fix(cpworker): serve concurrent and split control-socket requests

[timmy21]

Summary

• Concurrent clients (cpworker: control socket listen backlog is 1; concurrent Go clients fail with EAGAIN #260). The control socket listened with a backlog of 1. Go dials unix sockets non-blocking, so a second client connecting while the first was pending failed at once with EAGAIN (e.g. cpdaemon's stats poll overlapping a cpctl call). The backlog is now SOMAXCONN.

• Silent client. accept reads the handshake synchronously in the manager thread, and the socket had no receive timeout. A client that connected and never sent the handshake stalled the manager, and with it every other client. The handshake read is now bounded by SO_RCVTIMEO (1 s).

• Request split across writes (cpworker: control socket does not frame requests by newline; split or pipelined requests get no reply #261). The code meant to wait for the rest of a request, but:

  • select() was passed fd instead of fd + 1, so the fd was never examined and the wait always timed out;
  • the newline check used the last read's length instead of the buffered length;
  • a piece arriving on the third wait was read and then reported as incomplete.

  The loop is restructured to read until the newline; most of the diff in unix_client_recv is re-indentation.

The protocol stays strict request/response. Pipelined requests (several requests in one write, or the handshake and the first request together) are not supported; the only client, cpgolib/cpworker/unix_client.go, waits for each reply before sending the next request. The constraint is documented at unix_client_recv.

Tests

New integration test TestControlSocket (cpworker/tests/integration/control_socket_test.go):

subtest	before	after
concurrent_clients (32 clients at once)	15 of 32 failed	pass
request_split_across_writes	connection reset	pass
silent_client_does_not_block_others	i/o timeout	pass

Unit tests (9/9), the full integration suite and cpgolib tests pass.

Fixes #260

Refs #261