feat: add URMA transport support

[zchuango]

What problem does this PR solve?

About discussion：

#3167 —— Socket-Transport 架构改造共识
#3217 —— 路线 B（URMA-UrmaTransport）与路线 C（双后端）共识
#3226 —— OBMM/ShmTransport 实现方案
本提案对应 #3217 中的路线 B，并与#3226 的 OBMM-ShmTransport 形成路线 C（双后端协同）。

Issue Number: Related to #3401

Problem Summary:

brpc does not currently provide a remote-memory transport based on the UMDK
URMA API.

This PR adds URMA (Unified Remote Memory Access) as an optional transport in
the existing Transport framework. TCP is used for connection establishment
and URMA capability/resource negotiation. After both peers successfully
negotiate URMA support, the data path switches to URMA. When URMA is
unavailable or negotiation does not succeed, the connection falls back to TCP.

The initial implementation supports the baidu_std protocol only and is
disabled by default.

What is changed and the side effects?

Changed:

• Add SOCKET_MODE_URMA and UrmaTransport, and integrate them with the
  existing transport, socket, and message-processing framework.
• Add UrmaEndpoint for URMA resource management, SEND/RECV processing,
  completion handling, and credit-based flow control.
• Add TCP-based URMA capability and resource negotiation, including v2 binary
  and v3 protobuf handshake formats, with TCP fallback.
• Add a registered buffer pool for IOBuf blocks and APIs for explicit user
  memory registration.
• Support completion-event mode and optional busy-polling mode.
• Support the UMDK bonding extension when the corresponding provider extension
  is available.
• Add optional URMA build integration for CMake, Make, and Bazel, together with
  a link-time URMA mock for build and test environments without liburma or
  URMA hardware.
• Add URMA unit tests, English and Chinese documentation, and an
  urma_performance example.

Side effects:

• Performance effects:

  • URMA is optional and disabled by default. Existing socket modes and their
    default behavior remain unchanged.
  • Enabling URMA maps and registers a configurable IOBuf buffer pool. The
    default configuration is 65,536 buffers of 8 KiB each (512 MiB in total).
  • Busy-polling mode may increase CPU usage because completion pollers
    actively poll the JFC.
  • URMA performance depends on the provider, driver, hardware, and polling
    configuration. This PR does not claim a specific hardware performance
    improvement.

• Breaking backward compatibility:

  • No intended breaking changes.
  • SOCKET_MODE_URMA and the URMA memory-registration APIs are additive.
    Existing socket modes and their default behavior remain unchanged.

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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.

[wwbmmm]

I'm not familiar with URMA, but it seems that the code in src/brpc/urma is similar to src/brpc/rdma, is that anything we can reuse?

[Copilot]

Pull request overview

This PR adds an optional URMA (UMDK Unified Remote Memory Access) transport backend to bRPC’s existing Socket/Transport architecture, including a TCP-based negotiation/handshake with transparent TCP fallback when URMA is unavailable or negotiation fails.

Changes:

• Introduces SOCKET_MODE_URMA, UrmaTransport, and per-connection urma::UrmaEndpoint, plus handshake (v2 binary + v3 protobuf) and a credit-based flow-control model.
• Adds build-system integration for URMA across CMake/Make/Bazel (including fetching UMDK headers and a link-time URMA mock when liburma/hardware is unavailable).
• Adds URMA unit tests, documentation (EN/CN), and a new urma_performance example.

Reviewed changes

Copilot reviewed 36 out of 37 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
WORKSPACE	Adds Bazel external umdk repo for URMA headers.
MODULE.bazel	Adds bzlmod umdk repo rule for URMA headers.
BUILD.bazel	Wires URMA define and exposes URMA protos/headers to Bazel build.
bazel/config/BUILD.bazel	Adds brpc_with_urma config setting.
bazel/third_party/umdk/umdk.BUILD	Defines urma_headers cc_library for UMDK header export.
CMakeLists.txt	Adds WITH_URMA build option, header discovery/download, library detection, mock handling, and proto compilation.
Makefile	Includes src/brpc/urma sources conditionally; excludes mock when linking real liburma.
config_brpc.sh	Adds --with-urma configuration, header/lib detection, and mock selection.
src/brpc/socket_mode.h	Adds SOCKET_MODE_URMA.
src/brpc/transport_factory.h	Minor API/comment cleanup and parameter naming.
src/brpc/transport_factory.cpp	Adds URMA transport creation/context-init plumbing.
src/brpc/socket.h	Adds URMA forward decls and friend access for new endpoint/handshake types.
src/brpc/channel.h	Updates socket_mode comments to reference SocketMode generally.
src/brpc/channel.cpp	Incorporates URMA into channel signature hashing.
src/brpc/server.h	Clarifies socket_mode comment for accepted sockets.
src/brpc/input_messenger.h	Adds friend access for URMA endpoint.
src/brpc/input_messenger.cpp	Ensures URMA-delivered messages are executed off poller threads.
src/brpc/urma_transport.h	Declares UrmaTransport composing TcpTransport fallback and managing URMA state.
src/brpc/urma_transport.cpp	Implements transport dispatch, epoll-out waiting, message queueing, and context init constraints.
src/brpc/urma/urma_endpoint.h	Declares per-socket endpoint state machine, resources, polling/event CQ handling, and data-path APIs.
src/brpc/urma/urma_endpoint.cpp	Implements endpoint handshake, CQ polling, SEND/RECV processing, and connect driver.
src/brpc/urma/urma_helper.h	Declares global URMA init, polling-mode setup, and memory registration APIs.
src/brpc/urma/urma_helper.cpp	Implements global URMA init, buffer-pool + IOBuf allocator hijack, and user memory registration.
src/brpc/urma/urma_handshake.proto	Defines protobuf v3 handshake message (UrmaHello).
src/brpc/urma/urma_handshake.h	Declares handshake strategy abstraction + v2/v3 implementations and negotiation helpers.
src/brpc/urma/urma_handshake.cpp	Implements v2 binary + v3 protobuf handshake serialization/parsing and validation.
src/brpc/urma/urma_bonding.h	Optional provider-extension include gate for URMA bonding.
src/brpc/urma/mock_urma.cpp	Provides link-time URMA C-API mock for builds without liburma/hardware.
test/brpc_urma_unittest.cpp	Adds URMA handshake/helper/mock unit coverage.
example/cmake/BrpcExample.cmake	Updates example toolchain to handle newer Protobuf/Abseil and URMA include/defines.
example/urma_performance/CMakeLists.txt	Adds URMA performance example build (links liburma if present).
example/urma_performance/test.proto	Proto definitions for performance example RPC service.
example/urma_performance/server.cpp	URMA/TCP selectable server for performance testing.
example/urma_performance/client.cpp	URMA/TCP selectable benchmark client with warmup + worker bthreads.
docs/en/urma.md	English documentation for building/using UrmaTransport and configuration flags.
docs/cn/urma.md	Chinese documentation for building/using UrmaTransport and configuration flags.
.gitignore	Ignores local URMA proposal notes and graph artifacts.

Suppressed comments (1)

src/brpc/urma/urma_helper.cpp:103

• kIOBufBlockHeaderLen is currently hard-coded to 32, which assumes the current IOBuf::Block layout. Once butil/iobuf_inl.h is included, this can be derived from sizeof(butil::IOBuf::Block) to stay correct if the layout ever changes.

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