accounts: sign in with Discord, sharing the existing link callback (#115)

backend/accounts/models.py

@@ -34,13 +34,19 @@ class User(SQLAlchemyBaseUserTableUUID, Base):
     # is_superuser, is_verified. Optional extras:
     display_name: Mapped[str | None] = mapped_column(String(120), nullable=True)
     # Linked Discord account id (OAuth2 identify) — the key DM alerts reach the user
-    # by. NB: create_all only makes this on a fresh DB; an existing prod accounts.db
-    # needs the manual migration in deploy/migrations/ (no Alembic in this project).
+    # by, and the identity "Sign in with Discord" resolves an account from. Unique,
+    # so one Discord account can never own two Thermograph accounts.
     discord_id: Mapped[str | None] = mapped_column(String(32), unique=True, nullable=True)
     # Whether to also deliver alerts as a Discord DM. Set True on linking (an active
     # opt-in); the user can mute it while staying linked. Same migration caveat.
     discord_dm: Mapped[bool] = mapped_column(Boolean, nullable=False, default=False,
                                              server_default=false())
+    # True when the account was created by "Sign in with Discord" and so has no
+    # password its owner has ever seen (hashed_password is NOT NULL, so the row
+    # carries a generated one nobody knows). Load-bearing: unlinking Discord from
+    # such an account would remove its only way in, so discord_link.py refuses.
+    discord_only: Mapped[bool] = mapped_column(Boolean, nullable=False, default=False,
+                                               server_default=false())
 
 
 class AccessToken(SQLAlchemyBaseAccessTokenTableUUID, Base):

backend/accounts/schemas.py

@@ -24,6 +24,9 @@ class UserRead(schemas.BaseUser[uuid.UUID]):
     # flow (discord_link.py), not editable through the profile.
     discord_id: str | None = None
     discord_dm: bool = False
+    # True when Discord is the account's only credential, so the UI can explain
+    # why "Unlink Discord" is refused rather than just failing.
+    discord_only: bool = False
 
 
 class UserCreate(schemas.BaseUserCreate):

backend/accounts/users.py

@@ -140,3 +140,23 @@ fastapi_users = FastAPIUsers[User, uuid.UUID](get_user_manager, [auth_backend])
 # Dependencies handlers use to require / peek at the logged-in user.
 current_active_user = fastapi_users.current_user(active=True)
 current_user_optional = fastapi_users.current_user(active=True, optional=True)
+
+
+async def attach_session(response, user, user_manager, access_token_db, request=None):
+    """Log `user` in by copying a fresh session cookie onto `response`.
+
+    The library's routers own this for password login, but an OAuth callback has to
+    end in a *redirect* the browser follows, not the 204 the login router returns.
+    So mint the session the normal way and lift the Set-Cookie header off the
+    library's response rather than re-deriving the cookie's name/TTL/flags here —
+    duplicating those would let the two logins drift into issuing different cookies.
+    """
+    strategy = get_database_strategy(access_token_db)
+    issued = await auth_backend.login(strategy, user)
+    for key, value in issued.raw_headers:
+        if key.lower() == b"set-cookie":
+            response.raw_headers.append((key, value))
+    # The routers call this hook; a hand-rolled login has to as well, or Discord
+    # sign-ins go missing from the audit log that every password login appears in.
+    await user_manager.on_after_login(user, request, response)
+    return response

backend/alembic/versions/0003_user_discord_only.py

@@ -0,0 +1,50 @@
+"""user.discord_only — mark accounts whose only credential is Discord
+
+"Sign in with Discord" (notifications/discord_link.py) creates accounts for people
+who never chose a password. ``hashed_password`` is NOT NULL, so those rows carry a
+generated one nobody has ever seen; without a flag there is no way to tell such an
+account apart from a password account that merely linked Discord later. That
+distinction is load-bearing: unlinking Discord from a Discord-only account would
+delete its only way in, and no reset-password router is mounted to recover it.
+
+Conditional on purpose. Revision 0001 builds the schema with
+``Base.metadata.create_all``, which reads the *current* models.py — so a database
+created after this column was added to the ORM already has it, and an
+unconditional ``add_column`` would fail there with a duplicate-column error. Only
+a database stamped before this revision is missing it.
+
+Revision ID: 0003_user_discord_only
+Revises: 0002_climate_hypertables
+Create Date: 2026-07-26
+"""
+import sqlalchemy as sa
+from alembic import op
+
+revision = "0003_user_discord_only"
+down_revision = "0002_climate_hypertables"
+branch_labels = None
+depends_on = None
+
+
+def _has_column() -> bool:
+    cols = sa.inspect(op.get_bind()).get_columns("user")
+    return any(c["name"] == "discord_only" for c in cols)
+
+
+def upgrade() -> None:
+    if _has_column():
+        return
+    # server_default (not just a Python-side default) so the backfill of existing
+    # rows happens in the same statement: every account that predates Discord
+    # login has a password, so False is correct for all of them.
+    op.add_column(
+        "user",
+        sa.Column("discord_only", sa.Boolean(), nullable=False,
+                  server_default=sa.false()),
+    )
+
+
+def downgrade() -> None:
+    if not _has_column():
+        return
+    op.drop_column("user", "discord_only")

backend/notifications/discord_link.py

@@ -1,16 +1,33 @@
-"""Link a Thermograph account to a Discord account via OAuth2 (identify scope).
+"""Discord as an identity: sign in with it, and connect it to an account.
 
