Correctly handle calls from on install hook

src/charm.py

@@ -55,7 +55,11 @@ def _on_install(self, event) -> None:
         self._cluster.inhibit_default_cluster_creation()
 
         # Install the PostgreSQL and Patroni requirements packages.
-        self._install_apt_packages(event, ["postgresql", "python3-pip", "python3-psycopg2"])
+        try:
+            self._install_apt_packages(event, ["postgresql", "python3-pip", "python3-psycopg2"])
+        except (subprocess.CalledProcessError, apt.PackageNotFoundError):
+            self.unit.status = BlockedStatus("failed to install apt packages")
+            return
 
         try:
             resource_path = self.model.resources.fetch("patroni")
@@ -65,8 +69,12 @@ def _on_install(self, event) -> None:
             return
 
         # Build Patroni package path with raft dependency and install it.
-        patroni_package_path = f"{str(resource_path)}[raft]"
-        self._install_pip_packages([patroni_package_path])
+        try:
+            patroni_package_path = f"{str(resource_path)}[raft]"
+            self._install_pip_packages([patroni_package_path])
+        except subprocess.SubprocessError:
+            self.unit.status = BlockedStatus("failed to install Patroni python package")
+            return
 
         self.unit.status = WaitingStatus("waiting to start PostgreSQL")
 
@@ -120,24 +128,37 @@ def _get_postgres_password(self) -> str:
         return data.get("postgres-password", None)
 
     def _install_apt_packages(self, _, packages: List[str]) -> None:
-        """Simple wrapper around 'apt-get install -y."""
+        """Simple wrapper around 'apt-get install -y.
+
+        Raises:
+            CalledProcessError if it fails to update the apt cache.
+            PackageNotFoundError if the package is not in the cache.
+            PackageError if the packages could not be installed.
+        """
         try:
             logger.debug("updating apt cache")
             apt.update()
         except subprocess.CalledProcessError as e:
             logger.exception("failed to update apt cache, CalledProcessError", exc_info=e)
-            self.unit.status = BlockedStatus("failed to update apt cache")
-            return
+            raise
 
-        try:
-            logger.debug(f"installing apt packages: {', '.join(packages)}")
-            apt.add_package(packages)
-        except apt.PackageNotFoundError:
-            logger.error("a specified package not found in package cache or on system")
-            self.unit.status = BlockedStatus("failed to install packages")
+        for package in packages:
+            try:
+                apt.add_package(package)
+                logger.debug(f"installed package: {package}")
+            except apt.PackageNotFoundError:
+                logger.error(f"package not found: {package}")
+                raise
+            except apt.PackageError:
+                logger.error(f"package error: {package}")
+                raise
 
     def _install_pip_packages(self, packages: List[str]) -> None:
-        """Simple wrapper around pip install."""
+        """Simple wrapper around pip install.
+
+        Raises:
+            SubprocessError if the packages could not be installed.
+        """
         try:
             command = [
                 "pip3",
@@ -148,7 +169,7 @@ def _install_pip_packages(self, packages: List[str]) -> None:
             subprocess.check_call(command)
         except subprocess.SubprocessError:
             logger.error("could not install pip packages")
-            self.unit.status = BlockedStatus("failed to install pip packages")
+            raise
 
     def _new_password(self) -> str:
         """Generate a random password string.

tests/unit/test_charm.py

@@ -59,6 +59,41 @@ def test_on_install(
             WaitingStatus("waiting to start PostgreSQL"),
         )
 
+    @patch("charm.PostgresqlOperatorCharm._install_pip_packages")
+    @patch("charm.PostgresqlOperatorCharm._install_apt_packages")
+    @patch("charm.PostgresqlCluster.inhibit_default_cluster_creation")
+    def test_on_install_apt_failure(
+        self, _inhibit_default_cluster_creation, _install_apt_packages, _install_pip_packages
+    ):
+        # Mock the result of the call.
+        _install_apt_packages.side_effect = apt.PackageNotFoundError
+        # Trigger the hook.
+        self.charm.on.install.emit()
+        # Assert that the needed calls were made.
+        _inhibit_default_cluster_creation.assert_called_once()
+        _install_apt_packages.assert_called_once()
+        _install_pip_packages.assert_not_called()
+        self.assertTrue(isinstance(self.harness.model.unit.status, BlockedStatus))
+
+    @patch("charm.PostgresqlOperatorCharm._install_pip_packages")
+    @patch("charm.PostgresqlOperatorCharm._install_apt_packages")
+    @patch("charm.PostgresqlCluster.inhibit_default_cluster_creation")
+    def test_on_install_pip_failure(
+        self, _inhibit_default_cluster_creation, _install_apt_packages, _install_pip_packages
+    ):
+        # Mock the result of the call.
+        _install_pip_packages.side_effect = subprocess.CalledProcessError(
+            cmd="pip3 install patroni", returncode=1
+        )
+        # Add an empty file as Patroni resource just to check that the correct calls were made.
+        self.harness.add_resource("patroni", "")
+        self.charm.on.install.emit()
+        # Assert that the needed calls were made.
+        _inhibit_default_cluster_creation.assert_called_once()
+        _install_apt_packages.assert_called_once()
+        _install_pip_packages.assert_called_once()
+        self.assertTrue(isinstance(self.harness.model.unit.status, BlockedStatus))
+
     def test_on_leader_elected(self):
         # Assert that there is no password in the peer relation.
         self.harness.add_relation(self._peer_relation, self.charm.app.name)
@@ -162,30 +197,24 @@ def test_install_apt_packages(self, _update, _add_package):
         ]
 
         # Test for problem with apt update.
-        self.charm._install_apt_packages(mock_event, "postgresql")
-        _update.assert_called_once()
-        self.assertEqual(
-            self.harness.model.unit.status,
-            BlockedStatus("failed to update apt cache"),
-        )
+        with self.assertRaises(subprocess.CalledProcessError):
+            self.charm._install_apt_packages(mock_event, ["postgresql"])
+            _update.assert_called_once()
 
         # Test with a not found package.
         _add_package.side_effect = apt.PackageNotFoundError
-        self.charm._install_apt_packages(mock_event, "postgresql")
-        _update.assert_called()
-        _add_package.assert_called_once_with("postgresql")
-        self.assertEqual(
-            self.harness.model.unit.status,
-            BlockedStatus("failed to install packages"),
-        )
+        with self.assertRaises(apt.PackageNotFoundError):
+            self.charm._install_apt_packages(mock_event, ["postgresql"])
+            _update.assert_called()
+            _add_package.assert_called_with("postgresql")
 
         # Then test a valid one.
         _update.reset_mock()
         _add_package.reset_mock()
         _add_package.side_effect = None
-        self.charm._install_apt_packages(mock_event, "postgresql-12")
+        self.charm._install_apt_packages(mock_event, ["postgresql"])
         _update.assert_called_once()
-        _add_package.assert_called_once_with("postgresql-12")
+        _add_package.assert_called_with("postgresql")
 
     @patch("subprocess.call")
     def test_install_pip_packages(self, _call):
@@ -211,12 +240,8 @@ def test_install_pip_packages(self, _call):
         )
 
         # Then, test for an error.
-        self.charm._install_pip_packages(packages)
-        # Assert the status set by the event handler.
-        self.assertEqual(
-            self.harness.model.unit.status,
-            BlockedStatus("failed to install pip packages"),
-        )
+        with self.assertRaises(subprocess.SubprocessError):
+            self.charm._install_pip_packages(packages)
 
     def test_new_password(self):
         # Test the password generation twice in order to check if we get different passwords and