bug#54359: 29.0.50; [PATCH] Undo breaking change to project interface

[dick <dick.r.chiang@gmail.com>]

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From 793d1311b28701931a9343e5196adce82da7833d Mon Sep 17 00:00:00 2001
From: dickmao <dick.r.chiang@gmail.com>
Date: Sat, 12 Mar 2022 16:39:28 -0500
Subject: [PATCH] Avoid breaking the world's `project-current` calls

mitigate 86969f9.

* lisp/progmodes/project.el (cl-generic, cl-lib):
Rectify the requires
(project-try-vc): Cache the backend outside project "struct."
(project-root): Cache the backend outside project "struct."
(project-files): Cache the backend outside project "struct."
(project-ignores): Cache the backend outside project "struct."
* test/lisp/progmodes/project-tests.el
(project-sparing-backend-discovery): Test it.
---
 lisp/progmodes/project.el            | 67 ++++++++++++++--------------
 test/lisp/progmodes/project-tests.el | 15 +++++++
 2 files changed, 49 insertions(+), 33 deletions(-)

diff --git a/lisp/progmodes/project.el b/lisp/progmodes/project.el
index daaf86f3277..153e31552bc 100644
--- a/lisp/progmodes/project.el
+++ b/lisp/progmodes/project.el
@@ -156,7 +156,7 @@
 
 ;;; Code:
 
-(require 'cl-generic)
+(require 'cl-lib)
 (require 'seq)
 (eval-when-compile (require 'subr-x))
 
@@ -418,33 +418,32 @@ project-vc-external-roots-function
 backend implementation of `project-external-roots'.")
 
 (defun project-try-vc (dir)
-  (or (vc-file-getprop dir 'project-vc)
-      (let* ((backend (ignore-errors (vc-responsible-backend dir)))
-             (root
-              (pcase backend
-                ('Git
-                 ;; Don't stop at submodule boundary.
-                 (or (vc-file-getprop dir 'project-git-root)
-                     (let ((root (vc-call-backend backend 'root dir)))
-                       (vc-file-setprop
-                        dir 'project-git-root
-                        (if (and
-                             ;; FIXME: Invalidate the cache when the value
-                             ;; of this variable changes.
-                             (project--vc-merge-submodules-p root)
-                             (project--submodule-p root))
-                            (let* ((parent (file-name-directory
-                                            (directory-file-name root))))
-                              (vc-call-backend backend 'root parent))
-                          root)))))
-                ('nil nil)
-                (_ (ignore-errors (vc-call-backend backend 'root dir)))))
-             project)
-        (when root
-          (setq project (list 'vc backend root))
-          ;; FIXME: Cache for a shorter time.
-          (vc-file-setprop dir 'project-vc project)
-          project))))
+  (unless (vc-file-getprop dir 'project-vc)
+    (let* ((backend (ignore-errors (vc-responsible-backend dir)))
+           (root
+            (pcase backend
+              ('Git
+               ;; Don't stop at submodule boundary.
+               (or (vc-file-getprop dir 'project-git-root)
+                   (let ((root (vc-call-backend backend 'root dir)))
+                     (vc-file-setprop
+                      dir 'project-git-root
+                      (if (and
+                           ;; FIXME: Invalidate the cache when the value
+                           ;; of this variable changes.
+                           (project--vc-merge-submodules-p root)
+                           (project--submodule-p root))
+                          (let* ((parent (file-name-directory
+                                          (directory-file-name root))))
+                            (vc-call-backend backend 'root parent))
+                        root)))))
+              ('nil nil)
+              (_ (ignore-errors (vc-call-backend backend 'root dir))))))
+      (when root
+        ;; FIXME: Cache for a shorter time.
+        (vc-file-setprop dir 'project-vc (cons 'vc root))
+        (vc-file-setprop dir 'vc-backend backend))))
+  (vc-file-getprop dir 'project-vc))
 
 (defun project--submodule-p (root)
   ;; XXX: We only support Git submodules for now.
@@ -470,7 +469,7 @@ project--submodule-p
      (t nil))))
 
 (cl-defmethod project-root ((project (head vc)))
-  (nth 2 project))
+  (cdr project))
 
 (cl-defmethod project-external-roots ((project (head vc)))
   (project-subtract-directories
@@ -485,8 +484,9 @@ project-files
    (lambda (dir)
      (let ((ignores (project--value-in-dir 'project-vc-ignores dir))
            backend)
-       (if (and (file-equal-p dir (nth 2 project))
-                (setq backend (cadr project))
+       (if (and (file-equal-p dir (cdr project))
+                (setq backend (or (vc-file-getprop dir 'vc-backend)
+                                  (vc-responsible-backend dir)))
                 (cond
                  ((eq backend 'Hg))
                  ((and (eq backend 'Git)
@@ -598,11 +598,12 @@ project--git-submodules
     (file-missing nil)))
 
 (cl-defmethod project-ignores ((project (head vc)) dir)
-  (let* ((root (nth 2 project))
+  (let* ((root (cdr project))
          backend)
     (append
      (when (file-equal-p dir root)
-       (setq backend (cadr project))
+       (setq backend (or (vc-file-getprop dir 'vc-backend)
+                         (vc-responsible-backend root)))
        (delq
         nil
         (mapcar
diff --git a/test/lisp/progmodes/project-tests.el b/test/lisp/progmodes/project-tests.el
index d4b6bca7e8f..c001106e9f0 100644
--- a/test/lisp/progmodes/project-tests.el
+++ b/test/lisp/progmodes/project-tests.el
@@ -32,6 +32,7 @@
 (require 'ert-x) ; ert-with-temp-directory
 (require 'grep)
 (require 'xref)
+(require 'vc)
 
 (ert-deftest project/quoted-directory ()
   "Check that `project-files' and `project-find-regexp' deal with
@@ -110,4 +111,18 @@ project-ignores-bug-50240
                      (list
                       (expand-file-name "some-file" dir)))))))
 
+(ert-deftest project-sparing-backend-discovery ()
+  "Cache results of `vc-responsible-backend`."
+  (skip-unless (eq 'Git (ignore-errors
+                          (vc-responsible-backend default-directory))))
+  (let* ((default-directory (vc-call-backend 'Git 'root default-directory))
+         (project (project-current)))
+    (should (vc-file-getprop default-directory 'project-vc))
+    (should (eq 'Git (vc-file-getprop default-directory 'vc-backend)))
+    (cl-letf (((symbol-function 'vc-responsible-backend)
+               (lambda (&rest _args) (should nil))))
+      (project-files project)
+      (vc-file-clearprops default-directory)
+      (should-error (project-files project)))))
+
 ;;; project-tests.el ends here
-- 
2.26.2


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