Nonblocking warnings

[nakulj]

Background

In --parallel mode, Worker.warning_added sends each warning to the controller with a blocking XML-RPC report_warning call. The controller serves every worker on one thread, and that thread also runs hooks.test_distributed inside get_test, so a warning waits behind whatever the controller is doing for other workers. In monorail CI the Backslash plugin's test_distributed held that thread for 12–38 s, which timed out test steps that log warnings inside a wall-clock budget (parallelsystems/monorail#16187).

Description

• warning_added hands the pickled warning to a single background thread (ThreadPoolExecutor(max_workers=1)) and returns without waiting. Warnings are still pickled on the calling thread and sent in order.
• Each send creates its own ServerProxy, so no proxy is shared between threads. A failed send is logged instead of raised.
• report_session_error still blocks.

A warning logged in a child process forked from a worker is not sent. In tmux mode, warnings still queued when the controller stops waiting for a worker are dropped.

Verification

test_worker_warning_does_not_wait_for_server checks that warning_added returns while the controller is still handling the report, and that the warning arrives afterwards. tests/test_parallel.py and tests/test_plugin_parallel.py pass locally (62 tests).

🤖 Generated with Claude Code

[ps-groscoe]

LGTM!