fix(oauth): narrow context.lock scope in async_auth_flow

src/mcp/client/auth/oauth2.py

@@ -114,7 +114,18 @@ class OAuthContext:
     token_expiry_time: float | None = None
 
     # State
+    #
+    # `lock` guards short-lived reads/writes of provider state (initialization
+    # flag, token cache mutation, protocol_version assignment). It is held only
+    # while mutating state and is released before any HTTP request is yielded
+    # so a long-running request (e.g. GET SSE long-poll) does not block
+    # unrelated concurrent requests.
+    #
+    # `refresh_lock` provides single-flight semantics for token refresh: only
+    # one concurrent refresh fires; other waiters block on this lock, then
+    # re-check the token cache and proceed without re-refreshing.
     lock: anyio.Lock = field(default_factory=anyio.Lock)
+    refresh_lock: anyio.Lock = field(default_factory=anyio.Lock)
 
     def get_authorization_base_url(self, server_url: str) -> str:
         """Extract base URL by removing path component."""
@@ -452,7 +463,7 @@ async def _refresh_token(self) -> httpx.Request:
 
         return httpx.Request("POST", token_url, data=refresh_data, headers=headers)
 
-    async def _handle_refresh_response(self, response: httpx.Response) -> bool:  # pragma: no cover
+    async def _handle_refresh_response(self, response: httpx.Response) -> bool:
         """Handle token refresh response. Returns True if successful."""
         if response.status_code != 200:
             logger.warning(f"Token refresh failed: {response.status_code}")
@@ -504,7 +515,17 @@ async def _validate_resource_match(self, prm: ProtectedResourceMetadata) -> None
             raise OAuthFlowError(f"Protected resource {prm_resource} does not match expected {default_resource}")
 
     async def async_auth_flow(self, request: httpx.Request) -> AsyncGenerator[httpx.Request, httpx.Response]:
-        """HTTPX auth flow integration."""
+        """HTTPX auth flow integration.
+
+        Lock scope:
+          ``self.context.lock`` is held only while reading/mutating provider
+          state. The actual HTTP request yield (which may be a long-poll GET
+          SSE stream) runs outside any lock so concurrent unrelated requests
+          are not blocked. ``self.context.refresh_lock`` provides
+          single-flight semantics for token refresh.
+        """
+        # === Phase 1: state read + refresh decision (brief context.lock) ===
+        needs_refresh = False
         async with self.context.lock:
             if not self._initialized:
                 await self._initialize()  # pragma: no cover
@@ -513,20 +534,40 @@ async def async_auth_flow(self, request: httpx.Request) -> AsyncGenerator[httpx.
             self.context.protocol_version = request.headers.get(MCP_PROTOCOL_VERSION)
 
