fix(api): apply findings partition max age in months, not days (#12580)

api/changelog.d/findings-partition-max-age-unit.fixed.md

@@ -0,0 +1 @@
+`FINDINGS_TABLE_PARTITION_MAX_AGE_MONTHS` is now applied in months instead of days, and negative values are rejected

api/src/backend/api/partitions.py

@@ -6,6 +6,7 @@ from api.rls import RowLevelSecurityConstraint
 from api.uuid_utils import datetime_to_uuid7
 from dateutil.relativedelta import relativedelta
 from django.conf import settings
+from django.core.exceptions import ImproperlyConfigured
 from psqlextra.partitioning import (
     PostgresPartitioningError,
     PostgresPartitioningManager,
@@ -153,10 +154,17 @@ class PostgresUUIDv7PartitioningStrategy(PostgresRangePartitioningStrategy):
         )
 
 
-def relative_days_or_none(value):
-    if value is None:
+def relative_months_or_none(value):
+    # A negative value would set the cutoff in the future and delete every
+    # partition, so it is rejected rather than silently ignored.
+    if value is not None and value < 0:
+        raise ImproperlyConfigured(
+            "FINDINGS_TABLE_PARTITION_MAX_AGE_MONTHS must not be negative; "
+            "leave it unset or use 0 to keep partitions indefinitely"
+        )
+    if not value:
         return None
-    return relativedelta(days=value)
+    return relativedelta(months=value)
 
 
 #
@@ -173,7 +181,7 @@ manager = PostgresPartitioningManager(
                     months=settings.FINDINGS_TABLE_PARTITION_MONTHS
                 ),
                 count=settings.FINDINGS_TABLE_PARTITION_COUNT,
-                max_age=relative_days_or_none(
+                max_age=relative_months_or_none(
                     settings.FINDINGS_TABLE_PARTITION_MAX_AGE_MONTHS
                 ),
                 name_format="%Y_%b",
@@ -189,7 +197,7 @@ manager = PostgresPartitioningManager(
                     months=settings.FINDINGS_TABLE_PARTITION_MONTHS
                 ),
                 count=settings.FINDINGS_TABLE_PARTITION_COUNT,
-                max_age=relative_days_or_none(
+                max_age=relative_months_or_none(
                     settings.FINDINGS_TABLE_PARTITION_MAX_AGE_MONTHS
                 ),
                 name_format="%Y_%b",

api/src/backend/api/tests/test_partitions.py

@@ -0,0 +1,63 @@
+from datetime import UTC, datetime
+from itertools import islice
+
+import pytest
+from api.partitions import (
+    PostgresUUIDv7PartitioningStrategy,
+    relative_months_or_none,
+)
+from dateutil.relativedelta import relativedelta
+from django.core.exceptions import ImproperlyConfigured
+from psqlextra.partitioning import PostgresTimePartitionSize
+
+
+def build_strategy(max_age):
+    return PostgresUUIDv7PartitioningStrategy(
+        size=PostgresTimePartitionSize(months=1),
+        count=1,
+        start_date=datetime.now(UTC),
+        max_age=max_age,
+        name_format="%Y_%b",
+    )
+
+
+class TestRelativeMonthsOrNone:
+    @pytest.mark.parametrize("value", [None, 0])
+    def test_unset_or_zero_keeps_partitions_indefinitely(self, value):
+        assert relative_months_or_none(value) is None
+
+    @pytest.mark.parametrize("months", [1, 3, 12])
+    def test_value_is_interpreted_as_months(self, months):
+        assert relative_months_or_none(months) == relativedelta(months=months)
+
+    def test_value_is_not_interpreted_as_days(self):
+        assert relative_months_or_none(12) != relativedelta(days=12)
+
+    def test_negative_is_rejected(self):
+        with pytest.raises(ImproperlyConfigured):
+            relative_months_or_none(-12)
+
+
+class TestToDelete:
+    @pytest.mark.parametrize("max_age", [None, relative_months_or_none(0)])
+    def test_nothing_is_deleted_without_max_age(self, max_age):
+        strategy = build_strategy(max_age)
+
+        assert list(islice(strategy.to_delete(), 5)) == []
+
+    def test_first_deleted_partition_is_max_age_old(self):
+        months = 3
+        strategy = build_strategy(relative_months_or_none(months))
+
+        first = next(strategy.to_delete())
+
+        expected = strategy.get_start_datetime() - relativedelta(months=months)
+        assert first.name() == expected.strftime("%Y_%b").lower()
+
+    def test_deleted_partitions_go_further_back_in_time(self):
+        strategy = build_strategy(relative_months_or_none(3))
+
+        names = [p.name() for p in islice(strategy.to_delete(), 3)]
+        starts = [datetime.strptime(n, "%Y_%b") for n in names]
+
+        assert starts == sorted(starts, reverse=True)