run-test-in-systemd: watch output in real-time

This is good for usability, and might also prevent it from detecting
inactivity and killing our tests, which upstream developer Neil Williams
warned me might happen (although I haven't seen it on our installation).

Differential Revision: https://phabricator.apertis.org/D372
Reviewed-by: pwith

common/run-test-in-systemd

@@ -224,8 +224,10 @@ ${as_target_user} rm -f "${filename}.tmp"
 ${as_target_user} rm -f "${filename}"
 
 user_log_dir="$(${as_target_user} mktemp -d)"
-${as_target_user} touch "${user_log_dir}/stdout.log"
-exec_start="${tee_exec} ${user_log_dir}/stdout.log"
+# We write the output to a fifo (named pipe) so that we can watch it in
+# real-time, below. We start watching it before we start the test.
+${as_target_user} mkfifo "${user_log_dir}/stdout.fifo"
+exec_start="${tee_exec} ${user_log_dir}/stdout.fifo"
 
 while [ "$#" -gt 0 ]; do
     exec_start="${exec_start} $(systemd_escape_argument "$1")"
@@ -264,6 +266,12 @@ if [ -n "${lava_runes}" ]; then
 fi
 
 debug "starting test"
+
+# Watch the output in real time, both for developers' benefit and so that
+# LAVA does not think "it has become inactive" and kill us.
+${as_target_user} cat "${user_log_dir}/stdout.fifo" &
+cat_fifo_pid="$!"
+
 if ${time_adverb} ${systemctl} start ${service}; then
     debug "test passed"
     result=pass
@@ -279,14 +287,13 @@ else
     fi
 fi
 
-debug "getting command output..."
-
-${as_target_user} cat "${user_log_dir}/stdout.log" || result=fail
-rm -fr "${user_log_dir}" || result=fail
-
 debug "killing any leftover test processes"
 ${systemctl} stop ${service} || :
 
+debug "waiting for end-of-file on command output..."
+wait "$cat_fifo_pid" || result=fail
+rm -fr "${user_log_dir}" || result=fail
+
 if [ -n "${lava_runes}" ]; then
     echo "<LAVA_SIGNAL_ENDTC ${name}>"
     "lava-test-case" "${name}" --result "${result}"