feat: add pooled connection reuse health check

[StellarisW]

Summary

Adds a Linux-only, owner-synchronized (*TCPConnection).IsHealthyForReuse(ownerWaitTimeout) probe for idle pooled client connections.

• Keeps FDOperator ownership across the LinkBuffer inspection and a single non-consuming MSG_PEEK | MSG_DONTWAIT socket probe.
• Treats EINTR as fail-closed instead of retrying indefinitely, so both owner acquisition and the socket probe are bounded.
• Rejects pending buffered data, socket data, FIN/reset/other socket errors, inactive/unused operators, and the PollDetach → asynchronous OnHup lifecycle window.
• Does not modify the public netpoll.Connection interface.
• Does not consume application bytes.

Safety boundary

This is a pre-write eligibility check, not a reservation protocol: it rejects staleness visible while probing, but a peer may still close after this method returns and before a later write.

Functional validation

The tests are contained in this repository and use real loopback TCP connections created through NewDialer().DialConnection. The focused suite covers:

• an idle connection with no visible inbound data;
• a temporarily busy receive owner, where the check waits and retains a healthy connection;
• the deterministic ioread → InputAck handoff window;
• unread data in the socket or LinkBuffer, including verification that the probe does not consume the byte;
• peer FIN, peer reset, local close, and a nil receiver;
• PollDetach before asynchronous OnHup publishes the closing state;
• a local close that begins while the probe is waiting for receive ownership;
• a receive owner that remains busy past ownerWaitTimeout, where the check fails closed and does not leave the operator owned;
• an interrupted (EINTR) nonblocking probe, which fails closed without retrying.

Current-head commands run with Go 1.20.14 on Linux:

go test -mod=readonly ./...

go test -race -mod=readonly -count=20 \
  -run '^TestTCPConnectionIsHealthyForReuse' .

go vet -mod=readonly ./...

The full repository test suite passed, the timeout/EINTR-focused race lane passed all 20 repetitions, and go vet ./... passed at netpoll head 598bbe1df4d1772bceb7dffbb4bce6ade86dc905.

Performance benchmark

BenchmarkTCPConnectionIsHealthyForReuseIdle is included in connection_reuse_linux_test.go. It creates one real loopback TCP pair before timing, performs one warm-up check, resets the timer, and repeatedly invokes IsHealthyForReuse(1s) on that same idle connection. Every iteration fails the benchmark if the connection is classified as unhealthy.

taskset -c 0-3 go test -mod=readonly -run '^$' \
  -bench '^BenchmarkTCPConnectionIsHealthyForReuseIdle$' \
  -benchmem -benchtime=2s -count=10 -cpu=4 .

metric	10-sample median	sample range
ns/op	290.7	283.7–337.3
B/op	0	0–0
allocs/op	0	0–0

Benchmark machine

• Debian GNU/Linux 10; Linux 5.4.143.bsk.8-amd64; x86_64
• Intel Xeon Platinum 8336C @ 2.30 GHz
• 32 logical CPUs visible; benchmark pinned to CPUs 0-3
• 62.65 GiB RAM visible
• Go 1.20.14; GOMAXPROCS=4; loopback-only traffic

This benchmark measures the healthy idle IsHealthyForReuse(1s) probe itself. It is not an application-throughput or production-network benchmark.

Related work

• Hertz consumer PR: cloudwego/hertz#1523
• Release order: merge and release this owner capability first, then let Hertz pin the released netpoll version and publish its consumer release.