Reduce flydiff-update calls in diff-hl-flydiff-mode

[ermingol23]

When diff-hl-flydiff-mode is enabled, ensure that diff-hl-update function is only called when there are changes in the buffer and not every time the idle timer triggers. This reduces the number of calls to diff-hl-update and thus allows diff-hl-mode to run smoother on weaker (or older hardware).

The function diff-hl-flydiff/modified-p is rewritten so it uses buffer-chars-modified-tick internally instead of relying on buffer-modified-p. It is thus the only function that need to call buffer-chars-modified-tick and all other calls have been removed. Furthermore the buffer-local variable diff-hl-flydiff-modified-tick is only updated when it differs from buffer-chars-modified-tick.

Note that when diff-hl-flydiff/modified-p is called there is a guard that checks if the given state is one of 'added' or 'missing'. Unless one of those two states is given this will trigger a check using buffer-chars-modified-tick. This means that a nil state will trigger such a check.

The idle timer calls the diff-hl-update function, but an :around advice is used. That is the diff-hl-flydiff-update function decides if and when the original diff-hl-update function is called. To simplify the logic the existing 'unless' is replaced with 'when' and thus all checks are inverted; only when all conditions are met the update function diff-hl-update is called.

Customizeable variable diff-hl-flydiff-delay added to existing group diff-hl-flydiff.

Documentation strings written for existing functions diff-hl-flydiff-update and diff-hl-flydiff/modified-p.

NOTE: diff-hl-dired.el had a hard dependency to the vc-run-delayed-success function introduced in Emacs 31. A call to 'static-if' is added to check if such a function is bound or not. If it is not bound, then a direct call to vc-hg-after-dir-status is made instead. Not sure if this is the best way to support older Emacs versions.

[dgutov]

Things look great otherwise.

Could I ask if you are willing to sign FSF's copyright papers?

You submitted two PRs now, and while I could probably squeeze this one in, the two together are over the allowed limit for not assigned code.

[dgutov]

:group is usually not needed if the last defgroup in the file is the same.

[ermingol23]

I'll remove the ':group'!

[dgutov]

What happens if subsequent checks resolve to nil? But diff-hl-flydiff/modified-p already updated diff-hl-flydiff-modified-tick. Maybe we should do it here, before calling orig-fun.

[ermingol23]

Not entirely sure I follow you here. The construct looks like this

(when (and diff-hl-mode
             (diff-hl-flydiff/modified-p nil)
             (stringp buffer-file-name)
             (not (file-remote-p default-directory))
             (file-exists-p buffer-file-name))
    (apply orig-fun nil))

Since the 'and' function in Emacs LISP short-circuits, the following checks are never called if 'diff-hl-flydiff/modified-p' return 'nil'.

Are you thinking of a scenario where a buffer is created that is not associated with any file and then it is written to a file (function 'buffer-file-name' goes from returning 'nil' to a string containing name of file)? If you then insert/modify a character in the file 'diff-hl-flydiff/modified-p' would return non-'nil' and 'orig-fun' is called.

There might be a race condition though where a buffer is modified, the timer triggers a check, 'diff-hl-flydiff/modified-p' return non-'nil' and then the file is moved which causes the 'and'-clause to return 'nil' (due to that 'file-exists-p' would return 'nil'). If the file is moved back and the timer fires again the 'and'-clause would still return 'nil' due to that 'diff-hl-flydiff/modified-p' return 'nil' in this case.

One way of removing this race-condition would be to introduce an additional buffer-local variable (for instance 'diff-hl-flydiff--update-state') that is cleared after 'orig-fun' returns and 'diff-hl-flydiff/modified-p' do an 'or' between the value of this new variable and 'buffer-chars-modified-tick' like this

  (unless (memq state '(added missing))
    (let ((buffer-state (not (eq diff-hl-flydiff-modified-tick
                          (buffer-chars-modified-tick)))))
      (if (or buffer-state diff-hl-flydiff--update-state)
          (progn
            (setq diff-hl-flydiff-modified-tick (buffer-chars-modified-tick))
            (setq diff-hl-flydiff--update-state t)
            (setq buffer-state diff-hl-flydiff--update-state)))
      buffer-state)))

That is, if you time above attempt on causing a race condition as described the next time the timer fires 'diff-hl-flydiff-modified-tick' is still updated (even though the value has not changed) and non-'nil' is returned the next time the function is called. The following tick would then result in an update where 'orig-fun' is called and 'diff-hl-flydiff--update-state' is set to 'nil', that is

  (when (and diff-hl-mode
             (diff-hl-flydiff/modified-p nil)
             (stringp buffer-file-name)
             (not (file-remote-p default-directory))
             (file-exists-p buffer-file-name))
    (apply orig-fun nil)
    (setq diff-hl-flydiff--update-state nil))

Perhaps you were thinking of something else that I missed?

[ermingol23]

Could I ask if you are willing to sign FSF's copyright papers?
Sure! No problems! Please let me know how to do it (what forms to fill in, where to send them, and so on).

[dgutov]

Great! Here you go:

Please email the following information to assign@gnu.org, and we
will send you the assignment form for your past and future changes.

Please use your full legal name (in ASCII characters) as the subject
line of the message.
----------------------------------------------------------------------
REQUEST: SEND FORM FOR PAST AND FUTURE CHANGES

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