fix(endpoint-discovery): delete failed cache entry in blocking operations (#4011)

Author: kuhe

packages/middleware-endpoint-discovery/src/updateDiscoveredEndpointInCache.spec.ts

@@ -120,7 +120,8 @@ describe(updateDiscoveredEndpointInCache.name, () => {
           );
         }
         verifyCallsOnCacheUndefined();
-        expect(mockDelete).not.toHaveBeenCalled();
+        expect(mockDelete).toHaveBeenCalledTimes(1);
+        expect(mockDelete).toHaveBeenCalledWith(cacheKey);
         expect(mockSet).toHaveBeenCalledTimes(1);
         expect(mockSet).toHaveBeenCalledWith(cacheKey, placeholderEndpoints);
       });
@@ -133,7 +134,8 @@ describe(updateDiscoveredEndpointInCache.name, () => {
         await updateDiscoveredEndpointInCache(config, options);
 
         verifyCallsOnCacheUndefined();
-        expect(mockDelete).not.toHaveBeenCalled();
+        expect(mockDelete).toHaveBeenCalledTimes(1);
+        expect(mockDelete).toHaveBeenCalledWith(cacheKey);
         expect(mockSet).toHaveBeenCalledTimes(2);
         expect(mockSet).toHaveBeenNthCalledWith(1, cacheKey, placeholderEndpoints);
         expect(mockSet).toHaveBeenNthCalledWith(2, cacheKey, placeholderEndpoints);

packages/middleware-endpoint-discovery/src/updateDiscoveredEndpointInCache.ts

@@ -54,10 +54,11 @@ export const updateDiscoveredEndpointInCache = async (
           resolve();
         })
         .catch((error: any) => {
-          if (error.name === "InvalidEndpointException" || error.$metadata?.httpStatusCode === 421) {
-            // Endpoint is invalid, delete the cache entry.
-            endpointCache.delete(cacheKey);
-          }
+          // The cache entry must be deleted 
+          // because a subsequent blocking request will be stuck
+          // in a waiting state if it sees the cache entry
+          // but we have already flushed the request queue. 
+          endpointCache.delete(cacheKey);
 
           const errorToThrow = Object.assign(
             new Error(