Allow direct executors in cronet.

components/cronet/android/api/src/org/chromium/net/DirectPreventingExecutor.java

+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+package org.chromium.net;
+
+import java.util.concurrent.Executor;
+
+/**
+ * Executor that detects and throws if its mDelegate runs a submitted runnable inline.
+ */
+final class DirectPreventingExecutor implements Executor {
+    private final Executor mDelegate;
+
+    DirectPreventingExecutor(Executor delegate) {
+        this.mDelegate = delegate;
+    }
+
+    @Override
+    public void execute(Runnable command) {
+        Thread currentThread = Thread.currentThread();
+        InlineCheckingRunnable runnable = new InlineCheckingRunnable(command, currentThread);
+        mDelegate.execute(runnable);
+        if (runnable.mExecutedInline != null) {
+            throw runnable.mExecutedInline;
+        } else {
+            // It's possible that this method is being called on an executor, and the runnable that
+            // was just queued will run on this thread after the current runnable returns. By
+            // nulling out the mCallingThread field, the InlineCheckingRunnable's current thread
+            // comparison will not fire.
+            runnable.mCallingThread = null;
+        }
+    }
+
+    private static final class InlineCheckingRunnable implements Runnable {
+        private final Runnable mCommand;
+        private Thread mCallingThread;
+        private InlineExecutionProhibitedException mExecutedInline = null;
+
+        private InlineCheckingRunnable(Runnable command, Thread callingThread) {
+            this.mCommand = command;
+            this.mCallingThread = callingThread;
+        }
+
+        @Override
+        public void run() {
+            if (Thread.currentThread() == mCallingThread) {
+                // Can't throw directly from here, since the delegate executor could catch this
+                // exception.
+                mExecutedInline = new InlineExecutionProhibitedException();
+                return;
+            }
+            mCommand.run();
+        }
+    }
+}