feat: add Spotify client and auth routes

backend/app/adapters/spotify/client.py

@@ -0,0 +1,231 @@
+"""Thin asynchronous client for the Spotify Web API."""
+
+import asyncio
+
+import httpx2
+
+from app.adapters.spotify.auth import refresh_access_token
+from app.adapters.spotify.errors import (
+    SpotifyAuthenticationError,
+    SpotifyRateLimitedError,
+    SpotifyRequestError,
+    SpotifyUnavailableError,
+)
+from app.adapters.spotify.mapping import (
+    CreatedPlaylist,
+    CurrentUser,
+    parse_created_playlist,
+    parse_current_user,
+    parse_search_tracks,
+)
+from app.adapters.spotify.session import SpotifySession
+from app.config import Settings
+from app.domain.models import Track
+
+
+class SpotifyClient:
+    """Call the supported Spotify endpoints with bounded retry behavior."""
+
+    def __init__(
+        self,
+        http: httpx2.AsyncClient,
+        session: SpotifySession,
+        settings: Settings,
+    ) -> None:
+        """Bind the shared transport to one authenticated Spotify session."""
+        self.http = http
+        self.session = session
+        self.settings = settings
+
+    async def search_tracks(self, query: str, limit: int = 10) -> list[Track]:
+        """Search Spotify tracks and return only valid mapped results."""
+        response = await self._request(
+            "GET",
+            "/search",
+            params={"q": query, "type": "track", "limit": limit},
+        )
+        return parse_search_tracks(response.json())
+
+    async def fetch_current_user(self) -> CurrentUser:
+        """Fetch the authenticated Spotify user's stable identity."""
+        response = await self._request("GET", "/me")
+        return parse_current_user(response.json())
+
+    async def create_playlist(self, name: str, description: str) -> CreatedPlaylist:
+        """Create a private playlist for the authenticated user."""
+        response = await self._request(
+            "POST",
+            "/me/playlists",
+            json={"name": name, "description": description, "public": False},
+        )
+        return parse_created_playlist(response.json())
+
+    async def add_tracks_to_playlist(self, playlist_id: str, track_uris: list[str]) -> None:
+        """Add tracks to a playlist without retrying an ambiguous write."""
+        await self._request(
+            "POST",
+            f"/playlists/{playlist_id}/items",
+            json={"uris": track_uris},
+        )
+
+    async def _request(
+        self,
+        method: str,
+        path: str,
+        *,
+        params: dict[str, str | int] | None = None,
+        json: dict[str, object] | None = None,
+    ) -> httpx2.Response:
+        access_token = await self._access_token()
+        response = await self._send(
+            method,
+            path,
+            access_token,
+            params=params,
+            json=json,
+        )
+        response = await self._retry_once_if_unauthorized(
+            response,
+            method,
+            path,
+            access_token,
+            params=params,
+            json=json,
+        )
+        response = await self._retry_once_if_rate_limited(
+            response,
+            method,
+            path,
+            params=params,
+            json=json,
+        )
+        return self._raise_for_error(response)
+
+    async def _send(
+        self,
+        method: str,
+        path: str,
+        access_token: str,
+        *,
+        params: dict[str, str | int] | None,
+        json: dict[str, object] | None,
+    ) -> httpx2.Response:
+        return await self.http.request(
+            method,
+            f"{self.settings.spotify_api_base_url.rstrip('/')}{path}",
+            params=params,
+            json=json,
+            headers={"Authorization": f"Bearer {access_token}"},
+        )
+
+    async def _retry_once_if_unauthorized(
+        self,
+        response: httpx2.Response,
+        method: str,
+        path: str,
+        access_token: str,
+        *,
+        params: dict[str, str | int] | None,
+        json: dict[str, object] | None,
+    ) -> httpx2.Response:
+        if response.status_code != 401:
+            return response
+
+        await self._refresh_if_current(access_token)
+        return await self._send(
+            method,
+            path,
+            self.session.tokens.access_token,
+            params=params,
+            json=json,
+        )
+
+    async def _retry_once_if_rate_limited(
+        self,
+        response: httpx2.Response,
+        method: str,
+        path: str,
+        *,
+        params: dict[str, str | int] | None,
+        json: dict[str, object] | None,
+    ) -> httpx2.Response:
+        if response.status_code != 429:
+            return response
+
+        retry_after_seconds = _parse_retry_after(response)
+        _, reason = _parse_error_details(response)
+        if (
+            method != "GET"
+            or reason == "QUOTA_EXCEEDED"
+            or retry_after_seconds is None
+            or retry_after_seconds > self.settings.spotify_retry_after_cap_seconds
+        ):
+            return response
+
+        await asyncio.sleep(retry_after_seconds)
+        return await self._send(
+            method,
+            path,
+            self.session.tokens.access_token,
+            params=params,
+            json=json,
+        )
+
+    def _raise_for_error(self, response: httpx2.Response) -> httpx2.Response:
+        if response.status_code < 400:
+            return response
+
+        message, reason = _parse_error_details(response)
+        if response.status_code == 401:
+            raise SpotifyAuthenticationError("Spotify rejected refreshed authentication")
+        if response.status_code == 429:
+            raise SpotifyRateLimitedError(_parse_retry_after(response), reason)
+        if response.status_code >= 500:
+            raise SpotifyUnavailableError(response.status_code, message)
+        raise SpotifyRequestError(response.status_code, message)
+
+    async def _access_token(self) -> str:
+        access_token = self.session.tokens.access_token
+        if self.session.tokens.is_expired:
+            await self._refresh_if_current(access_token)
+        return self.session.tokens.access_token
+
+    async def _refresh_if_current(self, access_token: str) -> None:
+        async with self.session.refresh_lock:
+            if self.session.tokens.access_token != access_token:
+                return
+            self.session.tokens = await refresh_access_token(
+                self.http,
+                client_id=self.settings.spotify_client_id,
+                tokens=self.session.tokens,
+            )
+
+
+def _parse_retry_after(response: httpx2.Response) -> float | None:
+    value = response.headers.get("Retry-After")
+    if value is None:
+        return None
+    try:
+        retry_after_seconds = float(value)
+    except ValueError:
+        return None
+    return retry_after_seconds if retry_after_seconds >= 0 else None
+
+
+def _parse_error_details(response: httpx2.Response) -> tuple[str | None, str | None]:
+    try:
+        payload: object = response.json()
+    except ValueError:
+        return None, None
+    if not isinstance(payload, dict):
+        return None, None
+
+    error = payload.get("error")
+    if not isinstance(error, dict):
+        return None, None
+    message = error.get("message")
+    reason = error.get("reason")
+    return (
+        message if isinstance(message, str) else None,
+        reason if isinstance(reason, str) else None,
+    )

