Previously we had a "getProvider" function type used to implement plugin
fetching. Here we replace that with an interface type, initially with
just a "Get" function.
For now this just simplifies the interface by allowing the target
directory and protocol version to be members of the struct rather than
passed as arguments.
A later change will extend this interface to also include a method to
purge unused plugins, so that upgrading frequently doesn't leave behind
a trail of unused executable files.
The semver library we were using doesn't have support for a "pessimistic
constraint" where e.g. the user wants to accept only minor or patch
version upgrades. This is important for providers since users will
generally want to pin their dependencies to not inadvertantly accept
breaking changes.
So here we switch to hashicorp's home-grown go-version library, which
has the ~> constraint operator for this sort of constraint.
Given how much the old version object was already intruding into the
interface and creating dependency noise in callers, this also now wraps
the "raw" go-version objects in package-local structs, thus keeping the
details encapsulated and allowing callers to deal just with this package's
own types.
The .terraformrc file allows the user to override the executable location
for certain plugins. This mechanism allows us to retain that behavior for
a deprecation period by treating such executables as an unversioned
plugin for the given name and excluding all other candidates for that
name, thus ensuring that the override will "win".
Users must eventually transition away from using this mechanism and use
vendor directories instead, because these unversioned overrides will never
match for a provider referenced with non-zero version constraints.
With forthcoming support for versioned plugins we need to be able to
answer questions like what versions of plugins are currently installed,
what's the newest version of a given plugin available, etc.
PluginMetaSet gives us a building block for this sort of plugin version
wrangling.
Martin Atkins