Normalize Read result for empty collections

This workaround is aimed to reduce dirty refresh incidence on diffs between empty and null collections that is benign in
TF but projects worse to Pulumi.

pkg/tfbridge/normalize.go

@@ -0,0 +1,74 @@
+// Copyright 2016-2024, 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 tfbridge
+
+import (
+	"context"
+
+	"github.com/pulumi/pulumi-terraform-bridge/v3/pkg/tfbridge/info"
+	shim "github.com/pulumi/pulumi-terraform-bridge/v3/pkg/tfshim"
+	"github.com/pulumi/pulumi/sdk/v3/go/common/resource"
+)
+
+// Compensate for a TF problem causing spurious diffs between null and empty collections highlighted as dirty refresh in
+// Pulumi. When applying resource changes TF will call normalizeNullValues with apply=true which may erase the
+// distinction between empty and null collections. This makes state returned from Read differ from the state returned by
+// Create and Update and written to the state store. While the problem is present in both TF and Pulumi bridged
+// providers, it is worse in Pulumi because refresh is emitting a warning that something is changing.
+//
+// To compensate for this problem, this method corrects the Pulumi Read result during `pulumi refresh` to also erase
+// empty collections from the Read result if the old input is missing or null.
+//
+// This currently only handles top-level properties.
+//
+// See: https://github.com/pulumi/terraform-plugin-sdk/blob/upstream-v2.33.0/helper/schema/grpc_provider.go#L1514
+func normalizeNullValues(
+	ctx context.Context,
+	schemaMap shim.SchemaMap,
+	schemaInfos map[string]*info.Schema,
+	oldInputs, inputs resource.PropertyMap,
+) resource.PropertyMap {
+	if oldInputs == nil {
+		return inputs
+	}
+	copy := inputs.Copy()
+	for _, k := range inputs.StableKeys() {
+		v := inputs[k]
+		oldInput, gotOldInput := oldInputs[k]
+		if gotOldInput && !oldInput.IsNull() {
+			continue
+		}
+		if !(v.IsArray() && len(v.ArrayValue()) == 0) {
+			continue
+		}
+		tfName := PulumiToTerraformName(string(k), schemaMap, schemaInfos)
+		schema, ok := schemaMap.GetOk(tfName)
+		if !ok {
+			continue
+		}
+		t := schema.Type()
+		isCollection := t == shim.TypeList || t == shim.TypeMap || t == shim.TypeSet
+		if !isCollection || IsMaxItemsOne(schema, schemaInfos[tfName]) {
+			continue
+		}
+		// An empty collection (not MaxItems=1) with missing/null oldInput is getting replaced to match.
+		if gotOldInput {
+			copy[k] = oldInput
+		} else {
+			delete(copy, k)
+		}
+	}
+	return copy
+}

pkg/tfbridge/provider.go

@@ -1296,6 +1296,10 @@ func (p *Provider) Read(ctx context.Context, req *pulumirpc.ReadRequest) (*pulum
 			return nil, err
 		}
 
+		if isRefresh {
+			inputs = normalizeNullValues(oldInputs, res.TF.Schema(), res.Schema.Fields, inputs)
+		}
+
 		cleanInputs := deconflict(ctx, res.TF.Schema(), res.Schema.Fields, inputs)
 
 		minputs, err := plugin.MarshalProperties(cleanInputs, plugin.MarshalOptions{