ec2/vpc_endpoint: Fix erroneous diffs with equivalent policies

.changelog/22137.txt

@@ -0,0 +1,3 @@
+```release-note:bug
+resource/aws_vpc_endpoint: Fix erroneous diffs in `policy` when no changes made or policies are equivalent
+```

internal/service/ec2/vpc_endpoint.go

@@ -10,6 +10,7 @@ import (
 	"github.com/aws/aws-sdk-go/aws/arn"
 	"github.com/aws/aws-sdk-go/service/ec2"
 	"github.com/hashicorp/aws-sdk-go-base/tfawserr"
+	awspolicy "github.com/hashicorp/awspolicyequivalence"
 	"github.com/hashicorp/terraform-plugin-sdk/v2/helper/schema"
 	"github.com/hashicorp/terraform-plugin-sdk/v2/helper/structure"
 	"github.com/hashicorp/terraform-plugin-sdk/v2/helper/validation"
@@ -271,11 +272,21 @@ func resourceVPCEndpointRead(d *schema.ResourceData, meta interface{}) error {
 		return fmt.Errorf("error setting network_interface_ids: %s", err)
 	}
 	d.Set("owner_id", vpce.OwnerId)
-	policy, err := structure.NormalizeJsonString(aws.StringValue(vpce.PolicyDocument))
+
+	policyToSet, err := verify.SecondJSONUnlessEquivalent(d.Get("policy").(string), aws.StringValue(vpce.PolicyDocument))
+
+	if err != nil {
+		return fmt.Errorf("while setting policy (%s), encountered: %w", policyToSet, err)
+	}
+
+	policyToSet, err = structure.NormalizeJsonString(policyToSet)
+
 	if err != nil {
-		return fmt.Errorf("policy contains an invalid JSON: %s", err)
+		return fmt.Errorf("policy (%s) is invalid JSON: %w", policyToSet, err)
 	}
-	d.Set("policy", policy)
+
+	d.Set("policy", policyToSet)
+
 	d.Set("private_dns_enabled", vpce.PrivateDnsEnabled)
 	err = d.Set("route_table_ids", flex.FlattenStringSet(vpce.RouteTableIds))
 	if err != nil {
@@ -326,15 +337,19 @@ func resourceVPCEndpointUpdate(d *schema.ResourceData, meta interface{}) error {
 		}
 
 		if d.HasChange("policy") {
-			policy, err := structure.NormalizeJsonString(d.Get("policy"))
-			if err != nil {
-				return fmt.Errorf("policy contains an invalid JSON: %s", err)
-			}
-
-			if policy == "" {
-				req.ResetPolicy = aws.Bool(true)
-			} else {
-				req.PolicyDocument = aws.String(policy)
+			o, n := d.GetChange("policy")
+
+			if equivalent, err := awspolicy.PoliciesAreEquivalent(o.(string), n.(string)); err != nil || !equivalent {
+				policy, err := structure.NormalizeJsonString(d.Get("policy"))
+				if err != nil {
+					return fmt.Errorf("policy contains an invalid JSON: %s", err)
+				}
+
+				if policy == "" {
+					req.ResetPolicy = aws.Bool(true)
+				} else {
+					req.PolicyDocument = aws.String(policy)
+				}
 			}
 		}
 

internal/service/ec2/vpc_endpoint_test.go

@@ -182,6 +182,31 @@ func TestAccEC2VPCEndpoint_gatewayPolicy(t *testing.T) {
 	})
 }
 
+func TestAccEC2VPCEndpoint_ignoreEquivalent(t *testing.T) {
+	var endpoint ec2.VpcEndpoint
+	resourceName := "aws_vpc_endpoint.test"
+	rName := sdkacctest.RandomWithPrefix(acctest.ResourcePrefix)
+
+	resource.ParallelTest(t, resource.TestCase{
+		PreCheck:     func() { acctest.PreCheck(t) },
+		ErrorCheck:   acctest.ErrorCheck(t, ec2.EndpointsID),
+		Providers:    acctest.Providers,
+		CheckDestroy: testAccCheckVpcEndpointDestroy,
+		Steps: []resource.TestStep{
+			{
+				Config: testAccVpcEndpointOrderPolicyConfig(rName),
+				Check: resource.ComposeTestCheckFunc(
+					testAccCheckVpcEndpointExists(resourceName, &endpoint),
+				),
+			},
+			{
+				Config:   testAccVpcEndpointNewOrderPolicyConfig(rName),
+				PlanOnly: true,
+			},
+		},
+	})
+}
+
 func TestAccEC2VPCEndpoint_interfaceBasic(t *testing.T) {
 	var endpoint ec2.VpcEndpoint
 	resourceName := "aws_vpc_endpoint.test"
@@ -1007,3 +1032,81 @@ resource "aws_vpc_endpoint" "test" {
 }
 `, rName))
 }
+
+func testAccVpcEndpointOrderPolicyConfig(rName string) string {
+	return fmt.Sprintf(`
+data "aws_vpc_endpoint_service" "test" {
+  service = "dynamodb"
+}
+
+resource "aws_vpc" "test" {
+  cidr_block = "10.0.0.0/16"
+
+  tags = {
+    Name = %[1]q
+  }
+}
+
+resource "aws_vpc_endpoint" "test" {
+  policy = jsonencode({
+    Version = "2012-10-17"
+    Statement = [{
+      Sid       = "ReadOnly"
+      Principal = "*"
+      Action = [
+        "dynamodb:DescribeTable",
+        "dynamodb:ListTables",
+        "dynamodb:ListTagsOfResource",
+      ]
+      Effect   = "Allow"
+      Resource = "*"
+    }]
+  })
+  service_name = data.aws_vpc_endpoint_service.test.service_name
+  vpc_id       = aws_vpc.test.id
+
+  tags = {
+    Name = %[1]q
+  }
+}
+`, rName)
+}
+
+func testAccVpcEndpointNewOrderPolicyConfig(rName string) string {
+	return fmt.Sprintf(`
+data "aws_vpc_endpoint_service" "test" {
+  service = "dynamodb"
+}
+
+resource "aws_vpc" "test" {
+  cidr_block = "10.0.0.0/16"
+
+  tags = {
+    Name = %[1]q
+  }
+}
+
+resource "aws_vpc_endpoint" "test" {
+  policy = jsonencode({
+    Version = "2012-10-17"
+    Statement = [{
+      Sid       = "ReadOnly"
+      Principal = "*"
+      Action = [
+        "dynamodb:ListTables",
+        "dynamodb:ListTagsOfResource",
+        "dynamodb:DescribeTable",
+      ]
+      Effect   = "Allow"
+      Resource = "*"
+    }]
+  })
+  service_name = data.aws_vpc_endpoint_service.test.service_name
+  vpc_id       = aws_vpc.test.id
+
+  tags = {
+    Name = %[1]q
+  }
+}
+`, rName)
+}