Bug 1921759: Restart Windows Explorer at start of browser_bookmark_co…

…py_folder_tree.js a=dmeehan

In a new Windows update, explorer.exe is grabs the clipboard mutex and holds
it for a long time, which is causing test failures.  Killing explorer.exe
restarts it and forces it to give up the mutex, allowing the test to pass.

browser/components/places/tests/browser/browser_bookmark_copy_folder_tree.js

@@ -14,6 +14,33 @@ add_task(async function () {
     await PlacesUtils.bookmarks.eraseEverything();
   });
 
+  if (AppConstants.platform === "win") {
+    if (Services.env.get("MOZ_AUTOMATION")) {
+      // When running in CI, pre-emptively kill Windows Explorer, using system
+      // from the standard library, since it sometimes holds the clipboard for
+      // long periods, thereby breaking the test (bug 1921759).
+      const { ctypes } = ChromeUtils.importESModule(
+        "resource://gre/modules/ctypes.sys.mjs"
+      );
+      let libc = ctypes.open("ucrtbase.dll");
+      let exec = libc.declare(
+        "system",
+        ctypes.default_abi,
+        ctypes.int,
+        ctypes.char.ptr
+      );
+      let rv = exec(
+        '"powershell -command "&{&Stop-Process -ProcessName explorer}"'
+      );
+      libc.close();
+      is(rv, 0, "Launched powershell to stop explorer.exe");
+    } else {
+      info(
+        "Skipping terminating Windows Explorer since we are not running in automation"
+      );
+    }
+  }
+
   await withSidebarTree("bookmarks", async function (tree) {
     const selectedNodeComparator = {
       equalTitle: itemNode => {