Previously the set of providers was fixed early on in the command package
processing. In order to be version-aware we need to defer this work until
later, so this interface exists so we can hold on to the possibly-many
versions of plugins we have available and then later, once we've finished
determining the provider dependencies, select the appropriate version of
each provider to produce the final set of providers to use.
This commit establishes the use of this new mechanism, and thus populates
the provider factory map with only the providers that result from the
dependency resolution process.
This disables support for internal provider plugins, though the
mechanisms for building and launching these are still here vestigially,
to be cleaned up in a subsequent commit.
This also adds a new awkward quirk to the "terraform import" workflow
where one can't import a resource from a provider that isn't already
mentioned (implicitly or explicitly) in config. We will do some UX work
in subsequent commits to make this behavior better.
This breaks many tests due to the change in interface, but to keep this
particular diff reasonably easy to read the test fixes are split into
a separate commit.
Having this as a method of PluginMeta felt most natural, but unfortunately
that means that discovery must depend on plugin and plugin in turn
depends on core Terraform, thus making the discovery package hard to use
without creating dependency cycles.
To resolve this, we invert the dependency and make the plugin package be
responsible for instantiating clients given a meta, using a top-level
function.
Previously we did plugin discovery in the main package, but as we move
towards versioned plugins we need more information available in order to
resolve plugins, so we move this responsibility into the command package
itself.
For the moment this is just preserving the existing behavior as long as
there are only internal and unversioned plugins present. This is the
final state for provisioners in 0.10, since we don't want to support
versioned provisioners yet. For providers this is just a checkpoint along
the way, since further work is required to apply version constraints from
configuration and support additional plugin search directories.
The automatic plugin discovery behavior is not desirable for tests because
we want to mock the plugins there, so we add a new backdoor for the tests
to use to skip the plugin discovery and just provide their own mock
implementations. Most of this diff is thus noisy rework of the tests to
use this new mechanism.
Martin Atkins