-The Discord user id we store here is the durable key a later feature (DM alerts)
-uses to reach the person. The flow is the standard authorization-code grant:
+Two flows over one authorization-code grant, told apart by the signed `state`:
 
-  /link/start    -> redirect the logged-in user to Discord's consent screen
-  /link/callback -> exchange the code, read the Discord user id, store it
-  /unlink        -> forget it
+  link  — /discord/link/start   an already-signed-in user attaches a Discord
+                                account to the session they're in
+  login — /discord/login/start  an anonymous visitor authenticates, creating or
+                                resolving a Thermograph account from the Discord
+                                identity
+
+**Both come back to /discord/link/callback.** Discord only honours redirect URIs
+pre-registered in the developer portal, so a second callback path would mean an
+operator has to add one there before login could work anywhere. Sharing the
+registered one keeps this deployable with no portal change; the flows separate on
+the state's purpose, which is inside the HMAC and so can't be flipped by hand.
 
 `state` is signed with the app's auth secret (stdlib hmac, no new dependency) and
-carries the Thermograph user id, so the callback can't be replayed or bound to a
-different account. Everything requires an active session, so linking always acts on
-the person who is actually logged in.
+carries the purpose plus, for a link, the Thermograph user id — so a callback can't
+be replayed, bound to a different account, or replayed *as the other flow*.
+
+How a login resolves to an account, in order:
+
+  1. A user already carrying this `discord_id` — the durable key, no email needed.
+  2. Otherwise the Discord email, but **only if Discord reports it verified**.
+     That flag is the sole evidence the person owns the address; matching an
+     existing Thermograph account on an unverified one would hand that account to
+     whoever typed the address into Discord. Unverified is refused outright.
+  3. No account for that email: create one. It has no password its owner has ever
+     seen, so it is flagged `discord_only` and unlink is refused (see `unlink`) —
+     otherwise unlinking would delete the only way back in.
 """
 from __future__ import annotations
 
@@ -23,15 +40,23 @@ import time
 import urllib.parse
 
 import httpx
-from fastapi import APIRouter, Depends, Request
+from fastapi import APIRouter, Depends, HTTPException, Request
 from fastapi.responses import RedirectResponse
 from pydantic import BaseModel
-from sqlalchemy import update
+from sqlalchemy import select, update
 from sqlalchemy.ext.asyncio import AsyncSession
 
 from accounts.db import get_async_session
 from accounts.models import User
-from accounts.users import SECRET, current_active_user
+from accounts.users import (
+    SECRET,
+    attach_session,
+    current_active_user,
+    current_user_optional,
+    get_access_token_db,
+    get_user_manager,
+)
+from core import audit
 
 router = APIRouter(tags=["discord"])
 
@@ -43,6 +68,11 @@ _AUTHORIZE = "https://discord.com/api/oauth2/authorize"
 _TOKEN = "https://discord.com/api/oauth2/token"
 _ME = "https://discord.com/api/users/@me"
 
+# Linking only needs the account id. Signing in also needs the address to match or
+# create a Thermograph account by, which is a separate scope Discord prompts for.
+_SCOPE_LINK = "identify"
+_SCOPE_LOGIN = "identify email"
+
 # How long a start->callback round trip may take before the signed state expires.
 _STATE_TTL = 600
 _TIMEOUT = httpx.Timeout(10.0)
@@ -61,15 +91,22 @@ def _unb64(s: str) -> bytes:
     return base64.urlsafe_b64decode(s + "=" * (-len(s) % 4))
 
 
-def _sign_state(uid: str) -> str:
-    payload = _b64(json.dumps({"u": uid, "t": int(time.time())}).encode())
+def _sign_state(uid: str | None, purpose: str = "link") -> str:
+    payload = _b64(json.dumps(
+        {"u": uid or "", "t": int(time.time()), "p": purpose}
+    ).encode())
     mac = hmac.new(SECRET.encode(), payload.encode(), hashlib.sha256).hexdigest()[:32]
     return f"{payload}.{mac}"
 
 
-def _verify_state(state: str) -> str | None:
-    """The Thermograph user id the state was minted for, or None if it's missing,
-    tampered, or older than the TTL."""
+def _verify_state(state: str, purpose: str = "link") -> str | None:
+    """The Thermograph user id this state was minted for, or None if it's missing,
+    tampered, expired, or was minted for a *different* purpose.
+
+    A login state has no user id yet, so it verifies as ``""`` — falsy, but
+    distinct from the None that means "reject". Callers must test against None,
+    not truthiness, or a valid login state reads as a failure.
+    """
     try:
         payload, mac = state.split(".", 1)
     except (ValueError, AttributeError):
@@ -83,7 +120,11 @@ def _verify_state(state: str) -> str | None:
         return None
     if time.time() - float(data.get("t", 0)) > _STATE_TTL:
         return None
-    return data.get("u")
+    # States minted before the purpose field existed are links; they age out in
+    # _STATE_TTL, so this fallback only spans a deploy.
+    if data.get("p", "link") != purpose:
+        return None
+    return data.get("u") or ""
 
 
 def _redirect_uri(request: Request) -> str:
@@ -94,52 +135,30 @@ def _redirect_uri(request: Request) -> str:
     return f"{proto}://{host}{BASE}/api/v2/discord/link/callback"
 