             if not self.context.is_token_valid() and self.context.can_refresh_token():
-                # Try to refresh token
-                refresh_request = await self._refresh_token()  # pragma: no cover
-                refresh_response = yield refresh_request  # pragma: no cover
-
-                if not await self._handle_refresh_response(refresh_response):  # pragma: no cover
-                    # Refresh failed, need full re-authentication
-                    self._initialized = False
-
-            if self.context.is_token_valid():
-                self._add_auth_header(request)
-
-            response = yield request
-
-            if response.status_code == 401:
+                needs_refresh = True
+
+        # === Phase 2: single-flight token refresh (yield outside context.lock) ===
+        if needs_refresh:
+            async with self.context.refresh_lock:
+                # Re-check under context.lock: another coroutine may already have
+                # refreshed while we were waiting on refresh_lock.
+                refresh_request: httpx.Request | None = None
+                async with self.context.lock:
+                    if not self.context.is_token_valid() and self.context.can_refresh_token():
+                        refresh_request = await self._refresh_token()
+                if refresh_request is not None:
+                    # yield runs outside any lock so a long network round trip
+                    # does not block unrelated concurrent requests.
+                    refresh_response = yield refresh_request
+                    async with self.context.lock:
+                        if not await self._handle_refresh_response(refresh_response):
+                            # Refresh failed; fall through to 401 handling below.
+                            self._initialized = False
+
+        # === Phase 3: send request (no lock; safe for long-poll GET SSE) ===
+        if self.context.is_token_valid():
+            self._add_auth_header(request)
+
+        response = yield request
+
+        # === Phase 4: 401 / 403 full OAuth flow ===
+        # NOTE: Phase 4 yields multiple sub-requests (discovery, registration,
+        # token exchange) under context.lock. This is the existing behavior and
+        # is acceptable because the 401 path is exceptional and not concurrent
+        # with steady-state traffic. A future refactor could narrow the lock
+        # here in the same pattern as Phase 1-2.
+        if response.status_code == 401:
+            async with self.context.lock:
                 # Perform full OAuth flow
                 try:
                     # OAuth flow must be inline due to generator constraints
@@ -619,7 +660,8 @@ async def async_auth_flow(self, request: httpx.Request) -> AsyncGenerator[httpx.
                 # Retry with new tokens
                 self._add_auth_header(request)
                 yield request
-            elif response.status_code == 403:
+        elif response.status_code == 403:
+            async with self.context.lock:
                 # Step 1: Extract error field from WWW-Authenticate header
                 error = extract_field_from_www_auth(response, "error")
 

tests/client/test_auth.py

@@ -1,10 +1,12 @@
 """Tests for refactored OAuth client authentication implementation."""
 
 import base64
+import contextlib
 import time
 from unittest import mock
 from urllib.parse import parse_qs, quote, unquote, urlparse
 
