Add API for instrumentation to learn the starting wall time of bg threads

Summary: Computing things like "page faults since this thread was created" or "cpu time spent on this thread since it was created" is pretty easy - just measure it when you want it.  However, if you want to know "how much wall time has elapsed since this thread was created" you need to record some timing info when the thread is created.  This diff adds a an API for querying that from the RN thread holder abstraction.

Reviewed By: alexeylang

Differential Revision: D13246235

fbshipit-source-id: d36af61dbe27f662980fe508b2644e9d5255bb7e

ReactAndroid/src/main/java/com/facebook/react/bridge/queue/MessageQueueThread.java

@@ -58,4 +58,11 @@ public interface MessageQueueThread {
    */
   @DoNotStrip
   void quitSynchronous();
+
+  /**
+   * Returns the time in milliseconds at which this thread was started.  This
+   * method is intended to be used for instrumentation purposes.
+   */
+  @DoNotStrip
+  long getStartTimeMillis();
 }

ReactAndroid/src/main/java/com/facebook/react/bridge/queue/MessageQueueThreadImpl.java

@@ -10,9 +10,10 @@ package com.facebook.react.bridge.queue;
 import java.util.concurrent.Callable;
 import java.util.concurrent.Future;
 
+import android.os.SystemClock;
 import android.os.Looper;
 import android.os.Process;
-
+import android.util.Pair;
 import com.facebook.common.logging.FLog;
 import com.facebook.proguard.annotations.DoNotStrip;
 import com.facebook.react.bridge.AssertionException;
@@ -31,15 +32,25 @@ public class MessageQueueThreadImpl implements MessageQueueThread {
   private final Looper mLooper;
   private final MessageQueueThreadHandler mHandler;
   private final String mAssertionErrorMessage;
+  private long mStartTimeMillis;
   private volatile boolean mIsFinished = false;
 
   private MessageQueueThreadImpl(
       String name,
       Looper looper,
       QueueThreadExceptionHandler exceptionHandler) {
+        this(name, looper, exceptionHandler, -1);
+  }
+
+  private MessageQueueThreadImpl(
+      String name,
+      Looper looper,
+      QueueThreadExceptionHandler exceptionHandler,
+      long startTimeMillis) {
     mName = name;
     mLooper = looper;
     mHandler = new MessageQueueThreadHandler(looper, exceptionHandler);
+    mStartTimeMillis = startTimeMillis;
     mAssertionErrorMessage = "Expected to be called from the '" + getName() + "' thread!";
   }
 
@@ -126,6 +137,12 @@ public class MessageQueueThreadImpl implements MessageQueueThread {
     }
   }
 
+  @DoNotStrip
+  @Override
+  public long getStartTimeMillis() {
+    return mStartTimeMillis;
+  }
+
   public Looper getLooper() {
     return mLooper;
   }
@@ -180,21 +197,21 @@ public class MessageQueueThreadImpl implements MessageQueueThread {
       final String name,
       long stackSize,
       QueueThreadExceptionHandler exceptionHandler) {
-    final SimpleSettableFuture<Looper> looperFuture = new SimpleSettableFuture<>();
+    final SimpleSettableFuture<Pair<Looper, Long>> dataFuture = new SimpleSettableFuture<>();
+    long startTimeMillis;
     Thread bgThread = new Thread(null,
         new Runnable() {
           @Override
           public void run() {
             Process.setThreadPriority(Process.THREAD_PRIORITY_DISPLAY);
             Looper.prepare();
-
-            looperFuture.set(Looper.myLooper());
+            dataFuture.set(new Pair<>(Looper.myLooper(), SystemClock.uptimeMillis()));
             Looper.loop();
           }
         }, "mqt_" + name, stackSize);
     bgThread.start();
 
-    Looper myLooper = looperFuture.getOrThrow();
-    return new MessageQueueThreadImpl(name, myLooper, exceptionHandler);
+    Pair<Looper, Long> pair = dataFuture.getOrThrow();
+    return new MessageQueueThreadImpl(name, pair.first, exceptionHandler, pair.second);
   }
 }