PG: Handle locks without relation like transactionid locks (DataDog#21393)

In postgres, a transaction can wait for another transaction to end by trying
to acquire a sharelock on the target transactionid's lock. We currently
exclude those due to the join on relname/schema/db as they have null
values.

This PR convert pg_locks to a dynamic query and handles locks with null
relname/schema/db.

Author: bonnefoa

postgres/changelog.d/21393.changed

@@ -0,0 +1 @@
+PG: Handle locks without relations like transactionid or virtualxid locks

postgres/datadog_checks/postgres/relationsmanager.py

@@ -29,16 +29,7 @@
 
 # The view pg_locks provides access to information about the locks held by active processes within the database server.
 LOCK_METRICS = {
-    'descriptors': [
-        ('mode', 'lock_mode'),
-        ('locktype', 'lock_type'),
-        ('nspname', 'schema'),
-        ('datname', 'db'),
-        ('relname', 'table'),
-        ('granted', 'granted'),
-        ('fastpath', 'fastpath'),
-    ],
-    'metrics': {'lock_count': ('locks', AgentCheck.gauge)},
+    'name': 'pg_locks',
     'query': """
 SELECT mode,
        locktype,
@@ -47,17 +38,25 @@
        pc.relname,
        granted,
        fastpath,
-       count(*) AS {metrics_columns}
+       count(*)
   FROM pg_locks l
-  JOIN pg_database pd ON (l.database = pd.oid)
-  JOIN pg_class pc ON (l.relation = pc.oid)
+  LEFT JOIN pg_database pd ON (l.database = pd.oid)
+  LEFT JOIN pg_class pc ON (l.relation = pc.oid)
   LEFT JOIN pg_namespace pn ON (pn.oid = pc.relnamespace)
- WHERE {relations}
+ WHERE (pc IS NULL OR ({relations} AND pc.relname NOT LIKE 'pg^_%%' ESCAPE '^'))
    AND l.mode IS NOT NULL
-   AND pc.relname NOT LIKE 'pg^_%%' ESCAPE '^'
- GROUP BY pd.datname, pc.relname, pn.nspname, locktype, mode, granted, fastpath""",
-    'relation': True,
-    'name': 'lock_metrics',
+ GROUP BY pd.datname, pc.relname, pn.nspname, locktype, mode, granted, fastpath
+""",
+    'columns': [
+        {'name': 'lock_mode', 'type': 'tag'},
+        {'name': 'lock_type', 'type': 'tag'},
+        {'name': 'schema', 'type': 'tag_not_null'},
+        {'name': 'db', 'type': 'tag_not_null'},
+        {'name': 'table', 'type': 'tag_not_null'},
+        {'name': 'granted', 'type': 'tag'},
+        {'name': 'fastpath', 'type': 'tag'},
+        {'name': 'locks', 'type': 'gauge'},
+    ],
 }
 
 
@@ -414,8 +413,8 @@
     'name': 'index_bloat_metrics',
 }
 
-RELATION_METRICS = [LOCK_METRICS, STATIO_METRICS]
-DYNAMIC_RELATION_QUERIES = [QUERY_PG_CLASS, QUERY_PG_CLASS_SIZE, IDX_METRICS]
+RELATION_METRICS = [STATIO_METRICS]
+DYNAMIC_RELATION_QUERIES = [QUERY_PG_CLASS, QUERY_PG_CLASS_SIZE, IDX_METRICS, LOCK_METRICS]
 
 
 class RelationsManager(object):

postgres/tests/test_relations.py

@@ -411,30 +411,86 @@ def test_vacuum_age(aggregator, integration_check, pg_instance):
             [{"relation_regex": "perso.*", "relkind": ["r"]}],
             1,
             "persons",
-            None,
+            [
+                "db:datadog_test",
+                "lock_mode:AccessExclusiveLock",
+                "lock_type:relation",
+                "granted:True",
+                "fastpath:False",
+                "table:persons",
+                "schema:public",
+            ],
             id="test with matching relkind should return 1",
         ),
         pytest.param(
             [{"relation_regex": "perso.*", "relkind": ["i"]}],
             0,
             "persons",
-            None,
+            [
+                "db:datadog_test",
+                "lock_mode:AccessExclusiveLock",
+                "lock_type:relation",
+                "granted:True",
+                "fastpath:False",
+                "table:persons",
+                "schema:public",
+            ],
             id="test without matching relkind should return 0",
         ),
         pytest.param(
             ["pgtable"],
             1,
             "pgtable",
-            None,
+            [
+                "db:datadog_test",
+                "lock_mode:AccessExclusiveLock",
+                "lock_type:relation",
+                "granted:True",
+                "fastpath:False",
+                "table:pgtable",
+                "schema:public",
+            ],
             id="pgtable should be included in lock metrics",
         ),
         pytest.param(
             ["pg_newtable"],
             0,
             "pg_newtable",
-            None,
+            [
+                "db:datadog_test",
+                "lock_mode:AccessExclusiveLock",
+                "lock_type:relation",
+                "granted:True",
+                "fastpath:False",
+                "table:pg_newtable",
+                "schema:public",
+            ],
             id="pg_newtable should be excluded from query since it starts with `pg_`",
         ),
+        pytest.param(
+            ['persons'],
+            1,
+            'persons',
+            [
+                "lock_mode:ExclusiveLock",
+                "lock_type:transactionid",
+                "granted:True",
+                "fastpath:False",
+            ],
+            id="transactionid lock should be visible",
+        ),
+        pytest.param(
+            ['persons'],
+            1,
+            'persons',
+            [
+                "lock_mode:ExclusiveLock",
+                "lock_type:virtualxid",
+                "granted:True",
+                "fastpath:True",
+            ],
+            id="virtualxid lock should be visible",
+        ),
     ],
 )
 def test_locks_metrics(aggregator, integration_check, pg_instance, relations, lock_count, lock_table_name, tags):