feat: add dex_oidc_config library (#208)

* feat: add dex_oidc_config library

Add a library to handle the dex-oidc-config relation interface. This library
allows dex-auth to share its OIDC configuration with OIDC clients such as the
oidc-gatekeeper charm. The library is designed to be extended to share any
information about Dex, but right now it is only sharing Dex's issuer url.

Closes #203

lib/charms/dex_auth/v0/dex_oidc_config.py

@@ -0,0 +1,373 @@
+#!/usr/bin/env python3
+# Copyright 2024 Canonical Ltd.
+# See LICENSE file for licensing details.
+
+"""Library for sharing Dex's OIDC configuration with OIDC clients.
+
+This library offers a Python API for providing and requesting information about
+Dex's OIDC configuration.
+The default relation name is `dex-oidc-config` and it's recommended to use that name,
+though if changed, you must ensure to pass the correct name when instantiating the
+provider and requirer classes, as well as in `metadata.yaml`.
+
+## Getting Started
+
+### Fetching the library with charmcraft
+
+Using charmcraft you can:
+```shell
+charmcraft fetch-lib charms.dex_auth.v0.dex_oidc_config
+
+
+## Using the library as requirer
+
+### Add relation to metadata.yaml
+```yaml
+requires:
+  dex-oidc-config:
+    interface: dex-oidc-config
+    limit: 1
+```
+
+### Instantiate the DexOidcConfigRequirer class in charm.py
+
+```python
+from ops.charm import CharmBase
+from charms.dex_auth.v0.dex_oidc_config import DexOidcConfigRequirer, DexOidcConfigRelationError
+
+class RequirerCharm(CharmBase):
+    def __init__(self, *args):
+        self._dex_oidc_config_requirer = DexOidcConfigRequirer(self)
+        self.framework.observe(self.on.some_event_emitted, self.some_event_function)
+        self.framework.observe(self._dex_oidc_config_requirer.on.update, self.some_event_function)
+
+    def some_event_function():
+        # use the getter function wherever the info is needed
+        try:
+            k8s_svc_info_data = self._dex_oidc_config_requirer.get_data()
+        except DexOidcConfigRelationError as error:
+            "your error handler goes here"
+```
+
+## Using the library as provider
+
+### Add relation to metadata.yaml
+```yaml
+provides:
+  dex-oidc-config:
+    interface: dex-oidc-config
+```
+
+### Instantiate the DexOidcConfigProvider class in charm.py
+
+```python
+from ops.charm import CharmBase
+from charms.dex_auth.v0.dex_oidc_config import DexOidcConfigProvider, DexOidcConfigRelationError
+
+class ProviderCharm(CharmBase):
+    def __init__(self, *args, **kwargs):
+        ...
+        self._dex_oidc_config_provider = DexOidcConfigProvider(self)
+        self.observe(self.on.some_event, self._some_event_handler)
+
+    def _some_event_handler(self, ...):
+        # This will update the relation data bag with the issuer URL 
+        try:
+            self._dex_oidc_config_provider.send_data(issuer_url)
+        except DexOidcConfigRelationError as error:
+            "your error handler goes here"
+```
+
+Alternatively, if the provider is just broadcasting known data, it can be:
+
+```python
+from ops.charm import CharmBase
+from charms.dex_auth.v0.dex_oidc_config import DexOidcConfigProvider, DexOidcConfigRelationError
+
+class ProviderCharm(CharmBase):
+    def __init__(self, *args, **kwargs):
+        ...
+        self._dex_oidc_config_provider = DexOidcConfigProvider(self)
+```
+
+## Relation data
+
+The data shared by this library is:
+* issuer-url: the canonical URL for the issuer, OIDC cliets use this to refer to Dex
+"""
+import logging
+from typing import List, Optional, Union
+
+from ops.charm import CharmBase, RelationEvent
+from ops.framework import BoundEvent, EventSource, Object, ObjectEvents
+from ops.model import Relation
+from pydantic import BaseModel
+
+# The unique Charmhub library identifier, never change it
+LIBID = "eb5a471989b246e4977399bc8cf9ae6f"
+
+# 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
+
+# Default relation and interface names. If changed, consistency must be kept
+# across the provider and requirer.
+DEFAULT_RELATION_NAME = "dex-oidc-config"
+DEFAULT_INTERFACE_NAME = "dex-oidc-config"
+REQUIRED_ATTRIBUTES = ["issuer-url"]
+
+logger = logging.getLogger(__name__)
+
+
+class DexOidcConfigRelationError(Exception):
+    """Base exception class for any relation error handled by this library."""
+
+    pass
+
+
+class DexOidcConfigRelationMissingError(DexOidcConfigRelationError):
+    """Exception to raise when the relation is missing on either end."""
