Previous to this change, large `EscalateRequest` payloads exceeded the
kernel send buffer, causing our single `sendmsg(2)` call (with attached
FDs) to be split and retried without proper control handling; this led
to `EINVAL`/broken pipe in the
`handle_escalate_session_respects_run_in_sandbox_decision()` test when
using an `env` with large contents.
**Before:** `AsyncSocket::send_with_fds()` called `send_json_message()`,
which called `send_message_bytes()`, which made one `socket.sendmsg()`
call followed by additional `socket.send()` calls, as necessary:
2e4a402521/codex-rs/exec-server/src/posix/socket.rs (L198-L209)
**After:** `AsyncSocket::send_with_fds()` now calls
`send_stream_frame()`, which calls `send_stream_chunk()` one or more
times. Each call to `send_stream_chunk()` calls `socket.sendmsg()`.
In the previous implementation, the subsequent `socket.send()` writes
had no control information associated with them, whereas in the new
`send_stream_chunk()` implementation, a fresh `MsgHdr` (using
`with_control()`, as appropriate) is created for `socket.sendmsg()` each
time.
Additionally, with this PR, stream sending attaches `SCM_RIGHTS` only on
the first chunk, and omits control data when there are no FDs, allowing
oversized payloads to deliver correctly while preserving FD limits and
error checks.
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