pull, build and other commands should be idemnpotent

[Didier Roche-Tolomelli]

The fact that for instance "build()" would erase and redeploy a new build directory is an issue when inheriting and inserting hooks in plugin.

Here is an example of the trick needed due to this bug: https://github.com/ubuntu/snappy-playpen/blob/master/vlc/parts/plugins/x_vlcautotools.py

[Kyle Fazzari (kyrofa)]

Technically one could argue that it is idempotent. Running it twice does not result in different things: build(build(x)) == build(x).

However, I see the complication you're encountering when trying to extend a plugin; I've run into the same issues myself. The reason the build directory is removed at the beginning of the build step is in the case of a previously failed build. We're working toward being able to resume such builds, but it requires some other things to happen first.