The b9853ae stdin=DEVNULL fix wasn't sufficient. End-to-end
testing against a live VM in tmux revealed a second crash path:
libkrun spits "load \`config.json\`: parse error: trailing
garbage { \"ociVersion\": \"1.0.2\", ... }" and the main exec
dies (rc=1 or SIGKILL/rc=137, depending on race scheduling).
Root cause: each `smolvm machine exec` writes a per-invocation
OCI config.json to the same smolvm state dir during its bringup.
The wrapper's startup sync() fires within 1ms of Popen-ing the
main exec — both invocations write config.json concurrently,
libkrun loads one mid-write, and gets garbage. Trivial inner
commands (`sh -c "echo hi"`) finished before the overlap
mattered, masking the race in earlier tests. claude's slower
startup hits the race every time, and only inside tmux because
the outside-tmux foreground-handoff path takes a different
bringup sequence that happens to dodge the window.
Fix: schedule the initial sync on a 2-second `threading.Timer`
instead of calling it synchronously. By 2s the main exec is
past its bringup window, so the side-channel's config.json
write doesn't collide. Daemon thread so the timer doesn't
block exit when the child finishes quickly.
Trade-off: the in-VM PTY uses smolvm's default size for the
first ~2s, then snaps to the host pane size when the timer
fires. Verified end-to-end against a live VM in tmux: claude
renders at the default size during bringup, then redraws at
full pane width once the deferred sync lands. Operator-driven
resizes (SIGWINCH) still bridge in real time via the
already-installed signal handler.
Also drop the diagnostic log added in 9c83ea6 — we have the
fix.
Regression test:
`TestStartupSyncDeferred.test_main_schedules_timer_does_not_
call_sync_synchronously` mocks Popen + Timer + _push_size and
asserts `main()` schedules the timer with the documented
delay constant and never invokes _push_size synchronously.
Catches a "let's just inline the sync() call" regression
immediately.
638 unit tests pass.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
didericis-claude
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