Return last ring index instead of min_ring_index in prefetch_register_bufferv

[knizhnik]

Problem

See #12018

Now prefetch_register_bufferv calculates min_ring_index of all vector requests.
But because of pump prefetch state or connection failure, previous slots can be already proceeded and reused.

Summary of changes

Instead of returning minimal index, this function should return last slot index.
Actually result of this function is used only in two places. A first place just fort checking (and this check is redundant because the same check is done in prefetch_register_bufferv itself.
And in the second place where index of filled slot is actually used, there is just one request.
Sp fortunately this bug can cause only assert failure in debug build.

[github-actions]

If this PR added a GUC in the Postgres fork or neon extension,
please regenerate the Postgres settings in the cloud repo:

make NEON_WORKDIR=path/to/neon/checkout \
  -C goapp/internal/shareddomain/postgres generate

If you're an external contributor, a Neon employee will assist in
making sure this step is done.

[alexanderlaw]

@knizhnik , could you please clarify why can't this usage of prefetch_register_bufferv() affect the release build?:

                        if (entry == NULL)
                        {
                                ring_index = prefetch_register_bufferv(hashkey.buftag, reqlsns, 1, NULL, false);
                                Assert(ring_index != UINT64_MAX);
                                slot = GetPrfSlot(ring_index);
                        }

Doesn't it matter which slot received here?

[github-actions]

8481 tests run: 7899 passed, 0 failed, 582 skipped (full report)

---

Flaky tests (1)

Postgres 17

• test_timeline_offloading[False]: release-arm64-without-lfc

Code coverage* (full report)

• functions: 32.1% (9011 of 28033 functions)
• lines: 48.4% (80033 of 165360 lines)

* collected from Rust tests only

---

_{The comment gets automatically updated with the latest test results
c83bd28 at 2025-06-10T09:57:37.817Z :recycle:}

[knizhnik]

@knizhnik , could you please clarify why can't this usage of prefetch_register_bufferv() affect the release build?:

                        if (entry == NULL)
                        {
                                ring_index = prefetch_register_bufferv(hashkey.buftag, reqlsns, 1, NULL, false);
                                Assert(ring_index != UINT64_MAX);
                                slot = GetPrfSlot(ring_index);
                        }

Doesn't it matter which slot received here?

as you can see only one page is requested here. The problem ca happen only when there are more two or more requests: during processing of second request first slot can be invalidated.
But if some errors happens during processing of first slot, we just repeat operation and return the right slot.

[alexanderlaw]

I got it. Thank you for the explanation!

[MMeent]

LGTM