[DBMON-6589] Eliminate reference cycle in Postgres diagnose (DataDog#23647)

* Eliminate reference cycle in diagnose

* Add changelog

Author: eric-weaver

postgres/changelog.d/23647.fixed

@@ -0,0 +1 @@
+Eliminate reference cycle in diagnostic instrumentation

postgres/datadog_checks/postgres/diagnose.py

@@ -32,6 +32,11 @@
 RECOMMENDED_TRACK_ACTIVITY_QUERY_SIZE = 4096
 
 
+def run_diagnostics(check):
+    """Entry point for ``Diagnosis.register()``; creates a short-lived worker per invocation."""
+    PostgresDiagnose(check)._run()
+
+
 class PostgresDiagnose:
     """Explicit pre-flight diagnostics for `datadog-agent diagnose`."""
 
@@ -42,21 +47,6 @@ def __init__(self, check):
         # when shared_preload_libraries is empty). Reset at the top of the first orchestrator.
         self._failed = set()
 
-    # -- registration ---------------------------------------------------------
-
-    def register(self):
-        """Register the diagnostic entry point with the check's Diagnosis object.
-
-        Idempotent: re-invoking `register` on the same Diagnosis object is a no-op.
-        ``Diagnosis.register`` extends an internal list, so without this guard a
-        repeated call would stack the entry point and produce N× the diagnostics.
-        """
-        d = self._check.diagnosis
-        if getattr(d, '_postgres_diagnostics_registered', False):
-            return
-        d._postgres_diagnostics_registered = True
-        d.register(self._run)
-
     # -- orchestrator ---------------------------------------------------------
 
     def _run(self):

postgres/datadog_checks/postgres/postgres.py

@@ -47,7 +47,7 @@
 
 from .__about__ import __version__
 from .config import build_config, sanitize
-from .diagnose import PostgresDiagnose
+from .diagnose import run_diagnostics
 from .util import (
     ANALYZE_PROGRESS_METRICS,
     AWS_RDS_HOSTNAME_SUFFIX,
@@ -191,8 +191,7 @@ def __init__(self, name, init_config, instances):
             ttl=self._config.database_instance_collection_interval,
         )  # type: TTLCache
 
-        # Register explicit pre-flight diagnostics for `datadog-agent diagnose`.
-        PostgresDiagnose(self).register()
+        self.diagnosis.register(functools.partial(run_diagnostics, self))
 
     def _submit_initialization_health_event(self):
         try: