fix(io_context): decouple concurrency_hint == 1 from lockless mode (#…#316
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An automated preview of the documentation is available at https://316.corosio.prtest3.cppalliance.org/index.html If more commits are pushed to the pull request, the docs will rebuild at the same URL. 2026-07-15 15:42:05 UTC |
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…er forces lockless mode (cppalliance#310) `concurrency_hint` is now purely a performance hint (how many threads are expected to call `run()`); it never changes the thread-safety contract. In particular `concurrency_hint == 1` keeps full thread safety and full multi-thread utilization. Thread safety is governed by a new `io_context_options::locking` tier (`locking_mode` enum), replacing the `single_threaded` bool and mirroring Boost.Asio's SAFE / UNSAFE_IO / UNSAFE: - `safe` (default): all locks enabled; any thread may run and post. - `unsafe_io`: per-descriptor I/O locks off, scheduler locking kept; the context must be single-threaded, but resolver and POSIX file services remain available. - `unsafe`: all locks off; single-threaded, and DNS/POSIX file I/O return `operation_not_supported`. Implementation: - Scheduler threading is configured through a decomposed `threading_config` (scheduler_locking / reactor_io_locking / one_thread), replacing the coarse `configure_single_threaded` / `is_single_threaded`. The reactor, io_uring, and IOCP schedulers honor it. - `one_thread` engages a single-run-thread fast path (eliding inter-run-thread wakeups); it is set only for the lockless tiers, so the safe tier retains full multi-thread utilization at any hint. - The effective concurrency hint is normalized to 1 for the lockless tiers, driving the reactor inline-completion budget heuristic and, on IOCP, `NumberOfConcurrentThreads`. - Resolver and POSIX file services gate `operation_not_supported` on scheduler locking being disabled (the `unsafe` tier only), so they stay available under `unsafe_io`. - io_uring `SINGLE_ISSUER`/`DEFER_TASKRUN` and the eventfd wake elision are keyed on the ring being a single user (the lockless tiers). - The default constructor uses `max(1u, hardware_concurrency())`. Tests cover the tiers (unsafe_io keeps services available; unsafe restricts them), hint==1 thread safety and peer utilization across multiple run threads, and effective-hint normalization. Benchmarks, guide pages, and docstrings are updated for the new option. Fixes cppalliance#310.
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…310)
A concurrency_hint of 1 silently enabled full single-threaded lockless mode: scheduler mutex/condvar, per-descriptor locks, IOCP dispatch mutex, and io_uring ring/dispatch mutexes were all disabled, plus io_uring got SINGLE_ISSUER/DEFER_TASKRUN. This made cross-thread post() undefined behavior, contrary to asio (where a plain integer hint never disables locking) and to what migrating users expect.
A hint now only tunes performance and never changes the safety contract:
Tests: the resolver/stream_file single-threaded restriction tests switch to explicit opts.single_threaded_lockless; new testHintOneIsThreadSafe proves cross-thread post() into a hint == 1 context is TSan-clean across all backends (verified: fails under TSan with the coupling restored).
Docs: header Doxygen (option, Thread Safety, default ctor) plus the io-context and configuration guide pages updated.
Fixes #310