+import anyio
 import httpx
 import pytest
 from inline_snapshot import Is, snapshot
@@ -2618,3 +2620,201 @@ async def callback_handler() -> tuple[str, str | None]:
             await auth_flow.asend(final_response)
         except StopAsyncIteration:
             pass
+
+
+@pytest.mark.anyio
+async def test_concurrent_request_not_blocked_by_pending_long_running_request(
+    oauth_provider: OAuthClientProvider, valid_tokens: OAuthToken
+):
+    """Regression for #1326: a second request reaches its yield while the
+    first is still suspended (= simulating a server-side long-poll).
+
+    Before the lock-scope fix, ``async_auth_flow`` held ``context.lock``
+    across ``yield request``. A GET SSE long-poll would therefore hold the
+    lock for the entire SSE lifetime, blocking any concurrent request
+    waiting on the same provider's lock.
+    """
+    # Set up valid tokens so neither refresh (Phase 2) nor full OAuth
+    # flow (Phase 4) is triggered — we exercise the steady-state Phase 3
+    # yield path that previously held the lock.
+    oauth_provider.context.current_tokens = valid_tokens
+    oauth_provider.context.token_expiry_time = time.time() + 1800
+    oauth_provider.context.client_info = OAuthClientInformationFull(
+        client_id="test_client_id",
+        client_secret="test_client_secret",
+        redirect_uris=[AnyUrl("http://localhost:3030/callback")],
+    )
+    oauth_provider._initialized = True
+
+    # Flow 1: drive to yield, then leave suspended (simulating long-poll).
+    slow_request = httpx.Request("GET", "https://api.example.com/v1/mcp")
+    slow_flow = oauth_provider.async_auth_flow(slow_request)
+    yielded_slow = await slow_flow.__anext__()
+    assert yielded_slow.headers.get("Authorization") == "Bearer test_access_token"
+
+    # Flow 2: concurrent request. With the fix this reaches its yield
+    # immediately; without the fix it would block on context.lock.
+    fast_request = httpx.Request("POST", "https://api.example.com/v1/mcp")
+    fast_flow = oauth_provider.async_auth_flow(fast_request)
+    with anyio.fail_after(5):
+        yielded_fast = await fast_flow.__anext__()
+    assert yielded_fast.headers.get("Authorization") == "Bearer test_access_token"
+
+    with contextlib.suppress(StopAsyncIteration):
+        await fast_flow.asend(httpx.Response(200, request=yielded_fast))
+    with contextlib.suppress(StopAsyncIteration):
+        await slow_flow.asend(httpx.Response(200, request=yielded_slow))
+
+
+@pytest.mark.anyio
+async def test_refresh_lock_double_check_skips_redundant_refresh(
+    oauth_provider: OAuthClientProvider, valid_tokens: OAuthToken
+):
+    """Two flows enter Phase 2 with an expired token. After the first
+    completes a refresh, the second observes the fresh token via the
+    Phase 2 double-check inside ``refresh_lock`` (or directly in Phase 1
+    if it arrives late) and skips its own refresh.
+    """
+    oauth_provider.context.current_tokens = valid_tokens
+    oauth_provider.context.token_expiry_time = time.time() - 100  # expired
+    oauth_provider.context.client_info = OAuthClientInformationFull(
+        client_id="test_client_id",
+        client_secret="test_client_secret",
+        redirect_uris=[AnyUrl("http://localhost:3030/callback")],
+    )
+    oauth_provider._initialized = True
+
+    # Flow A: drive to refresh yield, then complete refresh.
+    request_a = httpx.Request("GET", "https://api.example.com/v1/mcp")
+    flow_a = oauth_provider.async_auth_flow(request_a)
+    refresh_a = await flow_a.__anext__()
+    assert "grant_type=refresh_token" in refresh_a.read().decode()
+
+    refresh_response = httpx.Response(
+        200,
+        content=(
+            b'{"access_token": "new_access_token", "token_type": "Bearer", '
+            b'"expires_in": 3600, "refresh_token": "new_refresh_token"}'
+        ),
+        request=refresh_a,
+    )
+    request_a_post = await flow_a.asend(refresh_response)
+    assert request_a_post.headers.get("Authorization") == "Bearer new_access_token"
+
+    # Flow B: state already refreshed; Phase 1 sees valid token, skips Phase 2.
+    request_b = httpx.Request("POST", "https://api.example.com/v1/mcp")
+    flow_b = oauth_provider.async_auth_flow(request_b)
+    with anyio.fail_after(5):
+        request_b_yielded = await flow_b.__anext__()
+    assert request_b_yielded.method == "POST"
+    assert request_b_yielded.headers.get("Authorization") == "Bearer new_access_token"
+
+    with contextlib.suppress(StopAsyncIteration):
+        await flow_b.asend(httpx.Response(200, request=request_b_yielded))
+    with contextlib.suppress(StopAsyncIteration):
+        await flow_a.asend(httpx.Response(200, request=request_a_post))
+
+
+@pytest.mark.anyio
+async def test_refresh_with_failed_status_clears_tokens(oauth_provider: OAuthClientProvider, valid_tokens: OAuthToken):
