command: Accept a "provider source" from the main package

Following the same approach we use for other CLI-Config-able objects like
the service discovery system, the main package is responsible for
producing a suitable implementation of this interface which the command
package can then use.

When unit testing in the command package we can then substitute mocks as
necessary, following the dependency inversion principle.

command/meta.go

@@ -23,6 +23,7 @@ import (
 	"github.com/hashicorp/terraform/configs/configload"
 	"github.com/hashicorp/terraform/helper/experiment"
 	"github.com/hashicorp/terraform/helper/wrappedstreams"
+	"github.com/hashicorp/terraform/internal/getproviders"
 	"github.com/hashicorp/terraform/providers"
 	"github.com/hashicorp/terraform/provisioners"
 	"github.com/hashicorp/terraform/terraform"
@@ -74,6 +75,11 @@ type Meta struct {
 	// into the given directory.
 	PluginCacheDir string
 
+	// ProviderSource allows determining the available versions of a provider
+	// and determines where a distribution package for a particular
+	// provider version can be obtained.
+	ProviderSource getproviders.Source
+
 	// OverrideDataDir, if non-empty, overrides the return value of the
 	// DataDir method for situations where the local .terraform/ directory
 	// is not suitable, e.g. because of a read-only filesystem.

commands.go

@@ -12,6 +12,7 @@ import (
 	"github.com/hashicorp/terraform/command"
 	"github.com/hashicorp/terraform/command/cliconfig"
 	"github.com/hashicorp/terraform/command/webbrowser"
+	"github.com/hashicorp/terraform/internal/getproviders"
 	pluginDiscovery "github.com/hashicorp/terraform/plugin/discovery"
 )
 
@@ -37,7 +38,7 @@ const (
 	OutputPrefix = "o:"
 )
 
-func initCommands(config *cliconfig.Config, services *disco.Disco) {
+func initCommands(config *cliconfig.Config, services *disco.Disco, providerSrc getproviders.Source) {
 	var inAutomation bool
 	if v := os.Getenv(runningInAutomationEnvName); v != "" {
 		inAutomation = true
@@ -67,6 +68,7 @@ func initCommands(config *cliconfig.Config, services *disco.Disco) {
 		Ui:               Ui,
 
 		Services:        services,
+		ProviderSource:  providerSrc,
 		BrowserLauncher: webbrowser.NewNativeLauncher(),
 
 		RunningInAutomation: inAutomation,

main.go

@@ -17,6 +17,7 @@ import (
 	"github.com/hashicorp/terraform/command/format"
 	"github.com/hashicorp/terraform/helper/logging"
 	"github.com/hashicorp/terraform/httpclient"
+	"github.com/hashicorp/terraform/internal/getproviders"
 	"github.com/hashicorp/terraform/version"
 	"github.com/mattn/go-colorable"
 	"github.com/mattn/go-shellwords"
@@ -164,12 +165,18 @@ func wrappedMain() int {
 	services := disco.NewWithCredentialsSource(credsSrc)
 	services.SetUserAgent(httpclient.TerraformUserAgent(version.String()))
 
+	// For the moment, we just always use the registry source to install
+	// direct from a registry. In future there should be a mechanism to
+	// configure providers sources from the CLI config, which will then
+	// change how we construct this object.
+	providerSrc := getproviders.NewRegistrySource(services)
+
 	// Initialize the backends.
 	backendInit.Init(services)
 
 	// In tests, Commands may already be set to provide mock commands
 	if Commands == nil {
-		initCommands(config, services)
+		initCommands(config, services, providerSrc)
 	}
 
 	// Run checkpoint