Add Version information to plugin APIs

[wfarner]

Publishing just to share the approach taken so far. Feedback welcome.

Closes #309

[GordonTheTurtle]

Please sign your commits following these rules:
https://github.com/docker/docker/blob/master/CONTRIBUTING.md#sign-your-work
The easiest way to do this is to amend the last commit:

$ git clone -b "api-version" git@github.com:wfarner/infrakit.git somewhere
$ cd somewhere
$ git rebase -i HEAD~842354112880
editor opens
change each 'pick' to 'edit'
save the file and quit
$ git commit --amend -s --no-edit
$ git rebase --continue # and repeat the amend for each commit
$ git push -f

Amending updates the existing PR. You DO NOT need to open a new one.

[codecov-io]

Current coverage is 67.89% (diff: 91.11%)

Merging #318 into master will increase coverage by 15.60%

@@             master       #318   diff @@
==========================================
  Files            12         30    +18   
  Lines           610       1545   +935   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits            319       1049   +730   
- Misses          246        396   +150   
- Partials         45        100    +55   

Powered by Codecov. Last update a1e94af...a9f948f

[chungers]

The comment - do you mean handshakeResult?

[wfarner]

Yep, thanks. Fixing.

[chungers]

As a nit -- would you mind rename API to something like Interface? API looks jarring when there are names like APIsRequest and APIsResponse or APIs().

Also for the method names APIs(), Implements() looks a little nicer to me. It also matches how Docker engine does activation with the field Implements: https://docs.docker.com/engine/extend/plugin_api/#/pluginactivate

So how about

type Interface struct {
   Name string
   Version string
}

type Plugin interface {
    Implements() ([]Interface, error)
}

Then the RPC response can be named HandshakeResponse and it will look like

{
     "Implements" : [ ....]
     "Interfaces" : [  /* list of interface descriptions like example input / output via reflection */ ]
}

[wfarner]

No strong opinions here, the names you gave sound fine to me.

[chungers]

Thank you for the flexibility. I agonize over naming and shape of characters when they are laid out next to each other.

[chungers]

LGTM