KKcorps commented on PR #18733:
URL: https://github.com/apache/pinot/pull/18733#issuecomment-6336601328
This comes from a model of upsert thread orders that StarTree runs on upsert
PRs.
**This clears none of the model's known bad orders and adds one: the row
with a lower comparison value that `doAddRecord` now accepts for an expired key
is undone when its segment is committed or loaded again.**
I replayed the model's 45 known bad orders with and without the change, and
none changed. A 62 minute search, 402 runs over 10 metadataTTL table shapes,
then found 44 runs that break only with the change. All but one are this order,
and the last is a smaller merge edge after a consuming segment is reset.
**The order it adds.** Only the consuming path has the new branch.
`doAddOrReplaceSegment` still compares, so at commit the expired row takes the
key back. It needs no race, only a late row for a key that is already past
metadataTTL.
```mermaid
sequenceDiagram
participant S as Stream
participant C as Consuming replica
participant L as Server loading N1
S->>C: A at 4, watermark is now 4
S->>C: B at 1, doc 0 of segment N1
S->>C: B at 0, doc 1 of N1
C->>C: B at 1 is below 4 minus metadataTTL 2, so doc 1 wins with no
compare
Note over C: serves B at 0
C->>L: N1 commits, then replaceSegment or addSegment walks its docs
L->>L: doc 0, B at 1, takes the key
L->>L: doc 1, B at 0, loses the compare to doc 0
Note over L: serves B at 1 again
```
In the new test the late row lands in a later segment, so the commit keeps
it. A new replica, or a restart that does not preload from an up to date
snapshot, still serves the expired row, and replicas disagree until the key
gets a newer row.
<details><summary>Model history</summary>
```
('append', 'A', (4,), 0, {'v': 0}, False)
('commit_remote', 0, 0)
('append', 'B', (1,), 1, {'v': 0}, False)
('append', 'B', (0,), 2, {'v': 0}, False)
('commit_remote', 1, 2)
('add_immutable', 'N0')
('step', 'add_immutable', 'N0', 'start_add')
('step', 'add_immutable', 'N0', 'add_row:A@0')
('add_immutable', 'N1')
('append', 'A', (0,), 3, {'v': 0}, False)
('step', 'add_immutable', 'N0', 'register')
('step', 'add_immutable', 'N1', 'start_add')
('step', 'add_immutable', 'N1', 'add_row:B@1')
('step', 'add_immutable', 'N1', 'add_row:B@2')
('step', 'add_immutable', 'N1', 'register')
```
</details>
The model covers one replica and the handoff slice of one partition, and it
searched only metadataTTL tables.
<!-- upsert-model-check head=5085afb0e358ca0a8f783eff120124ae54a83847 -->
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