Fix flaky DuplicateStreamIdThrows tests

Summary:
The DuplicateStreamIdThrows tests in SinkServiceTest and StreamServiceTest
were flaky due to a race condition in the server's connection close flow.

When the server detects a duplicate stream ID, it calls close() which sends
a connection-level error frame and sets the state to CLOSING. However,
closeIfNeeded() won't fully close the connection while there are inflight
requests. The handler for the first (valid) frame can still complete and
send a response before the connection fully closes. If the client receives
this response before the connection-level error frame, co_await completes
successfully instead of throwing TTransportException.

Fixed both tests to handle both race outcomes:
1. If co_await throws TTransportException - pass (duplicate detected before
   response arrived)
2. If co_await succeeds - verify the connection is broken by asserting a
   follow-up request throws TTransportException

Both outcomes correctly verify the server handles duplicate stream IDs.

Reviewed By: evanjzou

Differential Revision: D94856980

fbshipit-source-id: f8e463d3fcbf319d94c0cb6ff0e8921b1ea8650e

Author: sazonovkirill

third-party/thrift/src/thrift/lib/cpp2/async/tests/SinkServiceTest.cpp

@@ -403,8 +403,18 @@ TEST_F(SinkServiceTest, DuplicateStreamIdThrows) {
       [](Client<TestSinkService>& client) -> folly::coro::Task<void> {
         // dummy request to send setup frame
         co_await client.co_test();
-        // sink request frame will now be sent twice with the same stream id
-        EXPECT_THROW(co_await client.co_range(0, 100), TTransportException);
+        // sink request frame will now be sent twice with the same stream id.
+        // The server detects the duplicate and closes the connection, but due
+        // to a race the initial response may arrive before the connection-level
+        // error. In either case the connection should be broken afterwards.
+        try {
+          co_await client.co_range(0, 100);
+        } catch (const TTransportException&) {
+          co_return;
+        }
+        // Initial response arrived before connection error; verify the
+        // connection is now broken.
+        EXPECT_THROW(co_await client.co_test(), TTransportException);
       });
 }
 

third-party/thrift/src/thrift/lib/cpp2/async/tests/StreamServiceTest.cpp

@@ -143,8 +143,18 @@ TYPED_TEST(StreamServiceTest, DuplicateStreamIdThrows) {
       [](Client<TestStreamService>& client) -> folly::coro::Task<void> {
         // dummy request to send setup frame
         co_await client.co_test();
-        // sink request frame will now be sent twice with the same stream id
-        EXPECT_THROW(co_await client.co_range(0, 100), TTransportException);
+        // stream request frame will now be sent twice with the same stream id.
+        // The server detects the duplicate and closes the connection, but due
+        // to a race the initial response may arrive before the connection-level
+        // error. In either case the connection should be broken afterwards.
+        try {
+          co_await client.co_range(0, 100);
+        } catch (const TTransportException&) {
+          co_return;
+        }
+        // Initial response arrived before connection error; verify the
+        // connection is now broken.
+        EXPECT_THROW(co_await client.co_test(), TTransportException);
       });
 }