Subject: Re: [PATCH 1/4] rebase -i: demonstrate obscure loose object cache bug

From: Ævar Arnfjörð Bjarmason <[email-masked]>
Date: Wed, 13 Mar 2019 17:11:44 +0100

On Wed, Mar 13 2019, Johannes Schindelin via GitGitGadget wrote:

> From: Johannes Schindelin <[email-masked]>
>
> We specifically support `exec` commands in `git rebase -i`'s todo lists
> to rewrite the very same todo list. Of course, we need to validate that
> todo list when re-reading it.
>
> It is also totally legitimate to extend the todo list by `pick` lines
> using short names of commits that were created only after the rebase
> started.
>
> And this is where the loose object cache interferes with this feature:
> if *some* loose object was read whose hash shares the same first two
> digits with a commit that was not yet created when that loose object was
> created, then we fail to find that new commit by its short name in
> `get_oid()`, and the interactive rebase fails with an obscure error
> message like:
>
> 	error: invalid line 1: pick 6568fef
> 	error: please fix this using 'git rebase --edit-todo'.

As a further improvement, is there a good reason for why we wouldn't
pass something down to the oid machinery to say "we're only interested
in commits". I have a WIP series somewhere to generalize that more, but
e.g.  here locally:

    $ git rev-parse 80b06
    error: short SHA1 80b06 is ambiguous
    hint: The candidates are:
    hint:   80b06b942e commit 2019-03-13 - Revert "this patch fail whales"
    hint:   80b063bb9b blob
    hint:   80b06f0714 blob
    80b06
    $ git rev-parse 80b06^{commit}
    80b06b942ed33e597a0152b3e6ba45b7d8ead94b

I can't remember if there's some caveat with that particular peel syntax
I meant to fix, but I mean if we could pass something down to say "no
blobs or trees" shouldn't we?

Then stuff like this wouldn't die:
    
    $ git rebase -i
    hint: Waiting for your editor to close the file... Waiting for Emacs...
    error: short SHA1 80b06 is ambiguous
    hint: The candidates are:
    hint:   80b06b942e commit 2019-03-13 - Revert "this patch fail whales"
    hint:   80b063bb9b blob
    hint:   80b06f0714 blob
    error: invalid line 2: p 80b06 Revert "this patch fail whales"
    You can fix this with 'git rebase --edit-todo' and then run 'git rebase --continue'.

> [...]
> +test_expect_failure SHA1 'loose object cache vs re-reading todo list' '
> +	GIT_REBASE_TODO=.git/rebase-merge/git-rebase-todo &&
> +	export GIT_REBASE_TODO &&
> +	write_script append-todo.sh <<-\EOS &&
> +	# For values 5 and 6, this yields SHA-1s with the same first two digits
> +	echo "pick $(git rev-parse --short \
> +		$(printf "%s\\n" \
> +			"tree $EMPTY_TREE" \
> +			"author A U Thor <[email-masked]> $1 +0000" \
> +			"committer A U Thor <[email-masked]> $1 +0000" \
> +			"" \
> +			"$1" |
> +		  git hash-object -t commit -w --stdin))" >>$GIT_REBASE_TODO
> +
> +	shift
> +	test -z "$*" ||
> +	echo "exec $0 $*" >>$GIT_REBASE_TODO
> +	EOS
> +
> +	git rebase HEAD -x "./append-todo.sh 5 6"
> +'
> +
>  test_done

This is a test for rebase, but perhaps it would be best put in
t1512-rev-parse-disambiguation.sh. Then when we finally port that
somehow to SHA256 we'll have all this SHA-1 golfing in the same file &
can fix it at once. Just a thought...