test_respawn_pane_replaces_shell races the respawned exec under CI parallelism

[tony]

test_respawn_pane_replaces_shell reads pane_current_command immediately after respawn_pane returns, with nothing waiting for the new process to exec. It normally wins that race and passes; under CI's parallelism it sometimes loses, consuming reruns and masking itself behind --reruns=2.

Where

tests/test_pane_tools.py#L1938-L1954 — respawn_pane(...) is called and assert "sleep" in result.pane_current_command runs against whatever tmux reports at that instant.

Pre-existing, not introduced by #100 — the test body is unchanged since d5362a9 (2026-04-20) apart from a parameter rename in bec9ab1.

Field evidence

Observed in tests run 30168336319, where it reran on two independent tmux jobs in the same run:

build (3.14, 3.5)   [gw1] [ 43%] RERUN tests/test_pane_tools.py::test_respawn_pane_replaces_shell
build (3.14, 3.3a)  [gw0] [ 42%] RERUN tests/test_pane_tools.py::test_respawn_pane_replaces_shell

--reruns=2 is in addopts, so this does not turn CI red — it just quietly costs attempts. A green run is therefore weaker evidence than it looks.

Recreation

respawn-pane returns as soon as tmux has forked; pane_current_command reports the new command only once it has exec'd. Reading with no delay in between shows how wide the window is:

./repro_respawn.sh 200

repro_respawn.sh

#!/usr/bin/env bash
# Reproduce the race in test_respawn_pane_replaces_shell: the test reads
# pane_current_command immediately after respawn_pane returns, with no
# wait for the new process to exec. Mirrors the test exactly -- a session
# on its default shell, a SPLIT pane running the old command, then a
# respawn of that split.
set -uo pipefail
SOCK="repro-respawn-$$"
N=${1:-200}
new=0; stale=0; other=0
tmux -L "$SOCK" kill-server 2>/dev/null
tmux -L "$SOCK" new-session -d -s s
for _ in $(seq 1 "$N"); do
  pane=$(tmux -L "$SOCK" split-window -d -P -F '#{pane_id}' -t s "sleep 3600")
  tmux -L "$SOCK" respawn-pane -k -t "$pane" "sleep 7200"
  cmd=$(tmux -L "$SOCK" display-message -p -t "$pane" '#{pane_current_command}')
  case "$cmd" in
    sleep)  new=$((new+1))   ;;   # test passes
    "")     other=$((other+1));;
    *)      stale=$((stale+1));;  # test FAILS: 'sleep' not in cmd
  esac
  tmux -L "$SOCK" kill-pane -t "$pane" 2>/dev/null
done
tmux -L "$SOCK" kill-server 2>/dev/null; rm -f "/tmp/tmux-1000/$SOCK"
echo "reports 'sleep'  (test passes): $new/$N"
echo "reports stale    (test FAILS):  $stale/$N"
echo "reports empty:                  $other/$N"

Measured on tmux 3.7b:

reports 'sleep'  (test passes): 3/200
reports stale    (test FAILS):  197/200
reports empty:                  0/200

That is the mechanism in isolation, not the test's real rate. The Python path does more work between the respawn and the read — pane resolution and result construction — and normally that is enough: the real test passed 40/40 at idle on a6e4117d39ffe633d50eb1ad3a6fcd55a71e5e60. The failures only appear under CI's -n auto parallelism, which is exactly what the reruns above show.

for i in $(seq 1 40); do env -u VIRTUAL_ENV uv run pytest tests/test_pane_tools.py -q --reruns 0 -p no:randomly -k test_respawn_pane_replaces_shell; done

Background

pane_current_command resolves through tmux's process-name lookup for the pane's foreground process group, so it reflects whatever is running at the moment of the query — see osdep-linux.c and the pane_current_command entry in format.c (locally: ~/study/c/tmux/osdep-linux.c, ~/study/c/tmux/format.c, checkout at tag 3.7b). There is no tmux event that says "the respawned command has started", so a poll is the only correct way to observe it.

Suggested fix

Poll instead of asserting on the first read. The project already uses retry_until from libtmux.test.retry for exactly this shape elsewhere in the same file:

retry_until(lambda: "sleep" in (new_pane.pane_current_command or ""), 5, raises=True)

Keep an assertion on result.pane_id unconditionally — that part is not racy. Only the command read needs settling.

Worth checking whether any other test asserts on pane_current_command straight after a spawn or respawn; the same shape would have the same race.