Wait for RELRO before starting WebViewZygote at boot.

We start WebViewZygote at boot time to ensure the first app to use
WebView doesn't have to wait, but we were not waiting for RELRO creation
to finish before doing so. Move the background task which starts the
zygote to WebViewUpdateServiceImpl and have it wait for the RELRO first
to ensure that we get the memory saving of RELRO sharing whenever
possible, instead of only when the timing happens to work out by chance.

Fixes: 130305037
Test: manual, check logs and relro sharing status after boot
Change-Id: I55c3f80b0db1dc82727b90c70f777618ca77a942

core/java/android/webkit/WebViewZygote.java

@@ -17,7 +17,6 @@
 package android.webkit;
 
 import android.content.pm.PackageInfo;
-import android.os.AsyncTask;
 import android.os.Build;
 import android.os.ChildZygoteProcess;
 import android.os.Process;
@@ -81,17 +80,9 @@ public class WebViewZygote {
         synchronized (sLock) {
             sMultiprocessEnabled = enabled;
 
-            // When toggling between multi-process being on/off, start or stop the
-            // zygote. If it is enabled and the zygote is not yet started, launch it.
-            // Otherwise, kill it. The name may be null if the package information has
-            // not yet been resolved.
-            if (enabled) {
-                // Run on a background thread as this waits for the zygote to start and we don't
-                // want to block the caller on this. It's okay if this is delayed as anyone trying
-                // to use the zygote will call it first anyway.
-                AsyncTask.THREAD_POOL_EXECUTOR.execute(WebViewZygote::getProcess);
-            } else {
-                // No need to run this in the background, it's very brief.
+            // When multi-process is disabled, kill the zygote. When it is enabled,
+            // the zygote will be started when it is first needed in getProcess().
+            if (!enabled) {
                 stopZygoteLocked();
             }
         }

services/core/java/com/android/server/webkit/SystemImpl.java

@@ -265,6 +265,11 @@ public class SystemImpl implements SystemInterface {
         WebViewZygote.setMultiprocessEnabled(enableMultiProcess);
     }
 
+    @Override
+    public void ensureZygoteStarted() {
+        WebViewZygote.getProcess();
+    }
+
     @Override
     public boolean isMultiProcessDefaultEnabled() {
         // Multiprocess is enabled for all 64-bit devices, since the ability to run the renderer

services/core/java/com/android/server/webkit/SystemInterface.java

@@ -61,5 +61,7 @@ public interface SystemInterface {
     public int getMultiProcessSetting(Context context);
     public void setMultiProcessSetting(Context context, int value);
     public void notifyZygote(boolean enableMultiProcess);
+    /** Start the zygote if it's not already running. */
+    public void ensureZygoteStarted();
     public boolean isMultiProcessDefaultEnabled();
 }

services/core/java/com/android/server/webkit/WebViewUpdateServiceImpl.java

@@ -17,6 +17,7 @@ package com.android.server.webkit;
 
 import android.content.Context;
 import android.content.pm.PackageInfo;
+import android.os.AsyncTask;
 import android.os.UserHandle;
 import android.webkit.WebViewProviderInfo;
 import android.webkit.WebViewProviderResponse;
@@ -81,6 +82,14 @@ public class WebViewUpdateServiceImpl {
         migrateFallbackStateOnBoot();
         mWebViewUpdater.prepareWebViewInSystemServer();
         mSystemInterface.notifyZygote(isMultiProcessEnabled());
+        AsyncTask.THREAD_POOL_EXECUTOR.execute(this::startZygoteWhenReady);
+    }
+
+    void startZygoteWhenReady() {
+        // Wait on a background thread for RELRO creation to be done. We ignore the return value
+        // because even if RELRO creation failed we still want to start the zygote.
+        waitForAndGetProvider();
+        mSystemInterface.ensureZygoteStarted();
     }
 
     void handleNewUser(int userId) {

services/tests/servicestests/src/com/android/server/webkit/TestSystemImpl.java

@@ -184,6 +184,9 @@ public class TestSystemImpl implements SystemInterface {
     @Override
     public void notifyZygote(boolean enableMultiProcess) {}
 
+    @Override
+    public void ensureZygoteStarted() {}
+
     @Override
     public boolean isMultiProcessDefaultEnabled() {
         return mMultiProcessDefault;