Re: [jgit-dev] GIT replace

[Shawn Pearce <spearce@xxxxxxxxxxx>]

On Wed, Sep 27, 2017 at 10:29 PM, Duft Markus
<Markus.Duft@xxxxxxxxxxxxxxxx> wrote:
>
> We are thinking about splitting out repositories history, as it is getting
> quite large recently. To allow history browsing for our developers, we will
> provide a script that ‘git replace’s the old history onto the new repo.
> Unfortunately it seems that JGIT does not support reading git replace refs.
> Now my question to the experts: Do you think this is much effort to
> implement? I do not need to be able to use git replace from the IDE, but the
> history view and annotations in the editor should show the replaced history…
>
>
>
> I am willing to give it a shot and try to implement it if I get some hints
> on how/where to start J

Replace is a bit challenging because the reader needs to scan the
refs/replace/ namespace before it does anything. That scan has to
build up a mapping table to quickly map an ObjectId to its
replacement, so that individual object accesses can be checked against
the map.

I think this logic winds up in the ObjectReader. So an ObjectDatabase
implementation needs to check if there are refs/replace/ in its
sibling RefDatabase, build an ObjectIdOwnerMap to maintain the mapping
of old -> replace, and pass this mapping to any ObjectReader it makes.

ObjectReader's methods like open() and has() need to consult the
mapping, and if a replace entry exists, swap the caller's AnyObjectId
with the replacement id before proceeding to locate the object in the
ObjectDatabase.

IIRC PackWriter falls into a weird corner case where by default its
not supposed to honor refs/replace/. So you also need a flag on the
ObjectReader that allows a caller to disable/clear the replacement
mapping and let PackWriter set this on any ObjectReader its using to
ensure its packing the original graph and not the replacement
structure.