Merge ~kwmonroe/layer-snap:bug/set-core-flag into ~stub/layer-snap:master

[Kevin W Monroe]

We know any snap install will ensure the 'core' snap gets installed. Set a 'snap.installed.core' flag in the install() method in case consumers want to use things like get/set for core system options.

[Stuart Bishop (stub)]

I'm currently thinking we should explicitly install the core snap before installing any other snaps. This gives us a way of reporting 'snaps don't work on this machine' differently to 'requested snaps won't install for some reason'.

[Stuart Bishop (stub)]

This is landable and useful as-is.

review: Approve

diff --git a/lib/charms/layer/snap.py b/lib/charms/layer/snap.py
index be32056..efe4893 100644
--- a/lib/charms/layer/snap.py
+++ b/lib/charms/layer/snap.py
@@ -50,6 +50,12 @@ def install(snapname, **kw):
             _install_store(snapname, **kw)
         reactive.set_state(installed_state)
 
+    # Installing any snap will first ensure that 'core' is installed. Set an
+    # appropriate flag for consumers that want to get/set core options.
+    core_installed = 'snap.installed.core'
+    if not reactive.is_state(core_installed):
+        reactive.set_state(core_installed)
+
 
 def refresh(snapname, **kw):
     '''Update a snap.