process state even after provider.Apply errors

Terraform core expects a sane state even when the provider returns an
error. Make sure at the prior state is always the default value to
return, and then alway attempt to process any state returned by
provider.Apply.

builtin/providers/test/resource.go

@@ -1,6 +1,7 @@
 package test
 
 import (
+	"errors"
 	"fmt"
 
 	"github.com/hashicorp/terraform/helper/schema"
@@ -123,6 +124,11 @@ func testResource() *schema.Resource {
 					},
 				},
 			},
+			"apply_error": {
+				Type:        schema.TypeString,
+				Optional:    true,
+				Description: "return and error during apply",
+			},
 		},
 	}
 }
@@ -130,6 +136,11 @@ func testResource() *schema.Resource {
 func testResourceCreate(d *schema.ResourceData, meta interface{}) error {
 	d.SetId("testId")
 
+	errMsg, _ := d.Get("apply_error").(string)
+	if errMsg != "" {
+		return errors.New(errMsg)
+	}
+
 	// Required must make it through to Create
 	if _, ok := d.GetOk("required"); !ok {
 		return fmt.Errorf("Missing attribute 'required', but it's required!")
@@ -156,6 +167,10 @@ func testResourceRead(d *schema.ResourceData, meta interface{}) error {
 }
 
 func testResourceUpdate(d *schema.ResourceData, meta interface{}) error {
+	errMsg, _ := d.Get("apply_error").(string)
+	if errMsg != "" {
+		return errors.New(errMsg)
+	}
 	return nil
 }
 

builtin/providers/test/resource_test.go

@@ -2,6 +2,7 @@ package test
 
 import (
 	"reflect"
+	"regexp"
 	"strings"
 	"testing"
 
@@ -624,3 +625,55 @@ resource "test_resource" "foo" {
 		},
 	})
 }
+
+func TestResource_updateError(t *testing.T) {
+	resource.UnitTest(t, resource.TestCase{
+		Providers:    testAccProviders,
+		CheckDestroy: testAccCheckResourceDestroy,
+		Steps: []resource.TestStep{
+			resource.TestStep{
+				Config: strings.TrimSpace(`
+resource "test_resource" "foo" {
+  required     = "first"
+  required_map = {
+    a = "a"
+  }
+}
+`),
+			},
+			resource.TestStep{
+				Config: strings.TrimSpace(`
+resource "test_resource" "foo" {
+  required     = "second"
+  required_map = {
+    a = "a"
+  }
+  apply_error = "update_error"
+}
+`),
+				ExpectError: regexp.MustCompile("update_error"),
+			},
+		},
+	})
+}
+
+func TestResource_applyError(t *testing.T) {
+	resource.UnitTest(t, resource.TestCase{
+		Providers:    testAccProviders,
+		CheckDestroy: testAccCheckResourceDestroy,
+		Steps: []resource.TestStep{
+			resource.TestStep{
+				Config: strings.TrimSpace(`
+resource "test_resource" "foo" {
+  required     = "second"
+  required_map = {
+    a = "a"
+  }
+  apply_error = "apply_error"
+}
+`),
+				ExpectError: regexp.MustCompile("apply_error"),
+			},
+		},
+	})
+}

helper/plugin/grpc_provider.go

@@ -656,7 +656,10 @@ func (s *GRPCProviderServer) PlanResourceChange(_ context.Context, req *proto.Pl
 }
 
 func (s *GRPCProviderServer) ApplyResourceChange(_ context.Context, req *proto.ApplyResourceChange_Request) (*proto.ApplyResourceChange_Response, error) {
-	resp := &proto.ApplyResourceChange_Response{}
+	resp := &proto.ApplyResourceChange_Response{
+		// Start with the existing state as a fallback
+		NewState: req.PriorState,
+	}
 
 	res := s.provider.ResourcesMap[req.TypeName]
 	block := res.CoreConfigSchema()
@@ -753,15 +756,17 @@ func (s *GRPCProviderServer) ApplyResourceChange(_ context.Context, req *proto.A
 	}
 
 	newInstanceState, err := s.provider.Apply(info, priorState, diff)
+	// we record the error here, but continue processing any returned state.
 	if err != nil {
 		resp.Diagnostics = convert.AppendProtoDiag(resp.Diagnostics, err)
-		return resp, nil
 	}
 
 	newStateVal := cty.NullVal(block.ImpliedType())
 
-	// always return a nul value for destroy
-	if newInstanceState == nil || destroy {
+	// Always return a null value for destroy.
+	// While this is usually indicated by a nil state, check for missing ID or
+	// attributes in the case of a provider failure.
+	if destroy || newInstanceState == nil || newInstanceState.Attributes == nil || newInstanceState.ID == "" {
 		newStateMP, err := msgpack.Marshal(newStateVal, block.ImpliedType())
 		if err != nil {
 			resp.Diagnostics = convert.AppendProtoDiag(resp.Diagnostics, err)