From: Johannes Schindelin <Johannes.Schindelin@gmx.de>
To: Jeff King <peff@peff.net>
Cc: "Ævar Arnfjörð Bjarmason" <avarab@gmail.com>,
"Johannes Schindelin via GitGitGadget" <gitgitgadget@gmail.com>,
git@vger.kernel.org, "Junio C Hamano" <gitster@pobox.com>
Subject: Re: [PATCH 1/4] rebase -i: demonstrate obscure loose object cache bug
Date: Wed, 13 Mar 2019 23:40:54 +0100 (STD) [thread overview]
Message-ID: <nycvar.QRO.7.76.6.1903132327500.41@tvgsbejvaqbjf.bet> (raw)
In-Reply-To: <20190313165320.GA717@sigill.intra.peff.net>
Hi Peff,
On Wed, 13 Mar 2019, Jeff King wrote:
>
> 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 $GIT_REBASE_TODO, not stdout.
Indeed, because we want to append a `pick` command to the todo list.
> > + shift
> > + test -z "$*" ||
> > + 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.
> 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
next prev parent reply other threads:[~2019-03-13 22:41 UTC|newest]
Thread overview: 32+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-03-13 10:16 [PATCH 0/4] get_oid: cope with a possibly stale loose object cache Johannes Schindelin via GitGitGadget
2019-03-13 10:16 ` [PATCH 1/4] rebase -i: demonstrate obscure loose object cache bug Johannes Schindelin via GitGitGadget
2019-03-13 16:11 ` Ævar Arnfjörð Bjarmason
2019-03-13 16:35 ` Jeff King
2019-03-13 16:53 ` Jeff King
2019-03-13 22:40 ` Johannes Schindelin [this message]
2019-03-14 0:07 ` Jeff King
2019-03-13 22:27 ` Johannes Schindelin
2019-03-14 0:05 ` Jeff King
2019-03-14 6:40 ` Johannes Sixt
2019-03-14 13:06 ` Johannes Schindelin
2019-03-14 1:12 ` Junio C Hamano
2019-03-14 12:44 ` Johannes Schindelin
2019-03-13 10:16 ` [PATCH 2/4] sequencer: improve error message when an OID could not be parsed Johannes Schindelin via GitGitGadget
2019-03-14 1:14 ` Junio C Hamano
2019-03-13 10:16 ` [PATCH 3/4] sequencer: move stale comment into correct location Johannes Schindelin via GitGitGadget
2019-03-13 10:16 ` [PATCH 4/4] get_oid(): when an object was not found, try harder Johannes Schindelin via GitGitGadget
2019-03-14 1:29 ` Junio C Hamano
2019-03-14 2:22 ` Jeff King
2019-03-14 2:40 ` Jeff King
2019-03-14 4:05 ` Junio C Hamano
2019-03-14 18:55 ` Jeff King
2019-03-14 3:49 ` Junio C Hamano
2019-03-14 13:17 ` Johannes Schindelin
2019-03-14 18:57 ` Jeff King
2019-03-14 19:55 ` Johannes Schindelin
2019-03-13 15:33 ` [PATCH 0/4] get_oid: cope with a possibly stale loose object cache Jeff King
2019-03-14 15:33 ` [PATCH v2 0/4] get_short_oid: " Johannes Schindelin via GitGitGadget
2019-03-14 15:33 ` [PATCH v2 1/4] rebase -i: demonstrate obscure loose object cache bug Johannes Schindelin via GitGitGadget
2019-03-14 15:33 ` [PATCH v2 2/4] sequencer: improve error message when an OID could not be parsed Johannes Schindelin via GitGitGadget
2019-03-14 15:33 ` [PATCH v2 3/4] sequencer: move stale comment into correct location Johannes Schindelin via GitGitGadget
2019-03-14 15:33 ` [PATCH v2 4/4] get_oid(): when an object was not found, try harder Johannes Schindelin via GitGitGadget
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