The Live tab showed nothing while a turn ran, and the agent's own account of it
went to stderr on the node and nowhere a user could reach. This is the path that
carries it.
The blocker was the guest agent. `fcagent` handled one connection at a time,
inline, so during an hour-long turn the VM accepted nothing — which is why every
existing probe (subagents, stop-gate blocks, cap) runs AFTER the turn rather than
during it. It now spawns a thread per connection, wrapped in `catch_unwind`
because this process is pid 1: a panic used to take the accept loop with it, and
an unbootable VM is a far worse outcome than a missing log. A failed spawn logs
and keeps accepting rather than dropping the listener.
PROVED against a live VM before building on it, since "sound reasoning about this
system" and "measurement" have diverged repeatedly today. Patched rootfs, booted
under Firecracker, ran an 8s exec and a concurrent tail:
exec took 8.0s ok=True
+0.0s 'line1\nline2\n' +1.2s 'line4\n' +3.2s 'line6\n' +6.0s 'DONE\n'
VERDICT: CONCURRENT — tail returned data before exec finished
The rest is the pattern the terminal already uses. New `tail` op streams a file
by OFFSET (so a dropped link resumes instead of replaying, and the tail always
terminates — one that never returns pins a thread for the life of the VM). The
node follows the log alongside the turn and pushes `Uplink::VmOut { run_id, at,
data }` over the WebSocket it already holds, mirroring `PtyOut`. The server does
what `PtyOut` deliberately does not: it APPENDS to the run's checkpoint as well
as fanning out, because a terminal has no history worth keeping and a mission log
is the record of what the agent did. `run_events_sse` emits the new bytes as
`step` events, which the live pane already renders — no frontend change.
The turn is `tee`d, not redirected: the file feeds the live stream and stdout
still becomes `VmOutcome::summary`. A redirect would have produced a live view
and an empty summary, which is the same green-and-empty shape as the bug this
fixes. Tested, along with the log living outside the collected tree so it never
lands in a user's delivered diff.
246 lib tests, 20 binaries; node and fcagent build clean.
Omar Sobh
0b89b8316c