+
+    def __init__(self):
+        self.message = "Missing relation with a Dex OIDC config provider."
+        super().__init__(self.message)
+
+
+class DexOidcConfigRelationDataMissingError(DexOidcConfigRelationError):
+    """Exception to raise when there is missing data in the relation data bag."""
+
+    def __init__(self, message):
+        self.message = message
+        super().__init__(self.message)
+
+
+class DexOidcConfigUpdatedEvent(RelationEvent):
+    """Indicates the Dex OIDC config data was updated."""
+
+
+class DexOidcConfigEvents(ObjectEvents):
+    """Events for the Dex OIDC config library."""
+
+    updated = EventSource(DexOidcConfigUpdatedEvent)
+
+
+class DexOidcConfigObject(BaseModel):
+    """Representation of a Dex OIDC config object.
+
+    Args:
+        issuer_url: This is the canonical URL that OIDC clients MUST use to refer to dex.
+    """
+
+    issuer_url: str
+
+
+class DexOidcConfigRequirer(Object):
+    """Implement the Requirer end of the Dex OIDC config relation.
+
+    This library emits:
+    * DexOidcConfigUpdatedEvent: when data received on the relation is updated.
+
+    Args:
+        charm (CharmBase): the provider application
+        refresh_events: (list, optional): list of BoundEvents that this manager should handle.
+                       Use this to update the data sent on this relation on demand.
+        relation_name (str, optional): the name of the relation
+
+    Attributes:
+        charm (CharmBase): variable for storing the requirer application
+        relation_name (str): variable for storing the name of the relation
+    """
+
+    on = DexOidcConfigEvents()
+
+    def __init__(
+        self,
+        charm: CharmBase,
+        refresh_events: Optional[List[BoundEvent]] = None,
+        relation_name: Optional[str] = DEFAULT_RELATION_NAME,
+    ):
+        super().__init__(charm, relation_name)
+        self._charm = charm
+        self._relation_name = relation_name
+        self._requirer_wrapper = DexOidcConfigRequirerWrapper(self._charm, self._relation_name)
+
+        self.framework.observe(
+            self._charm.on[self._relation_name].relation_changed, self._on_relation_changed
+        )
+
+        self.framework.observe(
+            self._charm.on[self._relation_name].relation_broken, self._on_relation_broken
+        )
+
+        if refresh_events:
+            for evt in refresh_events:
+                self.framework.observe(evt, self._on_relation_changed)
+
+    def get_data(self) -> DexOidcConfigObject:
+        """Return a DexOidcConfigObject."""
+        return self._requirer_wrapper.get_data()
+
+    def _on_relation_changed(self, event: BoundEvent) -> None:
+        """Handle relation-changed event for this relation."""
+        self.on.updated.emit(event.relation)
+
+    def _on_relation_broken(self, event: BoundEvent) -> None:
+        """Handle relation-broken event for this relation."""
+        self.on.updated.emit(event.relation)
+
+
+class DexOidcConfigRequirerWrapper(Object):
+    """Wrapper for the relation data getting logic.
+
+    Args:
+        charm (CharmBase): the requirer application
+        relation_name (str, optional): the name of the relation
+
+    Attributes:
+        relation_name (str): variable for storing the name of the relation
+    """
+
+    def __init__(self, charm, relation_name: Optional[str] = DEFAULT_RELATION_NAME):
+        super().__init__(charm, relation_name)
+        self.relation_name = relation_name
+
+    @staticmethod
+    def _validate_relation(relation: Optional[Relation]) -> None:
+        """Series of checks for the relation and relation data.
+
+        Args:
+            relation (optional, Relation): the relation object to run the checks on.
+              This object must always come from a call of get_relation, which
+              can either return a Relation object or None.
+
+        Raises:
+            DexOidcConfigRelationDataMissingError if data is missing or incomplete
+            DexOidcConfigRelationMissingError: if there is no related application
+        """
+        # Raise if there is no related application
+        if not relation:
+            raise DexOidcConfigRelationMissingError()
+
+        # Extract remote app information from relation
+        remote_app = relation.app
+        # Get relation data from remote app
+        relation_data = relation.data[remote_app]
+
+        # Raise if there is no data found in the relation data bag
