Bug 1858082 - Wrap the drag service getter into a try block r=whimboo,webdriver-reviewers

The drag service is never instantiated if it's in the headless mode.
And in this case, `NS_ERROR_XPC_GS_RETURNED_FAILURE` exception is thrown.
Therefore, we need to wrap the service getter with a `try` block and
return `null` if it fails.

Differential Revision: https://phabricator.services.mozilla.com/D190580

remote/marionette/event.sys.mjs

@@ -16,9 +16,6 @@ export const event = {};
 ChromeUtils.defineLazyGetter(lazy, "dblclickTimer", () => {
   return Cc["@mozilla.org/timer;1"].createInstance(Ci.nsITimer);
 });
-ChromeUtils.defineLazyGetter(event, "dragService", () => {
-  return Cc["@mozilla.org/widget/dragservice;1"].getService(Ci.nsIDragService);
-});
 
 const _eventUtils = new WeakMap();
 

remote/marionette/interaction.sys.mjs

@@ -23,6 +23,22 @@ ChromeUtils.defineLazyGetter(lazy, "logger", () =>
   lazy.Log.get(lazy.Log.TYPES.MARIONETTE)
 );
 
+// dragService may be null if it's in the headless mode (e.g., on Linux).
+// It depends on the platform, though.
+ChromeUtils.defineLazyGetter(lazy, "dragService", () => {
+  try {
+    return Cc["@mozilla.org/widget/dragservice;1"].getService(
+      Ci.nsIDragService
+    );
+  } catch (e) {
+    // If we're in the headless mode, the drag service may be never
+    // instantiated.  In this case, an exception is thrown.  Let's ignore
+    // any exceptions since without the drag service, nobody can create a
+    // drag session.
+    return null;
+  }
+});
+
 /** XUL elements that support disabled attribute. */
 const DISABLED_ATTRIBUTE_SUPPORTED_XUL = new Set([
   "ARROWSCROLLBOX",
@@ -185,7 +201,7 @@ async function webdriverClickElement(el, a11y) {
       win
     );
 
-    if (lazy.event.dragService.getCurrentSession()) {
+    if (lazy.dragService?.getCurrentSession()) {
       // Special handling is required if the mousemove started a drag session.
       // In this case, mousedown event shouldn't be fired, and the mouseup should
       // end the session.  Therefore, we should synthesize only mouseup.

testing/mochitest/tests/SimpleTest/EventUtils.js

@@ -579,10 +579,23 @@ function synthesizeTouch(aTarget, aOffsetX, aOffsetY, aEvent, aWindow) {
   );
 }
 
+/**
+ * Return the drag service.  Note that if we're in the headless mode, this
+ * may return null because the service may be never instantiated (e.g., on
+ * Linux).
+ */
 function getDragService() {
-  return _EU_Cc["@mozilla.org/widget/dragservice;1"].getService(
-    _EU_Ci.nsIDragService
-  );
+  try {
+    return _EU_Cc["@mozilla.org/widget/dragservice;1"].getService(
+      _EU_Ci.nsIDragService
+    );
+  } catch (e) {
+    // If we're in the headless mode, the drag service may be never
+    // instantiated.  In this case, an exception is thrown.  Let's ignore
+    // any exceptions since without the drag service, nobody can create a
+    // drag session.
+    return null;
+  }
 }
 
 /**