Subject: Re: Bugs in git filter-branch (git replace related)

From: Anatoly Borodin <[email-masked]>
Date: Fri, 29 Jan 2016 18:24:07 +0000 (UTC)

Hi Jeff,

I've created a gist with the script
https://gist.github.com/anatolyborodin/6505a364a68584f13846

I've added some changes and a second test (will be discussed in the
comments).

Jeff King <[email-masked]> wrote:
> I'm not sure if this is a bug or not. The "empty commit" check works by
> checking the tree sha1s, without doing a full diff respecting replace
> refs.
> 
> You're expecting git to notice a tree change, even though it never even
> examined the tree in the first place (because you didn't give it a tree
> or index filter).

When git-filter-branch(1) says "If you have any grafts or replacement
refs defined, running this command will make them permanent.", and it
doesn't work like that because of some optimization, it *is* a bug.

> Try:
> 
>   git filter-branch --prune-empty --tree-filter true master
> 
> which will force git to go through the motions of checking out the
> replaced content and re-examining it.

Thank you, I've added this command to the script, and it works! I think
it should be added to git-filter-branch(1), if there is no other way to
rewrite the optimization.

>> Bug 2: the replace refs are not ignored (they can epresent blobs, trees etc,
>> but even if they represent commits - should they be rewritten?).
> 
> You told it "--all", which is passed to rev-list, where it means "all
> refs". I agree that running filter-branch on refs/replace is probably
> not going to yield useful results, but I'm not sure if it is
> filter-branch's responsibility to second-guess the rev-list options.

Look how `git log --all` works (see the second test in the script): it
ignores (without any messages) the blobs, and writes only the commits.
For example, the same commit log message is printed twice in the second
testcase.

Maybe it makes sence, I don't know. I would suggest that all
refs/replace/* heads should be ignored by `git log`. `git rev-list
--no-replace` maybe?

> Probably the documentation for filter-branch should recommend
> "--branches --tags" instead of "--all", though.

Or redefine `--all` as "all refs excepting refs/replace/*". Who would
really want to run `--all` the way it works now?

The blobs replaces should be ignored, as in `git log --all`. Is there
any reason to rewrite refs/rebase/hash if it's a replace commit?

-- 
Mit freundlichen Grüßen,
Anatoly Borodin