[akasakariko]

Fixes #2018

Problem and Mechanism

poll(2) currently clears registered interests after readiness and relies on IoSourceState::do_io to rearm them on WouldBlock
Raw unix::SourceFd registrations do not pass through that wrapper so later events are lost

This fix adds an internal per-registration rearm_on_io flag in the poll selector
Mio-owned I/O retains its existing rearm behavior on WouldBlock
Registrations via unix::SourceFd keep their interests enabled with level-triggered readiness
SourceFd documentation is updated to explain this behavior
No public API change

Regression Coverage

Two regression tests in tests/unix_sourcefd.rs use std UnixStream descriptors registered through SourceFd:

• Repeated readable readiness after WouldBlock
• Repeated writable readiness after WouldBlock, changing token and interests via reregister, and deregistration

Tests exclude Miri and Emscripten due to existing Unix socketpair limitations

Validation

• Before fix both new regressions fail with forced poll and pass with native epoll/kqueue, after fix both pass
• Linux aarch64 via Colima with Rust 1.96.1: cargo test --all-features and cargo test --release --all-features both PASS, with native epoll and with RUSTFLAGS='--cfg mio_unsupported_force_poll_poll'
• macOS aarch64 Rust 1.98: cargo test --all-features PASS on native kqueue, targeted forced-poll regressions PASS
• macOS native cargo clippy --all-targets --all-features -- -D warnings PASS
• rustfmt --check on every Rust file and git diff --check PASS
• cargo hack check --feature-powerset and cargo hack check --tests --feature-powerset --features log PASS with native and forced-poll backends on macOS
• cargo +1.71.1 check --all-features PASS with native and forced-poll backends

Limitations

• macOS forced-poll full suite still has pre-existing failures, each failing test also failed on an unmodified baseline run
• Linux Clippy and macOS forced-poll Clippy fail on existing diagnostics reproduced on baseline
• Test feature-powerset without log fails on existing tests importing log on baseline too, library powerset passes and test powerset with log enabled passes
• Haiku runtime not available and not tested
• No GitHub CI has run yet

[akasakariko]

@Thomasdezeeuw Could you share why this was closed? I'd like to know whether the approach is wrong or you'd prefer a different fix

[Thomasdezeeuw]

I'm not going to review an AI pr without prior discussion.