Bug 1732875 - Call the ExperimentUserAgentUpdated() handler once direcly in nsHttpHandler::Init(). r=cpeterson,necko-reviewers,valentin

This is needed to make sure the experimental User Agent string is applied immediately after starting the browser, not just whenever the Nimbus data changes as part of the NimbusFeatures::OnUpdate handler.

Differential Revision: https://phabricator.services.mozilla.com/D126774

browser/components/resistfingerprinting/moz.build

@@ -9,6 +9,7 @@ with Files("**"):
 
 BROWSER_CHROME_MANIFESTS += [
     "test/browser/browser.ini",
+    "test/browser/experiment-useragent/browser.ini",
 ]
 
 MOCHITEST_MANIFESTS += [

browser/components/resistfingerprinting/test/browser/experiment-useragent/browser.ini

@@ -0,0 +1,9 @@
+[DEFAULT]
+support-files =
+  ../browser_navigator_header.sjs
+prefs =
+  nimbus.syncdatastore.firefox100.firefoxVersion=100
+
+[browser_experiment_useragent_startup.js]
+skip-if = os == "android"
+https_first_disabled = true

browser/components/resistfingerprinting/test/browser/experiment-useragent/browser_experiment_useragent_startup.js

@@ -0,0 +1,25 @@
+add_task(async function firefoxVersionExperimentOnStartup() {
+  const BASE =
+    "http://mochi.test:8888/browser/browser/components/resistfingerprinting/test/browser/";
+  const TEST_TARGET_URL = `${BASE}browser_navigator_header.sjs?`;
+  let tab = await BrowserTestUtils.openNewForegroundTab(
+    gBrowser,
+    TEST_TARGET_URL
+  );
+
+  let result = await SpecialPowers.spawn(tab.linkedBrowser, [], function() {
+    return content.document.body.textContent;
+  });
+
+  ok(
+    result.includes("rv:100.0"),
+    "User-Agent string does include the rv:100.0 segment"
+  );
+
+  ok(
+    result.includes("Firefox/100.0"),
+    "User-Agent string does include the Firefox/100.0 segment"
+  );
+
+  BrowserTestUtils.removeTab(tab);
+});

netwerk/protocol/http/nsHttpHandler.cpp

@@ -393,6 +393,9 @@ nsresult nsHttpHandler::Init() {
   // monitor Firefox Version Experiment enrollment
   NimbusFeatures::OnUpdate(UA_EXPERIMENT_NAME, UA_EXPERIMENT_VAR,
                            ExperimentUserAgentUpdated, &mExperimentUserAgent);
+
+  // Load the experiment state once for startup
+  ExperimentUserAgentUpdated("", &mExperimentUserAgent);
 #endif  // MOZ_BUILD_APP_IS_BROWSER && !ANDROID
 
   Telemetry::ScalarSet(Telemetry::ScalarID::NETWORKING_HTTP3_ENABLED,