Fix policy refresh window too small

When adding a CAgg refresh policy using `start_offset => '2 months'` and
`end_offset => '0 months'` was failing because the policy refresh window
is too small and it should cover at least two buckets in the valid time
range of timestamptz.

The problem was when calculating the bucket width for the variable
bucket size (like months) we're assuming 31 days for each month but when
converting the end offset to integer using `interval_to_int64` we use 30
days per month. Fixed it by aligning the variable bucket size
calculation to also use 30 days.

tsl/src/bgw_policy/continuous_aggregate_api.c

@@ -443,7 +443,7 @@ validate_window_size(const ContinuousAgg *cagg, const CaggPolicyConfig *config)
 		 * 2. Buckets with timezones
 		 * 3. Cases 1 and 2 at the same time
 		 *
-		 * For months we simply take 31 days as the worst case scenario and
+		 * For months we simply take 30 days like on interval_to_int64 and
 		 * multiply this number by the number of months in the bucket. This
 		 * reduces the task to days/hours/minutes scenario.
 		 *
@@ -459,7 +459,7 @@ validate_window_size(const ContinuousAgg *cagg, const CaggPolicyConfig *config)
 
 		/* Make a temporary copy of bucket_width */
 		Interval interval = *cagg->bucket_function->bucket_time_width;
-		interval.day += 31 * interval.month;
+		interval.day += 30 * interval.month;
 		interval.month = 0;
 		bucket_width = ts_interval_value_to_internal(IntervalPGetDatum(&interval), INTERVALOID);
 	}

tsl/test/expected/exp_cagg_monthly.out

@@ -1166,9 +1166,36 @@ ERROR:  policy refresh window too small
 SELECT add_continuous_aggregate_policy('conditions_summary_policy',
     start_offset => INTERVAL '65 days',
     end_offset => INTERVAL '1 day',
+    schedule_interval => INTERVAL '1 hour') AS job_id \gset
+SELECT delete_job(:job_id);
+ delete_job 
+------------
+
+(1 row)
+
+SELECT add_continuous_aggregate_policy('conditions_summary_policy',
+    start_offset => INTERVAL '2 months',
+    end_offset => INTERVAL '0 months',
+    schedule_interval => INTERVAL '1 hour') AS job_id \gset
+SELECT delete_job(:job_id);
+ delete_job 
+------------
+
+(1 row)
+
+\set ON_ERROR_STOP 0
+SELECT add_continuous_aggregate_policy('conditions_summary_policy',
+    start_offset => INTERVAL '2 months',
+    end_offset => INTERVAL '1 months',
+    schedule_interval => INTERVAL '1 hour');
+ERROR:  policy refresh window too small
+\set ON_ERROR_STOP 1
+SELECT add_continuous_aggregate_policy('conditions_summary_policy',
+    start_offset => INTERVAL '3 months',
+    end_offset => INTERVAL '1 months',
     schedule_interval => INTERVAL '1 hour');
  add_continuous_aggregate_policy 
 ---------------------------------
-                            1000
+                            1002
 (1 row)
 

tsl/test/sql/exp_cagg_monthly.sql

@@ -544,4 +544,25 @@ SELECT add_continuous_aggregate_policy('conditions_summary_policy',
 SELECT add_continuous_aggregate_policy('conditions_summary_policy',
     start_offset => INTERVAL '65 days',
     end_offset => INTERVAL '1 day',
+    schedule_interval => INTERVAL '1 hour') AS job_id \gset
+
+SELECT delete_job(:job_id);
+
+SELECT add_continuous_aggregate_policy('conditions_summary_policy',
+    start_offset => INTERVAL '2 months',
+    end_offset => INTERVAL '0 months',
+    schedule_interval => INTERVAL '1 hour') AS job_id \gset
+
+SELECT delete_job(:job_id);
+
+\set ON_ERROR_STOP 0
+SELECT add_continuous_aggregate_policy('conditions_summary_policy',
+    start_offset => INTERVAL '2 months',
+    end_offset => INTERVAL '1 months',
+    schedule_interval => INTERVAL '1 hour');
+\set ON_ERROR_STOP 1
+
+SELECT add_continuous_aggregate_policy('conditions_summary_policy',
+    start_offset => INTERVAL '3 months',
+    end_offset => INTERVAL '1 months',
     schedule_interval => INTERVAL '1 hour');