fix(grouping): Fix unmerged seer grouphash integrity error

[lobsterkatie]

We've been getting occasional integrity errors when trying to delete grouphash records, because of the seer_matched_grouphash foreign key in the GroupHashMetadata model. Normally, the process works like this:

1. An event creates grouphash_A and group_1.
2. A new event creates grouphash_B, which Seer then matches to grouphash_A. The new event goes in group_1, and grouphash_B.metadata.seer_matched_grouphash is set to `grouphash_A.
3. group_1 ages out, and we delete it. In this process deletions happen from the bottom up, so first the GroupHashMetadata records for both grouphash_A and grouphash_B are deleted, then ``grouphash_Aandgrouphash_B` are themselves deleted, and finally `group_1` is deleted. Because of that ordering, the `grouphash_B.metadata.seer_matched_grouphash` link to `grouphash_A` disappears before we try to delete `grouphash_A`, and no integrity errors are thrown.

However, occasionally it goes like this:

1. Same as above
2. Same as above
3. The user decides they disagree with Seer, and unmerges grouphash_B from group_1, such that it now points to group_2, which is newer than group_1 and therefore doesn't expire when group_1 does.
4. group_1 ages out, and deletion happens as before, except this time it's only the GroupHashMetadata record for grouphash_A which gets deleted ahead of grouphash_A's deletion. So even after grouphash_A is deleted, grouphash_B.metadata.seer_matched_grouphash still points to it, and boom: you've got yourself an integrity error.

This fixes the problem by updating the on_delete setting for the seer_matched_grouphash field to be SET_NULL, so that Django will automatically break the foreign key link whenever a seer_matched_grouphash is deleted. I chose to have that happen during deletion rather than unmerging because when unmerges happen it's nice for debugging to be able to compare Seer's result to the human-mediated one.

[JoshFerge]

would it be better to set on_delete=models.SET_NULL here instead?

sentry/src/sentry/models/grouphashmetadata.py

Line 81 in 72394d5

"sentry.GroupHash", related_name="seer_matchees", on_delete=models.DO_NOTHING, null=True

[lobsterkatie]

would it be better to set on_delete=models.SET_NULL here instead?

Good question. There's some disconnect between the on_delete value and what actually happens during deletion, though - setting on_delete=models.CASCADE for example doesn't actually work (which is why fixes like this one are necessary). But lemme try that and see if it works.

UPDATE: That worked! Have updated the PR.

[github-actions]

This PR has a migration; here is the generated SQL for src/sentry/migrations/0808_change_grouphash_metadata_seer_matched_grouphash_deletion_config.py ()

--
-- Alter field seer_matched_grouphash on grouphashmetadata
--
-- (no-op)

[wedamija]

lgtm, there shouldn't be too many GroupHashMetadata per GroupHash, correct?

[lobsterkatie]

It's technically possible for it to be a large number, but most often it's not.

Also, I assume (?) that the null setting happens only when delete is called on the linked object, yes? If so, then in cases where there's been no manual unmerge (the 99.9% case), all of the GroupHashMetadata records pointing at the GroupHash instance will already have been deleted by the time we get to the null setting, as they'll belong to sibling GroupHash records of the GroupHash record in question. (See point 3 in the first list in the PR description.) So only in the 0.1% case will there even be any remaining GroupHashMetadata records on which to update the seer_matched_grouphash value.

[JoshFerge]

looks great!