fix: clamp max_age metrics at 0

After the change in #89 to have `delayed` surface DB-accurate max_age
metrics, it appears that PostgreSQL will occasionally report a `run_at`
that is milliseconds _after_ the current NOW() in UTC time.

This is more than just clock drift between the DB and the server that
inserted the row -- in READ COMMITTED mode, statements can see rows
committed after the start of the transaction, while NOW() is frozen to
the transaction start time. (Even without an explicit `BEGIN`, bare
statements are wrapped in an implicit transaction.)

This PR:
- Switches PostgreSQL to `STATEMENT_TIMESTAMP()` to get even closer to
  the time at which the `SELECT` actually began.
- Clamps at 0 anyways because clock drift is still a possibility.

/no-platform

stack-info: PR: #94, branch: smudge/stack/5

Author: smudge

lib/delayed/monitor.rb

@@ -38,7 +38,7 @@ def query_for(metric)
     def self.sql_now_in_utc
       case ActiveRecord::Base.connection.adapter_name
       when 'PostgreSQL'
-        "TIMEZONE('UTC', NOW())"
+        "TIMEZONE('UTC', STATEMENT_TIMESTAMP())"
       when 'MySQL', 'Mysql2'
         "UTC_TIMESTAMP()"
       else
@@ -142,16 +142,16 @@ def failed_count_grouped
     end
 
     def max_lock_age_grouped
-      oldest_locked_at_query.transform_values { |j| db_now(j) - j.locked_at }
+      oldest_locked_at_query.transform_values { |j| time_ago(db_now(j), j.locked_at) }
     end
 
     def max_age_grouped
-      oldest_run_at_query.transform_values { |j| db_now(j) - j.run_at }
+      oldest_run_at_query.transform_values { |j| time_ago(db_now(j), j.run_at) }
     end
 
     def alert_age_percent_grouped
       oldest_run_at_query.each_with_object({}) do |((priority, queue), j), metrics|
-        max_age = db_now(j) - j.run_at
+        max_age = time_ago(db_now(j), j.run_at)
         alert_age = Priority.new(priority).alert_age
         metrics[[priority, queue]] = [max_age / alert_age * 100, 100].min if alert_age
       end
@@ -183,6 +183,10 @@ def db_now(record)
       self.class.parse_utc_time(record.db_now_utc)
     end
 
+    def time_ago(now, value)
+      [now - (value || now), 0].max
+    end
+
     def priority_case_statement
       [
         'CASE',

spec/delayed/__snapshots__/monitor_spec.rb.snap

@@ -149,7 +149,7 @@ GroupAggregate  (cost=...)
 ---------------------------------
 SELECT (CASE WHEN priority < 10 THEN 0 WHEN priority < 20 THEN 10 WHEN priority < 30 THEN 20 WHEN priority >= 30 THEN 30 END) AS priority,
        queue, MIN(locked_at) AS locked_at,
-       TIMEZONE('UTC', NOW()) AS db_now_utc
+       TIMEZONE('UTC', STATEMENT_TIMESTAMP()) AS db_now_utc
   FROM (SELECT priority, queue, MIN(locked_at) AS locked_at
   FROM \"delayed_jobs\"
   WHERE \"delayed_jobs\".\"locked_at\" >= '2025-11-10 16:59:43'
@@ -159,7 +159,7 @@ SELECT (CASE WHEN priority < 10 THEN 0 WHEN priority < 20 THEN 10 WHEN priority
   GROUP BY CASE WHEN priority < 10 THEN 0 WHEN priority < 20 THEN 10 WHEN priority < 30 THEN 20 WHEN priority >= 30 THEN 30 END, \"queue\"
 
