Windows: mark downloads as "from the internet"

ts/util/windowsZoneIdentifier.ts

@@ -0,0 +1,45 @@
+import * as fs from 'fs';
+import { isWindows } from '../OS';
+
+const ZONE_IDENTIFIER_CONTENTS = Buffer.from('[ZoneTransfer]\r\nZoneId=3');
+
+/**
+ * Internet Explorer introduced the concept of "Security Zones". For our purposes, we
+ * just need to set the security zone to the "Internet" zone, which Windows will use to
+ * offer some protections. This is customizable by the user (or, more likely, by IT).
+ *
+ * To do this, we write the "Zone.Identifier" for the NTFS alternative data stream.
+ *
+ * This can fail in a bunch of sitations:
+ *
+ * - The OS is not Windows.
+ * - The filesystem is not NTFS.
+ * - Writing the metadata file fails for some reason (permissions, for example).
+ * - The metadata file already exists. (We could choose to overwrite it.)
+ * - The original file is deleted between the time that we check for its existence and
+ *   when we write the metadata. This is a rare race condition, but is possible.
+ *
+ * Consumers of this module should probably tolerate failures.
+ */
+export async function writeWindowsZoneIdentifier(
+  filePath: string
+): Promise<void> {
+  if (!isWindows()) {
+    throw new Error('writeWindowsZoneIdentifier should only run on Windows');
+  }
+
+  // tslint:disable-next-line non-literal-fs-path
+  if (!fs.existsSync(filePath)) {
+    throw new Error(
+      'writeWindowsZoneIdentifier could not find the original file'
+    );
+  }
+
+  await fs.promises.writeFile(
+    `${filePath}:Zone.Identifier`,
+    ZONE_IDENTIFIER_CONTENTS,
+    {
+      flag: 'wx',
+    }
+  );
+}