Bugfix: RDMA client cannot fall back to TCP when RDMA is not enabled on the server

[chenBright]

What problem does this PR solve?

Issue Number: resolve

Problem Summary:

When an RDMA-enabled client connects to a server that supports TCP only,
the client sends its handshake hello ("RDMA"/"RDM3" magic + body) and
then blocks waiting for the server hello. The server does not recognize those
leading bytes as any known protocol and simply closes the connection, so the
client only sees EOF on the socket and has no signal to downgrade to TCP.

What is changed and the side effects?

Changed:

A new server-side-only pseudo-protocol PROTOCOL_RDMA_HANDSHAKE
(registered ahead of PROTOCOL_HTTP so the protocol selector tries it first)
implements a tiny handshake state machine:

1. Detect: If the first 4 bytes are "RDMA" or "RDM3", enter the fallback path;
   otherwise return TRY_OTHERS and let normal protocol parsing continue.

2. Drain client hello: Read and drain the full v2 or v3 hello.

3. Reply an un-negotiable hello:

• v2: well-formed framing (correct magic + msg_len) but hello_ver = 0xFFFF,
  which makes the client's ValidHelloMessage() fail.
• v3: a RdmaHello protobuf that parses cleanly (all required fields set) but fails
  ValidRdmaHello() because block_size = 0 < MIN_BLOCK_SIZE and qp_num = 0.
• In both cases the client sets negotiated = false and downgrades to TCP on the
  same connection rather than erroring at the IO layer.

4. Drain the client ACK: A per-socket parsing context records that we are now
   expecting the 4B handshake ACK; the next call drops those bytes, clears the
   context, and returns TRY_OTHERS so the subsequent real RPC payload goes
   through normal parsing.

Side effects:

• Performance effects:

• Breaking backward compatibility:

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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.
• Please follow Contributor Covenant Code of Conduct.

[Copilot]

Pull request overview

This PR fixes a downgrade-path bug where an RDMA-enabled client connecting to a TCP-only server could block waiting for an RDMA server hello and fail to fall back cleanly. It does so by adding a server-side pseudo-protocol that recognizes RDMA handshake magic, drains the client hello, replies an intentionally “un-negotiable” hello to trigger client-side downgrade to TCP on the same connection, then drains the client ACK and returns control to normal protocol parsing.

Changes:

• Add a server-side rdma_handshake fallback protocol to enable RDMA-client → TCP-server downgrade on the same TCP connection.
• Centralize RDMA handshake wire constants into a shared header and update RDMA handshake code/tests to use them.
• Extend RawPacker/RawUnpacker with 16-bit and raw-bytes packing helpers and update v2 handshake serialization to use them.

Reviewed changes

Copilot reviewed 13 out of 13 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
test/brpc_rdma_unittest.cpp	Updates tests to use shared handshake constants and verifies RDMA-client → TCP-server calls no longer fail.
src/butil/raw_pack.h	Adds pack16/unpack16 and pack/unpack raw-bytes helpers used by handshake serialization.
src/brpc/server.cpp	Ensures rdma_handshake isn’t filtered out by enabled_protocols whitelist logic.
src/brpc/rdma/rdma_handshake.h	Switches handshake constants to the shared constants header.
src/brpc/rdma/rdma_handshake.cpp	Refactors v2 hello (de)serialization and uses shared constants for versions/sizes/magic.
src/brpc/rdma/rdma_handshake_constants.h	New shared header for RDMA handshake wire constants (magic, lengths, versions, bounds, ACK format).
src/brpc/rdma/rdma_endpoint.cpp	Uses shared constants for magic-length handling in server handshake path.
src/brpc/policy/rdma_handshake_fallback_protocol.h	Declares the new server-side RDMA-handshake fallback protocol parser.
src/brpc/policy/rdma_handshake_fallback_protocol.cpp	Implements the fallback handshake state machine using socket parsing context.
src/brpc/parse_result.h	Adds missing stddef.h include (size_t).
src/brpc/options.proto	Introduces PROTOCOL_RDMA_HANDSHAKE and changes protocol numeric ordering.
src/brpc/input_messenger.cpp	Removes the old special-case RDMA “TRY_OTHERS” hack in input parsing.
src/brpc/global.cpp	Registers the new rdma_handshake protocol at startup.

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