dart:io | Add Link class, as sibling to File and Directory.

sdk/lib/io/link.dart

Index: sdk/lib/io/link.dart
diff --git a/sdk/lib/io/link.dart b/sdk/lib/io/link.dart
new file mode 100644
index 0000000000000000000000000000000000000000..00aeb2d6b7789338ae364533105fdc6c51154220
--- /dev/null
+++ b/sdk/lib/io/link.dart
@@ -0,0 +1,178 @@
+// Copyright (c) 2013, 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.
+
+part of dart.io;
+
+/**
+ * [Link] objects are references to filesystem links.
+ *
+ */
+abstract class Link extends FileSystemEntity {
+  /**
+   * Create a Link object.
+   */
+  factory Link(String path) => new _Link(path);
+
+  /**
+   * Create a Link object from a Path object.
+   */
+  factory Link.fromPath(Path path) => new _Link.fromPath(path);
+
+  /**
+   * Check if the link exists. The link may exist, even if its target
+   * is missing or deleted.
+   * Returns a [:Future<bool>:] that completes when the answer is known.
+   */
+  Future<bool> exists();
+
+  /**
+   * Synchronously check if the link exists. The link may exist, even if
+   * its target is missing or deleted.
+   */
+  bool existsSync();
+
+  /**
+   * Create a symbolic link. Returns a [:Future<Link>:] that completes with
+   * the link when it has been created. If the link exists, the function
+   * the future will complete with an error.
+   *
+   * On the Windows platform, this will only work with directories, and the
+   * target directory must exist. The link will be created as a Junction.
+   * Only absolute links will be created, and relative paths to the target
+   * will be converted to absolute paths.
+   */
+  Future<Link> create(String target);
+
+  /**
+   * Synchronously create the link. Calling [createSync] on an existing link
+   * will throw an exception.
+   *
+   * On the Windows platform, this will only work with directories, and the
+   * target directory must exist. The link will be created as a Junction.
+   * Only absolute links will be created, and relative paths to the target
+   * will be converted to absolute paths.
+   */
+  void createSync(String target);
+
+  /**
+   * Synchronously update the link. Calling [updateSync] on a non-existing link
+   * will throw an exception.
+   *
+   * If [linkRelative] is true, the target argument should be a relative path,
+   * and the link will interpret the target as a path relative to the link's
+   * directory.
+   *
+   * On the Windows platform, this will only work with directories, and the
+   * target directory must exist.
+   */
+  void updateSync(String target, {bool linkRelative: false });
+
+  /**
+   * Delete the link. Returns a [:Future<Link>:] that completes with
+   * the link when it has been deleted.  This does not delete, or otherwise
+   * affect, the target of the link.
+   */
+  Future<Link> delete();
+
+  /**
+   * Synchronously delete the link. This does not delete, or otherwise
+   * affect, the target of the link.
+   */
+  void deleteSync();
+}
+
+
+class _Link extends FileSystemEntity implements Link {
+  final String path;
+
+  _Link(String this.path);
+
+  _Link.fromPath(Path inputPath) : path = inputPath.toNativePath();
+
+  Future<bool> exists() {
+    // TODO(whesse): Replace with asynchronous version.
+    return new Future.immediate(existsSync());
+  }
+
+  bool existsSync() => FileSystemEntity.isLinkSync(path);
+
+  Future<Link> create(String target) {
+    // TODO(whesse): Replace with asynchronous version.
+    return new Future.of(() {
+      createSync(target);
+      return this;
+    });
+  }
+
+  void createSync(String target) {
+    if (Platform.operatingSystem == 'windows') {
+      target = _makeWindowsLinkTarget(target);
+    }
+    var result = _File._createLink(path, target);
+    if (result is Error) {
+      throw new LinkIOException("Error in Link.createSync", result);
+    }
+  }
+
+  // Put target into the form "\??\C:\my\target\dir".
+  String _makeWindowsLinkTarget(String target) {
+    if (target.startsWith('\\??\\')) {
+      return target;
+    }
+    if (!(target.length > 3 && target[1] == ':' && target[2] == '\\')) {
+      target = new File(target).fullPathSync();
+    }
+    if (target.length > 3 && target[1] == ':' && target[2] == '\\') {
+      target = '\\??\\$target';
+    } else {
+      throw new ArgumentError(
+          'Target $target of Link.create on Windows cannot be converted' +
+          ' to start with a drive letter.  Unexpected error.');
+    }
+    return target;
+  }
+
+  void updateSync(String target, {bool linkRelative: false }) {
+    // TODO(whesse): Replace with atomic update, where supported by platform.
+    deleteSync();
+    createSync(target);
+  }
+
+  Future<Link> delete() {
+    return new File(path).delete().then((_) => this);
+  }
+
+  void deleteSync() {
+    new File(path).deleteSync();
+  }
+}
+
+
+class LinkIOException implements Exception {
+  const LinkIOException([String this.message = "",
+                         String this.path = "",
+                         OSError this.osError = null]);
+  String toString() {
+    StringBuffer sb = new StringBuffer();
+    sb.write("LinkIOException");
+    if (!message.isEmpty) {
+      sb.write(": $message");
+      if (path != null) {
+        sb.write(", path = $path");
+      }
+      if (osError != null) {
+        sb.write(" ($osError)");
+      }
+    } else if (osError != null) {
+      sb.write(": $osError");
+      if (path != null) {
+        sb.write(", path = $path");
+      }
+    }
+    return sb.toString();
+  }
+  final String message;
+  final String path;
+  final OSError osError;
+}