[qxforever]

For internal review before this goes to apache/brpc. No upstream issue has been filed yet; the issue draft's root-cause section is included below.

What problem does this PR solve?

Issue Number: none yet (to be filed on apache/brpc)

Problem Summary:

With socket_mode = SOCKET_MODE_URMA, a server never notices that a client closed its connection or exited, whether normally or with kill -9, once the URMA handshake has started. The server-side Socket is never failed or recycled, so for every client connection that ever existed the server keeps:

• the TCP connection, in CLOSE_WAIT;
• the UrmaEndpoint and its URMA resources (jetty, JFC, JFR, imported peer segment).

/sockets/<id> still shows the socket as healthy: no broken Socket mark, ninprocess=1 (no EOF seen) and URMA state=ESTABLISHED.

Root cause: UrmaEndpoint::OnNewDataFromTcp (urma_endpoint.cpp#L1208-L1267) returns early in three branches: server UNINIT (after starting the UrmaServerHandshake bthread), client UNINIT, and state < ESTABLISHED (after waking _read_butex). None of them reaches MoreReadEvents() at the bottom of the loop, so the socket's _nevent never goes back to 0. Socket::OnInputEvent starts a read handler only when _nevent goes from 0 to 1. On the server, the first TCP event of an accepted connection always finds the endpoint in UNINIT, so no read handler runs on that socket again. The handshake still completes, because the handshake bthread polls the fd itself every 50 ms. After it, the client's FIN is never read, TryReadOnTcpDuringUrmaEst never calls SetEOF(), and the socket is never failed.

RdmaEndpoint::OnNewDataFromTcpAtClient (rdma_endpoint.cpp#L251-L279), and the RDMA server path before apache#3350, which URMA follows, fall through to MoreReadEvents() in these branches.

What is changed and the side effects?

Changed:

• OnNewDataFromTcp: the server UNINIT, client UNINIT and state < ESTABLISHED branches fall through to MoreReadEvents(), as in RdmaEndpoint::OnNewDataFromTcpAtClient. The loop starts at Socket::PROGRESS_INIT instead of 0, which cost one extra iteration. Only FALLBACK_TCP, ESTABLISHED and the !IsUrmaAvailable() fallback still return, because their handlers release the events themselves.
• TryReadOnTcp(): events that arrive during the handshake are now released without reading the fd. So when the handshake bthread finishes, it takes the read side through _nevent.fetch_add(1) the way Socket::OnInputEvent does, and reads only if it went from 0 to 1. This is what RdmaEndpoint::TryReadOnTcp() did before Support RDMA handshake protocol apache/brpc#3350. If OnNewDataFromTcp is still running, the increment makes it loop once more and see the final state. This also keeps the fallback's OnNewMessages() from running at the same time as one started by an event. Before, only the stuck _nevent prevented that.
• TryReadOnTcp() also reads in ESTABLISHED, and both handshakes call it right after they reach ESTABLISHED. Without this, a FIN that arrived between the last handshake read and the state change would be released unread and never read again.
• New test test/brpc_urma_connection_unittest.cpp: over the URMA mock, a short-connection channel makes three calls. Each time the client closes the connection, the server must fail its socket within 5 seconds. While the call is in progress, the test also checks that the connection is URMA (state=ESTABLISHED), so a TCP fallback cannot make it pass. It fails without this change. It is a separate binary because the end-to-end setup needs the real global URMA init (over the mock), while brpc_urma_unittest skips it in main().

Side effects:

• Performance effects: none on the data path. There is one extra read() on the TCP fd per connection, right after the handshake. During the handshake, TCP events now wake the handshake bthread instead of waiting for its 50 ms poll.

• Breaking backward compatibility: none. The wire format is unchanged.

Testing status

• src/brpc/urma/urma_endpoint.cpp, test/brpc_urma_connection_unittest.cpp and test/brpc_urma_unittest.cpp pass -fsyntax-only with clang++ and g++-14 against the UMDK headers, with no warnings.
• Not yet built or run. Next is a mock build on the UB hosts: cmake -DWITH_URMA=ON -DURMA_USE_MOCK=ON -DBUILD_UNIT_TESTS=ON, then brpc_urma_connection_unittest and brpc_urma_unittest.
• After that, a hardware rerun of the leak scenario: the server socket should show as recycled, with no CLOSE_WAIT.
• Upstream CI does not build URMA.

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Check List:

• Please make sure your changes are compilable.
• When providing us with a new feature, it is best to add related tests.
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