backend/app/adapters/spotify/login.py

@@ -0,0 +1,66 @@
+"""Spotify login workflow independent of HTTP routing."""
+
+import secrets
+
+import httpx2
+
+from app.adapters.spotify.auth import (
+    build_authorize_url,
+    derive_code_challenge,
+    exchange_authorization_code,
+    generate_code_verifier,
+)
+from app.adapters.spotify.client import SpotifyClient
+from app.adapters.spotify.errors import SpotifyError
+from app.adapters.spotify.session import PendingLogins, SessionStore, SpotifySession
+from app.config import Settings
+
+
+def begin_login(settings: Settings, pending_logins: PendingLogins) -> str:
+    """Store a pending PKCE login and return its Spotify authorization URL."""
+    state = secrets.token_urlsafe(32)
+    code_verifier = generate_code_verifier()
+    pending_logins.add(state, code_verifier)
+    return build_authorize_url(
+        settings.spotify_client_id,
+        settings.spotify_redirect_uri,
+        state,
+        derive_code_challenge(code_verifier),
+    )
+
+
+async def complete_login(
+    http: httpx2.AsyncClient,
+    settings: Settings,
+    pending_logins: PendingLogins,
+    session_store: SessionStore,
+    code: str,
+    state: str,
+) -> str | None:
+    """Complete a pending Spotify login and return its application session ID."""
+    code_verifier = pending_logins.pop(state)
+    if code_verifier is None:
+        return None
+
+    try:
+        tokens = await exchange_authorization_code(
+            http,
+            client_id=settings.spotify_client_id,
+            redirect_uri=settings.spotify_redirect_uri,
+            code=code,
+            code_verifier=code_verifier,
+        )
+        bootstrap_session = SpotifySession(tokens=tokens, account_id="", display_name="")
+        current_user = await SpotifyClient(
+            http,
+            bootstrap_session,
+            settings,
+        ).fetch_current_user()
+        session = SpotifySession(
+            tokens=bootstrap_session.tokens,
+            account_id=current_user.account_id,
+            display_name=current_user.display_name,
+        )
+        return session_store.create(session)
+    except (SpotifyError, ValueError):
+        return None

