Fix bug with slot for former leader not retained on failover (#3261)

the problem existed because _build_retain_slots() method was falsely relying on members being present in DCS, while on failover the member key for the former leader is expiring exactly at the same time.

.github/workflows/tests.yaml

@@ -154,7 +154,7 @@ jobs:
       with:
         name: behave-${{ matrix.os }}-${{ matrix.dcs }}-${{ matrix.python-version }}-logs
         path: |
-          features/output/*_failed/*postgres?.*
+          features/output/*_failed/*postgres-?.*
           features/output/*.log
         if-no-files-found: error
         retention-days: 5

features/permanent_slots.feature

@@ -81,4 +81,6 @@ Feature: permanent slots
     When I start postgres-0
     Then postgres-0 role is the replica after 20 seconds
     And physical replication slot named postgres_1 on postgres-0 has no xmin value after 10 seconds
-    And physical replication slot named postgres_2 on postgres-0 has no xmin value after 10 seconds
+    # postgres_2 and postgres_3 slots are retained, but postgres_2 will still have xmin value :(
+    And postgres-0 has a physical replication slot named postgres_2 after 10 seconds
+    And postgres-0 has a physical replication slot named postgres_3 after 10 seconds

features/steps/slots.py

@@ -99,8 +99,8 @@ def has_physical_replication_slot(context, pg_name, slot_name, time_limit):
 def physical_slot_no_xmin(context, pg_name, slot_name, time_limit):
     time_limit *= context.timeout_multiplier
     max_time = time.time() + int(time_limit)
-    query = "SELECT xmin FROM pg_catalog.pg_replication_slots WHERE slot_type = 'physical'"
-    f" AND slot_name = '{slot_name}'"
+    query = "SELECT xmin FROM pg_catalog.pg_replication_slots WHERE slot_type = 'physical'" +\
+        f" AND slot_name = '{slot_name}'"
     exists = False
     while time.time() < max_time:
         try:

patroni/dcs/__init__.py

@@ -1841,10 +1841,6 @@ class AbstractDCS(abc.ABC):
         """
         timestamp = time.time()
 
-        # DCS is a source of truth, therefore we take missing values from there
-        self._last_retain_slots.update({name: timestamp for name in self._last_status['retain_slots']
-                                        if (not slots or name not in slots) and name not in self._last_retain_slots})
-
         if slots:  # if slots is not empty it implies we are running v11+
             members: Set[str] = set()
             found_self = False
@@ -1929,6 +1925,23 @@ class AbstractDCS(abc.ABC):
         :returns: ``True`` if key has been created successfully.
         """
 
+    def acquire_leader_lock(self) -> bool:
+        """Attempt to acquire leader lock.
+
+        .. note::
+            This method wraps :meth:`~AbstractDCS.attempt_to_acquire_leader`: and is
+            used to reset retention time of physical replication slots that representing
+            members of the cluster when current node is to be promoted to the leader.
+
+        :returns: ``True`` if the leader key has been created successfully.
+        """
+        ret = self.attempt_to_acquire_leader()
+        if ret:
+            timestamp = time.time()
+            # every time we promote we need to reset retention time for slots recorded in the /status key
+            self._last_retain_slots = {name: timestamp for name in self._last_status['retain_slots']}
+        return ret
+
     @abc.abstractmethod
     def set_failover_value(self, value: str, version: Optional[Any] = None) -> bool:
         """Create or update ``/failover`` key.

patroni/ha.py

@@ -345,11 +345,11 @@ class Ha(object):
 
     def acquire_lock(self) -> bool:
         try:
-            ret = self.dcs.attempt_to_acquire_leader()
+            ret = self.dcs.acquire_leader_lock()
         except DCSError:
             raise
         except Exception:
-            logger.exception('Unexpected exception raised from attempt_to_acquire_leader, please report it as a BUG')
+            logger.exception('Unexpected exception raised from acquire_leader_lock, please report it as a BUG')
             ret = False
         self.set_is_leader(ret)
         return ret