Re: [PATCH 5/8] commit-graph: not compatible with replace objects

> On Wed, Jul 18, 2018 at 08:15:41AM -0700, Derrick Stolee via GitGitGadget wrote:
>
>> From: Derrick Stolee <dstolee@microsoft.com>
>>
>> Create new method commit_graph_compatible(r) to check if a given
>> repository r is compatible with the commit-graph feature. Fill the
>> method with a check to see if replace-objects exist. Test this
>> interaction succeeds, including ignoring an existing commit-graph and
>> failing to write a new commit-graph.
> I think this approach is sensible. These are optimizations, and it's not
> a big deal to just punt no cases we can't handle.
>
> I do have one complaint about the implementation, though:
>
>> +static int commit_graph_compatible(struct repository *r)
>> +{
>> +	prepare_replace_object(r);
>> +	if (hashmap_get_size(&r->objects->replace_map->map))
>> +		return 0;
>> +
>> +	return 1;
>> +}
> If I'm reading the code correctly, this will predicate the decision
> entirely on the presence of refs in refs/replace/. But you may have a
> situation where those refs exist, but you are not respecting them in
> this run.
>
> For instance, imagine[1] a server that hosts arbitrary repositories, but
> wants to use commit graphs to speed up server-side operations (e.g.,
> serving fetches, but also perhaps a web interface doing --contains,
> etc). If it runs all of the server-side commands with
> GIT_NO_REPLACE_OBJECTS=1, then there should be no problem. But if a user
> pushes anything to refs/replace (because they _do_ use replace refs
> locally, and want to share them with other clients), that would disable
> graphs on the server.
>
> So I think this should at least be loosened to:
>
>    if (check_replace_refs) {
> 	prepare_replace_object(r);
> 	if (...)
>    }
>
> which would make this case work. I'd even go so far as to say that for
> writing, we should just always ignore replace refs and generate the full
> graph (just like pack-objects does so when writing a pack). Then the
> resulting graph can be used selectively by disabling replace refs for
> particular commands. But for the scenario I described above, the
> distinction is mostly academic, as replacements would be disabled anyway
> during the write command anyway.
>
> -Peff
>
> [1] You can probably guess that this is how GitHub handles replace refs.
>      We ran into this long ago because replacements and grafts mess up
>      any other caches external to Git that rely on the immutability of
>      the hash.
>
>      We do it with a config option, though, which requires a trivial
>      patch. I'll post that momentarily.