Eglot: beware activation in fundamental-mode

In the specific situation of visiting a buffer via M-. with
eglot-extend-to-xref set to t, it was found that buffer was first
visited in fundamental mode, running after-change-major-mode-hook, and
then again in the proper major mode for the file.  The call to
eglot-current-server of the first visit returned non-nil which cause
two didOpen notifications to be issued for the same file.

Furthermore, in the first call, eglot--languageId to returned nil,
prompting an error from servers such as rust-analyzer.

See also: https://github.com/joaotavora/eglot/discussions/1330

* lisp/progmodes/eglot.el (eglot-current-server): Watch out for
fundamental-mode.

1 files changed, 9 insertions, 7 deletions

diff --git a/lisp/progmodes/eglot.el b/lisp/progmodes/eglot.el
index 608389f1c05..84c5e6639df 100644
--- a/lisp/progmodes/eglot.el
+++ b/lisp/progmodes/eglot.el
@@ -2034,13 +2034,15 @@ Use `eglot-managed-p' to determine if current buffer is managed.")
   "Return logical Eglot server for current buffer, nil if none."
   (setq eglot--cached-server
         (or eglot--cached-server
-            (cl-find-if #'eglot--languageId
-                        (gethash (eglot--current-project)
-                                 eglot--servers-by-project))
-            (and eglot-extend-to-xref
-                 buffer-file-name
-                 (gethash (expand-file-name buffer-file-name)
-                          eglot--servers-by-xrefed-file)))))
+            (and (not (eq major-mode 'fundamental-mode)) ; gh#1330
+                 (or
+                  (cl-find-if #'eglot--languageId
+                              (gethash (eglot--current-project)
+                                       eglot--servers-by-project))
+                  (and eglot-extend-to-xref
+                       buffer-file-name
+                       (gethash (expand-file-name buffer-file-name)
+                                eglot--servers-by-xrefed-file)))))))
 
 (defun eglot--current-server-or-lose ()
   "Return current logical Eglot server connection or error."