"""
Redis-backed credential persistence for Mail Intake.

---

## Summary

This module provides a Redis-based implementation of the
``CredentialStore`` abstraction, enabling credential persistence
across distributed and horizontally scaled deployments.

The Redis credential store is designed for environments where
authentication credentials must be shared safely across multiple
processes, containers, or nodes, such as container orchestration
platforms and microservice architectures.

Key characteristics:
- Distributed-safe, shared storage using Redis
- Explicit, caller-defined serialization and deserialization
- No reliance on unsafe mechanisms such as pickle
- Optional time-to-live (TTL) support for automatic credential expiry

This module is responsible solely for persistence concerns.
Credential validation, refresh, rotation, and acquisition remain the
responsibility of authentication provider implementations.
"""


from typing import Optional, TypeVar, Callable

from mail_intake.credentials.store import CredentialStore

T = TypeVar("T")


class RedisCredentialStore(CredentialStore[T]):
    """
    Redis-backed implementation of ``CredentialStore``.

    This store persists credentials in Redis and is suitable for
    distributed and horizontally scaled deployments where credentials
    must be shared across multiple processes or nodes.

    Notes:
        **Responsibilities:**

            - This class is responsible only for persistence and retrieval
            - It does not interpret, validate, refresh, or otherwise manage the lifecycle of the credentials being stored

        **Guarantees:**

            - The store is intentionally generic and delegates all serialization concerns to caller-provided functions
            - This avoids unsafe mechanisms such as pickle and allows credential formats to be explicitly controlled and audited
    """

    def __init__(
        self,
        redis_client,
        key: str,
        serialize: Callable[[T], bytes],
        deserialize: Callable[[bytes], T],
        ttl_seconds: Optional[int] = None,
    ):
        """
        Initialize a Redis-backed credential store.

        Args:
            redis_client (Any):
                An initialized Redis client instance (for example, ``redis.Redis`` or a compatible interface) used to communicate with the Redis server.

            key (str):
                The Redis key under which credentials are stored. Callers are responsible for applying appropriate namespacing to avoid collisions.

            serialize (Callable[[T], bytes]):
                A callable that converts a credential object of type ``T`` into a ``bytes`` representation suitable for storage in Redis.

            deserialize (Callable[[bytes], T]):
                A callable that converts a ``bytes`` payload retrieved from Redis back into a credential object of type ``T``.

            ttl_seconds (Optional[int]):
                Optional time-to-live (TTL) for the stored credentials, expressed in seconds. When provided, Redis will automatically expire the stored credentials after the specified duration. If ``None``, credentials are stored without an expiration.
        """
        self.redis = redis_client
        self.key = key
        self.serialize = serialize
        self.deserialize = deserialize
        self.ttl_seconds = ttl_seconds

    def load(self) -> Optional[T]:
        """
        Load credentials from Redis.

        Returns:
            Optional[T]:
                An instance of type ``T`` if credentials are present and
                successfully deserialized; otherwise ``None``.

        Notes:
            **Guarantees:**

                - If no value exists for the configured key, or if the stored payload cannot be successfully deserialized, this method returns ``None``
                - The store does not attempt to validate the returned credentials or determine whether they are expired or otherwise usable
        """
        raw = self.redis.get(self.key)
        if not raw:
            return None
        try:
            return self.deserialize(raw)
        except Exception:
            return None

    def save(self, credentials: T) -> None:
        """
        Persist credentials to Redis.

        Args:
            credentials (T):
                The credential object to persist.

        Notes:
            **Responsibilities:**

                - Any previously stored credentials under the same key are overwritten
                - If a TTL is configured, the credentials will expire automatically after the specified duration
        """
        payload = self.serialize(credentials)
        if self.ttl_seconds:
            self.redis.setex(self.key, self.ttl_seconds, payload)
        else:
            self.redis.set(self.key, payload)

    def clear(self) -> None:
        """
        Remove stored credentials from Redis.

        Notes:
            **Lifecycle:**

                - This operation deletes the configured Redis key if it exists
                - Implementations should treat this method as idempotent
        """
        self.redis.delete(self.key)