Bug 1722718 - [xpconnect] Allow Javascript to print to stderr. r=kmag

Differential Revision: https://phabricator.services.mozilla.com/D121094

js/xpconnect/idl/xpccomponents.idl

@@ -101,8 +101,13 @@ interface nsIScheduledGCCallback : nsISupports
 [scriptable, builtinclass, uuid(86003fe3-ee9a-4620-91dc-eef8b1e58815)]
 interface nsIXPCComponents_Utils : nsISupports
 {
+    /*
+     * Prints the provided message to stderr.
+     */
+    void printStderr(in AUTF8String message);
 
-    /* reportError is designed to be called from JavaScript only.
+    /*
+     * reportError is designed to be called from JavaScript only.
      *
      * It will report a JS Error object to the JS console, and return. It
      * is meant for use in exception handler blocks which want to "eat"

js/xpconnect/src/XPCComponents.cpp

@@ -1317,6 +1317,12 @@ nsXPCComponents_Utils::CreateServicesCache(JSContext* aCx,
   return NS_ERROR_FAILURE;
 }
 
+NS_IMETHODIMP
+nsXPCComponents_Utils::PrintStderr(const nsACString& message) {
+  printf_stderr("%s", PromiseFlatUTF8String(message).get());
+  return NS_OK;
+}
+
 NS_IMETHODIMP
 nsXPCComponents_Utils::ReportError(HandleValue error, HandleValue stack,
                                    JSContext* cx) {

js/xpconnect/tests/unit/test_print_stderr.js

@@ -0,0 +1,14 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+add_task(async function() {
+  Assert.throws(
+    () => Cu.printStderr(),
+    /Not enough arguments/,
+    "Without argument printStderr throws"
+  );
+
+  const types = ["", "foo", 42, true, null, {foo: "bar"}];
+  types.forEach(type => Cu.printStderr(type));
+});

js/xpconnect/tests/unit/xpcshell.ini

@@ -159,3 +159,4 @@ head = head_watchdog.js
 [test_wrapped_js_enumerator.js]
 [test_uawidget_scope.js]
 [test_uninitialized_lexical.js]
+[test_print_stderr.js]