Re: GSoC Git Proposal Draft - ZheNing Hu

>
> On Fri, Apr 02, 2021 at 05:03:17PM +0800, ZheNing Hu wrote:
>
> > * Because part of the feature of `git for-each-ref` is very similar to
> > that of `git cat-file`, I think `git cat-file` can learn some feasible
> > solutions from it.
> >
> > #### My possible solutions:
> >
> > 1. Same [solution](https://github.com/git/git/pull/568/commits/cc40c464e813fc7a6bd93a01661646114d694d76)
> > as Olga, add member `struct ref_format format` in `struct
> > batch_options`.
> > 2. Use the function
> > [`verify_ref_format()`](https://github.com/gitgitgadget/git/blob/84d06cdc06389ae7c462434cb7b1db0980f63860/ref-filter.c#L904)
> > to replace the first `expand_format()` for parsing format strings.
> > 3. Write a function like
> > [`format_ref_array_item()`](https://github.com/gitgitgadget/git/blob/84d06cdc06389ae7c462434cb7b1db0980f63860/ref-filter.c#L2392),
> > get information about objects, and use `get_object()` to grub the
> > information which we prefer (or just use `grab_common_value()`).
> > 4. The migration of `%(rest)` may require learning the handling of
> > `%(if)` ,`%(else)`.
>
> I think one thing to keep an eye on here is the performance of cat-file.
> The formatting code used by for-each-ref is rather slow (it may load
> more of the object details than is necessary, it is too eager to
> allocate intermediate strings, and so on). That's usually not _too_ big
> a problem for ref-filter, because the number of refs tends to be much
> smaller than the number of total objects. But I'd expect that moving to
> the ref-filter code would make something like:
>
>  git cat-file --batch-all-objects --batch-check='%(objectname) %(objecttype)'
>
> measurably slower.
>

> IMHO the right path forward is not to try porting cat-file to use the
> ref-filter code, but to start first with writing a universal formatting
> module that takes the best of both implementations (and the commit
> pretty-printer):
>
>   - separate the format-parsing stage from formatting actual items, as
>     ref-filter does. This lets us have more complex formats without
>     paying a per-item runtime cost while formatting. This should also
>     allow us to handle multiple syntaxes for the same thing (e.g.,
>     ref-filter %(authorname) vs pretty.c %an).
>

>   - figure out which data will be needed for each item based on the
>     parsed format, and then do the minimum amount of work to get that
>     data (using "oid_object_info_extended()" helps here, because it
>     likewise tries to do as little work as possible to satisfy the
>     request, but there are many elements that it doesn't know about)
>

>   - likewise avoid doing any intermediate work we can; as much as
>     possible, format the result directly into a result strbuf, rather
>     than allocating many sub-strings and assembling them (as cat-file
>     does).
>

>   - handle formats where the necessary item data may or may not be
>     present. E.g., if we're given a refname, then "%(refname)" makes
>     sense. But in cat-file we'd not have a refname, and just an object.
>     We should still be able to use the same formatting code to handle
>     "%(objecttype)", etc. Likewise for formats which require a specific
>     type (say %(authorname) for a commit, but the object is a blob).
>     Ref-filter does this to some degree for things like authorname, but
>     we'd be extending it to the case that we don't even have a refname.
>