Re: [PATCH 12/15] [GSOC] cat-file: reuse ref-filter logic

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> On Thu, Jul 01 2021, ZheNing Hu via GitGitGadget wrote:
>
> > From: ZheNing Hu <adlternative@gmail.com>
> >
> > In order to let cat-file use ref-filter logic, let's do the
> > following:
> >
> > 1. Change the type of member `format` in struct `batch_options`
> > to `ref_format`, we will pass it to ref-filter later.
> > 2. Let `batch_objects()` add atoms to format, and use
> > `verify_ref_format()` to check atoms.
> > 3. Use `format_ref_array_item()` in `batch_object_write()` to
> > get the formatted data corresponding to the object. If the
> > return value of `format_ref_array_item()` is equals to zero,
> > use `batch_write()` to print object data; else if the return
> > value is less than zero, use `die()` to print the error message
> > and exit; else if return value is greater than zero, only print
> > the error message, but don't exit.
> > 4. Use free_ref_array_item_value() to free ref_array_item's
> > value.
> >
> > Most of the atoms in `for-each-ref --format` are now supported,
> > such as `%(tree)`, `%(parent)`, `%(author)`, `%(tagger)`, `%(if)`,
> > `%(then)`, `%(else)`, `%(end)`. But these atoms will be rejected:
> > `%(refname)`, `%(symref)`, `%(upstream)`, `%(push)`, `%(worktreepath)`,
> > `%(flag)`, `%(HEAD)`, because these atoms are unique to those objects
> > that pointed to by a ref, "for-each-ref"'s family can naturally use
> > these atoms, but not all objects are pointed to be a ref, so "cat-file"
> > will not be able to use them.
> >
> > The performance for `git cat-file --batch-all-objects
> > --batch-check` on the Git repository itself with performance
> > testing tool `hyperfine` changes from 669.4 ms ±  31.1 ms to
> > 1.134 s ±  0.063 s.
> >
> > The performance for `git cat-file --batch-all-objects --batch
> >>/dev/null` on the Git repository itself with performance testing
> > tool `time` change from "27.37s user 0.29s system 98% cpu 28.089
> > total" to "33.69s user 1.54s system 87% cpu 40.258 total".
>
> This new feature is really nice, but that's a really bad performance
> regression. A lot of software in the wild relies on "cat-file --batch"
> to be *the* performant interface to git for mass-extrction of object
> data.
>