 
-def _account_redirect(status: str) -> RedirectResponse:
-    # Back to the alerts page, which surfaces the link state; 303 so the browser
-    # issues a GET after the callback.
-    return RedirectResponse(url=f"{BASE}/subscriptions?discord={status}", status_code=303)
-
-
-# --- routes ------------------------------------------------------------------
-@router.get("/discord/config")
-async def discord_config():
-    """Whether Discord account-linking is configured on this server. The account
-    menu uses it to show the "Link Discord" entry only when linking will actually
-    work — so an unconfigured server surfaces no dead-end Discord UI, and enabling
-    it later (setting the OAuth env vars) makes the entry appear on its own."""
-    return {"enabled": enabled()}
-
-
-@router.get("/discord/link/start")
-async def link_start(request: Request, user=Depends(current_active_user)):
-    if not enabled():
-        return _account_redirect("unavailable")
+def _authorize_redirect(request: Request, scope: str, state: str) -> RedirectResponse:
     params = {
         "client_id": CLIENT_ID,
         "redirect_uri": _redirect_uri(request),
         "response_type": "code",
-        "scope": "identify",
-        "state": _sign_state(str(user.id)),
+        "scope": scope,
+        "state": state,
         "prompt": "consent",
     }
-    return RedirectResponse(url=f"{_AUTHORIZE}?{urllib.parse.urlencode(params)}", status_code=303)
+    return RedirectResponse(url=f"{_AUTHORIZE}?{urllib.parse.urlencode(params)}",
+                            status_code=303)
 
 
-@router.get("/discord/link/callback")
-async def link_callback(
-    request: Request,
-    user=Depends(current_active_user),
-    session: AsyncSession = Depends(get_async_session),
-):
-    if not enabled():
-        return _account_redirect("unavailable")
-    # User declined on Discord's screen, or the state doesn't check out.
-    code = request.query_params.get("code")
-    state = request.query_params.get("state", "")
-    if not code:
-        return _account_redirect("cancelled")
-    if _verify_state(state) != str(user.id):
-        return _account_redirect("error")
+def _account_redirect(status: str) -> RedirectResponse:
+    # Back to the alerts page, which surfaces the outcome as a toast; 303 so the
+    # browser issues a GET after the callback. The frontend serves this page at
+    # /alerts — the route is named for the feature, not for subscriptions.html.
+    return RedirectResponse(url=f"{BASE}/alerts?discord={status}", status_code=303)
+
+
+# --- Discord API --------------------------------------------------------------
+async def _fetch_profile(request: Request, code: str) -> dict | None:
+    """Trade the authorization code for the Discord user object, or None if any
+    step fails. Callers treat None as a graceful error, never a crash."""
     try:
         async with httpx.AsyncClient(timeout=_TIMEOUT) as client:
             tok = await client.post(_TOKEN, data={
@@ -150,16 +169,121 @@ async def link_callback(
                 "redirect_uri": _redirect_uri(request),
             }, headers={"Content-Type": "application/x-www-form-urlencoded"})
             if tok.status_code >= 300:
-                return _account_redirect("error")
+                return None
             access = tok.json().get("access_token")
             me = await client.get(_ME, headers={"Authorization": f"Bearer {access}"})
             if me.status_code >= 300:
-                return _account_redirect("error")
-            discord_id = str(me.json().get("id") or "")
+                return None
+            profile = me.json()
     except Exception:  # noqa: BLE001 - any network/parse failure is a graceful error
+        return None
+    return profile if isinstance(profile, dict) else None
+
+
+async def _user_by_discord_id(session: AsyncSession, discord_id: str) -> User | None:
+    return (
+        await session.execute(select(User).where(User.discord_id == discord_id))
+    ).scalar_one_or_none()
+
+
+async def _create_from_discord(user_manager, email: str, discord_id: str) -> User:
+    """Create an account for a Discord identity that matched none of ours.
+
+    Deliberately goes around ``UserManager.create()``: there is no password to
+    validate, and its ``on_after_register`` mails a confirmation link for an
+    address Discord has already verified. The generated password exists only
+    because ``hashed_password`` is NOT NULL — nobody ever sees it, which is what
+    ``discord_only`` records.
+    """
+    password = user_manager.password_helper.generate()
+    user = await user_manager.user_db.create({
+        "email": email,
+        "hashed_password": user_manager.password_helper.hash(password),
+        "is_active": True,
+        "is_superuser": False,
+        "is_verified": True,
+        "discord_id": discord_id,
+        "discord_dm": True,
+        "discord_only": True,
+    })
+    audit.log_activity("auth.register", {"user_id": str(user.id), "via": "discord"})
+    return user
+
+
+# --- routes ------------------------------------------------------------------
+@router.get("/discord/config")
+async def discord_config():
+    """Whether Discord is configured on this server. The account menu and the sign-in
+    modal use it to offer Discord only when it will actually work — so an
+    unconfigured server surfaces no dead-end Discord UI, and enabling it later
+    (setting the OAuth env vars) makes both entries appear on their own."""
+    return {"enabled": enabled()}
+
+
+@router.get("/discord/link/start")
+async def link_start(request: Request, user=Depends(current_active_user)):
+    if not enabled():
+        return _account_redirect("unavailable")
+    return _authorize_redirect(request, _SCOPE_LINK, _sign_state(str(user.id), "link"))
+
+
+@router.get("/discord/login/start")
+async def login_start(request: Request, user=Depends(current_user_optional)):
+    """Sign in with Discord. Unauthenticated by design — this is how you get a
+    session, not something you do with one."""
+    if not enabled():
+        return _account_redirect("unavailable")
+    # Someone already signed in who lands here wants to connect, not to be
+    # switched into whichever account the Discord identity resolves to.
+    if user is not None:
+        return _authorize_redirect(request, _SCOPE_LINK, _sign_state(str(user.id), "link"))
+    return _authorize_redirect(request, _SCOPE_LOGIN, _sign_state(None, "login"))
+
+
+@router.get("/discord/link/callback")
+async def link_callback(
+    request: Request,
+    user=Depends(current_user_optional),
+    session: AsyncSession = Depends(get_async_session),
+    user_manager=Depends(get_user_manager),
+    access_token_db=Depends(get_access_token_db),
+):
+    """The single redirect target both flows return to (see the module docstring)."""
+    if not enabled():
+        return _account_redirect("unavailable")
+    # User declined on Discord's screen, or the state doesn't check out.
+    code = request.query_params.get("code")
+    state = request.query_params.get("state", "")
+    if not code:
+        return _account_redirect("cancelled")
+
+    # Which flow minted this state? The purpose is inside the HMAC, so a state
+    # can only verify under the one it was signed for.
+    uid = _verify_state(state, "link")
+    if uid is not None:
+        return await _finish_link(request, code, uid, user, session)
+    if _verify_state(state, "login") is not None:
+        return await _finish_login(request, code, session, user_manager, access_token_db)
+    return _account_redirect("error")
+
+
+async def _finish_link(request: Request, code: str, uid: str, user,
+                       session: AsyncSession) -> RedirectResponse:
+    # Linking always acts on the person actually logged in, and only on the one
+    # the state was minted for.
+    if user is None or str(user.id) != uid:
         return _account_redirect("error")
+    profile = await _fetch_profile(request, code)
+    if profile is None:
+        return _account_redirect("error")
+    discord_id = str(profile.get("id") or "")
     if not discord_id:
         return _account_redirect("error")
+    # discord_id is UNIQUE, so without this check the UPDATE would raise an
+    # IntegrityError and 500 instead of explaining itself.
+    owner = await _user_by_discord_id(session, discord_id)
+    if owner is not None and owner.id != user.id:
+        return _account_redirect("taken")
     # Linking is an active opt-in, so turn DM alerts on; the user can mute them
     # (POST /discord/dm) while staying linked.
     await session.execute(
@@ -169,11 +293,73 @@ async def link_callback(
     return _account_redirect("linked")
 
