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

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

[dick]

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[-- Attachment #1: 0001-Avoid-breaking-the-world-s-project-current-calls.patch --]
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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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bug#54359: 29.0.50; [PATCH] Undo breaking change to project interface

[Eli Zaretskii]

> From: dick <dick.r.chiang@gmail.com>
> Date: Sat, 12 Mar 2022 16:47:53 -0500
> 
> >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.

Please provide the rationale for this changeset.  In particular, what
did the offending commit break, how to reproduce the breakage, and why
the best solution is to revert that commit?

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

[Lars Ingebrigtsen]

dick <dick.r.chiang@gmail.com> writes:

> Subject: [PATCH] Avoid breaking the world's `project-current` calls
>
> mitigate 86969f9.

It's unclear what the problem here is, but it'll be obvious to Dmitry,
so I've added him to the CCs.

-- 
(domestic pets only, the antidote for overdose, milk.)
   bloggy blog: http://lars.ingebrigtsen.no

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

[Rudolf Adamkovič via Bug reports for GNU Emacs, the Swiss army knife of text editors]

Eli Zaretskii <eliz@gnu.org> writes:

> [...], what did the offending commit break, how to reproduce the
> breakage, and why the best solution is to revert that commit?

I did not post the patch, but I too noticed the breakage when I could
not open a project at work out of sudden.  I had to change my
configuration file as follows:

 (defun my/project-try-local (dir)
   "Find projects marked with a '.project' stamp file."
   (let ((project-dir (locate-dominating-file dir ".project")))
-    (if project-dir (cons 'vc project-dir) nil)))
+    (if project-dir `(vc Git ,project-dir) nil)))
 
 (add-hook 'project-find-functions #'my/project-try-local)

Rudy
-- 
"Thinking is a momentary dismissal of irrelevancies."
-- Richard Buckminster Fuller, 1969

Rudolf Adamkovič <salutis@me.com> [he/him]
Studenohorská 25
84103 Bratislava
Slovakia

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

[Lars Ingebrigtsen]

Rudolf Adamkovič <salutis@me.com> writes:

> I did not post the patch, but I too noticed the breakage when I could
> not open a project at work out of sudden.  I had to change my
> configuration file as follows:
>
>  (defun my/project-try-local (dir)
>    "Find projects marked with a '.project' stamp file."
>    (let ((project-dir (locate-dominating-file dir ".project")))
> -    (if project-dir (cons 'vc project-dir) nil)))
> +    (if project-dir `(vc Git ,project-dir) nil)))

Right.  Then I think Dick's patch seems like the right solution here,
but I'm largely unfamiliar with project.el.  Dmitry?

-- 
(domestic pets only, the antidote for overdose, milk.)
   bloggy blog: http://lars.ingebrigtsen.no

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

[Dmitry Gutov]

On 17.03.2022 13:28, Lars Ingebrigtsen wrote:
> Rudolf Adamkovič<salutis@me.com>  writes:
> 
>> I did not post the patch, but I too noticed the breakage when I could
>> not open a project at work out of sudden.  I had to change my
>> configuration file as follows:
>>
>>   (defun my/project-try-local (dir)
>>     "Find projects marked with a '.project' stamp file."
>>     (let ((project-dir (locate-dominating-file dir ".project")))
>> -    (if project-dir (cons 'vc project-dir) nil)))
>> +    (if project-dir `(vc Git ,project-dir) nil)))
> Right.  Then I think Dick's patch seems like the right solution here,
> but I'm largely unfamiliar with project.el.  Dmitry?

Of course not.

And my/project-try-local shouldn't reuse the data type belonging to the 
vc project type either.

If it used its own type tag and generic method implementations, it 
wouldn't see this kind of breakage.

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

[Dmitry Gutov]

On 14.03.2022 12:38, Lars Ingebrigtsen wrote:
> dick<dick.r.chiang@gmail.com>  writes:
> 
>> Subject: [PATCH] Avoid breaking the world's `project-current` calls
>>
>> mitigate 86969f9.
> It's unclear what the problem here is, but it'll be obvious to Dmitry,
> so I've added him to the CCs.

It might be able to guess what Dick meant, but it's only going to be a 
guess without a proper bug report description.

But to address the title: there was no breaking change to the public 
interface.

A data structure internal to the vc project backend has changed, though.

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

[dick]

> A data structure internal to the vc project backend has changed,
> though.

Apparently, you're discussing an unrelated project written in an
unrelated language.

We are talking about emacs lisp, which professes data hiding as honestly
as I profess integrity.

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

[Lars Ingebrigtsen]

Dmitry Gutov <dgutov@yandex.ru> writes:

> And my/project-try-local shouldn't reuse the data type belonging to
> the vc project type either.
>
> If it used its own type tag and generic method implementations, it
> wouldn't see this kind of breakage.

It seems that (at least) two users have bumped into this problem, so
it might be worth considering doing this caching a different way.

-- 
(domestic pets only, the antidote for overdose, milk.)
   bloggy blog: http://lars.ingebrigtsen.no

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

[dick]

Never one to let sleeping dogs lie, if the symbols-with-positions fiasco
has taught us anything, it's that lisp does not admit "internal data
structures."

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

[Dmitry Gutov]

On 18.03.2022 03:13, dick wrote:
> We are talking about emacs lisp, which professes data hiding as honestly
> as I profess integrity.

Emacs Lisp also allows us to use abstractions over data. See cl-generic 
for one such example.

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

[Dmitry Gutov]

On 18.03.2022 11:33, Lars Ingebrigtsen wrote:
> It seems that (at least) two users have bumped into this problem, so
> it might be worth considering doing this caching a different way.

It's an issue of improper usage. In a way that the documentation says 
not to do.

There is no bug.

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

[dick]

> There is no bug.

I noticed all emacs developers, myself included, say this about their
own code.

It's fine.  Of the four people in the world using project.el, I suspect
only half of them rely on `(cdr (project-current))` being an atom
instead of a list.

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

[Lars Ingebrigtsen]

Dmitry Gutov <dgutov@yandex.ru> writes:

> It's an issue of improper usage. In a way that the documentation says
> not to do.
>
> There is no bug.

You're the domain expert, of course, so I'm closing this bug report.

-- 
(domestic pets only, the antidote for overdose, milk.)
   bloggy blog: http://lars.ingebrigtsen.no

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

[Juri Linkov]

>> There is no bug.
>
> I noticed all emacs developers, myself included, say this about their
> own code.
>
> It's fine.  Of the four people in the world using project.el, I suspect
> only half of them rely on `(cdr (project-current))` being an atom
> instead of a list.

I'm the fifth user of project.el and after that change
just quietly replaced in my code

-                 (let ((project (cdr (project-current nil directory))))
-                   (when project

with more correct code that doesn't rely on internal data structure

+                 (let* ((project (project-current nil directory))
+                        (root (when project (project-root project))))

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

[Dmitry Gutov]

On 19.03.2022 16:19, Lars Ingebrigtsen wrote:
> You're the domain expert, of course, so I'm closing this bug report.

Thanks.

I'd like to add one more thing: it's been a change that should have been 
done earlier. Now that we have projects of different "shapes" in the 
core, it becomes harder to use project.el incorrectly, and so, 
automatically, extending it with more project types should become more 
frictionless as well.

Breakages are unfortunate, of course, but so far there has been very 
limited indication on that front (two reports here and one message on 
github).