Adds blocking file locks.

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.
+
+// 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
+// bytes and that a non-blocking lock fails such that a blocking lock must
+// be taken, which succeeds.
+
+import 'dart:async';
+import 'dart:convert';
+import 'dart:io';
+
+import "package:async_helper/async_helper.dart";
+import "package:expect/expect.dart";
+import "package:path/path.dart";
+
+// Check whether the file is locked or not.
+runPeer(String path, int len, FileLock mode) {
+  var script = Platform.script.resolve(
+      'file_blocking_lock_script.dart').toFilePath();
+  var arguments = []
+      ..addAll(Platform.executableArguments)
+      ..add(script)
+      ..add(path)
+      ..add(len.toString());
+  return Process.start(Platform.executable, arguments).then((process) {
+    process.stdout
+        .transform(UTF8.decoder)
+        .listen((data) { print(data); });
+    process.stderr
+        .transform(UTF8.decoder)
+        .listen((data) { print(data); });
+    return process;
+  });
+}
+
+testLockWholeFile() async {
+  const int length = 25;
+  asyncStart();
+  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);
+
+  // Wait a bit for the other process to get started. We'll synchronize on
+  // the file lock.
+  await new Future.delayed(const Duration(seconds: 1));
+
+  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++;
+    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);
+  }
+
+  await raf.setPosition(0);
+  for (int i = 1; i <= length; i++) {
+    Expect.equals(i, await raf.readByte());
+  }
+  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);
+
+  peer.exitCode.then((v) {
+    Expect.equals(0, v);
+    raf.closeSync();
+    directory.deleteSync(recursive: true);
+    asyncEnd();
+  });
+}
+
+main() {
+  testLockWholeFile();
+}