diff-hl-show-hunk* doesn't work correctly with jj

[gessen]

Most diff-hl commands work with vc-jj out of the box, e.g. diff-hl-diff-goto-hunk or diff-hl-next-hunk, however, the family of diff-hl-show-hunk* functions do not. The message says "No next hunk" or "There is no next change". This is probably because diff-hl compares with the @ revision (current revision) rather than @-. If I modify diff-hl-reference-revision from nil to "@-", showing hunk with JJ seems to work correctly.

[dgutov]

Hi!

What does @- mean in Jj? Revision before the last one? I think we're mean to compare with the last one, by default.

Anyway, can you try this?

diff --git a/diff-hl.el b/diff-hl.el
index 3d302e8..632f14e 100644
--- a/diff-hl.el
+++ b/diff-hl.el
@@ -488,7 +488,8 @@ It can be a relative expression as well, such as \"HEAD^\" with Git, or
          ((eq state 'removed)
           `((:working . ,`((1 0 ,(line-number-at-pos (point-max)) delete))))))))))
 
-(defvar diff-hl-head-revision-alist '((Git . "HEAD") (Bzr . "last:1") (Hg . ".")))
+(defvar diff-hl-head-revision-alist '((Git . "HEAD") (Bzr . "last:1") (Hg . ".")
+                                      (Jj . "")))
 
 (defun diff-hl-head-revision (backend)
   (or (assoc-default backend diff-hl-head-revision-alist)

[gessen]

@- in JJ means the previous revision, @ means the current revision.

I've tried changing this with few different combinations (vc-jj defines the backend as JJ) :

• (Jj . "")
• (Jj . "@")
• (Jj . "@-")
• (JJ . "")
• (JJ . "@")
• (JJ . "@-")

but all of them gave the same results.

This is the output from *diff-hl-show-hunk-diff-buffer* for Git backend (6c891d6e0fb13d509eaa4f9c852e97d1c7ba5a7c is the HEAD commit):

diff -U 0 --strip-trailing-cr --label /dev/shm/\#\!tmp\!sandbox\!src\!main.rs\#.\~6c891d6e0fb13d509eaa4f9c852e97d1c7ba5a7c\~ --label \#\<buffer\ main.rs\> /dev/shm/\#\!tmp\!sandbox\!src\!main.rs\#.\~6c891d6e0fb13d509eaa4f9c852e97d1c7ba5a7c\~ /dev/shm/buffer-content-s1ojVL
--- /dev/shm/#!tmp!sandbox!src!main.rs#.~6c891d6e0fb13d509eaa4f9c852e97d1c7ba5a7c~
+++ #<buffer main.rs>
@@ -2 +1,0 @@
-    println!("Hello, world!");

Diff finished.  Wed Oct  1 11:27:02 2025

For the JJ backend, the same buffer shows (xtnkkkwlzyomuqlxvwoxwnmronsyxuqz is the current revision):

diff -U 0 --strip-trailing-cr --label /dev/shm/\#\!tmp\!sandbox\!src\!main.rs\#.\~xtnkkkwlzyomuqlxvwoxwnmronsyxuqz\~ --label \#\<buffer\ main.rs\> /dev/shm/\#\!tmp\!sandbox\!src\!main.rs\#.\~xtnkkkwlzyomuqlxvwoxwnmronsyxuqz\~ /dev/shm/buffer-content-1XZCy9

Diff finished (no differences).  Wed Oct  1 11:30:42 2025

If I modify diff-hl-reference-revision to "@-" it will show:

diff -U 0 --strip-trailing-cr --label /dev/shm/\#\!tmp\!sandbox\!src\!main.rs\#.\~\@-\~ --label \#\<buffer\ main.rs\> /dev/shm/\#\!tmp\!sandbox\!src\!main.rs\#.\~\@-\~ /dev/shm/buffer-content-4voGLh
--- /dev/shm/#!tmp!sandbox!src!main.rs#.~@-~
+++ #<buffer main.rs>
@@ -2 +1,0 @@
-    println!("Hello, world!");

Diff finished.  Wed Oct  1 11:32:53 2025

As I understand it, it compares the content of the selected revision (for Git it's HEAD, for JJ it's @) with the current content of the buffer. The problem is that for JJ, the content of the current revision is the same as the current buffer because of how JJ works:

Working-copy-as-a-commit: Changes to files are recorded automatically as normal commits, and amended on every subsequent change. This "snapshot" design simplifies the user-facing data model (commits are the only visible object), simplifies internal algorithms, and completely subsumes features like Git's stashes or the index/staging-area.

Currently, as a workaround, I'm using dir-local variable diff-hl-reference-revision set to "@-" after marking is as a safe variable.

[dgutov]

All right. But do you get any diff indicators from diff-hl, then?

By design, if diff-hl-reference-revision is unset, the indicators on the side of the window should correspond to the diff between the buffer contents and the current committed revision.

And from what you're describing, those contents should be the same, leading to empty diff.

Do you get any output from M-x vc-diff?

[gessen]

With diff-hl-reference-revision set to nil, vc-diff and diff indicators on the side works as expected.

