master 2697123: Make vc-responsible-backend choose the most specific backend

[Lars Ingebrigtsen]

branch: master
commit 2697123933e3ac7ed4e21a6d12746a98ed7fa74a
Author: Lars Ingebrigtsen <larsi@gnus.org>
Commit: Lars Ingebrigtsen <larsi@gnus.org>

    Make vc-responsible-backend choose the most specific backend
    
    * lisp/vc/vc.el (vc-responsible-backend): Search through all the
    VC backends instead of the first one, and choose the one that's
    most specific (bug#42966).
---
 etc/NEWS      |  8 ++++++++
 lisp/vc/vc.el | 22 ++++++++++++++++------
 2 files changed, 24 insertions(+), 6 deletions(-)

diff --git a/etc/NEWS b/etc/NEWS
index 7dd993c..2674094 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -403,6 +403,14 @@ their 'default-directory' under VC.
 
 *** New command 'vc-dir-root' uses the root directory without asking.
 
+---
+*** The responsible VC backend is now the most specific one.
+'vc-responsible-backend' loops over the backends in
+'vc-handled-backends' to determine which backend is responsible for a
+specific (unregistered) file.  Previously the first matching backend
+was chosen, but now the one with the most specific path is chosen (in
+case there's a directory handled by one backend inside another).
+
 *** New commands 'vc-dir-mark-registered-files' (bound to '* r') and
 'vc-dir-mark-unregistered-files'.
 
diff --git a/lisp/vc/vc.el b/lisp/vc/vc.el
index 39d0fab..8def7da 100644
--- a/lisp/vc/vc.el
+++ b/lisp/vc/vc.el
@@ -979,12 +979,22 @@ be reported.
 If NO-ERROR is nil, signal an error that no VC backend is
 responsible for the given file."
   (or (and (not (file-directory-p file)) (vc-backend file))
-      (catch 'found
-       ;; First try: find a responsible backend.  If this is for registration,
-       ;; it must be a backend under which FILE is not yet registered.
-       (dolist (backend vc-handled-backends)
-         (and (vc-call-backend backend 'responsible-p file)
-              (throw 'found backend))))
+      ;; First try: find a responsible backend.  If this is for registration,
+      ;; it must be a backend under which FILE is not yet registered.
+      (let ((dirs (delq nil
+                        (mapcar
+                         (lambda (backend)
+                           (when-let ((dir (vc-call-backend
+                                            backend 'responsible-p file)))
+                             (cons backend dir)))
+                         vc-handled-backends))))
+        ;; Just a single response (or none); use it.
+        (if (< (length dirs) 2)
+            (caar dirs)
+          ;; Several roots; we seem to have one vc inside another's
+          ;; directory.  Choose the most specific.
+          (caar (sort dirs (lambda (d1 d2)
+                             (< (length (cdr d2)) (length (cdr d1))))))))
       (unless no-error
         (error "No VC backend is responsible for %s" file))))