refactor(auth): type auth providers and decouple Google auth from disk storage

- Make MailIntakeAuthProvider generic over credential type to enforce
  typed auth contracts between providers and adapters
- Refactor Google OAuth provider to use CredentialStore abstraction
  instead of filesystem-based pickle persistence
- Remove node-local state assumptions from Google auth implementation
- Clarify documentation to distinguish credential lifecycle from
  credential persistence responsibilities

This change enables distributed-safe authentication providers and
allows multiple credential persistence strategies without modifying
auth logic.

mail_intake/auth/base.py

@@ -6,45 +6,54 @@ adapters to obtain provider-specific credentials.
 
 Authentication concerns are intentionally decoupled from adapter logic.
 Adapters depend only on this interface and must not be aware of how
-credentials are acquired, refreshed, or stored.
+credentials are acquired, refreshed, or persisted.
 """
 
 from abc import ABC, abstractmethod
+from typing import Generic, TypeVar
+
+T = TypeVar("T")
 
 
-class MailIntakeAuthProvider(ABC):
+class MailIntakeAuthProvider(ABC, Generic[T]):
     """
-    Abstract authentication provider.
+    Abstract base class for authentication providers.
 
-    Mail adapters depend on this interface, not on concrete
+    This interface enforces a strict contract between authentication
-    OAuth or credential implementations.
+    providers and mail adapters by requiring providers to explicitly
+    declare the type of credentials they return.
 
-    Authentication providers encapsulate all logic required to acquire
+    Authentication providers encapsulate all logic required to:
-    valid credentials for a mail provider.
+    - Acquire credentials from an external provider
+    - Refresh or revalidate credentials as needed
+    - Handle authentication-specific failure modes
+    - Coordinate with credential persistence layers where applicable
 
-    Implementations may involve:
+    Mail adapters must treat returned credentials as opaque and
-    - OAuth flows
+    provider-specific, relying only on the declared credential type
-    - Service account credentials
+    expected by the adapter.
-    - Token refresh logic
-    - Secure credential storage
-
-    Adapters must treat the returned credentials as opaque and provider-specific.
     """
 
     @abstractmethod
-    def get_credentials(self):
+    def get_credentials(self) -> T:
         """
-        Return provider-specific credentials object.
+        Retrieve valid, provider-specific credentials.
 
-        This method is synchronous by design and must either
+        This method is synchronous by design and represents the sole
-        return valid credentials or raise MailIntakeAuthError.
+        entry point through which adapters obtain authentication
+        material.
+
+        Implementations must either return credentials of the declared
+        type ``T`` that are valid at the time of return or raise an
+        authentication-specific exception.
 
         Returns:
-            Provider-specific credentials object suitable for use by
+            Credentials of type ``T`` suitable for immediate use by the
-            the corresponding mail adapter.
+            corresponding mail adapter.
 
         Raises:
-            Exception: Authentication-specific errors defined by the
+            Exception:
-                implementation.
+                An authentication-specific exception indicating that
+                credentials could not be obtained or validated.
         """
         raise NotImplementedError

mail_intake/auth/google.py

@@ -8,19 +8,21 @@ It encapsulates all Google-specific authentication concerns, including:
 - Credential loading and persistence
 - Token refresh handling
 - Interactive OAuth flow initiation
+- Coordination with a credential persistence layer
 
 No Google authentication details should leak outside this module.
 """
 
 import os
-import pickle
 from typing import Sequence
 
 import google.auth.exceptions
 from google.auth.transport.requests import Request
 from google_auth_oauthlib.flow import InstalledAppFlow
+from google.oauth2.credentials import Credentials
 
 from mail_intake.auth.base import MailIntakeAuthProvider
+from mail_intake.credentials.store import CredentialStore
 from mail_intake.exceptions import MailIntakeAuthError
 
 
@@ -32,10 +34,10 @@ class MailIntakeGoogleAuth(MailIntakeAuthProvider):
     Google's OAuth 2.0 flow and credential management libraries.
 
     Responsibilities:
-    - Load cached credentials from disk when available
+    - Load cached credentials from a credential store when available
     - Refresh expired credentials when possible
     - Initiate an interactive OAuth flow only when required
-    - Persist refreshed or newly obtained credentials
+    - Persist refreshed or newly obtained credentials via the store
 
     This class is synchronous by design and maintains a minimal internal state.
     """
@@ -43,48 +45,47 @@ class MailIntakeGoogleAuth(MailIntakeAuthProvider):
     def __init__(
         self,
         credentials_path: str,
-        token_path: str,
+        store: CredentialStore[Credentials],
         scopes: Sequence[str],
     ):
         """
         Initialize the Google authentication provider.
 
