bug#36564: 27.0.50; Wrong number of errors in compilation mode-line

[Juri Linkov]

> Often compilation-mode displays wrong number of errors
> in the mode-line even when compilation is finished.
>
> compilation-mode is based on font-lock, so when the
> *compilation* buffer is not displayed during compilation,
> some parts of this buffer that contain error messages
> are not fontified, and thus these errors are not counted.
>
> This patch ensures the correct number of errors
> is displayed on the mode-line:

Actually maybe better to count errors not at the end
of compilation, but during compilation as output goes:

diff --git a/lisp/progmodes/compile.el b/lisp/progmodes/compile.el
index 1a0d9bdbb7..7b319e9947 100644
--- a/lisp/progmodes/compile.el
+++ b/lisp/progmodes/compile.el
@@ -2245,6 +2250,8 @@ compilation-filter
               (unless comint-inhibit-carriage-motion
                 (comint-carriage-motion (process-mark proc) (point)))
               (set-marker (process-mark proc) (point))
+              ;; Update the number of errors in compilation-mode-line-errors
+              (font-lock-ensure compilation-filter-start (point))
               ;; (set (make-local-variable 'compilation-buffer-modtime)
               ;;      (current-time))
               (run-hooks 'compilation-filter-hook))