fix: synchronize concurrent coroutines checking & updating key set (#2215)

# What does this PR do?

This PR adds a lock to coordinate concurrent coroutines passing through
the jwt verification. As _refresh_jwks() was setting _jwks to an empty
dict then repopulating it, having multiple coroutines doing this
concurrently risks losing keys. The PR also builds the updated dict as a
separate object and assigns it to _jwks once completed. This avoids
impacting any coroutines using the key set as it is being updated.

Signed-off-by: Gordon Sim <gsim@redhat.com>

llama_stack/distribution/server/auth_providers.py

@@ -7,6 +7,7 @@
 import json
 import time
 from abc import ABC, abstractmethod
+from asyncio import Lock
 from enum import Enum
 from urllib.parse import parse_qs
 
@@ -236,6 +237,7 @@ class OAuth2TokenAuthProvider(AuthProvider):
         self.config = config
         self._jwks_at: float = 0.0
         self._jwks: dict[str, str] = {}
+        self._jwks_lock = Lock()
 
     async def validate_token(self, token: str, scope: dict | None = None) -> TokenValidationResult:
         """Validate a token using the JWT token."""
@@ -271,17 +273,19 @@ class OAuth2TokenAuthProvider(AuthProvider):
         """Close the HTTP client."""
 
     async def _refresh_jwks(self) -> None:
-        if time.time() - self._jwks_at > self.config.cache_ttl:
-            async with httpx.AsyncClient() as client:
-                res = await client.get(self.config.jwks_uri, timeout=5)
-                res.raise_for_status()
-                jwks_data = res.json()["keys"]
-                self._jwks = {}
-                for k in jwks_data:
-                    kid = k["kid"]
-                    # Store the entire key object as it may be needed for different algorithms
-                    self._jwks[kid] = k
-                self._jwks_at = time.time()
+        async with self._jwks_lock:
+            if time.time() - self._jwks_at > self.config.cache_ttl:
+                async with httpx.AsyncClient() as client:
+                    res = await client.get(self.config.jwks_uri, timeout=5)
+                    res.raise_for_status()
+                    jwks_data = res.json()["keys"]
+                    updated = {}
+                    for k in jwks_data:
+                        kid = k["kid"]
+                        # Store the entire key object as it may be needed for different algorithms
+                        updated[kid] = k
+                    self._jwks = updated
+                    self._jwks_at = time.time()
 
 
 class CustomAuthProviderConfig(BaseModel):