1 files changed, 62 insertions, 0 deletions
diff --git a/modules/libpref/test/unit/test_warnings.js b/modules/libpref/test/unit/test_warnings.js
new file mode 100644
index 0000000000..33adb22579
--- /dev/null
+++ b/modules/libpref/test/unit/test_warnings.js
@@ -0,0 +1,62 @@
+/* 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/. */
+
+function makeBuffer(length) {
+  return new Array(length + 1).join("x");
+}
+
+/**
+ * @resolves |true| if execution proceeded without warning,
+ * |false| if there was a warning.
+ */
+function checkWarning(pref, buffer) {
+  return new Promise(resolve => {
+    let complete = false;
+    let listener = {
+      observe(event) {
+        let message = event.message;
+        if (
+          !(
+            message.startsWith("Warning: attempting to write") &&
+            message.includes(pref)
+          )
+        ) {
+          return;
+        }
+        if (complete) {
+          return;
+        }
+        complete = true;
+        info("Warning while setting " + pref);
+        Services.console.unregisterListener(listener);
+        resolve(true);
+      },
+    };
+    do_timeout(1000, function() {
+      if (complete) {
+        return;
+      }
+      complete = true;
+      info("No warning while setting " + pref);
+      Services.console.unregisterListener(listener);
+      resolve(false);
+    });
+    Services.console.registerListener(listener);
+    Services.prefs.setCharPref(pref, buffer);
+  });
+}
+
+add_task(async function() {
+  // Simple change, shouldn't cause a warning
+  info("Checking that a simple change doesn't cause a warning");
+  let buf = makeBuffer(100);
+  let warned = await checkWarning("string.accept", buf);
+  Assert.ok(!warned);
+
+  // Large change, should cause a warning
+  info("Checking that a large change causes a warning");
+  buf = makeBuffer(32 * 1024);
+  warned = await checkWarning("string.warn", buf);
+  Assert.ok(warned);
+});