Implement a way to chain plugins

[jck]

There's currently no easy way to write a plugin which leverages an existing plugin. I can think of a few cases where it would be very helpful. For example:

• A gh_repo plugin which accepts a simple field like buildinspace/peru, builds the github url and the passes on the information to the git plugin via env vars
• A gh_release plugin which queries the github API to find the latest release and the uses the curl plugin to download it.

I can think of two ways to support this:

• plugin.yaml has a pre exe (which manipulates the environment) and a chain plug or pass plug or something which specifies the plugin which runs after the pre exe
• We could provide another entry point called something like peru-plugin. Plugins can call this command to leverage other plugins. However, this is probably not a good idea because plugins could cause infinite loops.

Let me know if you think this idea would be worth pursuing, I'll try implementing a simple poc.

[oconnor663]

Sorry for the silence on this one, I missed it in my inbox when you first posted it.

Here are a couple things I'd want to think about before expanding peru's plugin APIs:

• Copying code. Is it easy to just copy the code of the existing git plugin, and make changes in there to implement things like gh_repo? Currently our plugins don't actually share any code with each other, so each one should be pretty self-contained.
• Shelling out to plugin files directly. If we exposed some functions from peru/plugin.py to make it easier to grab the path to a plugin executable, how would you feel about executing those directly from your own plugin code?