Add an experiment to launch a spare renderer when a BrowserContext is created on WebView.

Bug: 326578832

Change-Id: Iede803d042c1cecde1656258b64819f8cc4e458d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5318617
Reviewed-by: Richard (Torne) Coles <torne@chromium.org>
Commit-Queue: John Abd-El-Malek <jam@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1265526}

android_webview/browser/aw_browser_context_store.cc

@@ -13,6 +13,7 @@
 #include "android_webview/browser/aw_browser_process.h"
 #include "android_webview/browser_jni_headers/AwBrowserContextStore_jni.h"
 #include "base/check_op.h"
+#include "base/feature_list.h"
 #include "base/files/file_path.h"
 #include "base/files/file_util.h"
 #include "base/memory/raw_ptr.h"
@@ -26,6 +27,7 @@
 #include "components/prefs/pref_service.h"
 #include "components/prefs/scoped_user_pref_update.h"
 #include "content/public/browser/browser_thread.h"
+#include "content/public/browser/render_process_host.h"
 
 namespace android_webview {
 
@@ -36,6 +38,14 @@ constexpr char kProfilePathKey[] = "path";
 
 bool g_initialized = false;
 
+BASE_FEATURE(kCreateSpareRendererOnBrowserContextCreation,
+             "CreateSpareRendererOnBrowserContextCreation",
+             base::FEATURE_DISABLED_BY_DEFAULT);
+
+const base::FeatureParam<bool> kCreateSpareRendereForDefaultIfMultiProfile{
+    &kCreateSpareRendererOnBrowserContextCreation,
+    "create_spare_renderer_for_default_if_multi_profile", true};
+
 }  // namespace
 
 AwBrowserContextStore::AwBrowserContextStore(PrefService* pref_service)
@@ -101,6 +111,16 @@ AwBrowserContext* AwBrowserContextStore::Get(const std::string& name,
     const bool is_default = name == kDefaultContextName;
     entry->instance =
         std::make_unique<AwBrowserContext>(name, entry->path, is_default);
+    // Ensure this code path is only taken if the IO thread is already running,
+    // as it's needed for launching processes.
+    if (base::FeatureList::IsEnabled(
+            kCreateSpareRendererOnBrowserContextCreation) &&
+        content::BrowserThread::IsThreadInitialized(
+            content::BrowserThread::IO) &&
+        (!is_default || kCreateSpareRendereForDefaultIfMultiProfile.Get())) {
+      content::RenderProcessHost::WarmupSpareRenderProcessHost(
+          entry->instance.get());
+    }
   }
   return entry->instance.get();
 }

android_webview/java/src/org/chromium/android_webview/common/ProductionSupportedFlagList.java

@@ -907,6 +907,7 @@ public final class ProductionSupportedFlagList {
                 BlinkFeatures.NO_THROTTLING_VISIBLE_AGENT,
                 "Do not throttle Javascript timers to 1Hz on hidden cross-origin frames that are"
                         + " same-agent with a visible frame."),
+        Flag.baseFeature("CreateSpareRendererOnBrowserContextCreation"),
         // Add new commandline switches and features above. The final entry should have a
         // trailing comma for cleaner diffs.
     };

android_webview/javatests/src/org/chromium/android_webview/test/AwContentsClientOnRenderProcessGoneTest.java

@@ -164,6 +164,7 @@ public class AwContentsClientOnRenderProcessGoneTest extends AwParameterizedTest
 
     @Test
     @Feature({"AndroidWebView"})
+    @CommandLineFlags.Add({"disable-features=CreateSpareRendererOnBrowserContextCreation"})
     @SmallTest
     @OnlyRunIn(MULTI_PROCESS)
     public void testRenderProcessCanNotTerminateBeforeStart() throws Throwable {

android_webview/javatests/src/org/chromium/android_webview/test/AwMetricsIntegrationTest.java

@@ -325,6 +325,7 @@ public class AwMetricsIntegrationTest extends AwParameterizedTest {
     @Test
     @MediumTest
     @Feature({"AndroidWebView"})
+    @CommandLineFlags.Add({"disable-features=CreateSpareRendererOnBrowserContextCreation"})
     public void testMetadata_stability_rendererLaunchCount() throws Throwable {
         EmbeddedTestServer embeddedTestServer =
                 EmbeddedTestServer.createAndStartServer(

content/browser/renderer_host/spare_render_process_host_manager.cc

@@ -40,7 +40,11 @@ void SpareRenderProcessHostManager::WarmupSpareRenderProcessHost(
     return;  // Nothing to warm up.
   }
 
+  bool had_spare_renderer = !!spare_render_process_host_;
   CleanupSpareRenderProcessHost();
+  UMA_HISTOGRAM_BOOLEAN(
+      "BrowserRenderProcessHost.SpareProcessEvictedOtherSpare",
+      had_spare_renderer);
 
   // Don't create a spare renderer for a BrowserContext that is in the
   // process of shutting down.

tools/metrics/histograms/metadata/browser/histograms.xml

@@ -1023,6 +1023,17 @@ chromium-metrics-reviews@google.com.
   </summary>
 </histogram>
 
+<histogram name="BrowserRenderProcessHost.SpareProcessEvictedOtherSpare"
+    enum="Boolean" expires_after="2024-08-30">
+  <owner>jam@chromium.org</owner>
+  <owner>cduvall@chromium.org</owner>
+  <summary>
+    Records true if WarmupSpareRenderProcessHost evicted an existing spare
+    process that had a different BrowserContext, or false if there wasn't an
+    existing spare process.
+  </summary>
+</histogram>
+
 <histogram name="BrowserRenderProcessHost.SpareProcessMaybeTakeAction"
     enum="SpareProcessMaybeTakeAction" expires_after="2024-06-30">
   <owner>alexmos@chromium.org</owner>