[songgaoye]

What problem does this PR solve?

Issue Number: resolve
Problem Summary:

On Linux, each accepted connection currently uses accept() followed by
separate calls to set non-blocking and close-on-exec flags. This adds
unnecessary syscalls on the connection-accept hot path.

What is changed and the side effects?

Changed:

• Use accept4() with SOCK_NONBLOCK | SOCK_CLOEXEC for accepted sockets on
  Linux.
• Fall back to accept() when accept4() is unavailable (ENOSYS).
• Skip redundant flag-setting calls when the accepted fd already has the
  requested flags.
• Add a Linux-only regression test verifying accepted sockets have
  O_NONBLOCK and FD_CLOEXEC.

Side effects:

• Performance effects: reduces syscall overhead while accepting connections on
  Linux.
• Breaking backward compatibility: No. Non-Linux platforms and Linux kernels
  without accept4() retain the existing behavior.

---

Check List:

• Please make sure your changes are compilable.
  • cmake --build build --target brpc_socket_unittest -j4
• When providing us with a new feature, it is best to add related tests.
• Please follow Contributor Covenant Code of Conduct.

[Copilot]

🟢 Approval recommended

Static review found no blocking issues in flag propagation, fallback handling, or the regression test; tests were not executed during review.

No open issues 🎉

What changed in this PR

This PR reduces Linux connection-accept overhead by setting socket flags through accept4(), while preserving existing behavior on other platforms and older Linux kernels.

Changes:

• Use accept4() with non-blocking and close-on-exec flags, falling back to accept() on ENOSYS.
• Pass known descriptor flags through socket initialization to avoid redundant flag-setting calls.
• Add a Linux-only regression test for accepted socket flags.

File	Description
test/​brpc_socket_unittest.cpp	Verify accepted sockets have both required flags on Linux.
src/​brpc/​socket.h	Declare flag-aware socket initialization interfaces.
src/​brpc/​socket.cpp	Skip flag-setting calls when flags are already set.
src/​brpc/​acceptor.cpp	Use accept4() on Linux with an accept() fallback.

_{Review effort: Balanced}

[wwbmmm]

I don't think this is necessary. Connection-accept is not hot path in most scenarios.

[songgaoye]

close it