Add a finch trial for disabling library prefetching.

We suspect that library prefetching slows startup on low-end devices.
This CL adds an experiment to verify that hypothesis.

Bug: 735799
Change-Id: I5bc969e7d417f07520caef28fc4e0598e545d51b
Reviewed-on: https://chromium-review.googlesource.com/578711
Commit-Queue: Dmitry Skiba <dskiba@chromium.org>
Reviewed-by: Maria Khomenko <mariakhomenko@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Egor Pasko <pasko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#488427}

base/android/java/src/org/chromium/base/library_loader/LibraryLoader.java

@@ -7,6 +7,7 @@ package org.chromium.base.library_loader;
 import android.annotation.SuppressLint;
 import android.content.Context;
 import android.os.AsyncTask;
+import android.os.StrictMode;
 import android.os.SystemClock;
 
 import org.chromium.base.CommandLine;
@@ -48,6 +49,9 @@ public class LibraryLoader {
     // Guards all access to the libraries
     private static final Object sLock = new Object();
 
+    // SharedPreferences key for "don't prefetch libraries" flag
+    private static final String DONT_PREFETCH_LIBRARIES_KEY = "dont_prefetch_libraries";
+
     // The singleton instance of NativeLibraryPreloader.
     private static NativeLibraryPreloader sLibraryPreloader;
 
@@ -193,6 +197,37 @@ public class LibraryLoader {
         }
     }
 
+    /**
+     * Disables prefetching for subsequent runs. The value comes from "DontPrefetchLibraries"
+     * finch experiment, and is pushed on every run. I.e. the effect of the finch experiment
+     * lags by one run, which is the best we can do considering that prefetching happens way
+     * before finch is initialized. Note that since LibraryLoader is in //base, it can't depend
+     * on ChromeFeatureList, and has to rely on external code pushing the value.
+     *
+     * @param dontPrefetch whether not to prefetch libraries
+     */
+    public static void setDontPrefetchLibrariesOnNextRuns(boolean dontPrefetch) {
+        ContextUtils.getAppSharedPreferences()
+                .edit()
+                .putBoolean(DONT_PREFETCH_LIBRARIES_KEY, dontPrefetch)
+                .apply();
+    }
+
+    /**
+     * @return whether not to prefetch libraries (see setDontPrefetchLibrariesOnNextRun()).
+     */
+    private static boolean isNotPrefetchingLibraries() {
+        // This might be the first time getAppSharedPreferences() is used, so relax strict mode
+        // to allow disk reads.
+        StrictMode.ThreadPolicy oldPolicy = StrictMode.allowThreadDiskReads();
+        try {
+            return ContextUtils.getAppSharedPreferences().getBoolean(
+                    DONT_PREFETCH_LIBRARIES_KEY, false);
+        } finally {
+            StrictMode.setThreadPolicy(oldPolicy);
+        }
+    }
+
     /** Prefetches the native libraries in a background thread.
      *
      * Launches an AsyncTask that, through a short-lived forked process, reads a
@@ -203,6 +238,8 @@ public class LibraryLoader {
      * detrimental to the startup time.
      */
     public void asyncPrefetchLibrariesToMemory() {
+        if (isNotPrefetchingLibraries()) return;
+
         final boolean coldStart = mPrefetchLibraryHasBeenCalled.compareAndSet(false, true);
         new AsyncTask<Void, Void, Void>() {
             @Override

chrome/android/java/src/org/chromium/chrome/browser/ChromeFeatureList.java

@@ -177,6 +177,7 @@ public abstract class ChromeFeatureList {
     // Android.
     public static final String DATA_REDUCTION_MAIN_MENU = "DataReductionProxyMainMenu";
     public static final String DATA_REDUCTION_SITE_BREAKDOWN = "DataReductionProxySiteBreakdown";
+    public static final String DONT_PREFETCH_LIBRARIES = "DontPrefetchLibraries";
     public static final String DOWNLOAD_HOME_SHOW_STORAGE_INFO = "DownloadHomeShowStorageInfo";
     // When enabled, fullscreen WebContents will be moved to a new Activity. Coming soon...
     public static final String FULLSCREEN_ACTIVITY = "FullscreenActivity";

chrome/android/java/src/org/chromium/chrome/browser/util/FeatureUtilities.java

@@ -23,6 +23,7 @@ import org.chromium.base.FieldTrialList;
 import org.chromium.base.Log;
 import org.chromium.base.ThreadUtils;
 import org.chromium.base.VisibleForTesting;
+import org.chromium.base.library_loader.LibraryLoader;
 import org.chromium.chrome.browser.ChromeFeatureList;
 import org.chromium.chrome.browser.ChromeSwitches;
 import org.chromium.chrome.browser.firstrun.FirstRunGlueImpl;
@@ -188,6 +189,12 @@ public class FeatureUtilities {
         cacheChromeHomeEnabled();
         FirstRunGlueImpl.cacheFirstRunPrefs();
         OmniboxPlaceholderFieldTrial.cacheOmniboxPlaceholderGroup();
+
+        // Propagate DONT_PREFETCH_LIBRARIES feature value to LibraryLoader. This can't
+        // be done in LibraryLoader itself because it lives in //base and can't depend
+        // on ChromeFeatureList.
+        LibraryLoader.setDontPrefetchLibrariesOnNextRuns(
+                ChromeFeatureList.isEnabled(ChromeFeatureList.DONT_PREFETCH_LIBRARIES));
     }
 
