fix(aci): Handle incorrect first_seen times (#93860)

Some groups have resolution/regression activities associated with the
group that take place before the group's first seen. We can skip those
events and start the open periods at the first_seen time, so the first
open period would be `{"date_started": group.first_seen, "date_ended":
first resolution activity > group.first_seen}`.

Author: snigdhas

src/sentry/migrations/0925_backfill_open_periods.py

@@ -67,9 +67,15 @@ def get_open_periods_for_group(
             )
         ]
 
+    # Since activities can apparently exist from before the start date, we want to ensure the
+    # first open period starts at the first_seen date and ends at the first resolution activity after it.
+    start_index = 0
+    while activities and activities[start_index].type not in RESOLVED_ACTIVITY_TYPES:
+        start_index += 1
+
     open_periods = []
     regression_time: datetime | None = first_seen
-    for activity in activities:
+    for activity in activities[start_index:]:
         if activity.type == ActivityType.SET_REGRESSION.value and regression_time is None:
             regression_time = activity.datetime
 
@@ -134,6 +140,10 @@ def _backfill_group_open_periods(
         group_id__in=group_ids,
         type__in=[ActivityType.SET_REGRESSION.value, *RESOLVED_ACTIVITY_TYPES],
     ).order_by("datetime"):
+        # Skip activities before the group's first_seen date
+        if activity.datetime < activity.group.first_seen:
+            continue
+
         activities[activity.group_id].append(activity)
 
     open_periods = []

tests/sentry/migrations/test_0925_backfill_open_periods.py

@@ -312,6 +312,21 @@ def setup_before_migration(self, app):
             ("regressed_group_with_auto_resolved_cycles", group, starts, ends, activities)
         )
 
+        # Create a group with activities before the first_seen date
+        group, _, _, activities = self._create_resolved_group()
+        activities[0].datetime = group.first_seen - timedelta(days=4)
+        activities[0].save()
+
+        self.test_cases.append(
+            (
+                "resolved_group_with_activities_before_first_seen",
+                group,
+                [group.first_seen],
+                [activities[-1].datetime],
+                [activities[-1]],
+            )
+        )
+
     def test(self):
         for description, group, starts, ends, activities in self.test_cases:
             group.refresh_from_db()