 
+async def _finish_login(request: Request, code: str, session: AsyncSession,
+                        user_manager, access_token_db) -> RedirectResponse:
+    """Resolve the Discord identity to an account and sign in as it.
+
+    Deliberately ignores any session already present. A login state carries no
+    user id — it can't, there is no user yet — so it is replayable against whoever
+    happens to be signed in. Treating it as a link would therefore be a forced-
+    linking takeover: an attacker who gets a victim to open this callback with a
+    code from the *attacker's* Discord would attach their identity to the victim's
+    account, then sign in as them. Only /discord/link/start, whose state is bound
+    to a specific user id, may link. The worst a replayed login state can do is
+    sign someone into the attacker's own account.
+    """
+    profile = await _fetch_profile(request, code)
+    if profile is None:
+        return _account_redirect("error")
+    discord_id = str(profile.get("id") or "")
+    if not discord_id:
+        return _account_redirect("error")
+
+    created = False
+    user = await _user_by_discord_id(session, discord_id)
+    if user is None:
+        # No account carries this Discord id, so fall back to the email — the only
+        # other identity Discord gives us, and only when Discord vouches for it.
+        email = str(profile.get("email") or "").strip().lower()
+        if not email:
+            return _account_redirect("noemail")
+        if profile.get("verified") is not True:
+            return _account_redirect("unverified")
+        user = await user_manager.user_db.get_by_email(email)
+        if user is None:
+            user = await _create_from_discord(user_manager, email, discord_id)
+            created = True
+        elif user.discord_id:
+            # That address belongs to an account already tied to a different
+            # Discord identity; connecting would silently move it.
+            return _account_redirect("mismatch")
+        elif not user.is_active:
+            # Checked before the link, not just before the session: a deactivated
+            # account must not quietly acquire a Discord identity on the way out.
+            return _account_redirect("inactive")
+        else:
+            await session.execute(
+                update(User).where(User.id == user.id)
+                .values(discord_id=discord_id, discord_dm=True)
+            )
+            await session.commit()
+    if not user.is_active:
+        return _account_redirect("inactive")
+    response = _account_redirect("created" if created else "signedin")
+    return await attach_session(response, user, user_manager, access_token_db, request)
+
+
 @router.post("/discord/unlink", status_code=204)
 async def unlink(
     user=Depends(current_active_user),
     session: AsyncSession = Depends(get_async_session),
 ):
+    if user.discord_only:
+        # The account has no password anyone has ever seen, and no reset-password
+        # route is mounted to set one — unlinking here is unrecoverable.
+        raise HTTPException(
+            status_code=409,
+            detail="This account was created with Discord and has no password, "
+                   "so unlinking would leave no way to sign in.",
+        )
     await session.execute(
         update(User).where(User.id == user.id).values(discord_id=None, discord_dm=False)
     )

