move pertanent operations to storage_detached

src/charm.py

@@ -5,7 +5,6 @@
 import json
 import logging
 import re
-import time
 from typing import Any, Dict, List, Optional, Set
 
 import jinja2
@@ -75,7 +74,11 @@
 )
 
 from config import Config
-from exceptions import AdminUserCreationError, MissingSecretError
+from exceptions import (
+    AdminUserCreationError,
+    EarlyRemovalOfConfigServerError,
+    MissingSecretError,
+)
 
 logger = logging.getLogger(__name__)
 
@@ -117,7 +120,6 @@ def __init__(self, *args):
 
         self.framework.observe(self.on.get_password_action, self._on_get_password)
         self.framework.observe(self.on.set_password_action, self._on_set_password)
-        self.framework.observe(self.on.stop, self._on_stop)
 
         self.framework.observe(self.on.secret_remove, self._on_secret_remove)
         self.framework.observe(self.on.secret_changed, self._on_secret_changed)
@@ -502,6 +504,11 @@ def get_charm_internal_revision(self) -> str:
         with open(Config.CHARM_INTERNAL_VERSION_FILE, "r") as f:
             return f.read().strip()
 
+    @property
+    def _is_removing_last_replica(self) -> bool:
+        """Returns True if the last replica (juju unit) is getting removed."""
+        return self.app.planned_units() == 0 and len(self.peers_units) == 0
+
     # END: properties
 
     # BEGIN: generic helper methods
@@ -691,10 +698,6 @@ def _relation_changes_handler(self, event: RelationEvent) -> None:
         self._connect_mongodb_exporter()
         self._connect_pbm_agent()
 
-        if isinstance(event, RelationDepartedEvent):
-            if event.departing_unit.name == self.unit.name:
-                self.unit_peer_data.setdefault("unit_departed", "True")
-
         if not self.unit.is_leader():
             return
 
@@ -760,19 +763,56 @@ def _reconcile_mongo_hosts_and_users(self, event: RelationEvent) -> None:
                 logger.info("Deferring reconfigure: error=%r", e)
                 event.defer()
 
-    def _on_stop(self, event) -> None:
-        if "True" == self.unit_peer_data.get("unit_departed", "False"):
-            logger.debug(f"{self.unit.name} blocking on_stop")
-            is_in_replica_set = True
-            timeout = UNIT_REMOVAL_TIMEOUT
-            while is_in_replica_set and timeout > 0:
-                is_in_replica_set = self.is_unit_in_replica_set()
-                time.sleep(1)
-                timeout -= 1
-                if timeout < 0:
-                    raise Exception(f"{self.unit.name}.on_stop timeout exceeded")
-            logger.debug(f"{self.unit.name} releasing on_stop")
-            self.unit_peer_data["unit_departed"] = ""
+    def mongodb_storage_detaching(self, event) -> None:
+        """Before storage detaches, allow removing unit to remove itself from the set.
+
+        If the removing unit is primary also allow it to step down and elect another unit as
+        primary while it still has access to its storage.
+        """
+        if self.upgrade_in_progress:
+            # We cannot defer and prevent a user from removing a unit, log a warning instead.
+            logger.warning(
+                "Removing replicas during an upgrade is not supported. The charm may be in a broken, unrecoverable state"
+            )
+
+        # A single replica cannot step down as primary and we cannot reconfigure the replica set to
+        # have 0 members.
+        if self._is_removing_last_replica:
+            # removing config-server from a sharded cluster can be disaterous.
+            if self.is_role(Config.Role.CONFIG_SERVER) and self.config_server.has_shards():
+                current_shards = self.config_server.get_related_shards()
+                early_removal_message = f"Cannot remove config-server, still related to shards {', '.join(current_shards)}"
+                logger.error(early_removal_message)
+                raise EarlyRemovalOfConfigServerError(early_removal_message)
+
+            # cannot drain shard after storage detached.
+            if self.is_role(Config.Role.SHARD) and self.shard.has_config_server():
+                logger.info("Wait for shard to drain before detaching storage.")
+                self.status.set_and_share_status(MaintenanceStatus("Draining shard from cluster"))
+                mongos_hosts = self.shard.get_mongos_hosts()
+                self.shard.wait_for_draining(mongos_hosts)
+                logger.info("Shard successfully drained storage.")
+
+        try:
+            # retries over a period of 10 minutes in an attempt to resolve race conditions it is
+            # not possible to defer in storage detached.
+            logger.debug("Removing %s from replica set", self.unit_host(self.unit))
+            for attempt in Retrying(
+                stop=stop_after_attempt(10),
+                wait=wait_fixed(1),
+                reraise=True,
+            ):
+                with attempt:
+                    # remove_replset_member retries for 60 seconds
+                    with MongoDBConnection(self.mongodb_config) as mongo:
+                        mongo.remove_replset_member(self.unit_host(self.unit))
+
+        except NotReadyError:
+            logger.info(
+                "Failed to remove %s from replica set, another member is syncing", self.unit.name
+            )
+        except PyMongoError as e:
+            logger.error("Failed to remove %s from replica set, error=%r", self.unit.name, e)
 
     def _on_update_status(self, event: UpdateStatusEvent):
         # user-made mistakes might result in other incorrect statues. Prioritise informing users of

src/exceptions.py

@@ -16,6 +16,10 @@ class ApplicationHostNotFoundError(MongoError):
     """Raised when a queried host is not in the application peers or the current host."""
 
 
+class EarlyRemovalOfConfigServerError(Exception):
+    """Raised when there is an attempt to remove a config-server, while related to a shard."""
+
+
 class MongoSecretError(MongoError):
     """Common parent for all Mongo Secret Exceptions."""