command: Expose providercache package objects for use elsewhere

These new functions allow command implementations to get hold of the
providercache objects and installation source object derived from the
current CLI configuration.

command/meta_providers.go

@@ -0,0 +1,63 @@
+package command
+
+import (
+	"path/filepath"
+
+	"github.com/hashicorp/terraform/internal/getproviders"
+	"github.com/hashicorp/terraform/internal/providercache"
+)
+
+// providerLocalCacheDir returns an object representing the
+// configuration-specific local cache directory. This is the
+// only location consulted for provider plugin packages for Terraform
+// operations other than provider installation.
+//
+// Only the provider installer (in "terraform init") is permitted to make
+// modifications to this cache directory. All other commands must treat it
+// as read-only.
+func (m *Meta) providerLocalCacheDir() *providercache.Dir {
+	dir := filepath.Join(m.DataDir(), "plugins")
+	if dir == "" {
+		return nil // cache disabled
+	}
+	return providercache.NewDir(dir)
+}
+
+// providerGlobalCacheDir returns an object representing the shared global
+// provider cache directory, used as a read-through cache when installing
+// new provider plugin packages.
+//
+// This function may return nil, in which case there is no global cache
+// configured and new packages should be downloaded directly into individual
+// configuration-specific cache directories.
+func (m *Meta) providerGlobalCacheDir() *providercache.Dir {
+	dir := m.PluginCacheDir
+	if dir == "" {
+		return nil // cache disabled
+	}
+	return providercache.NewDir(dir)
+}
+
+// providerInstallSource returns an object that knows how to consult one or
+// more external sources to determine the availability of and package
+// locations for versions of Terraform providers that are available for
+// automatic installation.
+//
+// This returns the standard provider install source that consults a number
+// of directories selected either automatically or via the CLI configuration.
+// Users may choose to override this during a "terraform init" command by
+// specifying one or more -plugin-dir options, in which case the installation
+// process will construct its own source consulting only those directories
+// and use that instead.
+func (m *Meta) providerInstallSource() getproviders.Source {
+	// A provider source should always be provided in normal use, but our
+	// unit tests might not always populate Meta fully and so we'll be robust
+	// by returning a non-nil source that just always answers that no plugins
+	// are available.
+	if m.ProviderSource == nil {
+		// A multi-source with no underlying sources is effectively an
+		// always-empty source.
+		return getproviders.MultiSource(nil)
+	}
+	return m.ProviderSource
+}

internal/getproviders/types.go

@@ -35,6 +35,12 @@ func ParseVersion(str string) (Version, error) {
 	return versions.ParseVersion(str)
 }
 
+// ParseVersionConstraints parses a "Ruby-like" version constraint string
+// into a VersionConstraints value.
+func ParseVersionConstraints(str string) (VersionConstraints, error) {
+	return constraints.ParseRubyStyleMulti(str)
+}
+
 // Platform represents a target platform that a provider is or might be
 // available for.
 type Platform struct {

internal/providercache/installer.go

@@ -345,7 +345,7 @@ func (err InstallerError) Error() string {
 	b.WriteString("some providers could not be installed:\n")
 	for _, addr := range addrs {
 		providerErr := err.ProviderErrors[addr]
-		fmt.Fprintf(&b, "- %s: %s\n", addr, providerErr.Error())
+		fmt.Fprintf(&b, "- %s: %s\n", addr, providerErr)
 	}
 	return b.String()
 }