Default to prior state in responses.

Don't pass empty state values into response types. Instead, default to
the prior or current state, which is almost always what providers will
want.

(Partially?) resolves #51.

.changelog/74.txt

@@ -0,0 +1,3 @@
+```release-note:enhancement
+Responses now default to returning the current state, meaning state will only change when provider developers actively change it. Previously, an empty state value would be returned, which caused problems.
+```

tfsdk/serve.go

@@ -348,6 +348,7 @@ func (s *server) ReadResource(ctx context.Context, req *tfprotov6.ReadResourceRe
 	}
 	readResp := ReadResourceResponse{
 		State: State{
+			Raw:    state,
 			Schema: resourceSchema,
 		},
 		Diagnostics: resp.Diagnostics,
@@ -583,15 +584,12 @@ func (s *server) ApplyResourceChange(ctx context.Context, req *tfprotov6.ApplyRe
 		createResp := CreateResourceResponse{
 			State: State{
 				Schema: resourceSchema,
+				Raw:    priorState,
 			},
 			Diagnostics: resp.Diagnostics,
 		}
 		resource.Create(ctx, createReq, &createResp)
 		resp.Diagnostics = createResp.Diagnostics
-		// TODO: set partial state before returning error
-		if diagsHasErrors(resp.Diagnostics) {
-			return resp, nil
-		}
 		newState, err := tfprotov6.NewDynamicValue(resourceSchema.TerraformType(ctx), createResp.State.Raw)
 		if err != nil {
 			resp.Diagnostics = append(resp.Diagnostics, &tfprotov6.Diagnostic{
@@ -647,15 +645,12 @@ func (s *server) ApplyResourceChange(ctx context.Context, req *tfprotov6.ApplyRe
 		updateResp := UpdateResourceResponse{
 			State: State{
 				Schema: resourceSchema,
+				Raw:    priorState,
 			},
 			Diagnostics: resp.Diagnostics,
 		}
 		resource.Update(ctx, updateReq, &updateResp)
 		resp.Diagnostics = updateResp.Diagnostics
-		// TODO: set partial state before returning error
-		if diagsHasErrors(resp.Diagnostics) {
-			return resp, nil
-		}
 		newState, err := tfprotov6.NewDynamicValue(resourceSchema.TerraformType(ctx), updateResp.State.Raw)
 		if err != nil {
 			resp.Diagnostics = append(resp.Diagnostics, &tfprotov6.Diagnostic{
@@ -666,6 +661,7 @@ func (s *server) ApplyResourceChange(ctx context.Context, req *tfprotov6.ApplyRe
 			return resp, nil
 		}
 		resp.NewState = &newState
+		return resp, nil
 	case !create && !update && destroy:
 		destroyReq := DeleteResourceRequest{
 			State: State{
@@ -702,15 +698,12 @@ func (s *server) ApplyResourceChange(ctx context.Context, req *tfprotov6.ApplyRe
 		destroyResp := DeleteResourceResponse{
 			State: State{
 				Schema: resourceSchema,
+				Raw:    priorState,
 			},
 			Diagnostics: resp.Diagnostics,
 		}
 		resource.Delete(ctx, destroyReq, &destroyResp)
 		resp.Diagnostics = destroyResp.Diagnostics
-		// TODO: set partial state before returning error
-		if diagsHasErrors(resp.Diagnostics) {
-			return resp, nil
-		}
 		newState, err := tfprotov6.NewDynamicValue(resourceSchema.TerraformType(ctx), destroyResp.State.Raw)
 		if err != nil {
 			resp.Diagnostics = append(resp.Diagnostics, &tfprotov6.Diagnostic{
@@ -721,6 +714,7 @@ func (s *server) ApplyResourceChange(ctx context.Context, req *tfprotov6.ApplyRe
 			return resp, nil
 		}
 		resp.NewState = &newState
+		return resp, nil
 	default:
 		resp.Diagnostics = append(resp.Diagnostics, &tfprotov6.Diagnostic{
 			Severity: tfprotov6.DiagnosticSeverityError,
@@ -811,6 +805,10 @@ func (s *server) ReadDataSource(ctx context.Context, req *tfprotov6.ReadDataSour
 	readResp := ReadDataSourceResponse{
 		State: State{
 			Schema: dataSourceSchema,
+			// default to the config values
+			// they should be of the same type
+			// we just want SetAttribute to not find an empty value
+			Raw: config,
 		},
 		Diagnostics: resp.Diagnostics,
 	}

tfsdk/serve_test.go

@@ -1289,6 +1289,8 @@ func TestServerApplyResourceChange(t *testing.T) {
 				"name": tftypes.NewValue(tftypes.String, "hello, world"),
 				"favorite_colors": tftypes.NewValue(tftypes.List{ElementType: tftypes.String}, []tftypes.Value{
 					tftypes.NewValue(tftypes.String, "red"),
+					tftypes.NewValue(tftypes.String, "orange"),
+					tftypes.NewValue(tftypes.String, "yellow"),
 				}),
 				"created_timestamp": tftypes.NewValue(tftypes.String, "right now I guess"),
 			}),