1 files changed, 109 insertions, 0 deletions
diff --git a/tools/profiler/tests/browser/browser_test_feature_preferencereads.js b/tools/profiler/tests/browser/browser_test_feature_preferencereads.js
new file mode 100644
index 0000000000..ff1b869a47
--- /dev/null
+++ b/tools/profiler/tests/browser/browser_test_feature_preferencereads.js
@@ -0,0 +1,109 @@
+/* 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/. */
+
+requestLongerTimeout(10);
+
+function countDpiPrefReadsInThread(thread) {
+  let count = 0;
+  for (let payload of getPayloadsOfType(thread, "PreferenceRead")) {
+    if (payload.prefName === "layout.css.dpi") {
+      count++;
+    }
+  }
+  return count;
+}
+
+async function waitForPaintAfterLoad() {
+  return SpecialPowers.spawn(gBrowser.selectedBrowser, [], () => {
+    return new Promise(function(resolve) {
+      function listener() {
+        if (content.document.readyState == "complete") {
+          content.requestAnimationFrame(() => content.setTimeout(resolve, 0));
+        }
+      }
+      if (content.document.readyState != "complete") {
+        content.document.addEventListener("readystatechange", listener);
+      } else {
+        listener();
+      }
+    });
+  });
+}
+
+/**
+ * Test the PreferenceRead feature.
+ */
+add_task(async function test_profile_feature_preferencereads() {
+  if (!AppConstants.MOZ_GECKO_PROFILER) {
+    return;
+  }
+  Assert.ok(
+    !Services.profiler.IsActive(),
+    "The profiler is not currently active"
+  );
+
+  startProfiler({ features: ["threads", "leaf", "preferencereads"] });
+
+  const url = BASE_URL + "fixed_height.html";
+  await BrowserTestUtils.withNewTab(url, async contentBrowser => {
+    const contentPid = await SpecialPowers.spawn(
+      contentBrowser,
+      [],
+      () => Services.appinfo.processID
+    );
+
+    await waitForPaintAfterLoad();
+
+    // Check that some PreferenceRead profile markers were generated when the
+    // feature is enabled.
+    {
+      const { contentThread } = await stopProfilerNowAndGetThreads(contentPid);
+
+      const timesReadDpiInContent = countDpiPrefReadsInThread(contentThread);
+
+      Assert.greater(
+        timesReadDpiInContent,
+        0,
+        "PreferenceRead profile markers for layout.css.dpi were recorded " +
+          "when the PreferenceRead feature was turned on."
+      );
+    }
+
+    startProfiler({ features: ["threads", "leaf"] });
+    // Now reload the tab with a clean run.
+    await ContentTask.spawn(contentBrowser, null, () => {
+      return new Promise(resolve => {
+        addEventListener("pageshow", () => resolve(), {
+          capturing: true,
+          once: true,
+        });
+        content.location.reload();
+      });
+    });
+
+    await waitForPaintAfterLoad();
+
+    // Check that no PreferenceRead markers were recorded when the feature
+    // is turned off.
+    {
+      const {
+        parentThread,
+        contentThread,
+      } = await stopProfilerNowAndGetThreads(contentPid);
+      Assert.equal(
+        getPayloadsOfType(parentThread, "PreferenceRead").length,
+        0,
+        "No PreferenceRead profile markers for layout.css.dpi were recorded " +
+          "when the PreferenceRead feature was turned on."
+      );
+
+      Assert.equal(
+        getPayloadsOfType(contentThread, "PreferenceRead").length,
+        0,
+        "No PreferenceRead profile markers for layout.css.dpi were recorded " +
+          "when the PreferenceRead feature was turned on."
+      );
+    }
+  });
+});