 GroupAggregate  (cost=...)
-  Output: (CASE WHEN (subquery.priority < 10) THEN 0 WHEN (subquery.priority < 20) THEN 10 WHEN (subquery.priority < 30) THEN 20 WHEN (subquery.priority >= 30) THEN 30 ELSE NULL::integer END), subquery.queue, min(subquery.locked_at), timezone('UTC'::text, now())
+  Output: (CASE WHEN (subquery.priority < 10) THEN 0 WHEN (subquery.priority < 20) THEN 10 WHEN (subquery.priority < 30) THEN 20 WHEN (subquery.priority >= 30) THEN 30 ELSE NULL::integer END), subquery.queue, min(subquery.locked_at), timezone('UTC'::text, statement_timestamp())
   Group Key: (CASE WHEN (subquery.priority < 10) THEN 0 WHEN (subquery.priority < 20) THEN 10 WHEN (subquery.priority < 30) THEN 20 WHEN (subquery.priority >= 30) THEN 30 ELSE NULL::integer END), subquery.queue
   ->  Sort  (cost=...)
         Output: (CASE WHEN (subquery.priority < 10) THEN 0 WHEN (subquery.priority < 20) THEN 10 WHEN (subquery.priority < 30) THEN 20 WHEN (subquery.priority >= 30) THEN 30 ELSE NULL::integer END), subquery.queue, subquery.locked_at
@@ -181,7 +181,7 @@ GroupAggregate  (cost=...)
 ---------------------------------
 SELECT (CASE WHEN priority < 10 THEN 0 WHEN priority < 20 THEN 10 WHEN priority < 30 THEN 20 WHEN priority >= 30 THEN 30 END) AS priority,
        queue, MIN(run_at) AS run_at,
-       TIMEZONE('UTC', NOW()) AS db_now_utc
+       TIMEZONE('UTC', STATEMENT_TIMESTAMP()) AS db_now_utc
   FROM (SELECT priority, queue, MIN(run_at) AS run_at
   FROM \"delayed_jobs\"
   WHERE (\"delayed_jobs\".\"locked_at\" IS NULL
@@ -192,7 +192,7 @@ SELECT (CASE WHEN priority < 10 THEN 0 WHEN priority < 20 THEN 10 WHEN priority
   GROUP BY CASE WHEN priority < 10 THEN 0 WHEN priority < 20 THEN 10 WHEN priority < 30 THEN 20 WHEN priority >= 30 THEN 30 END, \"queue\"
 
 GroupAggregate  (cost=...)
-  Output: (CASE WHEN (subquery.priority < 10) THEN 0 WHEN (subquery.priority < 20) THEN 10 WHEN (subquery.priority < 30) THEN 20 WHEN (subquery.priority >= 30) THEN 30 ELSE NULL::integer END), subquery.queue, min(subquery.run_at), timezone('UTC'::text, now())
+  Output: (CASE WHEN (subquery.priority < 10) THEN 0 WHEN (subquery.priority < 20) THEN 10 WHEN (subquery.priority < 30) THEN 20 WHEN (subquery.priority >= 30) THEN 30 ELSE NULL::integer END), subquery.queue, min(subquery.run_at), timezone('UTC'::text, statement_timestamp())
   Group Key: (CASE WHEN (subquery.priority < 10) THEN 0 WHEN (subquery.priority < 20) THEN 10 WHEN (subquery.priority < 30) THEN 20 WHEN (subquery.priority >= 30) THEN 30 ELSE NULL::integer END), subquery.queue
   ->  Sort  (cost=...)
         Output: (CASE WHEN (subquery.priority < 10) THEN 0 WHEN (subquery.priority < 20) THEN 10 WHEN (subquery.priority < 30) THEN 20 WHEN (subquery.priority >= 30) THEN 30 ELSE NULL::integer END), subquery.queue, subquery.run_at
@@ -408,7 +408,7 @@ GroupAggregate  (cost=...)
 ---------------------------------
 SELECT (CASE WHEN priority < 10 THEN 0 WHEN priority < 20 THEN 10 WHEN priority < 30 THEN 20 WHEN priority >= 30 THEN 30 END) AS priority,
        queue, MIN(locked_at) AS locked_at,
-       TIMEZONE('UTC', NOW()) AS db_now_utc
+       TIMEZONE('UTC', STATEMENT_TIMESTAMP()) AS db_now_utc
   FROM (SELECT priority, queue, MIN(locked_at) AS locked_at
   FROM \"delayed_jobs\"
   WHERE \"delayed_jobs\".\"locked_at\" >= '2025-11-10 16:59:43'
@@ -418,7 +418,7 @@ SELECT (CASE WHEN priority < 10 THEN 0 WHEN priority < 20 THEN 10 WHEN priority
   GROUP BY CASE WHEN priority < 10 THEN 0 WHEN priority < 20 THEN 10 WHEN priority < 30 THEN 20 WHEN priority >= 30 THEN 30 END, \"queue\"
 
