add validator to block zone creation with trailing dot

docs/data-sources/dns_zone.md

@@ -28,7 +28,7 @@ data "stackit_dns_zone" "example" {
 
 ### Optional
 
-- `dns_name` (String) The zone name. E.g. `example.com`
+- `dns_name` (String) The zone name. E.g. `example.com` (must not end with a trailing dot).
 - `zone_id` (String) The zone ID.
 
 ### Read-Only

stackit/internal/services/dns/dns_acc_test.go

@@ -93,6 +93,20 @@ func TestAccDnsMinResource(t *testing.T) {
 				ConfigVariables: configVarsInvalid(testConfigVarsMin),
 				ExpectError:     regexp.MustCompile(`not a valid dns name. Need at least two levels`),
 			},
+			// Creation fail: trailing dot is rejected on purpose
+			{
+				Config: resourceMinConfig,
+				ConfigVariables: func() config.Variables {
+					vars := maps.Clone(testConfigVarsMin)
+
+					// Ensure it ends with a dot (even if the random value already had one, be explicit)
+					base := testutil.ConvertConfigVariable(vars["dns_name"])
+					vars["dns_name"] = config.StringVariable(base + ".")
+
+					return vars
+				}(),
+				ExpectError: regexp.MustCompile(`dns_name must not end with a trailing dot`),
+			},
 			// creation
 			{
 				Config:          resourceMinConfig,

stackit/internal/services/dns/zone/datasource.go

@@ -6,6 +6,7 @@ import (
 	"net/http"
 
 	"github.com/hashicorp/terraform-plugin-framework-validators/datasourcevalidator"
+	"github.com/hashicorp/terraform-plugin-framework-validators/stringvalidator"
 	"github.com/hashicorp/terraform-plugin-framework/path"
 	"github.com/stackitcloud/terraform-provider-stackit/stackit/internal/conversion"
 	dnsUtils "github.com/stackitcloud/terraform-provider-stackit/stackit/internal/services/dns/utils"
@@ -95,8 +96,16 @@ func (d *zoneDataSource) Schema(_ context.Context, _ datasource.SchemaRequest, r
 				Computed:    true,
 			},
 			"dns_name": schema.StringAttribute{
-				Description: "The zone name. E.g. `example.com`",
+				Description: "The zone name. E.g. `example.com` (must not end with a trailing dot).",
 				Optional:    true,
+				Validators: []validator.String{
+					stringvalidator.LengthAtLeast(1),
+					stringvalidator.LengthAtMost(253),
+					stringvalidator.RegexMatches(
+						dnsNameNoTrailingDotRegex,
+						"dns_name must not end with a trailing dot",
+					),
+				},
 			},
 			"description": schema.StringAttribute{
 				Description: "Description of the zone.",

stackit/internal/services/dns/zone/resource.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"fmt"
 	"math"
+	"regexp"
 	"strings"
 
 	dnsUtils "github.com/stackitcloud/terraform-provider-stackit/stackit/internal/services/dns/utils"
@@ -36,6 +37,9 @@ var (
 	_ resource.ResourceWithImportState = &zoneResource{}
 )
 
+// dnsNameNoTrailingDotRegex defines the zone dns_name without trailing dot
+var dnsNameNoTrailingDotRegex = regexp.MustCompile(`^.*[^.]$`)
+
 type Model struct {
 	Id                types.String `tfsdk:"id"` // needed by TF
 	ZoneId            types.String `tfsdk:"zone_id"`
@@ -144,6 +148,10 @@ func (r *zoneResource) Schema(_ context.Context, _ resource.SchemaRequest, resp
 				Validators: []validator.String{
 					stringvalidator.LengthAtLeast(1),
 					stringvalidator.LengthAtMost(253),
+					stringvalidator.RegexMatches(
+						dnsNameNoTrailingDotRegex,
+						"dns_name must not end with a trailing dot",
+					),
 				},
 			},
 			"description": schema.StringAttribute{

stackit/internal/services/dns/zone/resource_test.go

@@ -435,3 +435,45 @@ func TestToPayloadUpdate(t *testing.T) {
 		})
 	}
 }
+
+func TestDnsNameNoTrailingDot(t *testing.T) {
+	tests := []struct {
+		description string
+		input       string
+		match       bool
+	}{
+		{
+			"normal domain without trailing dot",
+			"example.com",
+			true,
+		},
+		{
+			"single layer without trailing dot",
+			"example",
+			true,
+		},
+		{
+			"domain with trailing dot",
+			"example.com.",
+			false,
+		},
+		{
+			"only trailing dot",
+			".",
+			false,
+		},
+		{
+			"single layer with trailing dot",
+			"example.",
+			false,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.description, func(t *testing.T) {
+			got := dnsNameNoTrailingDotRegex.MatchString(tt.input)
+			if got != tt.match {
+				t.Fatalf("dnsNameNoTrailingDotRegex.MatchString(%q) = %v, want %v", tt.input, got, tt.match)
+			}
+		})
+	}
+}