cmdlib: let qemu write command output on its stdout

Instead of using `tail -qF`, just write the output to a virtio-serial
port and tell qemu to output that on its stdout. That way, we don't have
to worry about cleaning up the `tail` process at all.

This also fixes an issues we've been seeing where `tail` is killed
before it had a chance to print the entire command output, which
crucially might omit the final error message.

Note we close stdin to qemu here otherwise it seems like it just waits
forever for input to be written into the virtio port.

src/cmdlib.sh

@@ -409,7 +409,8 @@ json_key() {
 # process.  It mounts the workdir via 9p.  If you need to add new packages into
 # the vm, update `vmdeps.txt`.
 # If you need to debug it, one trick is to change the `-serial file` below
-# into `-serial stdio` and then e.g. add `bash` into the init process.
+# into `-serial stdio`, drop the <&- and virtio-serial stuff and then e.g. add
+# `bash` into the init process.
 runvm() {
     local qemu_args=()
     while true; do
@@ -443,9 +444,6 @@ runvm() {
     # include COSA in the image
     find /usr/lib/coreos-assembler/ -type f > "${vmpreparedir}/hostfiles"
 
-    local cmdoutput
-    cmdoutput=${workdir}/tmp/runvm-cmd-output.txt
-
     # the reason we do a heredoc here is so that the var substition takes
     # place immediately instead of having to proxy them through to the VM
     cat > "${vmpreparedir}/init" <<EOF
@@ -460,7 +458,9 @@ export USER=$(id -u)
 export RUNVM_NONET=${RUNVM_NONET:-}
 $(cat "${DIR}"/supermin-init-prelude.sh)
 rc=0
-bash ${TMPDIR}/cmd.sh 1>${cmdoutput} 2>&1 || rc=\$?
+# tee to the virtio port so its output is also part of the supermin output in
+# case e.g. a key msg happens in dmesg when the command does a specific operation
+bash ${TMPDIR}/cmd.sh |& tee /dev/virtio-ports/cosa-cmdout || rc=\$?
 echo \$rc > ${workdir}/tmp/rc
 if [ -n "\${cachedev}" ]; then
     /sbin/fstrim -v ${workdir}/cache
@@ -487,28 +487,28 @@ EOF
 
     local runvm_console
     runvm_console="${workdir}/tmp/runvm-console.txt"
-    rm -f "${workdir}/tmp/rc" "${runvm_console}" "${cmdoutput}"
+    rm -f "${workdir}/tmp/rc" "${runvm_console}"
 
     touch "${runvm_console}"
-    setpriv --pdeathsig SIGTERM -- tail -qF "${cmdoutput}" --pid $$ &
     #shellcheck disable=SC2086
-    kola qemuexec -m 2048 --auto-cpus -U --workdir none -- -no-reboot -nodefaults -serial file:"${runvm_console}" \
+    if ! kola qemuexec -m 2048 --auto-cpus -U --workdir none -- -no-reboot -nodefaults -serial file:"${runvm_console}" \
         -kernel "${vmbuilddir}/kernel" -initrd "${vmbuilddir}/initrd" \
+        -device virtio-serial \
+        -device virtserialport,chardev=virtioserial0,name=cosa-cmdout \
+        -chardev stdio,id=virtioserial0 \
         -drive "if=virtio,format=raw,snapshot=on,file=${vmbuilddir}/root,index=1" \
         "${cachedisk[@]}" \
         -virtfs local,id=workdir,path="${workdir}",security_model=none,mount_tag=workdir \
         "${srcvirtfs[@]}" -append "root=/dev/vda console=${DEFAULT_TERMINAL} selinux=1 enforcing=0 autorelabel=1" \
-	"${qemu_args[@]}"
-    kill %1
+        "${qemu_args[@]}" <&-; then # the <&- here closes stdin otherwise qemu waits forever
+            cat "${runvm_console}"
+            fatal "Failed to run 'kola qemuexec'"
+    fi
 
     if [ ! -f "${workdir}"/tmp/rc ]; then
         cat "${runvm_console}"
         fatal "Couldn't find rc file; failure inside supermin init?"
     fi
-    if [ ! -f "${cmdoutput}" ]; then
-        cat "${runvm_console}"
-        fatal "Couldn't find command output file, failure inside supermin init?"
-    fi
     rc="$(cat "${workdir}"/tmp/rc)"
     rm -f "${workdir}/tmp/rc"
     return "${rc}"