Allow Varnish to reuse its shared object

[Cryer,Phil]

Currently Varnish requires a C compiler to be present on the machine
it's running on, since it needs to compile the VCL config file into a
shared object each time it starts. During shutdown, Varnish removes this
shared object since it will be rebuilt during the next start. This
routine repeats regardless of  if anything has changed in the VCL config
file, and serves as a road bump to getting Varnish into certain
production environments since traditionally development applications
(such as the C compiler) are not allowed in such instances. For now I am
putting aside the arguments as to why it's is acceptable to have
development applications in production instances, since that argument's
outcome will vary in different situations, and I am aiming for a
solution that will cover all instances.

My proposal is to allow Varnish to reuse its shared object, instead
creating it during on launch, and destroying it on shutdown. The reuse
function would be called if a flag was present during startup, either on
the command line as -r, or as defined in the DAEMON_OPTS variable. This
would cause Varnish to do two things differently than it does now:

1) upon startup it would not create a new shared object, rather it would
search for, and use, the first shared object it found in the following
location:

/var/lib/varnish/`hostname`/bin.*

2) when called to shutdown, Varnish would not remove the existing shared
object, thus leaving it to be reused, if called again with the -r flag

The shared object would need to be created (and recreated after any
changes to the VCL file) on a clone machine that has a C compiler, and
then installed to the production machine during a change window, which
would be no different from a normal config file change. The upside of
this would be that the C compiler requirement would be conditional upon
the use of the reuse flag, which would allow smoother integration of
Varnish into production environments.

Where in the code should look for the startup and shutdown hooks so I
can start investigating this further?
 
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