backend/tests/notifications/test_discord_login.py

@@ -0,0 +1,251 @@
+"""Sign in with Discord: account resolution, session issuance, and the guards that
+keep the login flow from becoming an account-takeover path. Discord HTTP is mocked;
+reuses the throwaway accounts DB from conftest."""
+import pytest
+from fastapi.testclient import TestClient
+
+from web import app as appmod
+from accounts import db
+from notifications import discord_link as dl
+
+V2 = "/thermograph/api/v2"
+PW = "supersecret123"
+
+
+@pytest.fixture(scope="module", autouse=True)
+def _tables():
+    db.Base.metadata.create_all(db.sync_engine)
+
+
+@pytest.fixture(autouse=True)
+def _configured(monkeypatch):
+    monkeypatch.setattr(dl, "CLIENT_ID", "app123")
+    monkeypatch.setattr(dl, "CLIENT_SECRET", "secret456")
+
+
+class _Resp:
+    def __init__(self, code, data): self.status_code = code; self._data = data
+    def json(self): return self._data
+
+
+def _mock_discord(monkeypatch, profile):
+    """Stand in for httpx.AsyncClient: token exchange, then /users/@me -> profile."""
+    class _Client:
+        def __init__(self, *a, **k): pass
+        async def __aenter__(self): return self
+        async def __aexit__(self, *a): return False
+        async def post(self, url, **k): return _Resp(200, {"access_token": "tok"})
+        async def get(self, url, **k): return _Resp(200, profile)
+    monkeypatch.setattr(dl.httpx, "AsyncClient", _Client)
+
+
+def _register(client, email):
+    r = client.post(f"{V2}/auth/register", json={"email": email, "password": PW})
+    assert r.status_code in (201, 400)
+
+
+def _login(client, email):
+    _register(client, email)
+    assert client.post(f"{V2}/auth/login",
+                       data={"username": email, "password": PW}).status_code == 204
+
+
+def _callback(client, state):
+    return client.get(f"{V2}/discord/link/callback?code=abc&state={state}",
+                      follow_redirects=False)
+
+
+# --- start -------------------------------------------------------------------
+
+def test_login_start_needs_no_session_and_asks_for_email():
+    c = TestClient(appmod.app)
+    r = c.get(f"{V2}/discord/login/start", follow_redirects=False)
+    assert r.status_code == 303
+    loc = r.headers["location"]
+    assert loc.startswith("https://discord.com/api/oauth2/authorize")
+    # The email scope is what makes account resolution possible at all.
+    assert "scope=identify+email" in loc and "state=" in loc
+
+
+def test_login_start_while_signed_in_links_instead_of_switching_account():
+    """Someone already signed in wants to connect Discord, not be switched into
+    whichever account the Discord identity happens to resolve to."""
+    c = TestClient(appmod.app)
+    _login(c, "already-in@example.com")
+    r = c.get(f"{V2}/discord/login/start", follow_redirects=False)
+    assert r.status_code == 303
+    loc = r.headers["location"]
+    assert "scope=identify&" in loc or loc.endswith("scope=identify")
+    state = loc.split("state=")[1].split("&")[0]
+    uid = c.get(f"{V2}/users/me").json()["id"]
+    assert dl._verify_state(state, "link") == uid
+
+
+def test_login_start_when_unconfigured_bounces_back(monkeypatch):
+    monkeypatch.setattr(dl, "CLIENT_ID", "")
+    monkeypatch.setattr(dl, "CLIENT_SECRET", "")
+    c = TestClient(appmod.app)
+    r = c.get(f"{V2}/discord/login/start", follow_redirects=False)
+    assert r.status_code == 303 and "discord=unavailable" in r.headers["location"]
+
+
+# --- state purposes are not interchangeable ----------------------------------
+
+def test_state_purposes_do_not_cross_over():
+    login_state = dl._sign_state(None, "login")
+    link_state = dl._sign_state("user-123", "link")
+    # A login state carries no user id, but verifies as "" — distinct from reject.
+    assert dl._verify_state(login_state, "login") == ""
+    assert dl._verify_state(login_state, "link") is None
+    assert dl._verify_state(link_state, "link") == "user-123"
+    assert dl._verify_state(link_state, "login") is None
+
+
+def test_a_login_state_cannot_link_the_signed_in_account(monkeypatch):
+    """Forced-linking guard. A login state carries no user id, so it is replayable
+    against whoever is signed in; if the callback treated it as a link, an attacker
+    could attach their own Discord identity to a victim's account and then sign in
+    as them. It must complete as a plain login instead."""
+    _mock_discord(monkeypatch, {"id": "d-crossover", "email": "attacker@example.com",
+                                "verified": True})
+    c = TestClient(appmod.app)
+    _login(c, "crossover@example.com")
+    # A state whose purpose says login, but naming the signed-in user — the shape
+    # a forced-link attempt would take.
+    r = _callback(c, dl._sign_state(c.get(f"{V2}/users/me").json()["id"], "login"))
+    # Resolved as a login into the Discord identity's own account, not as a link.
+    assert "discord=created" in r.headers["location"]
+    assert c.get(f"{V2}/users/me").json()["email"] == "attacker@example.com"
+    # The account that had been signed in is untouched.
+    victim = TestClient(appmod.app)
+    _login(victim, "crossover@example.com")
+    assert victim.get(f"{V2}/users/me").json()["discord_id"] is None
+
+
+# --- login resolves an account -----------------------------------------------
+
+def test_login_creates_an_account_and_signs_in(monkeypatch):
+    _mock_discord(monkeypatch, {"id": "d-new", "email": "New.User@Example.com",
+                                "verified": True})
+    c = TestClient(appmod.app)
+    r = _callback(c, dl._sign_state(None, "login"))
+    assert r.status_code == 303 and "discord=created" in r.headers["location"]
+    me = c.get(f"{V2}/users/me")
+    assert me.status_code == 200
+    body = me.json()
+    assert body["email"] == "new.user@example.com"      # normalised
+    assert body["discord_id"] == "d-new"
+    assert body["is_verified"] is True                   # Discord vouched for it
