Revision 1031875

Enable CouchDB to manage OS process daemons.

This is a simple feature that allows useres to configure CouchDB
so that it maintains a given OS level process alive. If the process
dies for any reason, CouchDB will restart it. If the process restarts
too often, then CouchDB will mark it has halted and not attempt
to restart it. The default max restart rate is three times in the
last five seconds. These parameters are adjustable.

Commands that are started in this manner will have access to a simple
API over stdio to request configuration parameters or to add log
statements to CouchDB's logs.

To configure an OS process as a CouchDB os_daemon, create a section
in your local.ini like such:

    [os_daemons]
    daemon_name = /path/to/command -with args

This will make CouchDB bring up the command and attempt to keep it
alive. To request a configuration parameter, an os_daemon can write
a simple JSON message to stdout like such:

    ["get", "os_daemons"]\n

which would return:

    {"daemon_name": "/path/to/command -with args"}

Or:

    ["get", "os_daemons", "daemon_name"]\n

which would return:

    "/path/to/command -with args"

There's no restriction on what configuration variables are visible.
There's also no method for altering the configuration.

If you would like your OS daemon to be restarted in the event that
the configuration changes, you can send the following messages:

    ["register", $(SECTION)]\n

When anyting in that section changes, your OS process will be
rebooted so it can pick up the new configuration settings. If you
want to listen for changes on a specific key, you can send something
like:

    ["register", $(SECTION), $(KEY)]\n

In this case, CouchDB will only restart your daemon if that exact
section/key pair changes, instead of anything in that entire section.

Logging commands look like:

    ["log", $(JSON_MESSAGE)]\n

Where $(JSON_MESSAGE) is arbitrary JSON data. These messages are
logged at the 'info' level. If you want to log at a different level
you can pass messages like such:

    ["log", $(JSON_MESSAGE), {"level": $(LEVEL)}]\n

Where $(LEVEL) is one of "debug", "info", or "error".

When implementing a daemon process to be managed by CouchDB you
should remember to use a method like checking the parent process
id or if stdin has been closed. These flags can tell you if
your daemon process has been orphaned so you can exit cleanly.