position is now optional for vm.attachDisk().

src/api/vm.coffee

@@ -842,32 +842,16 @@ exports.import = import_
 # FIXME: if position is used, all other disks after this position
 # should be shifted.
 attachDisk = $coroutine ({vm, vdi, position, mode, bootable}) ->
-  xapi = @getXAPI vm
-
-  VBD_ref = $wait xapi.call 'VBD.create', {
-    VM: vm.ref
-    VDI: vdi.ref
-    mode: mode
-    type: 'Disk'
-    userdevice: position
-    bootable: bootable ? false
-    empty: false
-    other_config: {}
-    qos_algorithm_type: ''
-    qos_algorithm_params: {}
-  }
-
-  $wait xapi.call 'VBD.plug', VBD_ref
-
-  return true
+  $wait @getXAPI(vm).attachVdiToVm(vdi.id, vm.id, {bootable, mode, position})
+  return
 
 attachDisk.params = {
   bootable: {
     type: 'boolean'
     optional: true
   }
-  mode: { type: 'string' }
-  position: { type: 'string' }
+  mode: { type: 'string', optional: true }
+  position: { type: 'string', optional: true }
   vdi: { type: 'string' }
   vm: { type: 'string' }
 }

src/xapi.js

@@ -496,6 +496,41 @@ export default class Xapi extends XapiBase {
     )
   }
 
+  async attachVdiToVm (vdiId, vmId, {
+    bootable = false,
+    mode = 'RW',
+    position
+  } = {}) {
+    const vdi = this.getObject(vdiId)
+    const vm = this.getObject(vmId)
+
+    if (position == null) {
+      forEach(vm.$VBDs, vbd => {
+        const curPos = +vbd.userdevice
+        if (!(position > curPos)) {
+          position = curPos
+        }
+      })
+
+      position = position == null ? 0 : position + 1
+    }
+
+    const vbdRef = await this.call('VBD.create', {
+      bootable,
+      empty: false,
+      mode,
+      other_config: {},
+      qos_algorithm_params: {},
+      qos_algorithm_type: '',
+      type: 'Disk',
+      userdevice: String(position),
+      VDI: vdi.$ref,
+      VM: vm.$ref
+    })
+
+    await this.call('VBD.plug', vbdRef)
+  }
+
   // =================================================================
 
   async createVirtualInterface (vmId, networkId, {