Re: [PATCH v4] ref-filter: restore prefix-scoped iteration

>
> dabecb9db2 (for-each-ref: introduce a '--start-after' option,
> 2025-07-15) changed branch, remote-tracking branch, and tag enumeration
> from constructing an iterator with the namespace prefix to constructing
> an unscoped iterator and seeking to the prefix.
>
> Review of --start-after noted that the construction prefix and seek
> position represent different state and are easy to conflate [1]. It also
> noted that future branch or tag support would need to retain the
> namespace prefix while moving the cursor [2].
>
> The files backend constructs its loose-ref iterator with cache priming
> enabled. cache_ref_iterator_begin() immediately applies the construction
> prefix through cache_ref_iterator_set_prefix(), reading loose refs
> beneath it before packed refs are opened. An empty prefix therefore
> reads every loose ref, and a later seek cannot undo that I/O.
>
> For the current single-kind filters, construct the iterator with the
> namespace prefix when start_after is not set. Leave the existing
> start_after path unchanged; no current command combines it with these
> filters, and future support must carry the prefix separately from the
> cursor.
>
> With 10,000 unrelated loose refs in the files backend, the p6300 tests
> improve as follows:
>
>                          before   after
>   branch                  2.74 s   0.11 s
>   branch --remotes        2.81 s   0.12 s
>   tag                     3.01 s   0.11 s
>
> [1] https://lore.kernel.org/r/aGZidwwlToWThkn8@pks.im/
> [2] https://lore.kernel.org/r/xmqqikjq7s16.fsf@gitster.g/
>
> Fixes: dabecb9db2b2 ("for-each-ref: introduce a '--start-after' option")
> Suggested-by: Karthik Nayak <karthik.188@gmail.com>
> Signed-off-by: Tamir Duberstein <tamird@gmail.com>
> ---
> The series is based on a89346e34a (maint) because the regression has
> been present in released versions since Git 2.51.0.
> ---
> Changes in v4:
> - Explain the historical references in the commit message.
> - Run the new performance cases with both ref backends.
> - Drop the Assisted-by trailer.
> - Link to v3: https://patch.msgid.link/20260610-fix-git-branch-regression-v3-1-6fd48fad7a53@gmail.com
>
> Changes in v3:
> - Construct the iterator directly with the namespace prefix.
> - Explain when the files backend primes its loose-ref cache.
> - Condense the commit message and performance results.
> - Link to v2: https://patch.msgid.link/20260608-fix-git-branch-regression-v2-1-fd82075a8520@gmail.com
>
> Changes in v2:
> - Extract local variable `store`.
> - Link to v1: https://patch.msgid.link/20260605-fix-git-branch-regression-v1-1-02f40ad40929@gmail.com
> ---