[release/10.0] Guard against empty remote address on macOS accept - #133567

…indows/Linux (refs #133778) (#133925)

Workflow artifact: ci-fix
Artifact kind: help
Linked KBE: #133778

> [!NOTE]
> This is an AI/Copilot-generated **best-effort** fix attempt that I
could not fully validate. It is a starting point for a maintainer, not a
finished change. Please review the analysis below before merging.

## Root cause (best analysis)


`System.Net.Sockets.Tests.AcceptDualStackResetTests.Accept_DualStackListener_PeerImmediatelyResets_ListenerStaysHealthy(useAsync:
True)` fails on `windows-x64`/`linux-x64` (`TestReadyToRun_Libraries`
and the runtime pipeline):

```
System.Net.Sockets.SocketException : An existing connection was forcibly closed by the remote host.
   at System.Net.Sockets.Tests.AcceptDualStackResetTests.Accept_DualStackListener_PeerImmediatelyResets_ListenerStaysHealthy(Boolean useAsync) in /_/src/libraries/System.Net.Sockets/tests/FunctionalTests/Accept.cs:line 527
```

Line 527 is the `AcceptAsync`/`Accept` call, **not** the receive. The
test (added by #131869) closes an IPv4 peer with
`SO_LINGER=0` so the kernel sends an immediate RST, then expects the
listener to still deliver the healthy IPv6 peer's byte. The `try { ... }
catch (SocketException)` block only wraps the `ReceiveAsync`; the
`accept` that precedes it sits **outside** the `try`. On Windows and
Linux the reset is surfaced by `accept` itself (`WSAECONNRESET` /
"connection forcibly closed"), which escapes the catch and fails the
test, even though the very next accept iteration would return the
healthy peer. macOS discards the reset at accept (which
#131869 hardened `EndPoint.Create` for), so it doesn't hit
this path — which is why the test passed there but not on Windows/Linux.

The test comment already anticipates this ("Some platforms surface the
reset connection from accept()"), but the code doesn't guard the accept
accordingly.

## Attempted fix

Move the `accept` inside the existing `try` block so a `SocketException`
from either the accept or the following receive is tolerated, and the
loop retries to accept the healthy peer on the next iteration (the loop
already runs up to 2 accepts). No production code changes; the
listener-stays-healthy assertion is preserved. This is **not** a
test-disable — the test still runs and still asserts the healthy peer's
message is received.

## What is unverified / where I need help

- I could not build and run `System.Net.Sockets.Tests` in this
environment, so I have not confirmed the test now passes on
windows-x64/linux-x64 while still catching a genuine listener
regression.
- Please confirm the intended contract: on Windows/Linux, should the RST
be observable at `accept` (making tolerating it here correct), or should
the runtime itself suppress the reset connection from the accept loop
the way macOS does? If the latter, the real fix belongs in the socket
accept path rather than the test.

## Validation
- Command: `not run because the sockets functional test suite could not
be built/executed within the run budget`
- Result: not run

## Evidence
- Failing build:
https://dev.azure.com/dnceng-public/cbb18261-c48f-4abb-8651-8cdcb5474649/_build/results?buildId=1593787
- First build it occurred: after #131869 (merged
2026-09-09, commit 7c35b7d), which introduced this test (computed
within the scanned window; may not be the true origin)
- Suspected regressing change: #131869 (added the test
with the accept outside the catch)

## Help wanted
- Likely author: `@wfurt` (authored #131869 that added the
test)
- Area owners (`area-System.Net.Sockets`): `@dotnet/ncl`

---
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Structured data:
```json
{
  "artifact_kind": "help",
  "linked_kbe": 133778,
  "workflow_artifact": "ci-fix"
}
```

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Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>
Co-authored-by: Copilot <[email protected]>

Use a release-compatible conditional property because the servicing branch's xUnit extensions do not define TestPlatforms.OpenBSD.

Co-authored-by: Copilot <[email protected]>

Copilot-Session: 7ec5412f-9a88-4e9f-a1c4-fcff00d61248