provider: update SetTagsDiff when setting the computed tags_all attribute

.changelog/18958.txt

@@ -0,0 +1,3 @@
+```release-note:bug
+provider: Prevent `Provider produced inconsistent final plan` errors when resource `tags` are not known until apply
+```

aws/resource_aws_subnet_test.go

@@ -521,6 +521,47 @@ func TestAccAWSSubnet_defaultAndIgnoreTags(t *testing.T) {
 	})
 }
 
+// TestAccAWSSubnet_updateTagsKnownAtApply ensures computed "tags_all"
+// attributes are correctly determined when the provider-level default_tags block
+// is left unused and resource tags are only known at apply time, thereby
+// eliminating "Inconsistent final plan" errors
+// Reference: https://github.com/hashicorp/terraform-provider-aws/issues/18366
+func TestAccAWSSubnet_updateTagsKnownAtApply(t *testing.T) {
+	var providers []*schema.Provider
+	var subnet ec2.Subnet
+	resourceName := "aws_subnet.test"
+
+	resource.ParallelTest(t, resource.TestCase{
+		PreCheck:          func() { testAccPreCheck(t) },
+		ErrorCheck:        testAccErrorCheck(t, ec2.EndpointsID),
+		ProviderFactories: testAccProviderFactoriesInternal(&providers),
+		CheckDestroy:      testAccCheckSubnetDestroy,
+		Steps: []resource.TestStep{
+			{
+				Config: testAccSubnetConfig_tagsComputedFromVpcDataSource1("key1", "value1"),
+				Check: resource.ComposeTestCheckFunc(
+					testAccCheckSubnetExists(resourceName, &subnet),
+					resource.TestCheckResourceAttr(resourceName, "tags.%", "1"),
+					resource.TestCheckResourceAttr(resourceName, "tags_all.%", "1"),
+				),
+			},
+			{
+				Config: testAccSubnetConfig_tagsComputedFromVpcDataSource2("key1", "value1", "key2", "value2"),
+				Check: resource.ComposeTestCheckFunc(
+					testAccCheckSubnetExists(resourceName, &subnet),
+					resource.TestCheckResourceAttr(resourceName, "tags.%", "2"),
+					resource.TestCheckResourceAttr(resourceName, "tags_all.%", "2"),
+				),
+			},
+			{
+				ResourceName:      resourceName,
+				ImportState:       true,
+				ImportStateVerify: true,
+			},
+		},
+	})
+}
+
 func TestAccAWSSubnet_ignoreTags(t *testing.T) {
 	var providers []*schema.Provider
 	var subnet ec2.Subnet
@@ -969,6 +1010,48 @@ resource "aws_subnet" "test" {
 `, rName, tagKey1, tagValue1, tagKey2, tagValue2)
 }
 
+const testAccSubnetComputedTagsBaseConfig = `
+resource "aws_vpc" "test" {
+  cidr_block = "10.0.0.0/16"
+  tags       = local.tags
+}
+
+data "aws_vpc" "test" {
+  id = aws_vpc.test.id
+}
+
+resource "aws_subnet" "test" {
+  cidr_block = cidrsubnet(aws_vpc.test.cidr_block, 8, 0)
+  vpc_id     = aws_vpc.test.id
+  tags       = data.aws_vpc.test.tags
+}
+`
+
+func testAccSubnetConfig_tagsComputedFromVpcDataSource1(tagKey1, tagValue1 string) string {
+	return composeConfig(
+		testAccSubnetComputedTagsBaseConfig,
+		fmt.Sprintf(`
+locals {
+  tags = {
+    %q = %q
+  }
+}
+`, tagKey1, tagValue1))
+}
+
+func testAccSubnetConfig_tagsComputedFromVpcDataSource2(tagKey1, tagValue1, tagKey2, tagValue2 string) string {
+	return composeConfig(
+		testAccSubnetComputedTagsBaseConfig,
+		fmt.Sprintf(`
+locals {
+  tags = {
+    %q = %q
+    %q = %q
+  }
+}
+`, tagKey1, tagValue1, tagKey2, tagValue2))
+}
+
 const testAccSubnetConfigPreIpv6 = `
 resource "aws_vpc" "test" {
   cidr_block                       = "10.10.0.0/16"

aws/tags.go

@@ -111,8 +111,18 @@ func SetTagsDiff(_ context.Context, diff *schema.ResourceDiff, meta interface{})
 
 	allTags := defaultTagsConfig.MergeTags(resourceTags).IgnoreConfig(ignoreTagsConfig)
 
-	if err := diff.SetNew("tags_all", allTags.Map()); err != nil {
-		return fmt.Errorf("error setting new tags_all diff: %w", err)
+	// To ensure "tags_all" is correctly computed with the value held in allTags,
+	// we explicitly set the attribute diff when values are known,
+	// otherwise mark the attribute only as "Computed"
+	// Reference: https://github.com/hashicorp/terraform-provider-aws/issues/18366
+	if len(allTags) > 0 {
+		if err := diff.SetNew("tags_all", allTags.Map()); err != nil {
+			return fmt.Errorf("error setting new tags_all diff: %w", err)
+		}
+	} else {
+		if err := diff.SetNewComputed("tags_all"); err != nil {
+			return fmt.Errorf("error setting tags_all to computed: %w", err)
+		}
 	}
 
 	return nil