Subject: Re: [PATCH] t3429: try to protect against a potential racy todo file problem

From: SZEDER Gábor <[email-masked]>
Date: Mon, 25 Nov 2019 14:18:33 +0100

On Sun, Nov 24, 2019 at 10:10:21PM +0100, SZEDER Gábor wrote:
> To notice a changed todo file the sequencer stores the file's stat
> data in its 'struct todo_list' instance, and compares it with the
> file's current stat data after 'reword', 'squash' and 'exec'
> instructions.  If the two stat data doesn't match, it re-reads the
> todo file.
> 
> Sounds simple, but there are some subtleties going on here:
> 
>   - The 'struct todo_list' holds the stat data from the time when the
>     todo file was last read.
> 
>   - This stat data in 'struct todo_list' is not updated when the
>     sequencer itself writes the todo file.
> 
>   - Before executing each instruction during an interactive rebase,
>     the sequencer always updates the todo file by removing the
>     just-about-to-be-executed instruction.  This changes the file's
>     size and inode [1].
> 
> Consequently, when the sequencer looks at the stat data after a
> 'reword', 'squash' or 'exec' instruction, it most likely finds that
> they differ, even when the user didn't modify the todo list at all!
> This is not an issue in practice, it just wastes a few cycles on
> re-reading the todo list that matches what the sequencer already has
> in memory anyway.

It can be much more than just a few cycles, because the total number
of parsed instructions from all the todo file re-reads can go
quadratic with the number of rebased commits.

The simple test below runs 'git rebase -i -x' on 1000 commits, which
takes over 14seconds to run.  If it doesn't re-read the todo file at
all (I simply deleted the whole condition block checking the stat data
and re-reading) it runs for only ~2.5secs.

Just another angle to consider...

  ---  >8  ---

test_expect_success 'test' '
	num_commits=1000 &&
	test_commit_bulk --filename=file $num_commits &&

	/usr/bin/time -f "Elapsed time: %e" \
		git rebase -i --root -x true 2>out &&

	grep "Executing: true" out >actual &&
	test_line_count = $num_commits actual &&

	# show the elapsed time
	tail -n2 out
'

  ---  >8  ---