tfprotov5+tfprotov6: Update to protocol versions 5.4/6.4 with GetMetadata RPC and GetProviderSchemaOptional server capability (#311)

Reference: #310

Protocol upgrades that impose new RPCs will either require:

- The `tfprotov5.ProviderServer`/`tfprotov6.ProviderServer` interface to require a new method
- Or, new "optional" interfaces be implemented (let's make up `ProviderServerWithGetMetadata` in this case)

terraform-plugin-go is a low level abstraction which is designed to _directly implement_ the protocol rather than introduce its own abstractions. Most provider developers will directly interface with higher level Go modules, such as terraform-plugin-sdk and terraform-plugin-framework. Except for advanced provider development using this Go module directly, this type of low level "breaking" change will be hidden by also upgrading those Go modules at the same time:

- hashicorp/terraform-plugin-sdk#1235
- hashicorp/terraform-plugin-framework#829
- hashicorp/terraform-plugin-mux#186

Therefore the change is implemented on the existing interface.

Author: bflad

.changes/unreleased/ENHANCEMENTS-20230707-100231.yaml

@@ -0,0 +1,7 @@
+kind: ENHANCEMENTS
+body: 'tfprotov5: Added `ServerCapabilities` type `GetProviderSchemaOptional` field,
+  which when enabled can signal that the provider supports RPC operations without
+  the `GetProviderSchema` RPC being called first'
+time: 2023-07-07T10:02:31.242394-04:00
+custom:
+  Issue: "310"

.changes/unreleased/ENHANCEMENTS-20230707-100339.yaml

@@ -0,0 +1,7 @@
+kind: ENHANCEMENTS
+body: 'tfprotov6: Added `ServerCapabilities` type `GetProviderSchemaOptional` field,
+  which when enabled can signal that the provider supports RPC operations without
+  the `GetProviderSchema` RPC being called first'
+time: 2023-07-07T10:03:39.802189-04:00
+custom:
+  Issue: "310"

.changes/unreleased/FEATURES-20230824-152200.yaml

@@ -0,0 +1,5 @@
+kind: FEATURES
+body: 'tfprotov5: Upgraded protocol to 5.4 and implemented `GetMetadata` RPC'
+time: 2023-08-24T15:22:00.773731-04:00
+custom:
+  Issue: "310"

.changes/unreleased/FEATURES-20230824-152220.yaml

@@ -0,0 +1,5 @@
+kind: FEATURES
+body: 'tfprotov6: Upgraded protocol to 6.4 and implemented `GetMetadata` RPC'
+time: 2023-08-24T15:22:20.285837-04:00
+custom:
+  Issue: "310"

.changes/unreleased/NOTES-20230824-153956.yaml

@@ -0,0 +1,7 @@
+kind: NOTES
+body: 'all: If using terraform-plugin-framework, terraform-plugin-mux, or terraform-plugin-sdk,
+  only upgrade this Go module when upgrading those Go modules or you may receive
+  a `missing GetMetadata method` error when compiling'
+time: 2023-08-24T15:39:56.697018-04:00
+custom:
+  Issue: "310"

internal/logging/keys.go

@@ -53,4 +53,10 @@ const (
 
 	// The protocol version being used, as a string, such as "6"
 	KeyProtocolVersion = "tf_proto_version"
+
+	// Whether the GetProviderSchemaOptional server capability is enabled
+	KeyServerCapabilityGetProviderSchemaOptional = "tf_server_capability_get_provider_schema_optional"
+
+	// Whether the PlanDestroy server capability is enabled
+	KeyServerCapabilityPlanDestroy = "tf_server_capability_plan_destroy"
 )

tfprotov5/data_source.go

@@ -7,6 +7,13 @@ import (
 	"context"
 )
 
+// DataSourceMetadata describes metadata for a data resource in the GetMetadata
+// RPC.
+type DataSourceMetadata struct {
+	// TypeName is the name of the data resource.
+	TypeName string
+}
+
 // DataSourceServer is an interface containing the methods a data source
 // implementation needs to fill.
 type DataSourceServer interface {

tfprotov5/internal/fromproto/data_source.go

@@ -8,6 +8,16 @@ import (
 	"github.com/hashicorp/terraform-plugin-go/tfprotov5/internal/tfplugin5"
 )
 
+func DataSourceMetadata(in *tfplugin5.GetMetadata_DataSourceMetadata) *tfprotov5.DataSourceMetadata {
+	if in == nil {
+		return nil
+	}
+
+	return &tfprotov5.DataSourceMetadata{
+		TypeName: in.TypeName,
+	}
+}
+
 func ValidateDataSourceConfigRequest(in *tfplugin5.ValidateDataSourceConfig_Request) (*tfprotov5.ValidateDataSourceConfigRequest, error) {
 	resp := &tfprotov5.ValidateDataSourceConfigRequest{
 		TypeName: in.TypeName,

tfprotov5/internal/fromproto/provider.go

@@ -8,6 +8,40 @@ import (
 	"github.com/hashicorp/terraform-plugin-go/tfprotov5/internal/tfplugin5"
 )
 
+func GetMetadataRequest(in *tfplugin5.GetMetadata_Request) (*tfprotov5.GetMetadataRequest, error) {
+	return &tfprotov5.GetMetadataRequest{}, nil
+}
+
+func GetMetadataResponse(in *tfplugin5.GetMetadata_Response) (*tfprotov5.GetMetadataResponse, error) {
+	if in == nil {
+		return nil, nil
+	}
+
+	resp := &tfprotov5.GetMetadataResponse{
+		DataSources:        make([]tfprotov5.DataSourceMetadata, 0, len(in.DataSources)),
+		Resources:          make([]tfprotov5.ResourceMetadata, 0, len(in.Resources)),
+		ServerCapabilities: ServerCapabilities(in.ServerCapabilities),
+	}
+
+	for _, datasource := range in.DataSources {
+		resp.DataSources = append(resp.DataSources, *DataSourceMetadata(datasource))
+	}
+
+	for _, resource := range in.Resources {
+		resp.Resources = append(resp.Resources, *ResourceMetadata(resource))
+	}
+
+	diags, err := Diagnostics(in.Diagnostics)
+
+	if err != nil {
+		return resp, err
+	}
+
+	resp.Diagnostics = diags
+
+	return resp, nil
+}
+
 func GetProviderSchemaRequest(in *tfplugin5.GetProviderSchema_Request) (*tfprotov5.GetProviderSchemaRequest, error) {
 	return &tfprotov5.GetProviderSchemaRequest{}, nil
 }

tfprotov5/internal/fromproto/resource.go

@@ -10,6 +10,16 @@ import (
 	"github.com/hashicorp/terraform-plugin-go/tfprotov5/internal/tfplugin5"
 )
 
+func ResourceMetadata(in *tfplugin5.GetMetadata_ResourceMetadata) *tfprotov5.ResourceMetadata {
+	if in == nil {
+		return nil
+	}
+
+	return &tfprotov5.ResourceMetadata{
+		TypeName: in.TypeName,
+	}
+}
+
 func ValidateResourceTypeConfigRequest(in *tfplugin5.ValidateResourceTypeConfig_Request) (*tfprotov5.ValidateResourceTypeConfigRequest, error) {
 	resp := &tfprotov5.ValidateResourceTypeConfigRequest{
 		TypeName: in.TypeName,