         Args:
-            credentials_path: Path to the Google OAuth client secrets file.
+            credentials_path:
-            token_path: Path where OAuth tokens will be cached.
+                Path to the Google OAuth client secrets file used to
-            scopes: OAuth scopes required for access.
+                initiate the OAuth 2.0 flow.
+
+            store:
+                Credential store responsible for persisting and
+                retrieving Google OAuth credentials.
+
+            scopes:
+                OAuth scopes required for Gmail access.
         """
         self.credentials_path = credentials_path
-        self.token_path = token_path
+        self.store = store
         self.scopes = list(scopes)
 
-    def get_credentials(self):
+    def get_credentials(self) -> Credentials:
         """
         Retrieve valid Google OAuth credentials.
 
         This method attempts to:
-        1. Load cached credentials from disk
+        1. Load cached credentials from the configured credential store
         2. Refresh expired credentials when possible
         3. Perform an interactive OAuth login as a fallback
         4. Persist valid credentials for future use
 
         Returns:
-            Google OAuth credentials object suitable for use with
+            A ``google.oauth2.credentials.Credentials`` instance suitable
-            Google API clients.
+            for use with Google API clients.
 
         Raises:
             MailIntakeAuthError: If credentials cannot be loaded, refreshed,
                 or obtained via interactive authentication.
         """
-        creds = None
+        creds = self.store.load()
-
-        # Attempt to load cached credentials
-        if os.path.exists(self.token_path):
-            try:
-                with open(self.token_path, "rb") as fh:
-                    creds = pickle.load(fh)
-            except Exception:
-                creds = None
 
         # Validate / refresh credentials
         if not creds or not creds.valid:
@@ -92,6 +93,7 @@ class MailIntakeGoogleAuth(MailIntakeAuthProvider):
                 try:
                     creds.refresh(Request())
                 except google.auth.exceptions.RefreshError:
+                    self.store.clear()
                     creds = None
 
             # Interactive login if refresh failed or creds missing
@@ -112,13 +114,12 @@ class MailIntakeGoogleAuth(MailIntakeAuthProvider):
                         "Failed to complete Google OAuth flow"
                     ) from exc
 
-            # Persist refreshed / new credentials
+            # Persist refreshed or newly obtained credentials
             try:
-                with open(self.token_path, "wb") as fh:
+                self.store.save(creds)
-                    pickle.dump(creds, fh)
             except Exception as exc:
                 raise MailIntakeAuthError(
-                    f"Failed to write token file: {self.token_path}"
+                    "Failed to persist Google OAuth credentials"
                 ) from exc
 
         return creds

mail_intake/credentials/__init__.py

@@ -0,0 +1,8 @@
+"""
+from mail_intake.credentials.store import CredentialStore
+
+from google.oauth2.credentials import Credentials
+
+GoogleCredentialStore = CredentialStore[Credentials]
+
+"""

mail_intake/credentials/pickle.py

@@ -0,0 +1,96 @@
+"""
+Local filesystem–based credential persistence for Mail Intake.
+
+This module provides a file-backed implementation of the
+``CredentialStore`` abstraction using Python's ``pickle`` module.
+
+The pickle-based credential store is intended for local development,
+single-node deployments, and controlled environments where credentials
+do not need to be shared across processes or machines.
+
+Due to the security and portability risks associated with pickle-based
+serialization, this implementation is not suitable for distributed or
+untrusted environments.
+"""
+
+import pickle
+from typing import Optional, TypeVar
+
+from mail_intake.credentials.store import CredentialStore
+
+T = TypeVar("T")
+
+
+class PickleCredentialStore(CredentialStore[T]):
+    """
+    Filesystem-backed credential store using pickle serialization.
+
+    This store persists credentials as a pickled object on the local
+    filesystem. It is a simple implementation intended primarily for
+    development, testing, and single-process execution contexts.
+
+    This implementation:
+    - Stores credentials on the local filesystem
+    - Uses pickle for serialization and deserialization
+    - Does not provide encryption, locking, or concurrency guarantees
+
+    Credential lifecycle management, validation, and refresh logic are
+    explicitly out of scope for this class.
+    """
+
+    def __init__(self, path: str):
+        """
+        Initialize a pickle-backed credential store.
+
+        Args:
+            path:
+                Filesystem path where credentials will be stored.
+                The file will be created or overwritten as needed.
+        """
+        self.path = path
+
+    def load(self) -> Optional[T]:
+        """
+        Load credentials from the local filesystem.
+
+        If the credential file does not exist or cannot be successfully
+        deserialized, this method returns ``None``.
+
+        The store does not attempt to validate or interpret the returned
+        credentials.
+
+        Returns:
+            An instance of type ``T`` if credentials are present and
+            successfully deserialized; otherwise ``None``.
+        """
+        try:
+            with open(self.path, "rb") as fh:
+                return pickle.load(fh)
+        except Exception:
+            return None
+
+    def save(self, credentials: T) -> None:
+        """
+        Persist credentials to the local filesystem.
+
+        Any previously stored credentials at the configured path are
+        overwritten.
+
+        Args:
+            credentials:
+                The credential object to persist.
+        """
+        with open(self.path, "wb") as fh:
+            pickle.dump(credentials, fh)
+
+    def clear(self) -> None:
+        """
+        Remove persisted credentials from the local filesystem.
+
+        This method deletes the credential file if it exists and should
+        be treated as an idempotent operation.
+        """
+        import os
+
+        if os.path.exists(self.path):
+            os.remove(self.path)

