[songgaoye]

What problem does this PR solve?

Issue Number: resolve

Problem Summary:

Server::StartInternal() creates temporary bthreads to run
ServerOptions::bthread_init_fn.

The initialization bthread writes BthreadInitArgs::done and repeatedly reads
BthreadInitArgs::stop, while the startup thread reads done and writes
stop. These fields were ordinary bools, so the concurrent accesses formed
C++ data races. In particular, observing completion did not safely publish the
init callback result, and the init bthread was not synchronized with the
shutdown request.

What is changed and the side effects?

Changed:

• Make BthreadInitArgs::done and stop atomic.
• Publish init completion with a release store after writing result.
• Consume completion with an acquire load before proceeding with startup.
• Synchronize the stop request with release/acquire operations.
• Add a barrier-based regression test covering the init callback completion,
  startup, stop, and join handoff.

The new test reports the original race under TSAN on the unmodified code. The
targeted BthreadInitEntry / StartInternal report disappears after this
change.

Side effects:

• Performance effects:

  This affects only temporary server-startup bthreads. It adds atomic accesses
  to their completion and stop polling paths, not RPC request processing.

• Breaking backward compatibility:

  None.

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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]

🔵 Needs a closer look

Human confirmation is needed because correctness depends on runtime scheduling and TSAN coverage across supported architectures.

No open issues 🎉

What changed in this PR

This PR fixes data races during server startup by synchronizing completion and shutdown handoff for temporary bthreads used by bthread_init_fn. It also adds a regression test covering the initialization, startup, stop, and join sequence.

Changes:

• Makes done and stop atomic with appropriate release/acquire ordering.
• Publishes the initialization callback result before startup proceeds.
• Adds a controlled regression test for the bthread initialization handoff.

File	Description
test/​brpc_server_unittest.cpp	Adds a regression test covering synchronized bthread initialization, startup, stop, and join sequencing.
src/​brpc/​server.cpp	Uses atomic completion and stop signaling to synchronize temporary initialization bthreads with server startup.

_{Review effort: Lite}