Sysctl library

lib/charms/operator_libs_linux/v0/sysctl.py

@@ -1,24 +1,84 @@
-"""TODO: Add a proper docstring here.
+# Copyright 2023 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.
 
-This is a placeholder docstring for this charm library. Docstrings are
-presented on Charmhub and updated whenever you push a new version of the
-library.
+"""Handler for the sysctl config.
 
-Complete documentation about creating and documenting libraries can be found
-in the SDK docs at https://juju.is/docs/sdk/libraries.
+This library allows your charm to create and update sysctl config options to the machine.
 
-See `charmcraft publish-lib` and `charmcraft fetch-lib` for details of how to
-share and consume charm libraries. They serve to enhance collaboration
-between charmers. Use a charmer's libraries for classes that handle
-integration with their charm.
+Validation and merge capabilities are added, for situations where more than one application
+are setting values. The following files can be created:
 
-Bear in mind that new revisions of the different major API versions (v0, v1,
-v2 etc) are maintained independently.  You can continue to update v0 and v1
-after you have pushed v3.
+- /etc/sysctl.d/90-juju-<app-name>
+    Requirements from one application requesting to update the values.
 
-Markdown is supported, following the CommonMark specification.
+- /etc/sysctl.d/95-juju-sysctl.conf
+    Merged file resulting from all other `90-juju-*` application files.
+
+
+A charm using the sysctl lib will need a data structure like the following:
+```yaml
+vm.swappiness:
+  value: 1
+vm.max_map_count:
+  value: 262144
+vm.dirty_ratio:
+  value: 80
+vm.dirty_background_ratio:
+  value: 5
+net.ipv4.tcp_max_syn_backlog:
+  value: 4096
+```
+
+Now, it can use that template within the charm, or just declare the values directly:
+
+```python
+from charms.operator_libs_linux.v0 import sysctl
+
+class MyCharm(CharmBase):
+
+    def __init__(self, *args):
+        ...
+        self.sysctl = sysctl.Config(self.meta.name)
+
+        self.framework.observe(self.on.install, self._on_install)
+        self.framework.observe(self.on.remove, self._on_remove)
+
+    def _on_install(self, _):
+        # Altenatively, read the values from a template
+        sysctl_data = {"net.ipv4.tcp_max_syn_backlog": {"value": 4096}}
+
+        try:
+            self.sysctl.update(config=sysctl_data)
+        except (sysctl.SysctlPermissionError, sysctl.ValidationError) as e:
+            logger.error(f"Error setting values on sysctl: {e.message}")
+            self.unit.status = BlockedStatus("Sysctl config not possible")
+        except sysctl.SysctlError:
+            logger.error("Error on sysctl")
+
+    def _on_remove(self, _):
+        self.sysctl.remove()
+```
 """
 
+import logging
+import re
+from pathlib import Path
+from subprocess import STDOUT, CalledProcessError, check_output
+from typing import Dict, List
+
+logger = logging.getLogger(__name__)
+
 # The unique Charmhub library identifier, never change it
 LIBID = "17a6cd4d80104d15b10f9c2420ab3266"
 
