Do not throw error when disk path is not known yet

If the path to an attached disk is interpolated, then sometimes we may
end up in a situation where the diff for the VM having the disks runs
before the disk itself has been processed. In this case since the path
has no value we throw an error.

This changes the behaviour so we check if the path has been calculated
before we consider it final or not.

vsphere/internal/virtualdevice/virtual_machine_disk_subresource.go

@@ -582,15 +582,21 @@ func DiskDiffOperation(d *schema.ResourceDiff, c *govmomi.Client) error {
 			return fmt.Errorf("disk: duplicate name %s", name)
 		}
 		// If attach is set, we need to validate that there's no other duplicate paths.
+		curDiskPath := fmt.Sprintf("disk.%d.path", ni)
+		pathKnown := d.NewValueKnown(curDiskPath)
 		if nm["attach"].(bool) {
 			path := diskPathOrName(nm)
-			if path == "" {
-				return fmt.Errorf("disk.%d: path or name cannot be empty when using attach", ni)
-			}
-			if _, ok := attachments[path]; ok {
-				return fmt.Errorf("disk: multiple entries trying to attach external disk %s", path)
+			if pathKnown {
+				if path == "" {
+					return fmt.Errorf("disk.%d: path or name cannot be empty when using attach", ni)
+				}
+				if _, ok := attachments[path]; ok {
+					return fmt.Errorf("disk: multiple entries trying to attach external disk %s", path)
+				}
+				attachments[path] = struct{}{}
+			} else {
+				log.Printf("[DEBUG] Disk path for disk %d is not known yet.", ni)
 			}
-			attachments[path] = struct{}{}
 		}
 
 		if _, ok := units[nm["unit_number"].(int)]; ok {

vsphere/resource_vsphere_virtual_machine_test.go

@@ -2985,6 +2985,26 @@ func TestAccResourceVSphereVirtualMachine_readVappChildResourcePool(t *testing.T
 		},
 	})
 }
+
+func TestAccResourceVSphereVirtualMachine_interpolatedDisk(t *testing.T) {
+	resource.Test(t, resource.TestCase{
+		PreCheck: func() {
+			testAccPreCheck(t)
+			testAccResourceVSphereVirtualMachinePreCheck(t)
+		},
+		Providers:    testAccProviders,
+		CheckDestroy: testAccResourceVSphereVirtualMachineCheckExists(false),
+		Steps: []resource.TestStep{
+			{
+				Config: testAccResourceVSphereVirtualMachineTestPathInterpolation(),
+				Check: resource.ComposeTestCheckFunc(
+					testAccResourceVSphereVirtualMachineCheckExists(true),
+				),
+			},
+		},
+	})
+}
+
 func testAccResourceVSphereVirtualMachinePreCheck(t *testing.T) {
 	// Note that VSPHERE_USE_LINKED_CLONE is also a variable and its presence
 	// speeds up tests greatly, but it's not a necessary variable, so we don't
@@ -13726,3 +13746,68 @@ resource "vsphere_virtual_machine" "vm" {
 		os.Getenv("VSPHERE_VAPP_RESOURCE_POOL"),
 	)
 }
+
+func testAccResourceVSphereVirtualMachineTestPathInterpolation() string {
+	return fmt.Sprintf(`
+data "vsphere_datacenter" "dc" {
+  name = "%s"
+}
+
+data "vsphere_datastore" "datastore" {
+  name          = "%s"
+  datacenter_id = data.vsphere_datacenter.dc.id
+}
+
+data "vsphere_compute_cluster" "cluster" {
+  name          = "%s"
+  datacenter_id = data.vsphere_datacenter.dc.id
+}
+
+data "vsphere_network" "n" {
+  name          = "%s"
+  datacenter_id = data.vsphere_datacenter.dc.id
+}
+
+resource "null_resource" "n" {
+  count = 2
+}
+
+resource "vsphere_virtual_disk" "d" {
+  count      = 2
+  size       = 1
+  vmdk_path  = "${null_resource.n[count.index].id}.vmdk"
+  datastore  = data.vsphere_datastore.datastore.name
+  datacenter = data.vsphere_datacenter.dc.name
+}
+
+
+resource "vsphere_virtual_machine" "vm" {
+  name             = "terraform-test"
+  resource_pool_id = data.vsphere_compute_cluster.cluster.resource_pool_id
+  datastore_id     = data.vsphere_datastore.datastore.id
+  guest_id         = "ubuntu64Guest"
+
+  network_interface { 
+    network_id = data.vsphere_network.n.id
+  }
+    disk {
+      attach       = true
+      label        = "disk0"
+      path         = vsphere_virtual_disk.d[0].vmdk_path
+      unit_number  = 0
+      datastore_id = data.vsphere_datastore.datastore.id
+    }
+
+    disk {
+      attach       = true
+      label        = "disk1"
+      path         = vsphere_virtual_disk.d[1].vmdk_path
+      unit_number  = 1
+      datastore_id = data.vsphere_datastore.datastore.id
+   }
+}`,
+		os.Getenv("VSPHERE_DATACENTER"),
+		os.Getenv("VSPHERE_DATASTORE"),
+		os.Getenv("VSPHERE_CLUSTER"),
+		os.Getenv("VSPHERE_NETWORK_LABEL"))
+}