provider/azurerm: fix servicebus_topic updating values

enable_partitioning set to ForceNew
requires_duplicate_detection set to ForceNew

max_size_in_megabytes would cause a loop if enable_partitioning was true as this
causes the value to be multiplied by 16 for it's effective value, this computed
value is then returned by the ARM API in the same field which caused Terraform
to always detect a change

```
TF_ACC=1 go test ./builtin/providers/azurerm -v -run TestAccAzureRMServiceBusTopic -timeout 120m
=== RUN   TestAccAzureRMServiceBusTopic_importBasic
--- PASS: TestAccAzureRMServiceBusTopic_importBasic (345.08s)
=== RUN   TestAccAzureRMServiceBusTopic_basic
--- PASS: TestAccAzureRMServiceBusTopic_basic (342.23s)
=== RUN   TestAccAzureRMServiceBusTopic_update
--- PASS: TestAccAzureRMServiceBusTopic_update (359.56s)
=== RUN   TestAccAzureRMServiceBusTopic_enablePartitioning
--- PASS: TestAccAzureRMServiceBusTopic_enablePartitioning (362.80s)
=== RUN   TestAccAzureRMServiceBusTopic_enableDuplicateDetection
--- PASS: TestAccAzureRMServiceBusTopic_enableDuplicateDetection (364.97s)
PASS
ok  	github.com/hashicorp/terraform/builtin/providers/azurerm	1774.657s
```

builtin/providers/azurerm/resource_arm_servicebus_topic.go

@@ -80,17 +80,20 @@ func resourceArmServiceBusTopic() *schema.Resource {
 			"enable_partitioning": {
 				Type:     schema.TypeBool,
 				Optional: true,
+				ForceNew: true,
 			},
 
 			"max_size_in_megabytes": {
-				Type:     schema.TypeInt,
-				Optional: true,
-				Computed: true,
+				Type:         schema.TypeInt,
+				Optional:     true,
+				Computed:     true,
+				ValidateFunc: validateArmServiceBusTopicMaxSize,
 			},
 
 			"requires_duplicate_detection": {
 				Type:     schema.TypeBool,
 				Optional: true,
+				ForceNew: true,
 			},
 
 			"support_ordering": {
@@ -199,10 +202,18 @@ func resourceArmServiceBusTopicRead(d *schema.ResourceData, meta interface{}) er
 	d.Set("enable_express", props.EnableExpress)
 	d.Set("enable_filtering_messages_before_publishing", props.FilteringMessagesBeforePublishing)
 	d.Set("enable_partitioning", props.EnablePartitioning)
-	d.Set("max_size_in_megabytes", int(*props.MaxSizeInMegabytes))
 	d.Set("requires_duplicate_detection", props.RequiresDuplicateDetection)
 	d.Set("support_ordering", props.SupportOrdering)
 
+	// if partitioning is enabled then the max size returned by the API will be
+	// 16 times greater than the value set by the user
+	if *props.EnablePartitioning {
+		const partitionCount = 16
+		d.Set("max_size_in_megabytes", int(*props.MaxSizeInMegabytes/partitionCount))
+	} else {
+		d.Set("max_size_in_megabytes", int(*props.MaxSizeInMegabytes))
+	}
+
 	return nil
 }
 
@@ -221,3 +232,12 @@ func resourceArmServiceBusTopicDelete(d *schema.ResourceData, meta interface{})
 
 	return err
 }
+
+func validateArmServiceBusTopicMaxSize(i interface{}, k string) (s []string, es []error) {
+	v := i.(int)
+	if v%1024 != 0 || v < 0 || v > 10240 {
+		es = append(es, fmt.Errorf("%q must be a multiple of 1024 up to and including 10240", k))
+	}
+
+	return
+}

builtin/providers/azurerm/resource_arm_servicebus_topic_test.go

@@ -58,6 +58,63 @@ func TestAccAzureRMServiceBusTopic_update(t *testing.T) {
 	})
 }
 
