all: Initial provider defined functions implementation (#1288)

Reference: hashicorp/terraform-plugin-go#351
Reference: https://developer.hashicorp.com/terraform/plugin/framework/migrating

The next versions of the plugin protocol (5.5/6.5) include support for provider defined functions. The terraform-plugin-sdk Go module will not be receiving this feature, however this Go module must be updated to handle the new RPCs with errors.

Provider developers can still implement provider defined functions in their terraform-plugin-sdk based providers by following the framework migration documentation to introduce terraform-plugin-mux, which then will enable terraform-plugin-framework based provider function implementations.

.changes/unreleased/NOTES-20231107-141609.yaml

@@ -0,0 +1,8 @@
+kind: NOTES
+body: 'helper/schema: While this Go module will not receive support for
+  provider-defined functions, the provider server is updated to handle the new
+  operations, which will be required to prevent errors when updating
+  terraform-plugin-framework or terraform-plugin-mux in the future.'
+time: 2023-11-07T14:16:09.783296-05:00
+custom:
+  Issue: "1288"

helper/schema/grpc_provider.go

@@ -80,6 +80,7 @@ func (s *GRPCProviderServer) GetMetadata(ctx context.Context, req *tfprotov5.Get
 
 	resp := &tfprotov5.GetMetadataResponse{
 		DataSources:        make([]tfprotov5.DataSourceMetadata, 0, len(s.provider.DataSourcesMap)),
+		Functions:          make([]tfprotov5.FunctionMetadata, 0),
 		Resources:          make([]tfprotov5.ResourceMetadata, 0, len(s.provider.ResourcesMap)),
 		ServerCapabilities: s.serverCapabilities(),
 	}
@@ -106,6 +107,7 @@ func (s *GRPCProviderServer) GetProviderSchema(ctx context.Context, req *tfproto
 
 	resp := &tfprotov5.GetProviderSchemaResponse{
 		DataSourceSchemas:  make(map[string]*tfprotov5.Schema, len(s.provider.DataSourcesMap)),
+		Functions:          make(map[string]*tfprotov5.Function, 0),
 		ResourceSchemas:    make(map[string]*tfprotov5.Schema, len(s.provider.ResourcesMap)),
 		ServerCapabilities: s.serverCapabilities(),
 	}
@@ -1271,6 +1273,36 @@ func (s *GRPCProviderServer) ReadDataSource(ctx context.Context, req *tfprotov5.
 	return resp, nil
 }
 
+func (s *GRPCProviderServer) CallFunction(ctx context.Context, req *tfprotov5.CallFunctionRequest) (*tfprotov5.CallFunctionResponse, error) {
+	ctx = logging.InitContext(ctx)
+
+	logging.HelperSchemaTrace(ctx, "Returning error for provider function call")
+
+	resp := &tfprotov5.CallFunctionResponse{
+		Diagnostics: []*tfprotov5.Diagnostic{
+			{
+				Severity: tfprotov5.DiagnosticSeverityError,
+				Summary:  "Function Not Found",
+				Detail:   fmt.Sprintf("No function named %q was found in the provider.", req.Name),
+			},
+		},
+	}
+
+	return resp, nil
+}
+
+func (s *GRPCProviderServer) GetFunctions(ctx context.Context, req *tfprotov5.GetFunctionsRequest) (*tfprotov5.GetFunctionsResponse, error) {
+	ctx = logging.InitContext(ctx)
+
+	logging.HelperSchemaTrace(ctx, "Getting provider functions")
+
+	resp := &tfprotov5.GetFunctionsResponse{
+		Functions: make(map[string]*tfprotov5.Function, 0),
+	}
+
+	return resp, nil
+}
+
 func pathToAttributePath(path cty.Path) *tftypes.AttributePath {
 	var steps []tftypes.AttributePathStep
 

helper/schema/grpc_provider_test.go

@@ -2111,6 +2111,7 @@ func TestGRPCProviderServerGetMetadata(t *testing.T) {
 						TypeName: "test_datasource2",
 					},
 				},
+				Functions: []tfprotov5.FunctionMetadata{},
 				Resources: []tfprotov5.ResourceMetadata{},
 				ServerCapabilities: &tfprotov5.ServerCapabilities{
 					GetProviderSchemaOptional: true,
@@ -2137,6 +2138,7 @@ func TestGRPCProviderServerGetMetadata(t *testing.T) {
 						TypeName: "test_datasource2",
 					},
 				},
+				Functions: []tfprotov5.FunctionMetadata{},
 				Resources: []tfprotov5.ResourceMetadata{
 					{
 						TypeName: "test_resource1",
@@ -2159,6 +2161,7 @@ func TestGRPCProviderServerGetMetadata(t *testing.T) {
 			},
 			Expected: &tfprotov5.GetMetadataResponse{
 				DataSources: []tfprotov5.DataSourceMetadata{},
+				Functions:   []tfprotov5.FunctionMetadata{},
 				Resources: []tfprotov5.ResourceMetadata{
 					{
 						TypeName: "test_resource1",