Re: [PATCH 00/24] SHA-256 test fixes, part 8

[Johannes Schindelin <Johannes.Schindelin@gmx.de>]

Hi brian,

On Mon, 13 Jan 2020, brian m. carlson wrote:

> I suspect that t3404 also has a bug, since the object IDs that are
> supposed to collide do not, according to my instrumentation of the test.
> I'm unsure what the intended collision was and consequently haven't
> fixed it.  However, it does work with SHA-256 as it stands and is no
> more or less functional than with SHA-1, so I've removed the
> prerequisite.

This test was first introduced in 66ae9a57b88 (t3404: rebase -i:
demonstrate short SHA-1 collision, 2013-08-23). This commit does, however,
not give the full history of events. The most interesting tidbit is in
this mail:
https://public-inbox.org/git/1377112378-45511-4-git-send-email-sunshine@sunshineco.com/

Sadly, I could not make the indicated revision compile in a quick and
dirty way, so I cannot hope to bisect this down. But I can do better and
try to fix it properly.

On the other hand, I can do _even better_ than that and demonstrate that
the test case is both incomplete _and_ still has the proper collision. Let
me first explain the idea behind it: we want to run an interactive rebase
on

	collide1 - collide2 - collide3

Forget about collide1, this will be the base commit for the rebase. Now,
the idea is that collide2 is reworded during the rebase so that it has the
same short SHA-1 as collide3. Which means that a `pick <collide3>` will
fail if the short name is used because it is ambiguous, it could both
refer to collide3 and to the reworded collide2.

And this is indeed the case: if I insert a `break` after the `reword` to
force the rebase to be interrupted and the execute manually the command
`git rebase --edit-todo`, I see this:

	$ git rebase --edit-todo
	error: short SHA1 6bcda37 is ambiguous
	hint: The candidates are:
	hint:   6bcda372 commit 2005-04-07 - collide3
	hint:   6bcda37f commit 2005-04-07 - collide2 ac4f2ee
	error: could not parse 'collide3
	'
	error: invalid line 1: pick 6bcda37 collide3

But wait! This should not happen. That is exactly what this regression
test wanted to safeguard against: the `git-rebase-todo` file should have
been written with full SHA-1s, even if `git rebase --edit-todo` should
shorten them before opening the editor, and expanding them after the
editor was closed.

Uh oh.

*clicketyclick for some hours* Aha! A recent change in the interactive
rebase avoids re-reading the todo list all the time, and unfortunately we
now _also_ do not re-read the todo list right after expanding the SHA-1s
in `todo_list_write_to_file()`.

*clicketyclick for another few hours* Okay, I think I understand the
issue, and why the test passed (even if it should not have passed). I
opened https://github.com/gitgitgadget/git/pull/529 and will continue with
this project tomorrow.

Ciao,
Dscho