1 files changed, 60 insertions, 0 deletions
diff --git a/testing/web-platform/tests/preload/resources/preload_helper.js b/testing/web-platform/tests/preload/resources/preload_helper.js
new file mode 100644
index 0000000000..5b7a6eb52b
--- /dev/null
+++ b/testing/web-platform/tests/preload/resources/preload_helper.js
@@ -0,0 +1,60 @@
+function stashPutUrl(token) {
+  return `/preload/resources/stash-put.py?key=${token}`;
+}
+
+function encodedStashPutUrl(token) {
+  return encodeURIComponent(stashPutUrl(token));
+}
+
+async function hasArrivedAtServer(token) {
+  const res = await fetch(`/preload/resources/stash-take.py?key=${token}`);
+  assert_true(res.status === 200 || res.status === 404,
+              'status must be either 200 or 404');
+  return res.status === 200;
+}
+
+function verifyPreloadAndRTSupport()
+{
+    var link = window.document.createElement("link");
+    assert_true(link.relList && link.relList.supports("preload"), "Preload not supported");
+    assert_true(!!window.PerformanceResourceTiming, "ResourceTiming not supported");
+}
+
+function getAbsoluteURL(url)
+{
+    return new URL(url, location.href).href;
+}
+
+function verifyNumberOfResourceTimingEntries(url, number)
+{
+    assert_equals(numberOfResourceTimingEntries(url), number, url);
+}
+
+function numberOfResourceTimingEntries(url)
+{
+    return performance.getEntriesByName(getAbsoluteURL(url)).length;
+}
+
+// Verifies that the resource is loaded, but not downloaded from network
+// more than once. This can be used to verify that a preloaded resource is
+// not downloaded again when used.
+function verifyLoadedAndNoDoubleDownload(url) {
+    var entries = performance.getEntriesByName(getAbsoluteURL(url));
+    // UA may create separate RT entries for preload and normal load,
+    // so we just check (entries.length > 0).
+    assert_greater_than(entries.length, 0, url + ' should be loaded');
+
+    var numDownloads = 0;
+    entries.forEach(entry => {
+        // transferSize is zero if the resource is loaded from cache.
+        if (entry.transferSize > 0) {
+            numDownloads++;
+        }
+    });
+    // numDownloads can be zero if the resource was already cached before running
+    // the test (for example, when the test is running repeatedly without
+    // clearing cache between runs).
+    assert_less_than_equal(
+        numDownloads, 1,
+        url + ' should be downloaded from network at most once');
+}