smoketest: write run_in_vm's log as the command runs

So a long command (install.sh takes an hour or more) can be followed with
tail -f; run_in_vm prints the log's path when it starts.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

smoketest/lib/scenario.sh

@@ -47,14 +47,15 @@ skip() {
 
 # run_in_vm NAME SCRIPT: run SCRIPT in the VM, keeping its output in $OUTPUT,
 # its exit status in $STATUS, and both in NN-NAME.log. Doesn't fail on its own:
-# check with assert_status.
+# check with assert_status. The log is written as SCRIPT runs, so a long one
+# (install.sh takes an hour or more) can be followed with tail -f.
 run_in_vm() {
   RUN_COUNT=$((RUN_COUNT + 1))
   LAST_RUN="$1"
   LAST_LOG="$SMOKETEST_RESULTS/$(printf '%02d' "$RUN_COUNT")-$1.log"
-  echo "== $1"
-  OUTPUT="$(vm_exec "$2" 2>&1)" && STATUS=0 || STATUS=$?
-  printf '%s\n' "$OUTPUT" > "$LAST_LOG"
+  echo "== $1 (to follow it: tail -f $LAST_LOG)"
+  vm_exec "$2" > "$LAST_LOG" 2>&1 && STATUS=0 || STATUS=$?
+  OUTPUT="$(cat "$LAST_LOG")"
   echo "== $1 exited $STATUS (log: $LAST_LOG)"
 }
 

tests/smoketest.bats

@@ -131,6 +131,12 @@ upstream_main() {
   [ -f "$SMOKETEST_RESULTS/02-second.log" ]
 }
 
+@test "run_in_vm writes its log while the script runs" {
+  run_in_vm slow 'echo early; cat "$SMOKETEST_RESULTS/01-slow.log" > "$HOME/seen"'
+  [ "$STATUS" -eq 0 ]
+  [ "$(cat "$HOME/seen")" = early ]
+}
+
 @test "cd_update passes its variables to update" {
   mkdir -p "$HOME/.local/bin"
   printf '%s\n' '#!/bin/bash' 'echo "force=$UPDATE_FORCE"' > "$HOME/.local/bin/update"