Inside tmux the dashboard's smolmachines launch crashed within
~100ms of the wrapper Popen-ing the main smolvm exec child —
sometimes with rc=137 (SIGKILL), sometimes with smolvm
spitting a runc-style "load `config.json`: cannot parse the
data: parse error: trailing garbage" and exiting 1. The same
wrapper ran fine outside tmux. Diagnostic logs showed the
SIGKILL landed ~100ms after the wrapper kicked off its
initial `sync()` (which fires the side-channel smolvm exec).
Root cause: the side-channel `subprocess.run([smolvm, machine,
exec, --, sh, -c, ...])` did not specify `stdin=`, so it
inherited the wrapper's stdin — the tmux pane PTY. The main
smolvm child (the agent session) also had that PTY as stdin.
Two concurrent smolvm processes sharing the PTY's
foreground-process-group / input plumbing caused smolvm to
abort one of them. iTerm's PTY plumbing apparently tolerated
this; tmux's didn't.
Fix is one line in `_push_size`: `stdin=subprocess.DEVNULL`.
The side-channel never needs stdin — it runs a fire-and-forget
`stty` and exits. Verified end-to-end: pre-fix the wrapper
crashed under `tmux respawn-pane` against a live VM; post-fix
the same invocation completes cleanly.
Also drop the diagnostic log added in 37bd11b — we have the
fix.
Regression test:
`test_side_channel_uses_devnull_stdin` locks the
`stdin=DEVNULL` invariant so a future "let's simplify the
subprocess.run kwargs" refactor surfaces this immediately.
637 unit tests pass.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
didericis-claude
b9853ae0c7