Simplifies the blocking file lock test

tests/standalone/io/file_blocking_lock_test.dart

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 // OtherResources=file_blocking_lock_script.dart
 
 // This test works by spawning a new process running
 // file_blocking_lock_script.dart, trading the file lock back and forth,
 // writing bytes 1 ... 25 in order to the file. There are checks to ensure
 // that the bytes are written in order, that one process doesn't write all the
@@ skipping 21 matching lines @@
     process.stdout
         .transform(UTF8.decoder)
         .listen((data) { print(data); });
     process.stderr
         .transform(UTF8.decoder)
         .listen((data) { print(data); });
     return process;
   });
 }
 
+const int peerTimeoutMilliseconds = 10000;
+
+Future<bool> waitForPeer(RandomAccessFile raf, int length) async {
+  Stopwatch s = new Stopwatch();
+  s.start();
+  while (true) {
+    await raf.unlock(0, length);
+    if (s.elapsedMilliseconds > peerTimeoutMilliseconds) {
+      s.stop();
+      return false;
+    }
+    try {
+      await raf.lock(FileLock.EXCLUSIVE, 0, length);
+    } on dynamic {
+      await raf.lock(FileLock.BLOCKING_EXCLUSIVE, 0, length);
+      break;
+    }
+  }
+  s.stop();
+  return true;
+}
+
 testLockWholeFile() async {
   const int length = 25;
   Directory directory = await Directory.systemTemp.createTemp('dart_file_lock');
   File file = new File(join(directory.path, "file"));
   await file.writeAsBytes(new List.filled(length, 0));
   var raf = await file.open(mode: APPEND);
-  await raf.setPosition(0);
   await raf.lock(FileLock.BLOCKING_EXCLUSIVE, 0, length);
   Process peer = await runPeer(file.path, length, FileLock.BLOCKING_EXCLUSIVE);
 
-  int nextToWrite = 1;
-  int at = 0;
-  List iWrote = new List.filled(length, 0);
-  bool nonBlockingFailed = false;
-  while (nextToWrite <= length) {
-    int p = await raf.position();
-    await raf.writeByte(nextToWrite);
-    await raf.flush();
-    // Record which bytes this process wrote so that we can check that the
-    // other process was able to take the lock and write some bytes.
-    iWrote[nextToWrite-1] = nextToWrite;
-    nextToWrite++;
-    // Let the other process get the lock at least once by spinning until the
-    // non-blocking lock fails.
-    while (!nonBlockingFailed) {
-      await raf.unlock(0, length);
-      try {
-        await raf.lock(FileLock.EXCLUSIVE, 0, length);
-      } catch(e) {
-        // Check that at some point the non-blocking lock fails.
-        nonBlockingFailed = true;
-        await raf.lock(FileLock.BLOCKING_EXCLUSIVE, 0, length);
-      }
-    }
-    while (true) {
-      p = await raf.position();
-      at = await raf.readByte();
-      if (at == 0 || at == -1) break;
-      nextToWrite++;
-    }
-    await raf.setPosition(p);
-  }
+  // Waits for the peer to take the lock, then takes the lock.
+  Expect.isTrue(await waitForPeer(raf, length));
 
+  // Check that the peer wrote to the file.
+  int p = 0;
   await raf.setPosition(0);
-  for (int i = 1; i <= length; i++) {
-    Expect.equals(i, await raf.readByte());
+  while (p < length) {
+    int at = await raf.readByte();
+    Expect.equals(1, at);
+    p++;
   }
   await raf.unlock(0, length);
 
-  bool wroteAll = true;
-  for (int i = 0; i < length; i++) {
-    // If there's a 0 entry, this process didn't write all bytes.
-    wroteAll = wroteAll && (iWrote[i] == 0);
-  }
-  Expect.equals(false, wroteAll);
-
-  Expect.equals(true, nonBlockingFailed);
-
-  await peer.exitCode.then((v) async {
-    Expect.equals(0, v);
-    await raf.close();
-    await directory.delete(recursive: true);
-  });
+  // Check that the peer exited successfully.
+  int v = await peer.exitCode;
+  Expect.equals(0, v);
+  await raf.close();
+  await directory.delete(recursive: true);
 }
 
 main() async {
   asyncStart();
   await testLockWholeFile();
   asyncEnd();
 }