+    """A non-2xx refresh response clears stored tokens and marks the provider
+    uninitialized so the next request triggers a full OAuth flow.
+    """
+    oauth_provider.context.current_tokens = valid_tokens
+    oauth_provider.context.token_expiry_time = time.time() - 100
+    oauth_provider.context.client_info = OAuthClientInformationFull(
+        client_id="test_client_id",
+        client_secret="test_client_secret",
+        redirect_uris=[AnyUrl("http://localhost:3030/callback")],
+    )
+    oauth_provider._initialized = True
+
+    request = httpx.Request("POST", "https://api.example.com/v1/mcp")
+    flow = oauth_provider.async_auth_flow(request)
+    refresh_request = await flow.__anext__()
+    assert "grant_type=refresh_token" in refresh_request.read().decode()
+
+    # Refresh server returns 401.
+    refresh_response = httpx.Response(401, content=b'{"error": "invalid_grant"}', request=refresh_request)
+    with contextlib.suppress(StopAsyncIteration):
+        # After failed refresh, the flow proceeds to Phase 3 yielding the
+        # original request without a fresh Authorization header. We don't
+        # exercise the subsequent 401/full OAuth path here.
+        await flow.asend(refresh_response)
+
+    assert oauth_provider.context.current_tokens is None
+
+
+@pytest.mark.anyio
+async def test_refresh_with_invalid_json_clears_tokens(oauth_provider: OAuthClientProvider, valid_tokens: OAuthToken):
+    """A 200 refresh response with a malformed body clears stored tokens —
+    the pydantic ValidationError branch is taken.
+    """
+    oauth_provider.context.current_tokens = valid_tokens
+    oauth_provider.context.token_expiry_time = time.time() - 100
+    oauth_provider.context.client_info = OAuthClientInformationFull(
+        client_id="test_client_id",
+        client_secret="test_client_secret",
+        redirect_uris=[AnyUrl("http://localhost:3030/callback")],
+    )
+    oauth_provider._initialized = True
+
+    request = httpx.Request("POST", "https://api.example.com/v1/mcp")
+    flow = oauth_provider.async_auth_flow(request)
+    refresh_request = await flow.__anext__()
+
+    # Body does not parse as OAuthToken.
+    refresh_response = httpx.Response(200, content=b"not json", request=refresh_request)
+    with contextlib.suppress(StopAsyncIteration):
+        await flow.asend(refresh_response)
+
+    assert oauth_provider.context.current_tokens is None
+
+
+@pytest.mark.anyio
+async def test_double_check_inside_refresh_lock_skips_second_refresh(
+    oauth_provider: OAuthClientProvider, valid_tokens: OAuthToken, monkeypatch: pytest.MonkeyPatch
+):
+    """Exercise the double-check branch inside ``refresh_lock``: ``is_token_valid``
+    returns False in Phase 1 (= the flow decides to refresh) but True inside
+    the inner ``context.lock`` block (= another coroutine refreshed while we
+    were waiting on ``refresh_lock``). The flow must skip ``_refresh_token``
+    and proceed straight to Phase 3.
+    """
+    oauth_provider.context.current_tokens = valid_tokens
+    oauth_provider.context.token_expiry_time = time.time() - 100  # expired
+    oauth_provider.context.client_info = OAuthClientInformationFull(
+        client_id="test_client_id",
+        client_secret="test_client_secret",
+        redirect_uris=[AnyUrl("http://localhost:3030/callback")],
+    )
+    oauth_provider._initialized = True
+
+    # Toggle is_token_valid: False on the first call (Phase 1 decision),
+    # True on the second (double-check inside refresh_lock).
+    call_count = {"n": 0}
+    original_is_valid = oauth_provider.context.__class__.is_token_valid
+
+    def fake_is_token_valid(self: object) -> bool:
+        call_count["n"] += 1
+        if call_count["n"] == 1:
+            return False
+        # By the second call, "another coroutine" refreshed; reset token expiry
+        # so callers downstream see a valid token.
+        oauth_provider.context.token_expiry_time = time.time() + 1800
+        return True
+
+    monkeypatch.setattr(oauth_provider.context.__class__, "is_token_valid", fake_is_token_valid)
+    try:
+        request = httpx.Request("POST", "https://api.example.com/v1/mcp")
+        flow = oauth_provider.async_auth_flow(request)
+        # No refresh yield is expected — the flow goes directly to its own
+        # request yield with the (now-valid) token header attached.
+        with anyio.fail_after(5):
+            yielded = await flow.__anext__()
+        assert yielded.method == "POST"
+        assert yielded.headers.get("Authorization") == "Bearer test_access_token"
+        with contextlib.suppress(StopAsyncIteration):
+            await flow.asend(httpx.Response(200, request=yielded))
+    finally:
+        monkeypatch.setattr(oauth_provider.context.__class__, "is_token_valid", original_is_valid)