+        if not relation_data:
+            raise DexOidcConfigRelationDataMissingError(
+                f"No data found in relation {relation.name} data bag."
+            )
+
+    def get_data(self) -> DexOidcConfigObject:
+        """Return a DexOidcConfigObject containing Dex's OIDC configuration.
+
+        Raises:
+            DexOidcConfigRelationDataMissingError: if data is missing entirely or some attributes
+            DexOidcConfigRelationMissingError: if there is no related application
+            ops.model.TooManyRelatedAppsError: if there is more than one related application
+        """
+        # Validate relation data
+        # Raises TooManyRelatedAppsError if related to more than one app
+        relation = self.model.get_relation(self.relation_name)
+
+        self._validate_relation(relation=relation)
+
+        # Get relation data from remote app
+        relation_data = relation.data[relation.app]
+
+        return DexOidcConfigObject(issuer_url=relation_data["issuer-url"])
+
+
+class DexOidcConfigProvider(Object):
+    """Implement the Provider end of the Dex OIDC config relation.
+
+    Observes relation events to send data to related applications.
+
+    Args:
+        charm (CharmBase): the provider application
+        issuer_url (str): This is the canonical URL that OIDC clients MUST use to refer to dex.
+        refresh_events: (list, optional): list of BoundEvents that this manager should handle.  Use this to update
+                       the data sent on this relation on demand.
+        relation_name (str, optional): the name of the relation
+
+    Attributes:
+        charm (CharmBase): variable for storing the provider application
+        relation_name (str): variable for storing the name of the relation
+    """
+
+    def __init__(
+        self,
+        charm: CharmBase,
+        issuer_url: str,
+        refresh_events: Optional[List[BoundEvent]] = None,
+        relation_name: Optional[str] = DEFAULT_RELATION_NAME,
+    ):
+        super().__init__(charm, relation_name)
+        self.charm = charm
+        self.relation_name = relation_name
+        self._provider_wrapper = DexOidcConfigProviderWrapper(self.charm, self.relation_name)
+        self._issuer_url = issuer_url
+
+        self.framework.observe(self.charm.on.leader_elected, self._send_data)
+
+        self.framework.observe(self.charm.on[self.relation_name].relation_created, self._send_data)
+
+        if refresh_events:
+            for evt in refresh_events:
+                self.framework.observe(evt, self._send_data)
+
+    def _send_data(self, _) -> None:
+        """Serve as an event handler for sending Dex's OIDC configuration."""
+        self._provider_wrapper.send_data(self._issuer_url)
+
+
+class DexOidcConfigProviderWrapper(Object):
+    """Wrapper for the relation data sending logic.
+
+    Args:
+        charm (CharmBase): the provider application
+        relation_name (str, optional): the name of the relation
+
+    Attributes:
+        charm (CharmBase): variable for storing the provider application
+        relation_name (str): variable for storing the name of the relation
+    """
+
+    def __init__(self, charm: CharmBase, relation_name: Optional[str] = DEFAULT_RELATION_NAME):
+        super().__init__(charm, relation_name)
+        self.charm = charm
+        self.relation_name = relation_name
+
+    def send_data(
+        self,
+        issuer_url: str,
+    ) -> None:
+        """Update the relation data bag with data from Dex's OIDC configuration.
+
+        This method will complete successfully even if there are no related applications.
+
+        Args:
+            issuer_url (str): This is the canonical URL that OIDC clients MUST use to refer to dex.
+        """
+        # Validate unit is leader to send data; otherwise return
+        if not self.charm.model.unit.is_leader():
+            logger.info(
+                "DexOidcConfigProvider handled send_data event when it is not the leader."
+                "Skipping event - no data sent."
+            )
+            return
+
+        # Update the relation data bag with Dex's OIDC configuration
+        relations = self.charm.model.relations[self.relation_name]
+
+        # Update relation data
+        for relation in relations:
+            relation.data[self.charm.app].update(
+                {
+                    "issuer-url": issuer_url,
+                }
+            )