@@ -27,6 +87,209 @@
 
 # Increment this PATCH version before using `charmcraft publish-lib` or reset
 # to 0 if you are raising the major API version
-LIBPATCH = 1
+LIBPATCH = 2
+
+CHARM_FILENAME_PREFIX = "90-juju-"
+SYSCTL_DIRECTORY = Path("/etc/sysctl.d")
+SYSCTL_FILENAME = SYSCTL_DIRECTORY / "95-juju-sysctl.conf"
+SYSCTL_HEADER = f"""# This config file was produced by sysctl lib v{LIBAPI}.{LIBPATCH}
+#
+# This file represents the output of the sysctl lib, which can combine multiple
+# configurations into a single file like.
+"""
+
+
+class Error(Exception):
+    """Base class of most errors raised by this library."""
+
+    @property
+    def message(self):
+        """Return the message passed as an argument."""
+        return self.args[0]
+
+
+class CommandError(Error):
+    """Raised when there's an error running sysctl command."""
+
+
+class ApplyError(Error):
+    """Raised when there's an error applying values in sysctl."""
+
+
+class ValidationError(Error):
+    """Exception representing value validation error."""
+
+
+class Config(Dict):
+    """Represents the state of the config that a charm wants to enforce."""
+
+    _apply_re = re.compile(r"sysctl: permission denied on key \"([a-z_\.]+)\", ignoring$")
+
+    def __init__(self, name: str) -> None:
+        self.name = name
+        self._data = self._load_data()
+
+    def __contains__(self, key: str) -> bool:
+        """Check if key is in config."""
+        return key in self._data
+
+    def __len__(self):
+        """Get size of config."""
+        return len(self._data)
+
+    def __iter__(self):
+        """Iterate over config."""
+        return iter(self._data)
+
+    def __getitem__(self, key: str) -> str:
+        """Get value for key form config."""
+        return self._data[key]
+
+    @property
+    def charm_filepath(self) -> Path:
+        """Name for resulting charm config file."""
+        return SYSCTL_DIRECTORY / f"{CHARM_FILENAME_PREFIX}{self.name}"
+
+    def update(self, config: Dict[str, dict]) -> None:
+        """Update sysctl config options with a desired set of config params.
+
+        Args:
+            config: dictionary with keys to update:
+        ```
+        {"vm.swappiness": {"value": 10}, ...}
+        ```
+        """
+        self._parse_config(config)
+
+        # NOTE: case where own charm calls update() more than once.
+        if self.charm_filepath.exists():
+            self._merge(add_own_charm=False)
+
+        conflict = self._validate()
+        if conflict:
+            raise ValidationError(f"Validation error for keys: {conflict}")
+
+        snapshot = self._create_snapshot()
+        logger.debug("Created snapshot for keys: %s", snapshot)
+        try:
+            self._apply()
+        except ApplyError:
+            self._restore_snapshot(snapshot)
+            raise
+
+        self._create_charm_file()
+        self._merge()
+
+    def remove(self) -> None:
+        """Remove config for charm.
+
+        The removal process won't apply any sysctl configuration. It will only merge files from
+        remaining charms.
+        """
+        self.charm_filepath.unlink(missing_ok=True)
+        logger.info("Charm config file %s was removed", self.charm_filepath)
+        self._merge()
+
+    def _validate(self) -> List[str]:
+        """Validate the desired config params against merged ones."""
+        common_keys = set(self._data.keys()) & set(self._desired_config.keys())
+        conflict_keys = []
+        for key in common_keys:
+            if self._data[key] != self._desired_config[key]:
+                logger.warning(
+                    "Values for key '%s' are different: %s != %s",
+                    key,
+                    self._data[key],
+                    self._desired_config[key],
+                )
+                conflict_keys.append(key)
+
+        return conflict_keys
+
+    def _create_charm_file(self) -> None:
+        """Write the charm file."""
+        charm_params = [f"# {self.name}\n"] + [
+            f"{key}={value}\n" for key, value in self._desired_config.items()
+        ]
+        with open(self.charm_filepath, "w") as f:
+            f.writelines(charm_params)
+
+    def _merge(self, add_own_charm=True) -> None:
+        """Create the merged sysctl file.
+
+        Args:
+            add_own_charm : bool, if false it will skip the charm file from the merge.
+        """
+        # get all files that start by 90-juju-
+        data = [SYSCTL_HEADER]
+        paths = set(SYSCTL_DIRECTORY.glob(f"{CHARM_FILENAME_PREFIX}*"))
+        if not add_own_charm:
+            paths.discard(self.charm_filepath.as_posix())
+
+        for path in paths:
+            with open(path, "r") as f:
+                data += f.readlines()
+        with open(SYSCTL_FILENAME, "w") as f:
+            f.writelines(data)
+
+        # Reload data with newly created file.
+        self._data = self._load_data()
+
+    def _apply(self) -> None:
+        """Apply values to machine."""
+        cmd = [f"{key}={value}" for key, value in self._desired_config.items()]
+        result = self._sysctl(cmd)
+        failed_values = [
+            self._apply_re.match(line) for line in result if self._apply_re.match(line)
+        ]
+        logger.debug("Failed values: %s", failed_values)
+
+        if failed_values:
+            msg = f"Unable to set params: {[f.group(1) for f in failed_values]}"
+            logger.error(msg)
+            raise ApplyError(msg)
+
+    def _create_snapshot(self) -> Dict[str, str]:
+        """Create a snaphot of config options that are going to be set."""
+        cmd = ["-n"] + list(self._desired_config.keys())
+        values = self._sysctl(cmd)
+        return dict(zip(list(self._desired_config.keys()), values))
+
+    def _restore_snapshot(self, snapshot: Dict[str, str]) -> None:
+        """Restore a snapshot to the machine."""
+        values = [f"{key}={value}" for key, value in snapshot.items()]
+        self._sysctl(values)
+
+    def _sysctl(self, cmd: List[str]) -> List[str]:
+        """Execute a sysctl command."""
+        cmd = ["sysctl"] + cmd
+        logger.debug("Executing sysctl command: %s", cmd)
+        try:
+            return check_output(cmd, stderr=STDOUT, universal_newlines=True).splitlines()
+        except CalledProcessError as e:
+            msg = f"Error executing '{cmd}': {e.stdout}"
+            logger.error(msg)
+            raise CommandError(msg)
+
+    def _parse_config(self, config: Dict[str, dict]) -> None:
+        """Parse a config passed to the lib."""
+        result = {}
+        for key, value in config.items():
+            result[key] = str(value["value"])
+        self._desired_config: Dict[str, str] = result
+
+    def _load_data(self) -> Dict[str, str]:
+        """Get merged config."""
+        config = {}
+        if not SYSCTL_FILENAME.exists():
+            return config
+
+        with open(SYSCTL_FILENAME, "r") as f:
+            for line in f:
+                if line.startswith(("#", ";")) or not line.strip() or "=" not in line:
+                    continue
+
+                key, _, value = line.partition("=")
+                config[key.strip()] = value.strip()
 
-# TODO: add your code here! Happy coding!
+        return config