ghostel-test-osc51-eval-unknown
+    ghostel-test-osc51-eval-catches-errors
+    ghostel-test-flush-pending-output-preserves-buffer
     ghostel-test-copy-mode-cursor
     ghostel-test-copy-mode-hl-line
     ghostel-test-project-buffer-name