[wwbmmm]

What problem does this PR solve?

Issue Number: resolve

Problem Summary:

ProcessNsheadRequest() wraps MethodStatus::OnRequested() in CHECK(), treating a rejection of the concurrency limiter as an impossible state. Both built-in limiters (ConstantConcurrencyLimiter, AutoConcurrencyLimiter) really return false under overload, so the first request that reaches the limit configured by the operator trips the check: with -crash_on_fatal_log=true the process aborts, and with the default configuration a FATAL log with a stack trace is emitted while the rejected request is served anyway, i.e. the configured limit is silently disabled and the logs are flooded at the request rate. Sibling protocols (thrift, baidu_std, http, mongo) fail the controller with ELIMIT instead. The same rejection was also discarded in NsheadPbServiceAdaptor, so the method-level limit was never enforced for nshead-based pb protocols.

What is changed and the side effects?

Changed:

• src/brpc/policy/nshead_protocol.cpp: the service-level MethodStatus::OnRequested() result is checked inside the existing do/while validation block and a rejection fails the controller with ELIMIT, the same way as the other checks of that block, so the user handler (or the pb adaptor) answers the client instead of the request being served. The concurrency accounting stays balanced: OnRequested() counts the request even when it is rejected, and ConcurrencyRemover in NsheadClosure::Run() counts it out.
• src/brpc/nshead_pb_service_adaptor.cpp: the method-level OnRequested() result is no longer ignored, an over-limit request fails with ELIMIT (pbdone->status is set before the check, so OnResponded() is still called on the failure path).
• test/brpc_server_unittest.cpp: two regression tests, one for a plain NsheadService and one for a pb service served through PublicPbrpcServiceAdaptor. Each test holds the only concurrency slot with one request and verifies that the second request is rejected with ELIMIT and that the slot is recycled afterwards, so later requests are served again.

Side effects:

• Performance effects: none, the check is called at the same place with the same cost.

• Breaking backward compatibility: none at the API level. Requests that reach a configured concurrency limit on nshead services are now answered with ELIMIT instead of being served (with a FATAL log, or aborting the process when -crash_on_fatal_log=true), which is the documented behavior of max_concurrency / method_max_concurrency.

Tests run:

• brpc_server_unittest (all 50 cases)
• brpc_public_pbrpc_protocol_unittest, brpc_nova_pbrpc_protocol_unittest, brpc_nshead_mcpack_protocol_unittest, brpc_interceptor_unittest
• brpc_channel_unittest (2 pre-existing failures unrelated to this change: ChannelTest.parse_hostname needs network access to www.baidu.com, ChannelTest.backuprequest_selective_response_race is flaky)

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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.

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🤖 This PR was automatically created by brpc-oncall

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What changed in this PR

This PR fixes nshead request handling under concurrency overload by rejecting over-limit requests with ELIMIT instead of CHECK-crashing/logging fatals and inadvertently serving the request anyway.

Changes:

• Update nshead server protocol to treat MethodStatus::OnRequested() rejection as a normal overload path and fail the controller with ELIMIT.
• Enforce method-level concurrency limiting in NsheadPbServiceAdaptor by honoring the return value of OnRequested().
• Add regression tests ensuring over-limit nshead and nshead-adapted PB requests are rejected and concurrency is properly recycled.

File	Description
src/​brpc/​policy/​nshead_protocol.cpp	Replace CHECK(OnRequested()) with an ELIMIT failure path under overload.
src/​brpc/​nshead_pb_service_adaptor.cpp	Stop ignoring method-level limiter rejections; fail with ELIMIT when over limit.
test/​brpc_server_unittest.cpp	Add regression tests covering nshead service-level and nshead-adapted PB method-level concurrency rejection and slot recycling.

_{Review effort: Lite}

[chenBright]

LGTM