Observability Libs

# Copyright 2021 Canonical Ltd.
# See LICENSE file for licensing details.

"""# KubernetesServicePatch Library.

This library is designed to enable developers to more simply patch the Kubernetes Service created
by Juju during the deployment of a sidecar charm. When sidecar charms are deployed, Juju creates a
service named after the application in the namespace (named after the Juju model). This service by
default contains a "placeholder" port, which is 65536/TCP.

When modifying the default set of resources managed by Juju, one must consider the lifecycle of the
charm. In this case, any modifications to the default service (created during deployment), will be
overwritten during a charm upgrade.

When initialised, this library binds a handler to the parent charm's `install` and `upgrade_charm`
events which applies the patch to the cluster. This should ensure that the service ports are
correct throughout the charm's life.

The constructor simply takes a reference to the parent charm, and a list of
[`lightkube`](https://github.com/gtsystem/lightkube) ServicePorts that each define a port for the
service. For information regarding the `lightkube` `ServicePort` model, please visit the
`lightkube` [docs](https://gtsystem.github.io/lightkube-models/1.23/models/core_v1/#serviceport).

Optionally, a name of the service (in case service name needs to be patched as well), labels,
selectors, and annotations can be provided as keyword arguments.

## Getting Started

To get started using the library, you just need to fetch the library using `charmcraft`. **Note
that you also need to add `lightkube` and `lightkube-models` to your charm's `requirements.txt`.**

```shell
cd some-charm
charmcraft fetch-lib charms.observability_libs.v1.kubernetes_service_patch
cat << EOF >> requirements.txt
lightkube
lightkube-models
EOF
```

Then, to initialise the library:

For `ClusterIP` services:

```python
# ...
from charms.observability_libs.v1.kubernetes_service_patch import KubernetesServicePatch
from lightkube.models.core_v1 import ServicePort

class SomeCharm(CharmBase):
  def __init__(self, *args):
    # ...
    port = ServicePort(443, name=f"{self.app.name}")
    self.service_patcher = KubernetesServicePatch(self, [port])
    # ...
```

For `LoadBalancer`/`NodePort` services:

```python
# ...
from charms.observability_libs.v1.kubernetes_service_patch import KubernetesServicePatch
from lightkube.models.core_v1 import ServicePort

class SomeCharm(CharmBase):
  def __init__(self, *args):
    # ...
    port = ServicePort(443, name=f"{self.app.name}", targetPort=443, nodePort=30666)
    self.service_patcher = KubernetesServicePatch(
        self, [port], "LoadBalancer"
    )
    # ...
```

Port protocols can also be specified. Valid protocols are `"TCP"`, `"UDP"`, and `"SCTP"`

```python
# ...
from charms.observability_libs.v1.kubernetes_service_patch import KubernetesServicePatch
from lightkube.models.core_v1 import ServicePort

class SomeCharm(CharmBase):
  def __init__(self, *args):
    # ...
    tcp = ServicePort(443, name=f"{self.app.name}-tcp", protocol="TCP")
    udp = ServicePort(443, name=f"{self.app.name}-udp", protocol="UDP")
    sctp = ServicePort(443, name=f"{self.app.name}-sctp", protocol="SCTP")
    self.service_patcher = KubernetesServicePatch(self, [tcp, udp, sctp])
    # ...
```

Bound with custom events by providing `refresh_event` argument:
For example, you would like to have a configurable port in your charm and want to apply
service patch every time charm config is changed.

```python
from charms.observability_libs.v1.kubernetes_service_patch import KubernetesServicePatch
from lightkube.models.core_v1 import ServicePort

class SomeCharm(CharmBase):
  def __init__(self, *args):
    # ...
    port = ServicePort(int(self.config["charm-config-port"]), name=f"{self.app.name}")
    self.service_patcher = KubernetesServicePatch(
        self,
        [port],
        refresh_event=self.on.config_changed
    )
    # ...
```