+    assert body["discord_only"] is True
+    assert body["discord_dm"] is True
+
+
+def test_login_recognises_a_previously_linked_account(monkeypatch):
+    """The discord_id is the durable key: it resolves the account with no email."""
+    _mock_discord(monkeypatch, {"id": "d-returning", "email": "link-me@example.com",
+                                "verified": True})
+    linker = TestClient(appmod.app)
+    _login(linker, "link-me@example.com")
+    uid = linker.get(f"{V2}/users/me").json()["id"]
+    assert "discord=linked" in _callback(linker, dl._sign_state(uid, "link")
+                                         ).headers["location"]
+    # A fresh client, no session, and Discord now returns no email at all.
+    _mock_discord(monkeypatch, {"id": "d-returning"})
+    c = TestClient(appmod.app)
+    r = _callback(c, dl._sign_state(None, "login"))
+    assert r.status_code == 303 and "discord=signedin" in r.headers["location"]
+    assert c.get(f"{V2}/users/me").json()["id"] == uid
+
+
+def test_login_connects_a_verified_email_to_an_existing_account(monkeypatch):
+    """This is the 'connect the account with that' path: an existing password
+    account picks up the Discord id and is signed in."""
+    c = TestClient(appmod.app)
+    _register(c, "existing@example.com")
+    _mock_discord(monkeypatch, {"id": "d-existing", "email": "existing@example.com",
+                                "verified": True})
+    fresh = TestClient(appmod.app)
+    r = _callback(fresh, dl._sign_state(None, "login"))
+    assert r.status_code == 303 and "discord=signedin" in r.headers["location"]
+    body = fresh.get(f"{V2}/users/me").json()
+    assert body["email"] == "existing@example.com"
+    assert body["discord_id"] == "d-existing"
+    # It kept its password, so it is not Discord-only and may unlink.
+    assert body["discord_only"] is False
+
+
+def test_login_refuses_an_unverified_discord_email(monkeypatch):
+    """Discord's verified flag is the only evidence the person owns the address.
+    Without it, this would hand over any account whose email someone typed in."""
+    c = TestClient(appmod.app)
+    _register(c, "victim@example.com")
+    _mock_discord(monkeypatch, {"id": "d-attacker", "email": "victim@example.com",
+                                "verified": False})
+    fresh = TestClient(appmod.app)
+    r = _callback(fresh, dl._sign_state(None, "login"))
+    assert r.status_code == 303 and "discord=unverified" in r.headers["location"]
+    assert fresh.get(f"{V2}/users/me").status_code == 401     # no session issued
+    # And the targeted account is untouched.
+    _login(c, "victim@example.com")
+    assert c.get(f"{V2}/users/me").json()["discord_id"] is None
+
+
+def test_login_without_an_email_cannot_create_an_account(monkeypatch):
+    _mock_discord(monkeypatch, {"id": "d-noemail"})
+    c = TestClient(appmod.app)
+    r = _callback(c, dl._sign_state(None, "login"))
+    assert r.status_code == 303 and "discord=noemail" in r.headers["location"]
+    assert c.get(f"{V2}/users/me").status_code == 401
+
+
+def test_login_will_not_move_an_account_between_discord_identities(monkeypatch):
+    """The email matches an account that is already linked to a different Discord
+    id — signing in would silently re-point it."""
+    owner = TestClient(appmod.app)
+    _login(owner, "owned@example.com")
+    uid = owner.get(f"{V2}/users/me").json()["id"]
+    _mock_discord(monkeypatch, {"id": "d-owner", "email": "owned@example.com",
+                                "verified": True})
+    assert "discord=linked" in _callback(owner, dl._sign_state(uid, "link")
+                                         ).headers["location"]
+    # A second Discord account claiming the same address.
+    _mock_discord(monkeypatch, {"id": "d-interloper", "email": "owned@example.com",
+                                "verified": True})
+    fresh = TestClient(appmod.app)
+    r = _callback(fresh, dl._sign_state(None, "login"))
+    assert r.status_code == 303 and "discord=mismatch" in r.headers["location"]
+    assert fresh.get(f"{V2}/users/me").status_code == 401
+    assert owner.get(f"{V2}/users/me").json()["discord_id"] == "d-owner"
+
+
+# --- guards on the connected account -----------------------------------------
+
+def test_linking_a_discord_account_someone_else_owns_is_refused(monkeypatch):
+    first = TestClient(appmod.app)
+    _login(first, "first-owner@example.com")
+    _mock_discord(monkeypatch, {"id": "d-contested", "email": "first-owner@example.com",
+                                "verified": True})
+    uid = first.get(f"{V2}/users/me").json()["id"]
+    assert "discord=linked" in _callback(first, dl._sign_state(uid, "link")
+                                         ).headers["location"]
+    # A different Thermograph account tries to claim the same Discord identity.
+    second = TestClient(appmod.app)
+    _login(second, "second-owner@example.com")
+    uid2 = second.get(f"{V2}/users/me").json()["id"]
+    r = _callback(second, dl._sign_state(uid2, "link"))
+    assert r.status_code == 303 and "discord=taken" in r.headers["location"]
+    assert second.get(f"{V2}/users/me").json()["discord_id"] is None
+
+
+def test_a_discord_only_account_cannot_unlink_itself_into_a_lockout(monkeypatch):
+    _mock_discord(monkeypatch, {"id": "d-onlyway", "email": "onlyway@example.com",
+                                "verified": True})
+    c = TestClient(appmod.app)
+    assert "discord=created" in _callback(c, dl._sign_state(None, "login")
+                                          ).headers["location"]
+    r = c.post(f"{V2}/discord/unlink")
+    assert r.status_code == 409
+    assert "no password" in r.json()["detail"]
+    assert c.get(f"{V2}/users/me").json()["discord_id"] == "d-onlyway"
+    # Muting DMs is still fine — that is not a lockout.
+    assert c.post(f"{V2}/discord/dm", json={"enabled": False}).status_code == 204