With diff-hl-reference-revision set to "@-", vc-diff works as before, which is not surprising given that the change was in a completely different package. However, I have now noticed that the diff indicators are not always shown where they should be. If the change is an addition or removal (e.g. a new line), the indicators do not behave as expected. For example, adding a single new line results in two indicators: one green with a plus sign (the correct one) and one green with no sign in the line below. Clearly, just setting diff-hl-reference-revision to "@-" is not a solution to this problem.

[gessen]

I believe the flow for diff-hl-show-hunk is like this:

1. diff-hl-diff-buffer-with-reference is called
2. diff-hl-working-revision is called which then calls vc-BACKEND-working-revision which returns the current revision (@ for JJ and HEAD for Git).
3. vc-BACKEND-find-revision is called with the working-revision to create a buffer with the content of that revision
   This clearly will work incorrectly for JJ as the diff should be against @- due to how JJ works.

For normal operations, diff-hl most probably calls diff-hl-diff-against-reference at some point. For JJ it eventually calls (vc-jj-diff (list FILE) nil nil BUFFER nil) (https://github.com/dgutov/diff-hl/blob/master/diff-hl.el#L428) which calls jj diff --from @- --to @.

The problem is the mismatch between vc-jj-working-revision (https://codeberg.org/emacs-jj-vc/vc-jj.el/src/branch/master/vc-jj.el#L392) which returns @ and vc-jj-diff which uses @-..@ diff. As a solution, diff-hl could use a special logic for JJ to get a working-revision - 1, either by using symbolc @- or with a command like jj log --no-graph --limit 1 --revisions @- --template change_id

I wrote a patch that seems to work but maybe you would find a better way to fix that

diff --git i/diff-hl.el w/diff-hl.el
index ac6d39c..7615d70 100644
--- i/diff-hl.el
+++ w/diff-hl.el
@@ -396,6 +396,7 @@ It can be a relative expression as well, such as \"HEAD^\" with Git, or
 (declare-function vc-git--rev-parse "vc-git")
 (declare-function vc-hg-command "vc-hg")
 (declare-function vc-bzr-command "vc-bzr")
+(declare-function vc-jj--process-lines "vc-jj")
 
 (defun diff-hl-changes-buffer (file backend &optional new-rev)
   (diff-hl-with-diff-switches
@@ -1399,6 +1400,12 @@ the user should be returned."
                    (vc-call-backend (or backend (vc-backend file))
                                     'working-revision file)))
 
+(defun diff-hl-jj-previous-revision (file)
+  (vc-file-setprop
+   file 'vc-previous-revision
+   (car (last (vc-jj--process-lines "log" "--no-graph" "-r" "@-"
+                                    "-T" "change_id" "-n" "1")))))
+
 (declare-function diff-no-select "diff")
 
 (defvar diff-hl-temporary-directory (if (and (eq system-type 'gnu/linux)
@@ -1427,7 +1434,9 @@ CONTEXT-LINES is the size of the unified diff context, defaults to 0."
               (diff-hl-create-revision
                file
                (or (diff-hl-resolved-reference-revision backend)
-                   (diff-hl-working-revision file backend)))))
+                   (if (eq backend 'JJ)
+                       (diff-hl-jj-previous-revision file)
+                     (diff-hl-working-revision file backend))))))
            (switches (format "-U %d --strip-trailing-cr" (or context-lines 0))))
       (diff-no-select rev (current-buffer) switches (not (diff-hl--use-async-p))
                       (get-buffer-create dest-buffer))
@@ -1455,6 +1464,11 @@ CONTEXT-LINES is the size of the unified diff context, defaults to 0."
                       "-r" diff-hl-reference-revision)
       (goto-char (point-min))
       (buffer-substring-no-properties (point) (line-end-position))))
+   ;; I'm not really sure if this is even needed?
+   ((eq backend 'JJ)
+    (car (last (vc-jj--process-lines "log" "--no-graph"
+                                     "-r" diff-hl-reference-revision
+                                     "-T" "change_id" "-n" "1"))))
    (t
     diff-hl-reference-revision)))

[dgutov]

The problem is the mismatch between vc-jj-working-revision (https://codeberg.org/emacs-jj-vc/vc-jj.el/src/branch/master/vc-jj.el#L392) which returns @ and vc-jj-diff which uses @-..@ diff.

Yeah, and it sounds to me like it might be a mismatch between Jj and the rest of the VC backends, where the value nil means the working revision usually.

With diff-hl-reference-revision set to "@-", vc-diff works as before, which is not surprising given that the change was in a completely different package. However, I have now noticed that the diff indicators are not always shown where they should be. If the change is an addition or removal (e.g. a new line), the indicators do not behave as expected. For example, adding a single new line results in two indicators: one green with a plus sign (the correct one) and one green with no sign in the line below. Clearly, just setting diff-hl-reference-revision to "@-" is not a solution to this problem.

Ah right, that's because we show the difference between working revision and the reference as a separate set of hunks now.

[dgutov]

If the indicators on the side work fine with the default value of reference revision, I'll see what can be done about the show-hunk feature.

It makes sense that it could behave differently, though, because it uses a different method -- the same as diff-hl-flydiff-mode does, which I'm guessing also doesn't work correctly by default.

[gessen]

It makes sense that it could behave differently, though, because it uses a different method -- the same as diff-hl-flydiff-mode does, which I'm guessing also doesn't work correctly by default.

Correct. Without the patch, diff-hl-flydiff-mode does not appear to work correctly. With the patch I haven't found any issues with its functionality.