fs/skeleton/bin/register_experiment

#!/bin/sh
#
# Tell the experiment subsystem that we support the named system-level
# (as opposed to catawampus-internal) experiment.
#
# Note: This is a little more subtle than it sounds.  The trick is that
# system-level experiments should be persisted across reboots (so they can
# take effect early on, as soon as /config is mounted), but even if they
# are requested, they should only take effect the *next* time the system
# executes the code that is affected by the experiment.  So "registering"
# an experiment actually means two things:
#  - create a file that tells cwmpd an experiment is available
#  - if cwmpd has requested this experiment already, inform it that this
#    experiment is now active.
# It's important that cwmpd not be informed an experiment is active until
# it has *actually* taken effect (sometimes not until the next reboot).
# Otherwise the results of A/B tests will end up partially lying, where
# cwmpd's periodic stats say an experiment is active even though we are using
# the non-experimental behaviour.
#
expname="$1"
if [ ! -w /tmp ]; then
  echo "WARNING: /tmp not writable: experiment '$expname' cannot activate."
  exit 0
fi
if [ ! -w /config ]; then
  echo "WARNING: /config not writable: experiment '$expname' cannot activate."
  exit 0
fi

mkdir -p /tmp/experiments /config/experiments &&
  [ -w /config ] && chmod 777 /tmp/experiments /config/experiments

: >"/tmp/experiments/$expname.available"
if [ -e "/config/experiments/$expname.requested" ]; then
  echo "Activating experiment '$expname'." >&2
  mv "/config/experiments/$expname.requested" \
     "/config/experiments/$expname.active"
elif [ -e "/config/experiments/$expname.unrequested" ]; then
  echo "Deactivating experiment '$expname'." >&2
  rm -f "/config/experiments/$expname.active" \
        "/config/experiments/$expname.unrequested"
fi