s6-rc-update does not create pipes when already-running service becomes a consumer

[Adam Joseph <adam@westernsemico.com>]

A simple script to demonstrate this problem is attached below.

It appears that s6-rc-update will neglect to create the needed pipes in the
following situation:

1. A service X is already running before s6-rc-update is invoked
2. Service X was not the consumer-for any other service prior to s6-rc-update
3. Service X is the consumer-for at least one service after s6-rc-update

A similar problem occurs with producer-for.

The fix is relatively straightforward; in fill_convtable_and_flags() a check is
needed to detect if olddb->services[i].x.longrun.nproducers==0 is different from
newdb->services[x].x.longrun.nproducers==0; if so, the
NEWSTATE_IS_BIJECTIVE_CONVERSION_TARGET flag should not be added to newstate[x]
and instead OLDSTATE_RESTART should be added to oldstate[i].  I have a patch
that implements this, and the corresponding check for longrun.consumer.

Below is a crude shell script which demonstrates the bug.  It does not require
root.  Before running the script below, you must run the following command on a
different terminal (you'll want to observe its output):

mkdir -p /tmp/live/service
s6-svscan -- /tmp/live/service/

Here is the script:

#!/bin/bash
mkdir -p source/consumer
echo longrun > source/consumer/type
echo -e '#!/bin/bash\ncat > /dev/null' > source/consumer/run
chmod +x source/consumer/run

mkdir source/producer
echo longrun > source/producer/type
echo -e '#!/bin/bash\nwhile true; do sleep 1; date; done\n' > source/producer/run
chmod +x source/producer/run

s6-rc-compile compiled source
s6-rc-init -l /tmp/live/compiled -c $(pwd)/compiled /tmp/live/service/
s6-rc -l /tmp/live/compiled start producer consumer s6rc-fdholder

echo <<EOF
at this point you will see the output of "producer" in the terminal
window where s6-svscan is running -- its output goes to the catch-all logger

next, we will add the producer/consumer funnel links.  however since the
services have already started, they will not be *re*started, and s6-fdholder
wont have the necessary file descriptors
EOF
sleep 5

echo producer > source/consumer/consumer-for
echo consumer > source/producer/producer-for
s6-rc-compile compiled2 source
s6-rc-update -l /tmp/live/compiled $(pwd)/compiled2

echo <<EOF
next we restart the producer (only); you should see messages like the following
on the catch-all logger:
  s6-fdholder-retrieve: fatal: unable to retrieve fd for id pipe:s6rc-w-consumer: No such file or directory
EOF
sleep 5
s6-rc -l /tmp/live/compiled stop producer
s6-rc -l /tmp/live/compiled start producer