New file has no highlighting when using `diff-hl-flydiff-mode` (and make Emacs slow)

[whhone]

Repro steps

1. Create a new file in a repo (Git or Hg), open it.
2. Enable diff-hl-flydiff-mode

Expected
I can see highlights (all green for each line since they are new)

Actual
There is no highlight. Also, it makes Emacs slow.

Causes

1. diff-hl tries to find the new file from the previous revision (around (diff-hl-create-revision file revision))
2. Since the file does not exist in the previous revision, it throws an error.
3. Hence, it does not show the diff highlights.
4. Also, on every file modification, it repeats calling the expensive call the find the file from a previous revision.

[dgutov]

Hi! Thanks for the report.

It seems there are a few other conditions needed to reproduce it. First, diff-hl-show-staged-changes needs to be nil. Second, (vc-backend buffer-file-name) needs to return non-nil - that normally doesn't happen in files that aren't registered yet.

So I managed to trigger this once, but don't have a stable scenario yet.

[dgutov]

I've pushed a change that seems logical - please see if the error is gone, and how the performance is now.

[whhone]

Thanks for the fix for Git. :) I forgot to mention that I was using Hg, not Git, when seeing the issue.

[dgutov]

Okay, could you clarify the scenario then?

I create a file, enable python-mode, the syntax highlighting works. No diff highlighting though - but no errors either.

We don't get to diff-hl-create-revision because (vc-backend buffer-file-name) returns nil - since the file is not registered yet. As you can see, for example, diff-hl-diff-buffer-with-reference calls vc-ensure-vc-buffer first, and it should error if this point of the execution was reached. At what point does vc-backend return Hg with the newly added file?

[whhone]

The scenario is the same as the issue description.

Issue 1: I can see "syntax highlight" but not "diff-hl highlight". See the screenshot below.

Issue 2: It can make Emacs slow, especially when the VC backend is slow, since it queries the backend for a non-existing file for every edit.

Note that the vc-backend returns Hg when it is in a Mercurial repository.

[dgutov]

Note that the vc-backend returns Hg when it is in a Mercurial repository.

But it returns nil if the file has just been created, doesn't it?