core: use ResourceProviderResolver to resolve providers

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.

command/meta.go

@@ -119,8 +119,8 @@ type PluginOverrides struct {
 }
 
 type testingOverrides struct {
-	Providers    map[string]terraform.ResourceProviderFactory
-	Provisioners map[string]terraform.ResourceProvisionerFactory
+	ProviderResolver terraform.ResourceProviderResolver
+	Provisioners     map[string]terraform.ResourceProvisionerFactory
 }
 
 // initStatePaths is used to initialize the default values for
@@ -237,10 +237,10 @@ func (m *Meta) contextOpts() *terraform.ContextOpts {
 	// and just work with what we've been given, thus allowing the tests
 	// to provide mock providers and provisioners.
 	if m.testingOverrides != nil {
-		opts.Providers = m.testingOverrides.Providers
+		opts.ProviderResolver = m.testingOverrides.ProviderResolver
 		opts.Provisioners = m.testingOverrides.Provisioners
 	} else {
-		opts.Providers = m.providerFactories()
+		opts.ProviderResolver = m.providerResolver()
 		opts.Provisioners = m.provisionerFactories()
 	}
 

command/plugins.go

@@ -1,6 +1,7 @@
 package command
 
 import (
+	"fmt"
 	"log"
 	"os/exec"
 	"strings"
@@ -11,7 +12,35 @@ import (
 	"github.com/hashicorp/terraform/terraform"
 )
 
-func (m *Meta) providerFactories() map[string]terraform.ResourceProviderFactory {
+// multiVersionProviderResolver is an implementation of
+// terraform.ResourceProviderResolver that matches the given version constraints
+// against a set of versioned provider plugins to find the newest version of
+// each that satisfies the given constraints.
+type multiVersionProviderResolver struct {
+	Available discovery.PluginMetaSet
+}
+
+func (r *multiVersionProviderResolver) ResolveProviders(
+	reqd discovery.PluginRequirements,
+) (map[string]terraform.ResourceProviderFactory, []error) {
+	factories := make(map[string]terraform.ResourceProviderFactory, len(reqd))
+	var errs []error
+
+	candidates := r.Available.ConstrainVersions(reqd)
+	for name := range reqd {
+		if metas := candidates[name]; metas != nil {
+			newest := metas.Newest()
+			client := tfplugin.Client(newest)
+			factories[name] = providerFactory(client)
+		} else {
+			errs = append(errs, fmt.Errorf("provider.%s: no suitable version installed", name))
+		}
+	}
+
+	return factories, errs
+}
+
+func (m *Meta) providerResolver() terraform.ResourceProviderResolver {
 	var dirs []string
 
 	// When searching the following directories, earlier entries get precedence
@@ -25,36 +54,9 @@ func (m *Meta) providerFactories() map[string]terraform.ResourceProviderFactory
 	plugins := discovery.FindPlugins("provider", dirs)
 	plugins, _ = plugins.ValidateVersions()
 
-	// For now our goal is to just find the latest version of each plugin
-	// we have on the system, emulating our pre-versioning behavior.
-	// TODO: Reorganize how providers are handled so that we can use
-	// version constraints from configuration to select which plugins
-	// we will use when multiple are available.
-
-	factories := make(map[string]terraform.ResourceProviderFactory)
-
-	// Wire up the internal provisioners first. These might be overridden
-	// by discovered providers below.
-	for name := range InternalProviders {
-		client, err := internalPluginClient("provider", name)
-		if err != nil {
-			log.Printf("[WARN] failed to build command line for internal plugin %q: %s", name, err)
-			continue
-		}
-		factories[name] = providerFactory(client)
-	}
-
-	byName := plugins.ByName()
-	for name, metas := range byName {
-		// Since we validated versions above and we partitioned the sets
-		// by name, we're guaranteed that the metas in our set all have
-		// valid versions and that there's at least one meta.
-		newest := metas.Newest()
-		client := tfplugin.Client(newest)
-		factories[name] = providerFactory(client)
+	return &multiVersionProviderResolver{
+		Available: plugins,
 	}
-
-	return factories
 }
 
 func (m *Meta) provisionerFactories() map[string]terraform.ResourceProvisionerFactory {

terraform/context.go

@@ -57,7 +57,7 @@ type ContextOpts struct {
 	Parallelism        int
 	State              *State
 	StateFutureAllowed bool
-	Providers          map[string]ResourceProviderFactory
+	ProviderResolver   ResourceProviderResolver
 	Provisioners       map[string]ResourceProvisionerFactory
 	Shadow             bool
 	Targets            []string
@@ -166,7 +166,6 @@ func NewContext(opts *ContextOpts) (*Context, error) {
 	//        set by environment variables if necessary. This includes
 	//        values taken from -var-file in addition.
 	variables := make(map[string]interface{})
-
 	if opts.Module != nil {
 		var err error
 		variables, err = Variables(opts.Module, opts.Variables)
@@ -175,6 +174,20 @@ func NewContext(opts *ContextOpts) (*Context, error) {
 		}
 	}
 
+	// Bind available provider plugins to the constraints in config
+	var providers map[string]ResourceProviderFactory
+	if opts.ProviderResolver != nil {
+		var err error
+		deps := moduleTreeDependencies(opts.Module, state)
+		reqd := deps.AllPluginRequirements()
+		providers, err = resourceProviderFactories(opts.ProviderResolver, reqd)
+		if err != nil {
+			return nil, err
+		}
+	} else {
+		providers = make(map[string]ResourceProviderFactory)
+	}
+
 	diff := opts.Diff
 	if diff == nil {
 		diff = &Diff{}
@@ -182,7 +195,7 @@ func NewContext(opts *ContextOpts) (*Context, error) {
 
 	return &Context{
 		components: &basicComponentFactory{
-			providers:    opts.Providers,
+			providers:    providers,
 			provisioners: opts.Provisioners,
 		},
 		destroy:   opts.Destroy,

terraform/resource_provider.go

@@ -1,6 +1,8 @@
 package terraform
 
 import (
+	"bytes"
+	"errors"
 	"fmt"
 
 	"github.com/hashicorp/terraform/plugin/discovery"
@@ -252,3 +254,25 @@ func ProviderHasDataSource(p ResourceProvider, n string) bool {
 
 	return false
 }
+
+// resourceProviderFactories matches available plugins to the given version
+// requirements to produce a map of compatible provider plugins if possible,
+// or an error if the currently-available plugins are insufficient.
+//
+// This should be called only with configurations that have passed calls
+// to config.Validate(), which ensures that all of the given version
+// constraints are valid. It will panic if any invalid constraints are present.
+func resourceProviderFactories(resolver ResourceProviderResolver, reqd discovery.PluginRequirements) (map[string]ResourceProviderFactory, error) {
+	ret, errs := resolver.ResolveProviders(reqd)
+	if errs != nil {
+		errBuf := &bytes.Buffer{}
+		errBuf.WriteString("Can't satisfy provider requirements with currently-installed plugins:\n\n")
+		for _, err := range errs {
+			fmt.Fprintf(errBuf, "* %s\n", err)
+		}
+		errBuf.WriteString("\nRun 'terraform init' to install the necessary provider plugins.\n")
+		return nil, errors.New(errBuf.String())
+	}
+
+	return ret, nil
+}