bug#73387: 30.0.90; C-x v v in diff-mode doesn't work after C-c C-n

[Sean Whitton <spwhitton@spwhitton.name>]

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Hello,

On Mon 30 Sep 2024 at 01:11pm +03, Dmitry Gutov wrote:

> Just these two points:
>
> - add a command which does the kill-all-but-this-hunk (or hunks in
>   region if mark active) thing -- it's generally useful.
>
> - make C-x v v on a narrowed buffer, by default, issue a message saying
>   "Cannot commit patch when narrowed, consider <binding of new command>"

Okay, what do you think to the attached?

I tested the (apply #'user-error ...) by applying your patch from
up-thread, though I think there may be a bug with that patch because
after applying it I was not able to use C-x v v to commit.

(FTAOD I think the attached is valid independently of your patch.)

-- 
Sean Whitton

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From 34f5c95c6d04cd5d07d11214a72516b0c9ff10e4 Mon Sep 17 00:00:00 2001
From: Sean Whitton <spwhitton@spwhitton.name>
Date: Mon, 30 Sep 2024 21:08:38 +0800
Subject: [PATCH] New command diff-delete-other-hunks

* lisp/vc/diff-mode.el (diff-delete-other-hunks): New
command (bug#73387).
(diff-mode-map): Bind the new command to C-c RET k.
(diff-mode-menu): New entry for the new command.
(vc-next-action): Stop, and warn, if the user attempts to commit
a patch from a narrowed buffer.
* doc/emacs/files.texi (Diff Mode):
* etc/NEWS: Document the new command.
---
 doc/emacs/files.texi |  5 +++++
 etc/NEWS             |  5 +++++
 lisp/vc/diff-mode.el | 35 +++++++++++++++++++++++++++++++++++
 lisp/vc/vc.el        | 10 ++++++++++
 4 files changed, 55 insertions(+)

diff --git a/doc/emacs/files.texi b/doc/emacs/files.texi
index a3a8c854aa6..a32689552b1 100644
--- a/doc/emacs/files.texi
+++ b/doc/emacs/files.texi
@@ -1703,6 +1703,11 @@ Diff Mode
 Apply all the hunks in the buffer (@code{diff-apply-buffer}).  If the
 diffs were applied successfully, save the changed buffers.
 
+@findex diff-delete-other-hunks
+@item C-c @key{RET} k
+Delete all hunks other than the current hunk.  If the region is active,
+then delete all hunks other than those the region overlaps.
+
 @findex diff-refine-hunk
 @item C-c C-b
 Highlight the changes of the hunk at point with a finer granularity
diff --git a/etc/NEWS b/etc/NEWS
index cdc7f47b7a9..a459ec18495 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -369,6 +369,11 @@ This command reverts the hunk at point (i.e., applies the reverse of the
 hunk), and then removes the hunk from the diffs.  This is useful to undo
 commits when you are in buffers generated by 'C-x v =' and 'C-x v D'.
 
++++
+*** New command 'diff-delete-other-hunks' bound to C-c RET k.
+This command deletes hunks other than the current hunk.
+It is useful to prepare a *vc-diff* buffer for committing a single hunk.
+
 ** php-ts-mode
 
 ---
diff --git a/lisp/vc/diff-mode.el b/lisp/vc/diff-mode.el
index 25c6238765d..ff141e19c50 100644
--- a/lisp/vc/diff-mode.el
+++ b/lisp/vc/diff-mode.el
@@ -220,6 +220,7 @@ diff-mode-map
   "C-c C-a" #'diff-apply-hunk
   "C-c M-r" #'diff-revert-and-kill-hunk
   "C-c C-m a" #'diff-apply-buffer
+  "C-c C-m k" #'diff-delete-other-hunks
   "C-c C-e" #'diff-ediff-patch
   "C-c C-n" #'diff-restrict-view
   "C-c C-s" #'diff-split-hunk
@@ -278,6 +279,8 @@ diff-mode-menu
      :help "Kill current hunk"]
     ["Kill current file's hunks" diff-file-kill
      :help "Kill all current file's hunks"]
+    ["Delete other hunks"       diff-delete-other-hunks
+     :help "Delete hunks other than the current hunk"]
     "-----"
     ["Previous Hunk"		diff-hunk-prev
      :help "Go to the previous count'th hunk"]
@@ -814,6 +817,38 @@ diff-hunk-kill
       (goto-char (car bounds))
       (ignore-errors (diff-beginning-of-hunk t)))))
 
+;; This is not `diff-kill-other-hunks' because we might need to make
+;; copies of file headers in order to ensure the new kill ring entry
+;; would be a patch with the same meaning.  That is not implemented
+;; because it does not seem like it would be useful to anyone.
+(defun diff-delete-other-hunks (&optional beg end)
+  "Delete hunks other than this one.
+Interactively, if the region is active, then delete all hunks that the
+region does not overlap.
+When called from Lisp, the region to act upon is specified by optional
+arguments BEG and END."
+  (interactive (list (use-region-beginning) (use-region-end)))
+  (when (buffer-narrowed-p)
+    (user-error "Command is not safe in a narrowed buffer"))
+  (let ((inhibit-read-only t))
+    (save-excursion
+      (cond ((xor beg end)
+             (error "Require exactly zero or two arguments"))
+            (beg
+             (goto-char beg)
+             (setq beg (car (diff-bounds-of-hunk)))
+             (goto-char end)
+             (setq end (cadr (diff-bounds-of-hunk))))
+            (t
+             (pcase-setq `(,beg ,end) (diff-bounds-of-hunk))))
+      (delete-region end (point-max))
+      (goto-char beg)
+      (diff-beginning-of-file)
+      (diff-hunk-next)
+      (delete-region (point) beg)
+      (diff-beginning-of-file-and-junk)
+      (delete-region (point-min) (point)))))
+
 (defun diff-beginning-of-file-and-junk ()
   "Go to the beginning of file-related diff-info.
 This is like `diff-beginning-of-file' except it tries to skip back over leading
diff --git a/lisp/vc/vc.el b/lisp/vc/vc.el
index 597a1622f5a..39e5ca424d4 100644
--- a/lisp/vc/vc.el
+++ b/lisp/vc/vc.el
@@ -1302,6 +1302,16 @@ vc-next-action
      ;; Fileset comes from a diff-mode buffer, see
      ;; 'diff-vc-deduce-fileset', and the buffer is the patch to apply.
      ((eq model 'patch)
+      (when (buffer-narrowed-p)
+        ;; If we used `diff-restrict-view' then we may not have the file
+        ;; header and the commit will not succeed (bug#73387).
+        (apply #'user-error
+               "Cannot commit patch when narrowed; consider %s %s %s"
+               (mapcar (lambda (c)
+                         (key-description (where-is-internal c nil t)))
+                       '(widen
+                         diff-delete-other-hunks
+                         vc-next-action))))
       (vc-checkin files backend nil nil nil (buffer-string)))
      ((or (null state) (eq state 'unregistered))
       (cond (verbose
-- 
2.45.2