+func TestAccAzureRMServiceBusTopic_enablePartitioning(t *testing.T) {
+	ri := acctest.RandInt()
+	preConfig := fmt.Sprintf(testAccAzureRMServiceBusTopic_basic, ri, ri, ri)
+	postConfig := fmt.Sprintf(testAccAzureRMServiceBusTopic_enablePartitioning, ri, ri, ri)
+
+	resource.Test(t, resource.TestCase{
+		PreCheck:     func() { testAccPreCheck(t) },
+		Providers:    testAccProviders,
+		CheckDestroy: testCheckAzureRMServiceBusTopicDestroy,
+		Steps: []resource.TestStep{
+			resource.TestStep{
+				Config: preConfig,
+				Check: resource.ComposeTestCheckFunc(
+					testCheckAzureRMServiceBusTopicExists("azurerm_servicebus_topic.test"),
+				),
+			},
+			resource.TestStep{
+				Config: postConfig,
+				Check: resource.ComposeTestCheckFunc(
+					resource.TestCheckResourceAttr(
+						"azurerm_servicebus_topic.test", "enable_partitioning", "true"),
+					// Ensure size is read back in it's original value and not the x16 value returned by Azure
+					resource.TestCheckResourceAttr(
+						"azurerm_servicebus_topic.test", "max_size_in_megabytes", "10240"),
+				),
+			},
+		},
+	})
+}
+
+func TestAccAzureRMServiceBusTopic_enableDuplicateDetection(t *testing.T) {
+	ri := acctest.RandInt()
+	preConfig := fmt.Sprintf(testAccAzureRMServiceBusTopic_basic, ri, ri, ri)
+	postConfig := fmt.Sprintf(testAccAzureRMServiceBusTopic_enableDuplicateDetection, ri, ri, ri)
+
+	resource.Test(t, resource.TestCase{
+		PreCheck:     func() { testAccPreCheck(t) },
+		Providers:    testAccProviders,
+		CheckDestroy: testCheckAzureRMServiceBusTopicDestroy,
+		Steps: []resource.TestStep{
+			resource.TestStep{
+				Config: preConfig,
+				Check: resource.ComposeTestCheckFunc(
+					testCheckAzureRMServiceBusTopicExists("azurerm_servicebus_topic.test"),
+				),
+			},
+			resource.TestStep{
+				Config: postConfig,
+				Check: resource.ComposeTestCheckFunc(
+					resource.TestCheckResourceAttr(
+						"azurerm_servicebus_topic.test", "requires_duplicate_detection", "true"),
+				),
+			},
+		},
+	})
+}
+
 func testCheckAzureRMServiceBusTopicDestroy(s *terraform.State) error {
 	client := testAccProvider.Meta().(*ArmClient).serviceBusTopicsClient
 
@@ -159,3 +216,48 @@ resource "azurerm_servicebus_topic" "test" {
     enable_express = true
 }
 `
+
+var testAccAzureRMServiceBusTopic_enablePartitioning = `
+resource "azurerm_resource_group" "test" {
+    name = "acctestRG-%d"
+    location = "West US"
+}
+
+resource "azurerm_servicebus_namespace" "test" {
+    name = "acctestservicebusnamespace-%d"
+    location = "West US"
+    resource_group_name = "${azurerm_resource_group.test.name}"
+    sku = "standard"
+}
+
+resource "azurerm_servicebus_topic" "test" {
+    name = "acctestservicebustopic-%d"
+    location = "West US"
+    namespace_name = "${azurerm_servicebus_namespace.test.name}"
+    resource_group_name = "${azurerm_resource_group.test.name}"
+    enable_partitioning = true
+	max_size_in_megabytes = 10240
+}
+`
+
+var testAccAzureRMServiceBusTopic_enableDuplicateDetection = `
+resource "azurerm_resource_group" "test" {
+    name = "acctestRG-%d"
+    location = "West US"
+}
+
+resource "azurerm_servicebus_namespace" "test" {
+    name = "acctestservicebusnamespace-%d"
+    location = "West US"
+    resource_group_name = "${azurerm_resource_group.test.name}"
+    sku = "standard"
+}
+
+resource "azurerm_servicebus_topic" "test" {
+    name = "acctestservicebustopic-%d"
+    location = "West US"
+    namespace_name = "${azurerm_servicebus_namespace.test.name}"
+    resource_group_name = "${azurerm_resource_group.test.name}"
+    requires_duplicate_detection = true
+}
+`

website/source/docs/providers/azurerm/r/servicebus_topic.html.markdown

@@ -82,12 +82,16 @@ The following arguments are supported:
 
 * `enable_partitioning` - (Optional) Boolean flag which controls whether to enable
     the topic to be partitioned across multiple message brokers. Defaults to false.
+    Changing this forces a new resource to be created.
 
 * `max_size_in_megabytes` - (Optional) Integer value which controls the size of
-    memory allocated for the topic.
+    memory allocated for the topic. Supported values are multiples of 1024 up to
+    10240, if `enable_partitioning` is enabled then 16 partitions will be created
+    per GB, making the maximum possible topic size 163840 (10240 * 16).
 
 * `requires_duplicate_detection` - (Optional) Boolean flag which controls whether
-    the Topic requires duplicate detection. Defaults to false.
+    the Topic requires duplicate detection. Defaults to false. Changing this forces
+    a new resource to be created.
 
 * `support_ordering` - (Optional) Boolean flag which controls whether the Topic
     supports ordering. Defaults to false.