Fix raw plan availability (#1521)

Making a surgical change to avoid nil pointer panics on accessing
RawPlan, coming from the AWS panic in
pulumi/pulumi-aws#2904

I have verified that this resolves 2904.

---------

Co-authored-by: Ian Wahbe <[email protected]>

pkg/tfshim/sdk-v2/provider_diff.go

@@ -165,7 +165,28 @@ func simpleDiffViaPlanState(
 	if state.RawConfig.IsNull() {
 		state.RawConfig = rawConfigVal
 	}
-	return res.SimpleDiff(ctx, state, planned, meta)
+	diff, err := res.SimpleDiff(ctx, state, planned, meta)
+	if err != nil {
+		return nil, err
+	}
+
+	// TF gRPC servers compensate for the fact that SimpleDiff may return
+	// terraform.NewInstanceDiff(), nil dropping any available information on RawPlan.
+	//
+	// See for example this code in PlanResourceChange:
+	//
+	// See https://github.com/hashicorp/terraform-plugin-sdk/blob/
+	//         28e631776d97f0a5a5942b3524814addbef90875/helper/schema/grpc_provider.go#L797
+	//
+	// In TF this is communicated from PlanResourceChange to ApplyResourceChange; unlike TF, in
+	// the current codebase InstanceDiff is passed directly to Apply. If RawPlan is not set on
+	// the diff it may cause nil panics in the provider.
+	if diff != nil && len(diff.Attributes) == 0 {
+		diff.RawPlan = priorStateVal
+		// TODO[pulumi/pulumi-terraform-bridge#1505] handle private state similar to upstream
+	}
+
+	return diff, nil
 }
 
 func showDiffChangeType(b byte) string {

pkg/tfshim/sdk-v2/provider_diff_test.go

@@ -0,0 +1,67 @@
+// Copyright 2016-2023, Pulumi Corporation.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package sdkv2
+
+import (
+	"testing"
+
+	"github.com/hashicorp/go-cty/cty"
+	"github.com/hashicorp/terraform-plugin-sdk/v2/helper/schema"
+	"github.com/hashicorp/terraform-plugin-sdk/v2/terraform"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestRawPlanSet(t *testing.T) {
+	r := &schema.Resource{
+		Schema: map[string]*schema.Schema{
+			"tags": {
+				Type:     schema.TypeMap,
+				Optional: true,
+				Elem:     &schema.Schema{Type: schema.TypeString},
+			},
+		},
+	}
+	p := &schema.Provider{
+		ResourcesMap: map[string]*schema.Resource{"myres": r},
+	}
+
+	wp := NewProvider(p, WithDiffStrategy(PlanState))
+
+	state := cty.ObjectVal(map[string]cty.Value{
+		"tags": cty.MapVal(map[string]cty.Value{"tag1": cty.StringVal("tag1v")}),
+	})
+
+	config := cty.ObjectVal(map[string]cty.Value{
+		"tags": cty.MapVal(map[string]cty.Value{"tag1": cty.StringVal("tag1v")}),
+	})
+
+	instanceState := terraform.NewInstanceStateShimmedFromValue(state, 0)
+	instanceState.ID = "oldid"
+	instanceState.Meta = map[string]interface{}{} // ignore schema versions for this test
+	resourceConfig := terraform.NewResourceConfigShimmed(config, r.CoreConfigSchema())
+
+	ss := v2InstanceState{
+		resource: r,
+		tf:       instanceState,
+	}
+
+	id, err := wp.Diff("myres", ss, v2ResourceConfig{
+		tf: resourceConfig,
+	})
+	require.NoError(t, err)
+
+	assert.False(t, id.(v2InstanceDiff).tf.RawPlan.IsNull(), "RawPlan should not be Null")
+}