Bump installer protocol version to 5 and separate client and server protocol references

command/import_test.go

@@ -14,7 +14,6 @@ import (
 
 	"github.com/hashicorp/terraform/configs/configschema"
 	"github.com/hashicorp/terraform/helper/copy"
-	"github.com/hashicorp/terraform/plugin"
 	"github.com/hashicorp/terraform/plugin/discovery"
 	"github.com/hashicorp/terraform/providers"
 	"github.com/hashicorp/terraform/terraform"
@@ -804,7 +803,7 @@ func TestImport_pluginDir(t *testing.T) {
 			Ui:         cli.NewMockUi(),
 		},
 		providerInstaller: &discovery.ProviderInstaller{
-			PluginProtocolVersion: plugin.Handshake.ProtocolVersion,
+			PluginProtocolVersion: discovery.PluginInstallProtocolVersion,
 		},
 	}
 	if code := initCmd.Run(nil); code != 0 {

command/init.go

@@ -21,7 +21,6 @@ import (
 	"github.com/hashicorp/terraform/configs/configupgrade"
 	"github.com/hashicorp/terraform/internal/earlyconfig"
 	"github.com/hashicorp/terraform/internal/initwd"
-	"github.com/hashicorp/terraform/plugin"
 	"github.com/hashicorp/terraform/plugin/discovery"
 	"github.com/hashicorp/terraform/states"
 	"github.com/hashicorp/terraform/terraform"
@@ -85,7 +84,7 @@ func (c *InitCommand) Run(args []string) int {
 		c.providerInstaller = &discovery.ProviderInstaller{
 			Dir:                   c.pluginDir(),
 			Cache:                 c.pluginCache(),
-			PluginProtocolVersion: plugin.Handshake.ProtocolVersion,
+			PluginProtocolVersion: discovery.PluginInstallProtocolVersion,
 			SkipVerify:            !flagVerifyPlugins,
 			Ui:                    c.Ui,
 			Services:              c.Services,

plugin/discovery/requirements.go

@@ -4,6 +4,12 @@ import (
 	"bytes"
 )
 
+// PluginInstallProtocolVersion is the protocol version TF-core
+// supports to communicate with servers, and is used to resolve
+// plugin discovery with terraform registry, in addition to
+// any specified plugin version constraints
+const PluginInstallProtocolVersion = 5
+
 // PluginRequirements describes a set of plugins (assumed to be of a consistent
 // kind) that are required to exist and have versions within the given
 // corresponding sets.

plugin/serve.go

@@ -7,11 +7,18 @@ import (
 	"github.com/hashicorp/terraform/terraform"
 )
 
-// The constants below are the names of the plugins that can be dispensed
-// from the plugin server.
 const (
+	// The constants below are the names of the plugins that can be dispensed
+	// from the plugin server.
 	ProviderPluginName    = "provider"
 	ProvisionerPluginName = "provisioner"
+
+	// DefaultProtocolVersion is the protocol version assumed for legacy clients that don't specify
+	// a particular version during their handshake. This is the version used when Terraform 0.10
+	// and 0.11 launch plugins that were built with support for both versions 4 and 5, and must
+	// stay unchanged at 4 until we intentionally build plugins that are not compatible with 0.10 and
+	// 0.11.
+	DefaultProtocolVersion = 4
 )
 
 // Handshake is the HandshakeConfig used to configure clients and servers.
@@ -21,7 +28,7 @@ var Handshake = plugin.HandshakeConfig{
 	// one or the other that makes it so that they can't safely communicate.
 	// This could be adding a new interface value, it could be how
 	// helper/schema computes diffs, etc.
-	ProtocolVersion: 4,
+	ProtocolVersion: DefaultProtocolVersion,
 
 	// The magic cookie values should NEVER be changed.
 	MagicCookieKey:   "TF_PLUGIN_MAGIC_COOKIE",