Additionally, you may wish to use mocks in your charm's unit testing to ensure that the library
does not try to make any API calls, or open any files during testing that are unlikely to be
present, and could break your tests. The easiest way to do this is during your test `setUp`:

```python
# ...

@patch("charm.KubernetesServicePatch", lambda x, y: None)
def setUp(self, *unused):
    self.harness = Harness(SomeCharm)
    # ...
```
"""

import logging
from types import MethodType
from typing import List, Literal, Optional, Union

from lightkube import ApiError, Client  # pyright: ignore
from lightkube.core import exceptions
from lightkube.models.core_v1 import ServicePort, ServiceSpec
from lightkube.models.meta_v1 import ObjectMeta
from lightkube.resources.core_v1 import Service
from lightkube.types import PatchType
from ops.charm import CharmBase
from ops.framework import BoundEvent, Object

logger = logging.getLogger(__name__)

# The unique Charmhub library identifier, never change it
LIBID = "0042f86d0a874435adef581806cddbbb"

# Increment this major API version when introducing breaking changes
LIBAPI = 1

# Increment this PATCH version before using `charmcraft publish-lib` or reset
# to 0 if you are raising the major API version
LIBPATCH = 9

ServiceType = Literal["ClusterIP", "LoadBalancer"]


class KubernetesServicePatch(Object):
    """A utility for patching the Kubernetes service set up by Juju."""

    def __init__(
        self,
        charm: CharmBase,
        ports: List[ServicePort],
        service_name: Optional[str] = None,
        service_type: ServiceType = "ClusterIP",
        additional_labels: Optional[dict] = None,
        additional_selectors: Optional[dict] = None,
        additional_annotations: Optional[dict] = None,
        *,
        refresh_event: Optional[Union[BoundEvent, List[BoundEvent]]] = None,
    ):
        """Constructor for KubernetesServicePatch.

        Args:
            charm: the charm that is instantiating the library.
            ports: a list of ServicePorts
            service_name: allows setting custom name to the patched service. If none given,
                application name will be used.
            service_type: desired type of K8s service. Default value is in line with ServiceSpec's
                default value.
            additional_labels: Labels to be added to the kubernetes service (by default only
                "app.kubernetes.io/name" is set to the service name)
            additional_selectors: Selectors to be added to the kubernetes service (by default only
                "app.kubernetes.io/name" is set to the service name)
            additional_annotations: Annotations to be added to the kubernetes service.
            refresh_event: an optional bound event or list of bound events which
                will be observed to re-apply the patch (e.g. on port change).
                The `install` and `upgrade-charm` events would be observed regardless.
        """
        super().__init__(charm, "kubernetes-service-patch")
        self.charm = charm
        self.service_name = service_name if service_name else self._app
        self.service = self._service_object(
            ports,
            service_name,
            service_type,
            additional_labels,
            additional_selectors,
            additional_annotations,
        )

        # Make mypy type checking happy that self._patch is a method
        assert isinstance(self._patch, MethodType)
        # Ensure this patch is applied during the 'install' and 'upgrade-charm' events
        self.framework.observe(charm.on.install, self._patch)
        self.framework.observe(charm.on.upgrade_charm, self._patch)
        self.framework.observe(charm.on.update_status, self._patch)

        # apply user defined events
        if refresh_event:
            if not isinstance(refresh_event, list):
                refresh_event = [refresh_event]

            for evt in refresh_event:
                self.framework.observe(evt, self._patch)

    def _service_object(
        self,
        ports: List[ServicePort],
        service_name: Optional[str] = None,
        service_type: ServiceType = "ClusterIP",
        additional_labels: Optional[dict] = None,
        additional_selectors: Optional[dict] = None,
        additional_annotations: Optional[dict] = None,
    ) -> Service:
        """Creates a valid Service representation.

