fix(deletions): Delete seer matched group hash metadata first (#102612)

When deleting a GroupHash row, all GroupHashMetadata rows pointing to it
via `seer_matched_grouphash` need updating (see code):

https://github.com/getsentry/sentry/blob/698262018e6009759d8562e2da63be749df7c32d/src/sentry/models/grouphashmetadata.py#L115-L118

Before #101720, we would only delete GroupHash rows and that would time
out because we would stomp queries longer than 30 seconds. In #101720 we
added the deletion of the GroupHashMetadata rows but we should have also
added the updating.

The new code will have these three stages:
```
GroupHashMetadata.objects.filter(seer_matched_grouphash_id__in=hash_ids).update(seer_matched_grouphash=None)
GroupHashMetadata.objects.filter(grouphash_id__in=hash_ids).delete()
GroupHash.objects.filter(id__in=hash_ids).delete()
```

Fixes [SENTRY-5ABJ](https://sentry.sentry.io/issues/6930113529/).

For posterity, this is the top of the stack trace:
```
OperationalError
canceling statement due to user request

SQL: UPDATE "sentry_grouphashmetadata" SET "seer_matched_grouphash_id" = NULL WHERE "sentry_grouphashmetadata"."seer_matched_grouphash_id" IN (%s, %s, %s, %s, %s, %s, %s, %s, %s, %s, %s, %s, %s, %s, %s, %s, %s, %s, %s, %s, %s, %s, %s, %s, %s, %s, %s, %s, %s, %s, %s, %s, %s, %s, %s, %s, %s, %s, %s, %s, %s, %s, %s, %s, %s, %s, %s, %s, %s, %s, %s, %s, %s, %s, %s, %s, %s, %s, %s, %s, %s, %s, %s, %s, %s, %s, %s, %s, %s, %s, %s, %s, %s, %s, %s, %s, %s, %s, %s, %s, %s, %s, %s, %s, %s, %s, %s, %s, %s, %s, %s, %s, %s, %s, %s, %s, %s, %s, %s, %s)
```

Author: armenzg

src/sentry/deletions/defaults/group.py

@@ -258,20 +258,16 @@ def delete_group_hashes(
             logger.warning("Error scheduling task to delete hashes from seer")
         finally:
             hash_ids = [gh[0] for gh in hashes_chunk]
-            # If we delete the grouphash metadata rows first we will not need to update the references to the other grouphashes.
-            # If we try to delete the group hashes first, then it will require the updating of the columns first.
-            #
-            # To understand this, let's say we have the following relationships:
-            # gh A -> ghm A -> no reference to another grouphash
-            # gh B -> ghm B -> gh C
-            # gh C -> ghm C -> gh A
-            #
-            # Deleting group hashes A, B & C (since they all point to the same group) will require:
-            # * Updating columns ghmB & ghmC to point to None
-            # * Deleting the group hash metadata rows
-            # * Deleting the group hashes
-            #
-            # If we delete the metadata first, we will not need to update the columns before deleting them.
+            # GroupHashMetadata rows can reference GroupHash rows via seer_matched_grouphash_id.
+            # Before deleting these GroupHash rows, we need to either:
+            # 1. Update seer_matched_grouphash to None first (to avoid foreign key constraint errors), OR
+            # 2. Delete the GroupHashMetadata rows entirely (they'll be deleted anyway)
+            # If we update the columns first, the deletion of the grouphash metadata rows will have less work to do,
+            # thus, improving the performance of the deletion.
+            if options.get("deletions.group-hashes-metadata.update-seer-matched-grouphash-ids"):
+                GroupHashMetadata.objects.filter(seer_matched_grouphash_id__in=hash_ids).update(
+                    seer_matched_grouphash=None
+                )
             GroupHashMetadata.objects.filter(grouphash_id__in=hash_ids).delete()
             GroupHash.objects.filter(id__in=hash_ids).delete()
 

src/sentry/options/defaults.py

@@ -342,6 +342,12 @@
     type=Int,
     flags=FLAG_AUTOMATOR_MODIFIABLE,
 )
+register(
+    "deletions.group-hashes-metadata.update-seer-matched-grouphash-ids",
+    default=False,
+    type=Bool,
+    flags=FLAG_AUTOMATOR_MODIFIABLE,
+)
 
 register(
     "deletions.group-history.use-bulk-deletion",