pt-stalk feature: execute external script on event

This looks useful. But I think we might want to do these slightly differently. Rather than providing a command to the tool's option, we could require the user to specify a file to be sourced, which must provide a specific function, similar to the way we source and call a trigger function. The function that's called should be executed in the background with & so that it does not hang the script entirely if the call is long-lived.

Perhaps an option called --plugin which takes a script that can define various hooks like:

after_collect() {
   <my code>
}

after_sleep() {
   <my code>
}

This would allow us to implement as many hooks in as many places as we want with the same option.

The tool would then be like:

after_collect() {
   # Default stub, does nothing
   :
}
...etc...

. "$OPT_PLUGIN"

after_collect ... # call plugin's if defined, else default stub

I think that sounds like a good idea. It will let us add generic plugin hooks for anything we want without increasing the complexity of the tool significantly. We'll just need to document the available hooks.

I rescind my comment about calling things with & to put them in the background. The plugin author can control that; if they do something wrong, then they can cause the tool to hang while the plugin works, but that's OK. If we background the plugin's functions, then we can get out-of-order execution and hard problems like "what if I want this to run only after that other thing is finished," which will be much harder for plugin authors to solve than the potential mistake of not backgrounding a blocking action.

I'd find this feature very useful, because I want to set up Rain Gauge from https://github.com/box/RainGauge but I don't want to patch pt-stalk.

Tentatively targeted to 2.1.5.

Since this isn't a bug, I've created https://blueprints.launchpad.net/percona-toolkit/+spec/pt-stalk-plugin instead.

Percona now uses JIRA for bug reports so this bug report is migrated to: https://jira.percona.com/browse/PT-313