        Args:
            ports: a list of ServicePorts
            service_name: allows setting custom name to the patched service. If none given,
                application name will be used.
            service_type: desired type of K8s service. Default value is in line with ServiceSpec's
                default value.
            additional_labels: Labels to be added to the kubernetes service (by default only
                "app.kubernetes.io/name" is set to the service name)
            additional_selectors: Selectors to be added to the kubernetes service (by default only
                "app.kubernetes.io/name" is set to the service name)
            additional_annotations: Annotations to be added to the kubernetes service.

        Returns:
            Service: A valid representation of a Kubernetes Service with the correct ports.
        """
        if not service_name:
            service_name = self._app
        labels = {"app.kubernetes.io/name": self._app}
        if additional_labels:
            labels.update(additional_labels)
        selector = {"app.kubernetes.io/name": self._app}
        if additional_selectors:
            selector.update(additional_selectors)
        return Service(
            apiVersion="v1",
            kind="Service",
            metadata=ObjectMeta(
                namespace=self._namespace,
                name=service_name,
                labels=labels,
                annotations=additional_annotations,  # type: ignore[arg-type]
            ),
            spec=ServiceSpec(
                selector=selector,
                ports=ports,
                type=service_type,
            ),
        )

    def _patch(self, _) -> None:
        """Patch the Kubernetes service created by Juju to map the correct port.

        Raises:
            PatchFailed: if patching fails due to lack of permissions, or otherwise.
        """
        try:
            client = Client()  # pyright: ignore
        except exceptions.ConfigError as e:
            logger.warning("Error creating k8s client: %s", e)
            return

        try:
            if self._is_patched(client):
                return
            if self.service_name != self._app:
                self._delete_and_create_service(client)
            client.patch(Service, self.service_name, self.service, patch_type=PatchType.MERGE)
        except ApiError as e:
            if e.status.code == 403:
                logger.error("Kubernetes service patch failed: `juju trust` this application.")
            else:
                logger.error("Kubernetes service patch failed: %s", str(e))
        else:
            logger.info("Kubernetes service '%s' patched successfully", self._app)

    def _delete_and_create_service(self, client: Client):
        service = client.get(Service, self._app, namespace=self._namespace)
        service.metadata.name = self.service_name  # type: ignore[attr-defined]
        service.metadata.resourceVersion = service.metadata.uid = None  # type: ignore[attr-defined]   # noqa: E501
        client.delete(Service, self._app, namespace=self._namespace)
        client.create(service)

    def is_patched(self) -> bool:
        """Reports if the service patch has been applied.

        Returns:
            bool: A boolean indicating if the service patch has been applied.
        """
        client = Client()  # pyright: ignore
        return self._is_patched(client)

    def _is_patched(self, client: Client) -> bool:
        # Get the relevant service from the cluster
        try:
            service = client.get(Service, name=self.service_name, namespace=self._namespace)
        except ApiError as e:
            if e.status.code == 404 and self.service_name != self._app:
                return False
            logger.error("Kubernetes service get failed: %s", str(e))
            raise

        # Construct a list of expected ports, should the patch be applied
        expected_ports = [(p.port, p.targetPort) for p in self.service.spec.ports]  # type: ignore[attr-defined]
        # Construct a list in the same manner, using the fetched service
        fetched_ports = [
            (p.port, p.targetPort) for p in service.spec.ports  # type: ignore[attr-defined]
        ]  # noqa: E501
        return expected_ports == fetched_ports

    @property
    def _app(self) -> str:
        """Name of the current Juju application.

        Returns:
            str: A string containing the name of the current Juju application.
        """
        return self.charm.app.name

    @property
    def _namespace(self) -> str:
        """The Kubernetes namespace we're running in.

        Returns:
            str: A string containing the name of the current Kubernetes namespace.
        """
        with open("/var/run/secrets/kubernetes.io/serviceaccount/namespace", "r") as f:
            return f.read().strip()