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. -Peff
