resource/aws_route53_resolver_rule: Correct handling for single perio…

…d (`.`) value in `domain_name` argument (#15015)

Output from acceptance testing:

```
--- PASS: TestAccAwsRoute53ResolverRule_basic (51.50s)
--- PASS: TestAccAwsRoute53ResolverRule_forward (302.02s)
--- PASS: TestAccAwsRoute53ResolverRule_forwardEndpointRecreate (468.20s)
--- PASS: TestAccAwsRoute53ResolverRule_justDotDomainName (53.81s)
--- PASS: TestAccAwsRoute53ResolverRule_tags (79.55s)
--- PASS: TestAccAwsRoute53ResolverRule_trailingDotDomainName (53.97s)
--- PASS: TestAccAwsRoute53ResolverRule_updateName (76.52s)

--- PASS: TestAccAwsRoute53ResolverRuleAssociation_basic (200.65s)

--- PASS: TestAccAWSRoute53Zone_basic (73.79s)
--- PASS: TestAccAWSRoute53Zone_Comment (88.35s)
--- PASS: TestAccAWSRoute53Zone_DelegationSetID (75.67s)
--- PASS: TestAccAWSRoute53Zone_disappears (67.61s)
--- PASS: TestAccAWSRoute53Zone_ForceDestroy (238.61s)
--- PASS: TestAccAWSRoute53Zone_ForceDestroy_TrailingPeriod (237.20s)
--- PASS: TestAccAWSRoute53Zone_multiple (78.13s)
--- PASS: TestAccAWSRoute53Zone_Tags (80.90s)
--- PASS: TestAccAWSRoute53Zone_VPC_Multiple (168.51s)
--- PASS: TestAccAWSRoute53Zone_VPC_Single (99.75s)
--- PASS: TestAccAWSRoute53Zone_VPC_Updates (262.79s)

--- PASS: TestAccAWSRoute53ZoneAssociation_basic (201.40s)
--- PASS: TestAccAWSRoute53ZoneAssociation_CrossRegion (205.99s)
--- PASS: TestAccAWSRoute53ZoneAssociation_disappears (205.47s)
--- PASS: TestAccAWSRoute53ZoneAssociation_disappears_VPC (191.07s)
--- PASS: TestAccAWSRoute53ZoneAssociation_disappears_Zone (192.61s)
```

aws/resource_aws_route53_resolver_rule_test.go

@@ -111,6 +111,64 @@ func TestAccAwsRoute53ResolverRule_basic(t *testing.T) {
 	})
 }
 
+func TestAccAwsRoute53ResolverRule_justDotDomainName(t *testing.T) {
+	var rule route53resolver.ResolverRule
+	resourceName := "aws_route53_resolver_rule.example"
+
+	resource.ParallelTest(t, resource.TestCase{
+		PreCheck:     func() { testAccPreCheck(t); testAccPreCheckAWSRoute53Resolver(t) },
+		Providers:    testAccProviders,
+		CheckDestroy: testAccCheckRoute53ResolverRuleDestroy,
+		Steps: []resource.TestStep{
+			{
+				Config: testAccRoute53ResolverRuleConfig("."),
+				Check: resource.ComposeTestCheckFunc(
+					testAccCheckRoute53ResolverRuleExists(resourceName, &rule),
+					resource.TestCheckResourceAttr(resourceName, "domain_name", "."),
+					resource.TestCheckResourceAttr(resourceName, "rule_type", "SYSTEM"),
+					resource.TestCheckResourceAttr(resourceName, "share_status", "NOT_SHARED"),
+					testAccCheckResourceAttrAccountID(resourceName, "owner_id"),
+					resource.TestCheckResourceAttr(resourceName, "tags.%", "0"),
+				),
+			},
+			{
+				ResourceName:      resourceName,
+				ImportState:       true,
+				ImportStateVerify: true,
+			},
+		},
+	})
+}
+
+func TestAccAwsRoute53ResolverRule_trailingDotDomainName(t *testing.T) {
+	var rule route53resolver.ResolverRule
+	resourceName := "aws_route53_resolver_rule.example"
+
+	resource.ParallelTest(t, resource.TestCase{
+		PreCheck:     func() { testAccPreCheck(t); testAccPreCheckAWSRoute53Resolver(t) },
+		Providers:    testAccProviders,
+		CheckDestroy: testAccCheckRoute53ResolverRuleDestroy,
+		Steps: []resource.TestStep{
+			{
+				Config: testAccRoute53ResolverRuleConfig("example.com."),
+				Check: resource.ComposeTestCheckFunc(
+					testAccCheckRoute53ResolverRuleExists(resourceName, &rule),
+					resource.TestCheckResourceAttr(resourceName, "domain_name", "example.com"),
+					resource.TestCheckResourceAttr(resourceName, "rule_type", "SYSTEM"),
+					resource.TestCheckResourceAttr(resourceName, "share_status", "NOT_SHARED"),
+					testAccCheckResourceAttrAccountID(resourceName, "owner_id"),
+					resource.TestCheckResourceAttr(resourceName, "tags.%", "0"),
+				),
+			},
+			{
+				ResourceName:      resourceName,
+				ImportState:       true,
+				ImportStateVerify: true,
+			},
+		},
+	})
+}
+
 func TestAccAwsRoute53ResolverRule_tags(t *testing.T) {
 	var rule route53resolver.ResolverRule
 	resourceName := "aws_route53_resolver_rule.example"
@@ -389,6 +447,15 @@ func testAccCheckRoute53ResolverRuleExists(n string, rule *route53resolver.Resol
 	}
 }
 
+func testAccRoute53ResolverRuleConfig(domainName string) string {
+	return fmt.Sprintf(`
+resource "aws_route53_resolver_rule" "example" {
+  domain_name = %[1]q
+  rule_type   = "SYSTEM"
+}
+`, domainName)
+}
+
 const testAccRoute53ResolverRuleConfig_basicNoTags = `
 resource "aws_route53_resolver_rule" "example" {
   domain_name = "example.com"

aws/resource_aws_route53_zone.go

@@ -397,17 +397,23 @@ func cleanZoneID(ID string) string {
 
 // trimTrailingPeriod is used to remove the trailing period
 // of "name" or "domain name" attributes often returned from
-// the Route53 API or provided as user input
+// the Route53 API or provided as user input.
+// The single dot (".") domain name is returned as-is.
 func trimTrailingPeriod(v interface{}) string {
 	var str string
 	switch value := v.(type) {
 	case *string:
-		str = *value
+		str = aws.StringValue(value)
 	case string:
 		str = value
 	default:
 		return ""
 	}
+
+	if str == "." {
+		return str
+	}
+
 	return strings.TrimSuffix(str, ".")
 }
 

aws/resource_aws_route53_zone_test.go

@@ -50,11 +50,19 @@ func TestCleanChangeID(t *testing.T) {
 
 func TestTrimTrailingPeriod(t *testing.T) {
 	cases := []struct {
-		Input, Output string
+		Input  interface{}
+		Output string
 	}{
 		{"example.com", "example.com"},
 		{"example.com.", "example.com"},
 		{"www.example.com.", "www.example.com"},
+		{"", ""},
+		{".", "."},
+		{aws.String("example.com"), "example.com"},
+		{aws.String("example.com."), "example.com"},
+		{(*string)(nil), ""},
+		{42, ""},
+		{nil, ""},
 	}
 
 	for _, tc := range cases {