Make install handler idempotent (#92)

Fixes #84

juju can call `install` twice
(https://chat.charmhub.io/charmhub/pl/maxrc5tasin3bjsoxbt3st9o1w)

Author: carlcsaposs-canonical

src/machine_charm.py

@@ -62,7 +62,7 @@ def _read_only_endpoint(self) -> str:
     # =======================
 
     def _on_install(self, _) -> None:
-        snap.install(unit=self.unit)
+        snap.install(unit=self.unit, model_uuid=self.model.uuid)
         self.unit.set_workload_version(self.get_workload(event=None).version)
 
     def _on_remove(self, _) -> None:

src/snap.py

@@ -23,9 +23,22 @@
 _UNIX_USERNAME = "snap_daemon"
 
 
-def install(*, unit: ops.Unit):
+def install(*, unit: ops.Unit, model_uuid: str):
     """Install snap."""
-    if _snap.present:
+    installed_by_unit = pathlib.Path(
+        "/var/snap", _SNAP_NAME, "common", "installed_by_mysql_router_charm_unit"
+    )
+    unique_unit_name = f"{model_uuid}_{unit.name}"
+    # This charm can override/use an existing snap installation only if the snap was previously
+    # installed by this charm.
+    # Otherwise, the snap could be in use by another charm (e.g. MySQL Server charm, a different
+    # MySQL Router charm).
+    if _snap.present and not (
+        installed_by_unit.exists() and installed_by_unit.read_text() == unique_unit_name
+    ):
+        logger.debug(
+            f"{installed_by_unit.exists() and installed_by_unit.read_text()=} {unique_unit_name=}"
+        )
         logger.error(f"{_SNAP_NAME} snap already installed on machine. Installation aborted")
         raise Exception(f"Multiple {_SNAP_NAME} snap installs not supported on one machine")
     logger.debug(f"Installing {_SNAP_NAME=}, {_REVISION=}")
@@ -45,6 +58,8 @@ def _set_retry_status(_) -> None:
     ):
         with attempt:
             _snap.ensure(state=snap_lib.SnapState.Present, revision=_REVISION)
+    installed_by_unit.write_text(unique_unit_name)
+    logger.debug(f"Wrote {unique_unit_name=} to {installed_by_unit.name=}")
     _snap.hold()
     logger.debug(f"Installed {_SNAP_NAME=}, {_REVISION=}")