frontend/static/account.js

@@ -10,7 +10,7 @@
 // units.js is (imported by each page's entry module).
 
 let currentUser = null;          // {id, email, display_name} or null
-let discordEnabled = false;      // is Discord linking configured on the server?
+let discordEnabled = false;      // is Discord configured on the server?
 let discordChecked = false;      // have we asked yet? (once per page load)
 const authCbs = [];              // notified on login/logout so pages can re-gate
 
@@ -89,10 +89,10 @@ async function refreshUser() {
     const res = await apiFetch(uv("users/me"));
     currentUser = res.ok ? await res.json() : null;
   } catch (e) { currentUser = null; }
-  // Learn once whether Discord linking is configured, so the account menu only
-  // offers "Link Discord" when it will actually work (checked lazily, and only
-  // for a signed-in user — the menu never shows it to anyone else).
-  if (currentUser && !discordChecked) {
+  // Learn once whether Discord is configured, so we only offer "Link Discord" and
+  // "Continue with Discord" when they'll actually work. Asked regardless of sign-in
+  // state: the sign-in modal needs the answer precisely when nobody is signed in.
+  if (!discordChecked) {
     discordChecked = true;
     try {
       const r = await apiFetch(uv("discord/config"));
@@ -126,6 +126,10 @@ export async function logout() {
 // --- auth modal --------------------------------------------------------------
 let modal = null, mode = "login";
 
+// Discord's brand mark ("Clyde"), inline like the other icons here so it needs no
+// extra request and inherits currentColor.
+const DISCORD_IC = `<svg viewBox="0 0 24 18" fill="currentColor" aria-hidden="true"><path d="M20.32 1.51A19.79 19.79 0 0 0 15.43 0a13.9 13.9 0 0 0-.63 1.28 18.4 18.4 0 0 0-5.6 0A13.9 13.9 0 0 0 8.57 0 19.74 19.74 0 0 0 3.68 1.51C.57 6.15-.28 10.68.14 15.14a19.9 19.9 0 0 0 6.05 3.05c.49-.66.92-1.37 1.3-2.11a12.9 12.9 0 0 1-2.05-.98c.17-.13.34-.26.5-.4a14.2 14.2 0 0 0 12.12 0c.16.14.33.27.5.4-.65.38-1.34.71-2.05.98.37.74.81 1.45 1.3 2.11a19.87 19.87 0 0 0 6.05-3.05c.49-5.17-.84-9.67-3.54-13.63ZM8.02 12.4c-1.18 0-2.15-1.08-2.15-2.41S6.82 7.58 8.02 7.58s2.17 1.09 2.15 2.41c0 1.33-.95 2.41-2.15 2.41Zm7.96 0c-1.18 0-2.15-1.08-2.15-2.41s.95-2.41 2.15-2.41 2.17 1.09 2.15 2.41c0 1.33-.95 2.41-2.15 2.41Z"/></svg>`;
+
 function buildModal() {
   modal = document.createElement("div");
   modal.className = "mp-overlay acct-overlay";
@@ -145,6 +149,12 @@ function buildModal() {
         </label>
         <p class="acct-error" role="alert" hidden></p>
         <button type="submit" class="acct-submit">Sign in</button>
+        <div class="acct-alt" hidden>
+          <span class="acct-or">or</span>
+          <a class="acct-discord-login" href="${uv("discord/login/start")}">
+            ${DISCORD_IC}<span>Continue with Discord</span>
+          </a>
+        </div>
         <p class="acct-switch"></p>
       </form>
     </div>`;
@@ -173,6 +183,9 @@ function setMode(m) {
   modal.querySelector(".acct-switch").innerHTML = isLogin
     ? 'Need an account? <button type="button">Create one</button>'
     : 'Already have an account? <button type="button">Sign in</button>';
+  // One label for both modes: Discord signs you in or creates the account,
+  // whichever applies, so making the user pick first would be a false choice.
+  modal.querySelector(".acct-alt").hidden = !discordEnabled;
   showError("");
 }
 
@@ -371,7 +384,11 @@ function renderHeader() {
           <a href="${APP_BASE}/alerts" class="acct-pop-link">My alerts</a>
           ${currentUser.discord_id
             ? `<button type="button" class="acct-pop-link acct-discord-dm">${currentUser.discord_dm ? "Discord alerts: on" : "Discord alerts: off"}</button>`
-              + '<button type="button" class="acct-pop-link acct-discord-unlink">Unlink Discord</button>'
+              // A Discord-created account has no password, so Discord is its only
+              // way in and the server refuses to unlink it — don't offer the button.
+              + (currentUser.discord_only
+                ? '<span class="acct-pop-note muted">Signed in with Discord</span>'
+                : '<button type="button" class="acct-pop-link acct-discord-unlink">Unlink Discord</button>')
             : (discordEnabled ? `<a href="${uv("discord/link/start")}" class="acct-pop-link">Link Discord</a>` : "")}
           <button type="button" class="acct-pop-link acct-signout">Sign out</button>
         </div>
@@ -403,7 +420,17 @@ function renderHeader() {
     const unlinkBtn = el.querySelector(".acct-discord-unlink");
     if (unlinkBtn) unlinkBtn.addEventListener("click", async () => {
       unlinkBtn.disabled = true;
-      try { await apiFetch(uv("discord/unlink"), { method: "POST" }); } catch (e) {}
+      try {
+        const res = await apiFetch(uv("discord/unlink"), { method: "POST" });
+        // 409: the account was created with Discord and has no password, so
+        // unlinking would lock it out. Say why instead of appearing to do nothing.
+        if (res.status === 409) {
+          const body = await res.json().catch(() => null);
+          showToast((body && body.detail) || "Discord can't be unlinked from this account.", true);
+          unlinkBtn.disabled = false;
+          return;
+        }
+      } catch (e) {}
       await refreshUser();
       emitAuth();              // repaint the popover in its unlinked state
     });
@@ -452,6 +479,30 @@ function showToast(msg, isError = false) {
   toastTimer = setTimeout(() => { el.hidden = true; }, 6000);
 }
 