mail_intake/credentials/redis.py

@@ -0,0 +1,142 @@
+"""
+Redis-backed credential persistence for Mail Intake.
+
+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.
+
+    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.
+
+    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.
+    """
+
+    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:
+                An initialized Redis client instance (for example,
+                ``redis.Redis`` or a compatible interface) used to
+                communicate with the Redis server.
+
+            key:
+                The Redis key under which credentials are stored.
+                Callers are responsible for applying appropriate
+                namespacing to avoid collisions.
+
+            serialize:
+                A callable that converts a credential object of type
+                ``T`` into a ``bytes`` representation suitable for
+                storage in Redis.
+
+            deserialize:
+                A callable that converts a ``bytes`` payload retrieved
+                from Redis back into a credential object of type ``T``.
+
+            ttl_seconds:
+                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.
+
+        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.
+
+        Returns:
+            An instance of type ``T`` if credentials are present and
+            successfully deserialized; otherwise ``None``.
+        """
+        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.
+
+        Any previously stored credentials under the same key are
+        overwritten. If a TTL is configured, the credentials will
+        expire automatically after the specified duration.
+
+        Args:
+            credentials:
+                The credential object to persist.
+        """
+        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.
+
+        This operation deletes the configured Redis key if it exists.
+        Implementations should treat this method as idempotent.
+        """
+        self.redis.delete(self.key)

mail_intake/credentials/store.py

@@ -0,0 +1,96 @@
+"""
+Credential persistence abstractions for Mail Intake.
+
+This module defines the generic persistence contract used to store and
+retrieve authentication credentials across Mail Intake components.
+
+The ``CredentialStore`` abstraction establishes a strict separation
+between credential *lifecycle management* and credential *storage*.
+Authentication providers are responsible for acquiring, validating,
+refreshing, and revoking credentials, while concrete store
+implementations are responsible solely for persistence concerns.
+
+By remaining agnostic to credential structure, serialization format,
+and storage backend, this module enables multiple persistence
+strategies—such as local files, in-memory caches, distributed stores,
+or secrets managers—without coupling authentication logic to any
+specific storage mechanism.
+"""
+
+
+from abc import ABC, abstractmethod
+from typing import Generic, Optional, TypeVar
+
+T = TypeVar("T")
+
+
+class CredentialStore(ABC, Generic[T]):
+    """
+    Abstract base class defining a generic persistence interface for
+    authentication credentials.
+
+    This interface separates *credential lifecycle management* from
+    *credential storage mechanics*. Implementations are responsible
+    only for persistence concerns, while authentication providers
+    retain full control over credential creation, validation, refresh,
+    and revocation logic.
+
+    The store is intentionally agnostic to:
+    - The concrete credential type being stored
+    - The serialization format used to persist credentials
+    - The underlying storage backend or durability guarantees
+
+    Type Parameters:
+        T:
+            The concrete credential type managed by the store. This may
+            represent OAuth credentials, API tokens, session objects,
+            or any other authentication material.
+    """
+
+    @abstractmethod
+    def load(self) -> Optional[T]:
+        """
+        Load previously persisted credentials.
+
+        Implementations should return ``None`` when no credentials are
+        present or when stored credentials cannot be successfully
+        decoded or deserialized.
+
+        The store must not attempt to validate, refresh, or otherwise
+        interpret the returned credentials.
+
+        Returns:
+            An instance of type ``T`` if credentials are available and
+            loadable; otherwise ``None``.
+        """
+
+    @abstractmethod
+    def save(self, credentials: T) -> None:
+        """
+        Persist credentials to the underlying storage backend.
+
+        This method is invoked when credentials are newly obtained or
+        have been refreshed and are known to be valid at the time of
+        persistence.
+
+        Implementations are responsible for:
+        - Ensuring durability appropriate to the deployment context
+        - Applying encryption or access controls where required
+        - Overwriting any previously stored credentials
+
+        Args:
+            credentials:
+                The credential object to persist.
+        """
+
+    @abstractmethod
+    def clear(self) -> None:
+        """
+        Remove any persisted credentials from the store.
+
+        This method is called when credentials are known to be invalid,
+        revoked, corrupted, or otherwise unusable, and must ensure that
+        no stale authentication material remains accessible.
+
+        Implementations should treat this operation as idempotent.
+        """