Enable strict mode for executors.

Any violations would kill the app in debug builds of firebase-common,
and log a warning in release builds. This is done to fail tests that
incorrectly use executors while not affecting 3p apps in release builds.

Additionally correctly set thread priorities in an Android specific way.

Author: vkryachko

firebase-common/src/main/java/com/google/firebase/concurrent/CustomThreadFactory.java

@@ -14,26 +14,38 @@
 
 package com.google.firebase.concurrent;
 
+import android.os.Process;
+import android.os.StrictMode;
 import java.util.Locale;
 import java.util.concurrent.Executors;
 import java.util.concurrent.ThreadFactory;
 import java.util.concurrent.atomic.AtomicLong;
+import javax.annotation.Nullable;
 
 class CustomThreadFactory implements ThreadFactory {
   private static final ThreadFactory DEFAULT = Executors.defaultThreadFactory();
   private final AtomicLong threadCount = new AtomicLong();
   private final String namePrefix;
   private final int priority;
+  private final StrictMode.ThreadPolicy policy;
 
-  CustomThreadFactory(String namePrefix, int priority) {
+  CustomThreadFactory(String namePrefix, int priority, @Nullable StrictMode.ThreadPolicy policy) {
     this.namePrefix = namePrefix;
     this.priority = priority;
+    this.policy = policy;
   }
 
   @Override
   public Thread newThread(Runnable r) {
-    Thread thread = DEFAULT.newThread(r);
-    thread.setPriority(priority);
+    Thread thread =
+        DEFAULT.newThread(
+            () -> {
+              Process.setThreadPriority(priority);
+              if (policy != null) {
+                StrictMode.setThreadPolicy(policy);
+              }
+              r.run();
+            });
     thread.setName(
         String.format(Locale.ROOT, "%s Thread #%d", namePrefix, threadCount.getAndIncrement()));
     return thread;

firebase-common/src/main/java/com/google/firebase/concurrent/ExecutorsRegistrar.java

@@ -15,7 +15,10 @@
 package com.google.firebase.concurrent;
 
 import android.annotation.SuppressLint;
+import android.os.Build;
 import android.os.Process;
+import android.os.StrictMode;
+import com.google.firebase.BuildConfig;
 import com.google.firebase.annotations.concurrent.Background;
 import com.google.firebase.annotations.concurrent.Blocking;
 import com.google.firebase.annotations.concurrent.Lightweight;
@@ -40,15 +43,17 @@ public class ExecutorsRegistrar implements ComponentRegistrar {
           () ->
               scheduled(
                   Executors.newFixedThreadPool(
-                      4, factory("Firebase Background", Process.THREAD_PRIORITY_BACKGROUND))));
+                      4,
+                      factory(
+                          "Firebase Background", Process.THREAD_PRIORITY_BACKGROUND, bgPolicy()))));
 
   private static final Lazy<ScheduledExecutorService> LITE_EXECUTOR =
       new Lazy<>(
           () ->
               scheduled(
                   Executors.newFixedThreadPool(
                       Math.max(2, Runtime.getRuntime().availableProcessors()),
-                      factory("Firebase Lite", Process.THREAD_PRIORITY_DEFAULT))));
+                      factory("Firebase Lite", Process.THREAD_PRIORITY_DEFAULT, litePolicy()))));
 
   private static final Lazy<ScheduledExecutorService> BLOCKING_EXECUTOR =
       new Lazy<>(
@@ -97,6 +102,33 @@ private static ScheduledExecutorService scheduled(ExecutorService delegate) {
   }
 
   private static ThreadFactory factory(String threadPrefix, int priority) {
-    return new CustomThreadFactory(threadPrefix, priority);
+    return new CustomThreadFactory(threadPrefix, priority, null);
+  }
+
+  private static ThreadFactory factory(
+      String threadPrefix, int priority, StrictMode.ThreadPolicy policy) {
+    return new CustomThreadFactory(threadPrefix, priority, policy);
+  }
+
+  private static StrictMode.ThreadPolicy bgPolicy() {
+    StrictMode.ThreadPolicy.Builder builder = new StrictMode.ThreadPolicy.Builder().detectNetwork();
+    if (Build.VERSION.SDK_INT >= 23) {
+      builder.detectResourceMismatches();
+      if (Build.VERSION.SDK_INT >= 26) {
+        builder.detectUnbufferedIo();
+      }
+    }
+    if (BuildConfig.DEBUG) {
+      builder.penaltyDeath();
+    }
+    return builder.penaltyLog().build();
+  }
+
+  private static StrictMode.ThreadPolicy litePolicy() {
+    StrictMode.ThreadPolicy.Builder builder = new StrictMode.ThreadPolicy.Builder().detectAll();
+    if (BuildConfig.DEBUG) {
+      builder.penaltyDeath();
+    }
+    return builder.penaltyLog().build();
   }
 }