bug#60956: 27.2; project-try-vc considers every nonempty directory a root when vc-handled-backends is nil

[Dmitry Gutov <dgutov@yandex.ru>]

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Hi!

On 20/01/2023 00:45, Jason Orendorff wrote:
> I observed that `eglot' would happily start dozens of language server
> processes, but each one of them only seemed to know about a tiny
> fraction of my code. I tracked this down and found a minor bug in
> project.el.
> 
> In `project-try-vc', when `vc-handled-backends' is nil, the value of
> `marker-re' computed here:
> 
> https://git.savannah.gnu.org/cgit/emacs.git/tree/lisp/progmodes/project.el?id=207901457c018d94b1ce9e13a897d8241b1f3af2#n516 <https://git.savannah.gnu.org/cgit/emacs.git/tree/lisp/progmodes/project.el?id=207901457c018d94b1ce9e13a897d8241b1f3af2#n516>
> 
> is the empty string.
> 
> If no vc backends are enabled, no directory should be treated as a vc
> root. But the empty regexp matches everything, so *all* directories are
> treated as vc roots.
> 
> Consequently, `eglot' spawned a lot of processes, because it starts one
> per project.
> 
> The workaround is to set `vc-handled-backends' to any other value. (I
> suppose I configured it to nil long ago, when working in a huge
> Mercurial repository. Back then, commonplace hg commands in that repo
> were unreasonably slow.)

Thank you for this report.

Please try the attached patch.

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diff --git a/lisp/progmodes/project.el b/lisp/progmodes/project.el
index dc87cb8e15d..4a2ca45e633 100644
--- a/lisp/progmodes/project.el
+++ b/lisp/progmodes/project.el
@@ -514,11 +514,14 @@ project-try-vc
                 (lambda (b) (assoc-default b backend-markers-alist))
                 vc-handled-backends)))
              (marker-re
-              (mapconcat
-               (lambda (m) (format "\\(%s\\)" (wildcard-to-regexp m)))
-               (append backend-markers
-                       (project--value-in-dir 'project-vc-extra-root-markers dir))
-               "\\|"))
+              (concat
+               "\\`"
+               (mapconcat
+                (lambda (m) (format "\\(%s\\)" (wildcard-to-regexp m)))
+                (append backend-markers
+                        (project--value-in-dir 'project-vc-extra-root-markers dir))
+                "\\|")
+               "\\'"))
              (locate-dominating-stop-dir-regexp
               (or vc-ignore-dir-regexp locate-dominating-stop-dir-regexp))
              last-matches