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

From: Johannes Schindelin <johannes.schindelin@gmx.de>
Date: Mar 13, 2019, 22:40 UTC
Message-ID: <nycvar.QRO.7.76.6.1903132327500.41@tvgsbejvaqbjf.bet>
In-Reply-To: <20190313165320.GA717@sigill.intra.peff.net>

Hi Peff,

On Wed, 13 Mar 2019, Jeff King wrote:

Show 19 quoted lines

> 
> By the way, while reading the test more carefully, I did notice two
> funny things:
> 
> > +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 <author@example.org> $1 +0000" \
> > +			"committer A U Thor <author@example.org> $1 +0000" \
> > +			"" \
> > +			"$1" |
> > +		  git hash-object -t commit -w --stdin))" >>$GIT_REBASE_TODO
> 
> Here we redirect the output into $GIT_REBASE_TODO, not stdout.

Indeed, because we want to append a `pick` command to the todo list.

Show 5 quoted lines

> > +	shift
> > +	test -z "$*" ||
> > +	echo "exec $0 $*" >>$GIT_REBASE_TODO
> 
> And here we do the same thing. That second redirection is unnecessary.

It is actually not unnecessary, but to the contrary quite necessary to achieve the intended effect: with this command, we append an `exec` line to the todo list that is guaranteed to be executed after the `pick` command that we added earlier.

Show 8 quoted lines

> I also find it interesting that it iterates over its arguments by
> recursive processes. Wouldn't:
> 
>   for i in "$@"; do
> 	echo "pick ..." >>$GIT_REBASE_TODO
>   done
> 
> be a bit more efficient (as well as more obvious?).

It would be more efficient, but it would also fail to test for the regression.

Remember: it is absolutely crucial for the regression test that the parent
process' loose object cache already has been initialized *before* the new
commit is created and then picked. Otherwise the cache would contain that
commit object already. The whole point of the regression test is that the
cache does *not* contain that object.

The only way we can guarantee that order in this test is if the first commit is created and picked *before* we `exec` to create the second commit and then append the `pick` line for that one.

Now, I could have tried to play some fake editor games because it is not strictly necessary to create the first commit via an `exec` line. Instead, I could have generated it before the rebase, and then initialized the todo list with the `pick` of the first commit and then an `exec` of the script that creates the second commit and then appends a `pick` line for that.

But the reality is that this would have resulted in more code! And not even easier-to-read code at that! (I know, because one of my unsent iterations did exactly that.)

So instead, I opted for using the `-x` option to modify the initial todo list to begin with (it consists of a single `noop`, obviously). This will add that `exec` line that calls the script that creates the first commit and appends the `pick` line.

It *also* adds an `exec` line to guarantee that the second commit is created, and a `pick` line for it is appended to the todo list, *after* the sequencer initalized the loose object cache by virtue of picking the first commit.

So yes, it is crucial that the `append-todo.sh` script is `exec`ed *twice*. Otherwise the first `pick` would initialize the loose object cache *after* the second commit was created, which Just Works, even without this here patch series.

Ciao, Dscho