Re: [PATCH] fetch-pack: approximate no_dependents with filter

[Junio C Hamano <gitster@pobox.com>]

Jonathan Tan <jonathantanmy@google.com> writes:

> Whenever a lazy fetch is performed for a tree object, any trees and
> blobs it directly or indirectly references will be fetched as well.
> There is a "no_dependents" argument in struct fetch_pack_args that
> indicates that objects that the wanted object references need not be
> sent, but it currently has no effect other than to inhibit usage of
> object flags.
>
> Extend the "no_dependents" argument to also exclude sending of objects
> as much as the current protocol allows: when fetching a tree, all trees
> it references will be sent (but not the blobs), and when fetching a
> blob, it will still be sent. (If this mechanism is used to fetch a
> commit or any other non-blob object, all referenced objects, except
> blobs, will be sent.) The client neither needs to know or specify the
> type of each object it wants.
>
> The necessary code change is done in fetch_pack() instead of somewhere
> closer to where the "filter" instruction is written to the wire so that
> only one part of the code needs to be changed in order for users of all
> protocol versions to benefit from this optimization.

It is very clear how you are churning the code, but it is utterly
unclear from the description what you perceived as a problem and why
this change is a good (if not the best) solution for that problem,
at least to me.

After reading the above description, I cannot shake the feeling that
this is tied too strongly to the tree:0 use case?  Does it help
other use cases (e.g. would it be useful or harmful if a lazy clone
was done to exclude blobs that are larger than certain threshold, or
objects of all types that are not referenced by commits younger than
certain threshold)?