 GroupAggregate  (cost=...)
-  Output: (CASE WHEN (subquery.priority < 10) THEN 0 WHEN (subquery.priority < 20) THEN 10 WHEN (subquery.priority < 30) THEN 20 WHEN (subquery.priority >= 30) THEN 30 ELSE NULL::integer END), subquery.queue, min(subquery.locked_at), timezone('UTC'::text, now())
+  Output: (CASE WHEN (subquery.priority < 10) THEN 0 WHEN (subquery.priority < 20) THEN 10 WHEN (subquery.priority < 30) THEN 20 WHEN (subquery.priority >= 30) THEN 30 ELSE NULL::integer END), subquery.queue, min(subquery.locked_at), timezone('UTC'::text, statement_timestamp())
   Group Key: (CASE WHEN (subquery.priority < 10) THEN 0 WHEN (subquery.priority < 20) THEN 10 WHEN (subquery.priority < 30) THEN 20 WHEN (subquery.priority >= 30) THEN 30 ELSE NULL::integer END), subquery.queue
   ->  Sort  (cost=...)
         Output: (CASE WHEN (subquery.priority < 10) THEN 0 WHEN (subquery.priority < 20) THEN 10 WHEN (subquery.priority < 30) THEN 20 WHEN (subquery.priority >= 30) THEN 30 ELSE NULL::integer END), subquery.queue, subquery.locked_at
@@ -440,7 +440,7 @@ GroupAggregate  (cost=...)
 ---------------------------------
 SELECT (CASE WHEN priority < 10 THEN 0 WHEN priority < 20 THEN 10 WHEN priority < 30 THEN 20 WHEN priority >= 30 THEN 30 END) AS priority,
        queue, MIN(run_at) AS run_at,
-       TIMEZONE('UTC', NOW()) AS db_now_utc
+       TIMEZONE('UTC', STATEMENT_TIMESTAMP()) AS db_now_utc
   FROM (SELECT priority, queue, MIN(run_at) AS run_at
   FROM \"delayed_jobs\"
   WHERE (\"delayed_jobs\".\"locked_at\" IS NULL
@@ -451,7 +451,7 @@ SELECT (CASE WHEN priority < 10 THEN 0 WHEN priority < 20 THEN 10 WHEN priority
   GROUP BY CASE WHEN priority < 10 THEN 0 WHEN priority < 20 THEN 10 WHEN priority < 30 THEN 20 WHEN priority >= 30 THEN 30 END, \"queue\"
 
 GroupAggregate  (cost=...)
-  Output: (CASE WHEN (subquery.priority < 10) THEN 0 WHEN (subquery.priority < 20) THEN 10 WHEN (subquery.priority < 30) THEN 20 WHEN (subquery.priority >= 30) THEN 30 ELSE NULL::integer END), subquery.queue, min(subquery.run_at), timezone('UTC'::text, now())
+  Output: (CASE WHEN (subquery.priority < 10) THEN 0 WHEN (subquery.priority < 20) THEN 10 WHEN (subquery.priority < 30) THEN 20 WHEN (subquery.priority >= 30) THEN 30 ELSE NULL::integer END), subquery.queue, min(subquery.run_at), timezone('UTC'::text, statement_timestamp())
   Group Key: (CASE WHEN (subquery.priority < 10) THEN 0 WHEN (subquery.priority < 20) THEN 10 WHEN (subquery.priority < 30) THEN 20 WHEN (subquery.priority >= 30) THEN 30 ELSE NULL::integer END), subquery.queue
   ->  Sort  (cost=...)
         Output: (CASE WHEN (subquery.priority < 10) THEN 0 WHEN (subquery.priority < 20) THEN 10 WHEN (subquery.priority < 30) THEN 20 WHEN (subquery.priority >= 30) THEN 30 ELSE NULL::integer END), subquery.queue, subquery.run_at

spec/helper.rb

@@ -228,10 +228,7 @@ def supports_block_expectations?
   diffable
 
   match do |block|
-    if @approximately && current_adapter != 'postgresql'
-      @expected_value = a_value_within([2, @expected_value.abs * 0.05].max).of(@expected_value)
-    end
-
+    @expected_value = a_value_within([2, @expected_value.abs * 0.05].max).of(@expected_value) if @approximately
     @expected = { event_name: expected_event_name, payload: expected_payload, value: @expected_value }
     @actuals = []
     callback = ->(name, _started, _finished, _unique_id, payload) do