Re: [PATCH 08/17] pack-objects: add --path-walk option

From: Derrick Stolee <stolee@gmail.com>
Date: Oct 31, 2024, 02:12 UTC
Message-ID: <59c0b17a-491e-48ed-840e-7eec1c2d1de2@gmail.com>
In-Reply-To: <20241028195404.4119175-1-jonathantanmy@google.com>

On 10/28/24 3:54 PM, Jonathan Tan wrote:

Show 24 quoted lines

> "Derrick Stolee via GitGitGadget" <gitgitgadget@gmail.com> writes:
>> This new walk is incompatible with some features and is ignored by
>> others:
>>
>>   * Object filters are not currently integrated with the path-walk API,
>>     such as sparse-checkout or tree depth. A blobless packfile could be
>>     integrated easily, but that is deferred for later.
>>
>>   * Server-focused features such as delta islands, shallow packs, and
>>     using a bitmap index are incompatible with the path-walk API.
>>
>>   * The path walk API is only compatible with the --revs option, not
>>     taking object lists or pack lists over stdin. These alternative ways
>>     to specify the objects currently ignores the --path-walk option
>>     without even a warning.
> 
> It might be better to declare --path-walk as "internal use only" and
> only supporting what send-pack.c (used by "git push") and "git repack"
> needs. (From this list, it seems that there is a lot of incompatibility,
> some of which can happen without a warning to the user, so it sounds
> better to be up-front and say that we only support what send-pack.c
> needs. This also makes reviewing easier, as we don't have to think about
> the possible interactions with every other rev-list feature - only what
> is used by send-pack.c.)

I do wonder what the value of doing this would be. I consider 'gitpack-objects' to already be a plumbing command, so marking any option as "internal use only" seems like overkill. It takes effort to combine the options carefully for the right effect. The tests in p5313 are not terribly simple, such as needing --no-reuse-delta to guarantee we are using the desired delta algorithm.

Show 6 quoted lines

> Also from a reviewer perspective, it might be better to restrict this
> patch set to what send-pack.c needs and leave "git repack" for a future
> patch set. This means that we would not need features such as blob
> and tree exclusions, and possibly not even bitmap use or delta reuse
> (assuming that the user would typically push recently-created objects
> that have not been repacked).

While I can understand that as being a potential place to split the patch series, the integration to add 'git repack --path-walk' is actually very simple. Repacking "everything" needs to happen to be able to push a repo to an empty remote, after all.

There are some subtleties around indexed objects, reflogs, and the like that add some complexity, but they also are handled in the path-walk API layer. Some of that complexity was helpful to know about during repack tests.

Finally, the 'git repack --path-walk' use case is a great one for demonstrating the benefits to threading the 'git pack-objects --path-walk' algorithm.

Show 17 quoted lines

>> +		/* Skip objects that do not exist locally. */
>> +		if (exclude_promisor_objects &&
>> +		    oid_object_info_extended(the_repository, oid, &oi,
>> +					     OBJECT_INFO_FOR_PREFETCH) < 0)
>> +			continue;
> 
> This functionality is typically triggered by --missing=allow;
> --exclude_promisor_objects means (among other things) that we allow
> a missing link only if that object is known to be a promisor object
> (because another promisor object refers to it) (see Documentation/
> rev-list-options.txt, and also get_reference() and elsewhere in
> revision.c - notice how is_promisor_object() is paired with it)
> 
> Having said that, we should probably just fail outright on missing
> objects, whether or not we have exclude_promisor_objects. If we have
> computed that we need to push an object, that object needs to exist.
> (Same for repack.)

I think that this is not a reasonable assumption that a hard fail should be expected. Someone could create a blobless clone with a sparse-checkout and then add a new file outside their sparse-checkout without ever having the previous version downloaded.

When pushing, they won't be able to use the previous version as a delta base, but they would certainly be confused about an object being downloaded during 'git push'.

While the example I bring up is somewhat contrived, I can easily imagine cases where missing objects are part of the commit boundary and could be marked as UNINTERESTING but would still be sent in the batch to be considered as a delta base.

Thanks, -Stolee