pin snap revision + bug fix (#214)

## Issue
1. snap revision wasn't pinned
2. charm starts pbm and mongodb exporter before database was started

lib/charms/mongodb/v0/mongodb_backups.py

@@ -192,6 +192,7 @@ def _resync_config_options(self, pbm_snap):
                 # if a resync is running restart the service
                 if isinstance(pbm_status, (WaitingStatus)):
                     pbm_snap.restart(services=["pbm-agent"])
+                    raise PBMBusyError
 
         # wait for re-sync and update charm status based on pbm syncing status. Need to wait for
         # 2 seconds for pbm_agent to receive the resync command before verifying.

src/charm.py

@@ -68,7 +68,7 @@
 # We expect the MongoDB container to use the default ports
 MONGODB_PORT = 27017
 MONGODB_EXPORTER_PORT = 9216
-SNAP_PACKAGES = [("charmed-mongodb", "5/edge")]
+SNAP_PACKAGES = [("charmed-mongodb", "5/edge", 82)]
 REL_NAME = "database"
 
 
@@ -540,13 +540,15 @@ def _install_snap_packages(self, packages: List[str]) -> None:
         Args:
             packages: list of packages to install.
         """
-        for snap_name, snap_channel in packages:
+        for snap_name, snap_channel, snap_revision in packages:
             try:
                 snap_cache = snap.SnapCache()
                 snap_package = snap_cache[snap_name]
 
                 if not snap_package.present:
-                    snap_package.ensure(snap.SnapState.Latest, channel=snap_channel)
+                    snap_package.ensure(
+                        snap.SnapState.Latest, channel=snap_channel, revision=snap_revision
+                    )
 
             except snap.SnapError as e:
                 logger.error(
@@ -594,6 +596,9 @@ def _push_tls_certificate_to_workload(self) -> None:
 
     def _connect_mongodb_exporter(self) -> None:
         """Exposes the endpoint to mongodb_exporter."""
+        if "db_initialised" not in self.app_peer_data:
+            return
+
         # must wait for leader to set URI before connecting
         if not self.get_secret("app", "monitor-password"):
             return
@@ -605,6 +610,9 @@ def _connect_mongodb_exporter(self) -> None:
 
     def _connect_pbm_agent(self) -> None:
         """Updates URI for pbm-agent."""
+        if "db_initialised" not in self.app_peer_data:
+            return
+
         # must wait for leader to set URI before any attempts to update are made
         if not self.get_secret("app", "backup-password"):
             return

tests/unit/test_mongodb_backups.py

@@ -165,7 +165,10 @@ def test_resync_config_restart(self, pbm_status, retry_stop, retry_wait, check_o
         retry_stop.return_value = wait_fixed(1)
         pbm_status.return_value = WaitingStatus()
         mock_snap = mock.Mock()
-        self.harness.charm.backups._resync_config_options(mock_snap)
+
+        with self.assertRaises(PBMBusyError):
+            self.harness.charm.backups._resync_config_options(mock_snap)
+
         mock_snap.restart.assert_called()
 
     @patch("charm.subprocess.check_output")