backend/app/adapters/spotify/mapping.py

@@ -0,0 +1,132 @@
+"""Map Spotify response data into application-owned models."""
+
+from collections.abc import Mapping
+from dataclasses import dataclass
+from typing import cast
+
+from app.domain.models import Track
+
+
+@dataclass(frozen=True)
+class CurrentUser:
+    """The stable identity fields returned for the current Spotify user."""
+
+    account_id: str
+    display_name: str
+
+
+@dataclass(frozen=True)
+class CreatedPlaylist:
+    """The application-owned result of creating a Spotify playlist."""
+
+    id: str
+    url: str
+
+
+def parse_search_tracks(payload: object) -> list[Track]:
+    """Map valid Spotify search items and discard malformed entries."""
+    root = _as_mapping(payload)
+    tracks = _as_mapping(root.get("tracks")) if root is not None else None
+    items = tracks.get("items") if tracks is not None else None
+    if not isinstance(items, list):
+        return []
+
+    parsed_tracks: list[Track] = []
+    for item in items:
+        parsed_track = _parse_track(item)
+        if parsed_track is not None:
+            parsed_tracks.append(parsed_track)
+    return parsed_tracks
+
+
+def parse_current_user(payload: object) -> CurrentUser:
+    """Map a Spotify current-user response into stable identity fields."""
+    root = _as_mapping(payload)
+    account_id = _required_string(root, "account_id")
+    display_name = _required_string(root, "display_name")
+    if account_id is None or display_name is None:
+        raise ValueError("Spotify returned an invalid current-user response")
+    return CurrentUser(account_id=account_id, display_name=display_name)
+
+
+def parse_created_playlist(payload: object) -> CreatedPlaylist:
+    """Map a Spotify playlist creation response."""
+    root = _as_mapping(payload)
+    playlist_id = _required_string(root, "id")
+    external_urls = _as_mapping(root.get("external_urls")) if root is not None else None
+    url = _required_string(external_urls, "spotify")
+    if playlist_id is None or url is None:
+        raise ValueError("Spotify returned an invalid playlist response")
+    return CreatedPlaylist(id=playlist_id, url=url)
+
+
+def _parse_track(payload: object) -> Track | None:
+    item = _as_mapping(payload)
+    track_id = _required_string(item, "id")
+    uri = _required_string(item, "uri")
+    title = _required_string(item, "name")
+    album = _as_mapping(item.get("album")) if item is not None else None
+    album_name = _required_string(album, "name")
+    artists = _parse_artists(item.get("artists")) if item is not None else None
+
+    if (
+        item is None
+        or track_id is None
+        or uri is None
+        or title is None
+        or album_name is None
+        or artists is None
+    ):
+        return None
+
+    return Track(
+        id=track_id,
+        uri=uri,
+        title=title,
+        artists=artists,
+        album_name=album_name,
+        album_art_url=_parse_album_art_url(album),
+        external_url=_parse_external_url(item),
+    )
+
+
+def _parse_artists(payload: object) -> tuple[str, ...] | None:
+    if not isinstance(payload, list) or not payload:
+        return None
+
+    names: list[str] = []
+    for artist_payload in payload:
+        name = _required_string(_as_mapping(artist_payload), "name")
+        if name is None:
+            return None
+        names.append(name)
+    return tuple(names)
+
+
+def _parse_album_art_url(album: Mapping[str, object] | None) -> str | None:
+    images = album.get("images") if album is not None else None
+    if not isinstance(images, list) or not images:
+        return None
+    return _optional_string(_as_mapping(images[0]), "url")
+
+
+def _parse_external_url(item: Mapping[str, object]) -> str | None:
+    return _optional_string(_as_mapping(item.get("external_urls")), "spotify")
+
+
+def _as_mapping(payload: object) -> Mapping[str, object] | None:
+    if not isinstance(payload, Mapping):
+        return None
+    return cast(Mapping[str, object], payload)
+
+
+def _required_string(payload: Mapping[str, object] | None, key: str) -> str | None:
+    value = _optional_string(payload, key)
+    return value if value else None
+
+
+def _optional_string(payload: Mapping[str, object] | None, key: str) -> str | None:
+    if payload is None:
+        return None
+    value = payload.get(key)
+    return value if isinstance(value, str) else None