     /**

chrome/browser/about_flags.cc

@@ -3240,6 +3240,12 @@ const FeatureEntry kFeatureEntries[] = {
      SINGLE_VALUE_TYPE(chromeos::switches::kEnableLockScreenApps)},
 #endif  // defined(OS_CHROMEOS)
 
+#if defined(OS_ANDROID)
+    {"dont-prefetch-libraries", flag_descriptions::kDontPrefetchLibrariesName,
+     flag_descriptions::kDontPrefetchLibrariesDescription, kOsAndroid,
+     FEATURE_VALUE_TYPE(chrome::android::kDontPrefetchLibraries)},
+#endif  // defined(OS_ANDROID)
+
     // NOTE: Adding new command-line switches requires adding corresponding
     // entries to enum "LoginCustomFlags" in histograms/enums.xml. See note in
     // enums.xml and don't forget to run AboutFlagsHistogramTest unit test.

chrome/browser/android/chrome_feature_list.cc

@@ -67,6 +67,7 @@ const base::Feature* kFeaturesExposedToJava[] = {
     &kContextualSuggestionsCarousel,
     &kCustomContextMenu,
     &kCustomFeedbackUi,
+    &kDontPrefetchLibraries,
     &kDownloadHomeShowStorageInfo,
     &data_reduction_proxy::features::kDataReductionMainMenu,
     &data_reduction_proxy::features::kDataReductionSiteBreakdown,
@@ -179,6 +180,9 @@ const base::Feature kCustomContextMenu{"CustomContextMenu",
 const base::Feature kCustomFeedbackUi{"CustomFeedbackUi",
                                       base::FEATURE_DISABLED_BY_DEFAULT};
 
+const base::Feature kDontPrefetchLibraries{"DontPrefetchLibraries",
+                                           base::FEATURE_DISABLED_BY_DEFAULT};
+
 const base::Feature kDownloadAutoResumptionThrottling{
     "DownloadAutoResumptionThrottling", base::FEATURE_ENABLED_BY_DEFAULT};
 

chrome/browser/android/chrome_feature_list.h

@@ -32,6 +32,7 @@ extern const base::Feature kContextualSearchUrlActions;
 extern const base::Feature kContextualSuggestionsCarousel;
 extern const base::Feature kCustomContextMenu;
 extern const base::Feature kCustomFeedbackUi;
+extern const base::Feature kDontPrefetchLibraries;
 extern const base::Feature kDownloadAutoResumptionThrottling;
 extern const base::Feature kDownloadHomeShowStorageInfo;
 extern const base::Feature kFullscreenActivity;

chrome/browser/flag_descriptions.cc

@@ -1455,6 +1455,10 @@ const char kContextualSearchName[] = "Contextual Search";
 const char kContextualSearchDescription[] =
     "Whether or not Contextual Search is enabled.";
 
+const char kDontPrefetchLibrariesName[] = "Don't Prefetch Libraries";
+const char kDontPrefetchLibrariesDescription[] =
+    "Don't prefetch libraries after loading.";
+
 const char kEnableAndroidPayIntegrationV1Name[] = "Enable Android Pay v1";
 const char kEnableAndroidPayIntegrationV1Description[] =
     "Enable integration with Android Pay using the first version of the API";

chrome/browser/flag_descriptions.h

@@ -902,6 +902,9 @@ extern const char kContextualSearchUrlActionsDescription[];
 extern const char kContextualSearchName[];
 extern const char kContextualSearchDescription[];
 
+extern const char kDontPrefetchLibrariesName[];
+extern const char kDontPrefetchLibrariesDescription[];
+
 extern const char kEnableAndroidPayIntegrationV1Name[];
 extern const char kEnableAndroidPayIntegrationV1Description[];
 

tools/metrics/histograms/enums.xml

@@ -22731,6 +22731,7 @@ uploading your change for review.  These are checked by presubmit scripts.
   <int value="-1578295753" label="UserMediaScreenCapturing:disabled"/>
   <int value="-1575984706" label="gl-composited-overlay-candidate-quad-border"/>
   <int value="-1575554415" label="AndroidPaymentAppsFilter:enabled"/>
+  <int value="-1575430234" label="DontPrefetchLibraries:disabled"/>
   <int value="-1572010356" label="enable-privet-v3"/>
   <int value="-1571841513" label="enable-devtools-experiments"/>
   <int value="-1559789642" label="RunAllFlashInAllowMode:enabled"/>
@@ -23351,6 +23352,7 @@ from previous Chrome versions.
   <int value="575394365" label="AndroidPaymentApps:disabled"/>
   <int value="581118445" label="enable-eol-notification"/>
   <int value="581355159" label="ContentSuggestionsCategoryRanker:disabled"/>
+  <int value="582187448" label="DontPrefetchLibraries:enabled"/>
   <int value="584541349" label="ContextualSearchSingleActions:disabled"/>
   <int value="586021329" label="VibrateRequiresUserGesture:enabled"/>
   <int value="592050831" label="disable-slimming-paint"/>