Unwrapping control scripts: Apache under daemontools

In Debian, if you start apache The Right Way, you're actually going through two indirection layers: /etc/init.d/apache2 start sets up some environment variables (e.g. by reading /etc/default/apache2) and eventually runs apache2ctl start — which again sets up some stuff and eventually runs apache2. You can't really safely skip either of them.

This poses some problems in case you want to run apache2 non-daemonized (in the foreground), say under the watchful eye of a process supervisor like daemontools (or runit, or s6, or any of the other clones / enhancements). We all know that apache never crashes and never segfaults, so there's no need to auto-restart it, but still.

We want to run apache2 in the exact environment that /etc/init.d/apache2 start and apache2ctl start create. You could stare at the scripts and extract environment variables by hand, but this is time-consuming and error-prone. The elegant way to replicate the actions of the scripts is to replace the final call to apache2 with a stub that saves the complete environment, and then exec /usr/sbin/apache2 in that environment from the daemontools run script. To achieve this, one can rewrite apache2ctl (call it apache2ctl_fake) to invoke our stub, then rewrite /etc/init.d/apache2 to invoke apache2ctl_fake instead of the real apache2ctl. The stub itself can simply use env to dump the environment into a file. Putting all this together, we get

exec 2>&1
# rewrite /etc/init.d/ script
cp "/etc/init.d/${NAME}" "/tmp/initd_${NAME}_fake"
perl -pi.bak -e \
  's@APACHE2CTL( start)@ENV /tmp/apache2ctl_fake$1@' \
# rewrite apache2ctl
{ echo '#!/bin/sh'; echo "APACHE_HTTPD=/tmp/${NAME}_fake";
  cat `which apache2ctl`; } >"/tmp/${NAME}ctl_fake"
# create stub
cat <<EOF >"/tmp/${NAME}_fake"
/usr/bin/env >"/tmp/${NAME}_env.txt"
# execute fake init.d script
chmod a+x "/tmp/${NAME}_fake" "/tmp/${NAME}ctl_fake"
chmod a+x "/tmp/initd_${NAME}_fake"
"/tmp/initd_${NAME}_fake" start >/dev/null 2>&1
# prefix all encironment assignments with export
perl -ni.bak -e 's/^/export /; print unless /^export PWD=/' \
# load environment
. "/tmp/${NAME}_env.txt"
# call the real apache2
exec /usr/sbin/apache2 -k start -DNO_DETACH -DNO_DAEMONIZE

Note that the final crude “environment reload” trick only works for environment variables with no spaces in their values, because env does not quote assignments and/or escape quotes, i.e. it doesn't output VAR="value with \"nasty\" stuff". For more thorough handling one could generate output in the style of daemontools' envdir and use that tool to exec apache2.