Add is_container charm library

lib/charms/hpc_libs/v0/is_container.py

@@ -0,0 +1,81 @@
+# Copyright 2024 Canonical Ltd.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""Detect if machine is a container instance.
+
+Even though Juju supports using LXD containers as the backing cloud for
+deploying charmed operators, not all HPC applications work within system containers,
+and some need additional configuration. This simple charm library provides utilities
+for identifying the virtualization runtime for a charmed operator.
+
+### Example Usage:
+
+```python3
+from charms.hpc_libs.v0.is_container import is_container
+
+class ApplicationCharm(CharmBase):
+
+    def __init__(self, *args):
+        super().__init__(*args)
+
+        self.framework.observe(self.on.install, self._on_install)
+
+    def _on_install(self, _: InstallEvent) -> None:
+        if is_container():
+            self.unit.status = BlockedStatus("app does not support container runtime")
+
+        # Proceed with installation.
+        ...
+```
+"""
+
+import shutil
+import subprocess
+
+# The unique Charmhub library identifier, never change it
+LIBID = "eb95ad73da1941c0af186ee670f96507"
+
+# Increment this major API version when introducing breaking changes
+LIBAPI = 0
+
+# Increment this PATCH version before using `charmcraft publish-lib` or reset
+# to 0 if you are raising the major API version
+LIBPATCH = 1
+
+
+class UnknownVirtStateError(Exception):
+    """Raise error if unknown virtualization state is returned."""
+
+    @property
+    def message(self) -> str:
+        """Return message passed as argument to exception."""
+        return self.args[0]
+
+
+def is_container() -> bool:
+    """Detect if the machine is a container instance.
+
+    Raises:
+        DetectVirtNotFoundError: Raised if `systemd-detect-virt` is not found on machine.
+    """
+    if shutil.which("systemd-detect-virt") is None:
+        raise UnknownVirtStateError(
+            (
+                "executable `systemd-detect-virt` not found. "
+                + "cannot determine if machine is a container instance"
+            )
+        )
+
+    result = subprocess.run(["systemd-detect-virt", "--container"])
+    return result.returncode == 0

tests/integration/is_container/test_is_container.py

@@ -0,0 +1,10 @@
+#!/usr/bin/env python3
+# Copyright 2024 Canonical Ltd.
+# See LICENSE file for licensing details.
+
+from lib.charms.hpc_libs.v0.is_container import is_container
+
+
+def test_is_container() -> None:
+    """Test that `is_container` properly detects the system container."""
+    assert is_container() is True

tests/integration/test_hpc_libs.yaml

@@ -26,6 +26,8 @@ acts:
         path: dev-requirements.txt
       - host-path: tests/integration/slurm_ops
         path: slurm_ops
+      - host-path: tests/integration/is_container
+        path: is_container
     scenes:
       - name: "Install dependencies in a virtual environment"
         run: |
@@ -34,10 +36,17 @@ acts:
           apt install -y python3-venv python3-yaml
           python3 -m venv venv --system-site-packages
           venv/bin/python3 -m pip install -r dev-requirements.txt
-      - name: "Run integration tests with pytest"
+      - name: "Run `slurm_ops` integration tests"
         run: |
           venv/bin/python3 -m pytest -v \
             -s \
             --tb native \
             --log-cli-level=INFO \
             slurm_ops
+      - name: "Run `is_container` integration tests"
+        run: |
+          venv/bin/python3 -m pytest -v \
+            -s \
+            --tb native \
+            --log-cli-level=INFO \
+            is_container

tests/unit/test_is_container.py

@@ -0,0 +1,40 @@
+#!/usr/bin/env python3
+# Copyright 2024 Canonical Ltd.
+# See LICENSE file for licensing details.
+
+"""Test `is_container` library."""
+
+from unittest import TestCase
+from unittest.mock import patch
+
+from charms.hpc_libs.v0.is_container import UnknownVirtStateError, is_container
+
+
+@patch("charms.hpc_libs.v0.is_container.shutil.which", return_value="/usr/bin/systemd-detect-virt")
+@patch("charms.hpc_libs.v0.is_container.subprocess.run")
+class TestIsContainer(TestCase):
+
+    def test_inside_container(self, run, _) -> None:
+        """Test that `is_container` returns True when inside a container."""
+        run.return_value.returncode = 0
+        self.assertTrue(is_container())
+
+    def test_inside_virtual_machine(self, run, _) -> None:
+        """Test that `is_container` returns False when inside a virtual machine."""
+        run.return_value.returncode = 1
+        self.assertFalse(is_container())
+
+    def test_detect_virt_not_found(self, _, which) -> None:
+        """Test that correct error is thrown if `systemd-detect-virt` is not found."""
+        which.return_value = None
+
+        try:
+            is_container()
+        except UnknownVirtStateError as e:
+            self.assertEqual(
+                e.message,
+                (
+                    "executable `systemd-detect-virt` not found. "
+                    + "cannot determine if machine is a container instance"
+                ),
+            )