The executor drained child stdout/stderr via GLib.IOChannel watches on the
main loop, so a stalled or wedged GUI stopped draining the pipes and the
child would block on a full (~64KB) pipe forever — the cascade that froze a
batch when the editor/terminal died.
Replace the main-loop watches with per-stream background reader threads that
drain pipes with blocking reads into thread-safe buffers; a GLib timer on the
main thread consumes the buffers and does all parsing/UI (threads never touch
GObject/GTK). Now the child can never block on a pipe we own, regardless of
main-loop state. Also: stdout can go straight to a file when requested, stdin
is fed from a file in a thread, the retained end-of-job log is bounded, and
wait_end is guarded to run once.
Tests (tests/test_executor_io.py): line parsing incl. CR progress lines,
2MB output with no loss/deadlock, completion with the main loop never running
(the wedged-GUI case), cancel, stdout-to-file, and exit-code propagation.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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