webview: metric for UI thread chosen in old apps.

Apps that target pre-JB MR2 have relaxed threading requirements for
WebView. Add a metric that tracks whether deferring the decision of
which thread is the UI thread was actually necessary or not.

This may help us determine how widely this compatibility behaviour is
still relied upon.

Bug: 1106486
Change-Id: Ia0b56930f491d3dd2d42f6cc3197aa1eb23311e1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2303771
Commit-Queue: Richard Coles <[email protected]>
Reviewed-by: Changwan Ryu <[email protected]>
Reviewed-by: Brian White <[email protected]>
Auto-Submit: Richard Coles <[email protected]>
Cr-Commit-Position: refs/heads/master@{#814200}

android_webview/glue/java/src/com/android/webview/chromium/WebViewChromium.java

@@ -306,6 +306,10 @@ public void init(final Map<String, Object> javaScriptInterfaces,
             } else if (!mFactory.hasStarted()) {
                 if (Looper.myLooper() == Looper.getMainLooper()) {
                     mFactory.startYourEngines(true);
+                } else {
+                    // Record which thread we're on now so we can track whether the final UI thread
+                    // decision differed.
+                    mFactory.getAwInit().setFirstWebViewConstructedOn(Looper.myLooper());
                 }
             }
 

android_webview/glue/java/src/com/android/webview/chromium/WebViewChromiumAwInit.java

@@ -16,6 +16,8 @@
 import android.webkit.WebStorage;
 import android.webkit.WebViewDatabase;
 
+import androidx.annotation.IntDef;
+
 import org.chromium.android_webview.AwBrowserContext;
 import org.chromium.android_webview.AwBrowserProcess;
 import org.chromium.android_webview.AwContents;
@@ -88,6 +90,7 @@ public class WebViewChromiumAwInit {
 
     // Read/write protected by mLock.
     private boolean mStarted;
+    private Looper mFirstWebViewConstructedOn;
 
     private final WebViewChromiumFactoryProvider mFactory;
 
@@ -243,6 +246,32 @@ private void setUpResources(int packageId, Context context) {
         }
     }
 
+    // Only called for apps which target <JB MR2, and which construct WebView on a non-main thread.
+    void setFirstWebViewConstructedOn(Looper looper) {
+        synchronized (mLock) {
+            if (!mStarted && mFirstWebViewConstructedOn == null) {
+                mFirstWebViewConstructedOn = looper;
+            }
+        }
+    }
+
+    // Used to record the UMA histogram Android.WebView.ActualUiThread. Since these values are
+    // persisted to logs, they should never be renumbered or reused.
+    @IntDef({ActualUiThread.FIRST_WEBVIEW_CONSTRUCTED, ActualUiThread.MAIN_LOOPER,
+            ActualUiThread.OTHER})
+    @interface ActualUiThread {
+        int FIRST_WEBVIEW_CONSTRUCTED = 0;
+        int MAIN_LOOPER = 1;
+        int OTHER = 2;
+
+        int COUNT = 3;
+    }
+
+    private static void recordActualUiThread(@ActualUiThread int value) {
+        RecordHistogram.recordEnumeratedHistogram(
+                "Android.WebView.ActualUiThread", value, ActualUiThread.COUNT);
+    }
+
     boolean hasStarted() {
         return mStarted;
     }
@@ -268,6 +297,23 @@ void startYourEngines(boolean onMainThread) {
                         + " looper " + looper);
         ThreadUtils.setUiThread(looper);
 
+        // For apps targeting <JBMR2 which aren't required to commit to a thread in
+        // WebViewChromium.init, record a metric stating which thread we picked.
+        if (mFirstWebViewConstructedOn != null) {
+            if (looper == mFirstWebViewConstructedOn) {
+                // Using the same thread that the first WebView was constructed on.
+                recordActualUiThread(ActualUiThread.FIRST_WEBVIEW_CONSTRUCTED);
+            } else if (looper == Looper.getMainLooper()) {
+                // Using the main looper.
+                recordActualUiThread(ActualUiThread.MAIN_LOOPER);
+            } else {
+                // Using some other thread.
+                recordActualUiThread(ActualUiThread.OTHER);
+            }
+            // Reset to null to avoid leaking the app's looper.
+            mFirstWebViewConstructedOn = null;
+        }
+
         if (ThreadUtils.runningOnUiThread()) {
             startChromiumLocked();
             return;

tools/metrics/histograms/enums.xml

@@ -2030,6 +2030,12 @@ Unknown properties are collapsed to zero. -->
   <int value="4" label="Undos Dismissed (Action) (Deprecated)"/>
 </enum>
 
+<enum name="AndroidWebViewActualUiThread">
+  <int value="0" label="The thread on which the first WebView was constructed"/>
+  <int value="1" label="The main looper"/>
+  <int value="2" label="Some other thread"/>
+</enum>
+
 <enum name="AndroidWebViewClientSafeBrowsingThreatType">
   <obsolete>
     Removed 04/2020. These metrics are tracked generally for SafeBrowsing, and

tools/metrics/histograms/histograms_xml/android/histograms.xml

@@ -2056,6 +2056,17 @@ reviews. Googlers can read more about this at go/gwsq-gerrit.
   </summary>
 </histogram>
 
+<histogram name="Android.WebView.ActualUiThread"
+    enum="AndroidWebViewActualUiThread" expires_after="2021-03-31">
+  <owner>[email protected]</owner>
+  <owner>src/android_webview/OWNERS</owner>
+  <summary>
+    Recorded once per startup for apps targeting releases before JB MR2 if they
+    create their first WebView off the main thread. This records which thread
+    they actually ended up using as their UI thread.
+  </summary>
+</histogram>
+
 <histogram name="Android.WebView.AndroidProtocolHandler.ResourceGetIdentifier"
     enum="BooleanSuccess" expires_after="2021-01-30">
   <owner>[email protected]</owner>