+// --- Discord OAuth outcomes --------------------------------------------------
+// The link/login callback can only talk back to us through a redirect, so it
+// lands on ?discord=<status>. Each status maps to [message, isError]; anything
+// unrecognised is treated as a generic failure rather than shown raw.
+const DISCORD_STATUS = {
+  linked:    ["Discord connected.", false],
+  signedin:  ["Signed in with Discord.", false],
+  created:   ["Account created with Discord — welcome to Thermograph.", false],
+  cancelled: ["Discord sign-in cancelled.", false],
+  taken:     ["That Discord account is already connected to another Thermograph account.", true],
+  mismatch:  ["That email belongs to an account connected to a different Discord account. "
+              + "Sign in with your password to change it.", true],
+  noemail:   ["Discord didn't share an email address, so there's no account to sign in to.", true],
+  unverified: ["Verify your email address with Discord first, then try again.", true],
+  inactive:  ["That account is deactivated.", true],
+  unavailable: ["Discord sign-in isn't available on this server.", true],
+};
+
+function showDiscordStatus(status) {
+  const [msg, isError] = DISCORD_STATUS[status]
+    || ["Something went wrong with Discord. Please try again.", true];
+  showToast(msg, isError);
+}
+
 // --- boot --------------------------------------------------------------------
 (async function initAccount() {
   ensureAcctEl();
@@ -462,10 +513,19 @@ function showToast(msg, isError = false) {
 
   const params = new URLSearchParams(location.search);
   const token = params.get("verify_token");
-  if (token) {
+  // Both of these are one-shot handoffs from a redirect; strip them together so a
+  // reload can't replay either, and so the two never fight over the URL.
+  const discordStatus = params.get("discord");
+  if (token || discordStatus) {
     params.delete("verify_token");
+    params.delete("discord");
     const clean = location.pathname + (params.toString() ? `?${params}` : "") + location.hash;
     history.replaceState(null, "", clean);
+  }
+  // refreshUser() above already ran, and the callback's redirect carried the new
+  // session cookie, so the header is painted signed-in before this toast lands.
+  if (discordStatus) showDiscordStatus(discordStatus);
+  if (token) {
     try {
       await verifyEmail(token);
       await refreshUser();

frontend/static/style.css

@@ -6,6 +6,9 @@
   --text: #e7ecf2;
   --muted: #9aa6b2;
   --accent: #f0803c;
+  /* Discord's brand blurple. Fixed in both schemes on purpose: it identifies the
+     provider, so it is their colour rather than one of ours to re-theme. */
+  --discord: #5865f2;
 
   /* temperature grades: 9-step diverging cold -> green -> hot (colorblind-safe).
      rec-* are the "Near Record" danger tiers — deliberately dark/saturated. */
@@ -422,6 +425,27 @@ main { max-width: 1200px; margin: 0 auto; padding: 20px 24px 60px; }
   color: var(--text); border: 1px solid var(--rec-hot, #d64545);
 }
 .acct-error[hidden] { display: none; }
+/* "or / Continue with Discord" under the submit button. Hidden unless the server
+   reports Discord configured (account.js sets [hidden]). */
+.acct-alt { display: flex; flex-direction: column; gap: 10px; }
+.acct-alt[hidden] { display: none; }
+.acct-or {
+  display: flex; align-items: center; gap: 10px;
+  font-size: 12px; color: var(--muted); text-transform: uppercase; letter-spacing: .06em;
+}
+.acct-or::before, .acct-or::after {
+  content: ""; flex: 1; height: 1px; background: var(--border);
+}
+.acct-discord-login {
+  display: flex; align-items: center; justify-content: center; gap: 9px;
+  padding: 12px 16px; min-height: 44px; border-radius: 10px;
+  background: var(--discord); color: #fff; font-weight: 700; font-size: 15px;
+  text-decoration: none; border: 1px solid transparent;
+}
+.acct-discord-login:hover { filter: brightness(1.08); }
+.acct-discord-login svg { width: 20px; height: 15px; flex: none; }
+
+.acct-pop-note { display: block; padding: 9px 12px; font-size: 13px; }
 .acct-switch { margin: 0; font-size: 13px; color: var(--muted); text-align: center; }
 .acct-switch button {
   background: none; border: 0; color: var(--accent); cursor: pointer;

infra/deploy/thermograph.env.example

@@ -210,10 +210,19 @@ THERMOGRAPH_BASE_URL=https://thermograph.org
 # Discord allows ONE gateway connection per bot token, so enable this in exactly
 # one environment — prod, the only vault with the token.
 #THERMOGRAPH_DISCORD_BOT=
-# Account linking (OAuth2 "identify"): lets a signed-in user connect their Discord
-# account, storing their Discord user id for DM alerts. CLIENT_ID is the same App
-# ID above. The CLIENT_SECRET is from OAuth2 -> Client Secret (a credential). In the
-# portal, add the redirect: https://thermograph.org/api/v2/discord/link/callback
+# Discord as an identity (notifications/discord_link.py). One pair of credentials
+# gates BOTH flows, and setting them makes both appear in the UI at once:
+#   - account linking (scope "identify") — a signed-in user connects their Discord
+#     account, storing their Discord user id for DM alerts;
+#   - sign in with Discord (scope "identify email") — an anonymous visitor
+#     authenticates, resolving or creating an account from the Discord identity.
+# CLIENT_ID is the same App ID above. The CLIENT_SECRET is from OAuth2 -> Client
+# Secret (a credential). In the portal, add the redirect:
+#   https://thermograph.org/api/v2/discord/link/callback
+# Both flows come back to that one path (the purpose is carried in the signed
+# state), so enabling sign-in needs no new redirect registered. Scopes are asked
+# for per request, not registered, so the email scope needs no portal change
+# either. An environment without these two vars simply shows no Discord UI.
 #THERMOGRAPH_DISCORD_CLIENT_SECRET=
 # Gateway bot (opt-in): holds a live websocket so the bot replies to messages that
 # @mention it (or DM it) with a city grade — e.g. "@Thermograph Phoenix". Reuses