fix: prevent race conditions in token operations and auth flows

Backend:
- Refresh token rotation: use atomic UPDATE...WHERE revoked_at IS NULL
  to prevent concurrent refresh requests from both succeeding
- Account invite codes: SELECT FOR UPDATE to prevent double-spend
- Platform invite codes: SELECT FOR UPDATE to prevent double-spend
- Password reset tokens: SELECT FOR UPDATE to prevent double-use
- Email verification tokens: SELECT FOR UPDATE to prevent double-use

Frontend:
- Token refresh subscriber arrays: swap before iterating so a throwing
  callback doesn't leave the queue in a dirty state

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/app/api/endpoints/auth.py

@@ -5,7 +5,7 @@ from typing import Annotated
 from fastapi import APIRouter, Depends, HTTPException, status, Request
 from fastapi.security import OAuth2PasswordRequestForm
 from sqlalchemy.ext.asyncio import AsyncSession
-from sqlalchemy import select
+from sqlalchemy import select, update as sa_update
 from app.core.config import settings
 from app.core.settings_manager import SettingsManager
 from app.core.database import get_db
@@ -78,13 +78,15 @@ async def register(
     After user creation, if no account invite was used, a personal Account
     and free Subscription are created automatically.
     """
-    # Check for account invite code FIRST — bypasses platform invite gate
+    # Check for account invite code FIRST — bypasses platform invite gate.
+    # SELECT FOR UPDATE prevents two concurrent registrations from both
+    # reading the same invite as unused and double-spending it.
     account_invite_record = None
     if user_data.account_invite_code:
         result = await db.execute(
-            select(AccountInvite).where(
-                AccountInvite.code == user_data.account_invite_code
-            )
+            select(AccountInvite)
+            .where(AccountInvite.code == user_data.account_invite_code)
+            .with_for_update()
         )
         account_invite_record = result.scalar_one_or_none()
 
@@ -116,9 +118,12 @@ async def register(
             )
 
         if user_data.invite_code:
-            # Look up invite code (case-insensitive) — applies plan/trial regardless of REQUIRE_INVITE_CODE
+            # Look up invite code (case-insensitive) — applies plan/trial regardless of REQUIRE_INVITE_CODE.
+            # FOR UPDATE prevents double-spend by concurrent registrations.
             result = await db.execute(
-                select(InviteCode).where(InviteCode.code == user_data.invite_code.upper())
+                select(InviteCode)
+                .where(InviteCode.code == user_data.invite_code.upper())
+                .with_for_update()
             )
             invite_code_record = result.scalar_one_or_none()
 
@@ -305,24 +310,29 @@ async def refresh_token(
     user_id = payload.get("sub")
     jti = payload.get("jti")
 
-    # Validate refresh token hasn't been revoked
+    # Atomically revoke the old refresh token (token rotation).
+    # Using a conditional UPDATE prevents the race where two concurrent
+    # refresh requests both read revoked_at=NULL and both succeed.
     if jti:
         token_hash = hash_token(jti)
         result = await db.execute(
-            select(RefreshToken).where(RefreshToken.token_hash == token_hash)
+            sa_update(RefreshToken)
+            .where(
+                RefreshToken.token_hash == token_hash,
+                RefreshToken.revoked_at.is_(None),
+            )
+            .values(revoked_at=datetime.now(timezone.utc))
+            .returning(RefreshToken.id, RefreshToken.user_id)
         )
-        stored_token = result.scalar_one_or_none()
+        revoked_row = result.fetchone()
 
-        if stored_token and stored_token.is_revoked:
+        if not revoked_row:
+            # Either the token doesn't exist or was already revoked/used
             raise HTTPException(
                 status_code=status.HTTP_401_UNAUTHORIZED,
                 detail="Refresh token has been revoked"
             )
 
-        # Revoke the old refresh token (token rotation)
-        if stored_token:
-            stored_token.revoked_at = datetime.now(timezone.utc)
-
     result = await db.execute(select(User).where(User.id == user_id))
     user = result.scalar_one_or_none()
 
@@ -552,9 +562,12 @@ async def reset_password(
             detail="Invalid reset token"
         )
 
-    # Validate token in DB (single-use)
+    # Validate token in DB (single-use).
+    # FOR UPDATE prevents two concurrent reset requests from both succeeding.
     result = await db.execute(
-        select(PasswordResetToken).where(PasswordResetToken.token_hash == hash_token(jti))
+        select(PasswordResetToken)
+        .where(PasswordResetToken.token_hash == hash_token(jti))
+        .with_for_update()
     )
     token_record = result.scalar_one_or_none()
 
@@ -674,10 +687,11 @@ async def verify_email(
             detail="Invalid verification token"
         )
 
+    # FOR UPDATE prevents two concurrent verification requests from both succeeding.
     result = await db.execute(
-        select(EmailVerificationToken).where(
-            EmailVerificationToken.token_hash == hash_token(jti)
-        )
+        select(EmailVerificationToken)
+        .where(EmailVerificationToken.token_hash == hash_token(jti))
+        .with_for_update()
     )
     token_record = result.scalar_one_or_none()
 

frontend/src/api/client.ts

@@ -75,15 +75,19 @@ let refreshSubscribers: ((token: string) => void)[] = []
 let refreshFailSubscribers: ((error: unknown) => void)[] = []
 
 function onRefreshed(token: string) {
-  refreshSubscribers.forEach(cb => cb(token))
+  // Swap arrays before iterating — if a callback throws, the arrays
+  // are already cleared so the next refresh cycle starts clean.
+  const subscribers = refreshSubscribers
   refreshSubscribers = []
   refreshFailSubscribers = []
+  subscribers.forEach(cb => cb(token))
 }
 
 function onRefreshFailed(error: unknown) {
-  refreshFailSubscribers.forEach(cb => cb(error))
+  const failSubscribers = refreshFailSubscribers
   refreshSubscribers = []
   refreshFailSubscribers = []
+  failSubscribers.forEach(cb => cb(error))
 }
 
 // Response interceptor - handle token refresh