Throw a StateError if read()/readAsString() is called multiple times

dart/pkg/shelf/lib/src/message.dart

 // Copyright (c) 2014, 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.
 
 library shelf.message;
 
 import 'dart:async';
 import 'dart:convert';
 
 import 'package:http_parser/http_parser.dart';
@@ skipping 19 matching lines @@
   /// since it's in the `shelf` package.
   ///
   /// The value is immutable.
   final Map<String, Object> context;
 
   /// The streaming body of the message.
   ///
   /// This can be read via [read] or [readAsString].
   final Stream<List<int>> _body;
 
+  /// This boolean indicates whether [_body] has been read.
+  ///
+  /// After calling [read], or [readAsString] (which internally calls [read]),
+  /// this will be `true`.
+  bool _bodyWasRead = false;
+
   /// Creates a new [Message].
   ///
   /// If [headers] is `null`, it is treated as empty.
   Message(this._body, {Map<String, String> headers,
         Map<String, Object> context})
       : this.headers = new ShelfUnmodifiableMap<String>(headers,
             ignoreKeyCase: true),
         this.context = new ShelfUnmodifiableMap<Object>(context,
             ignoreKeyCase: false);
 
@@ skipping 41 matching lines @@
     if (_contentTypeCache != null) return _contentTypeCache;
     if (!headers.containsKey('content-type')) return null;
     _contentTypeCache = new MediaType.parse(headers['content-type']);
     return _contentTypeCache;
   }
   MediaType _contentTypeCache;
 
   /// Returns a [Stream] representing the body.
   ///
   /// Can only be called once.
-  Stream<List<int>> read() => _body;
+  Stream<List<int>> read() {
+    if (_bodyWasRead) {
+      throw new StateError("The 'read' method can only be called once on a "
+                           "shelf.Request/shelf.Response object.");
+    }
+    _bodyWasRead = true;
+    return _body;
+  }
 
   /// Returns a [Future] containing the body as a String.
   ///
   /// If [encoding] is passed, that's used to decode the body.
   /// Otherwise the encoding is taken from the Content-Type header. If that
   /// doesn't exist or doesn't have a "charset" parameter, UTF-8 is used.
   ///
   /// This calls [read] internally, which can only be called once.
   Future<String> readAsString([Encoding encoding]) {
     if (encoding == null) encoding = this.encoding;
     if (encoding == null) encoding = UTF8;
     return Chain.track(encoding.decodeStream(read()));
   }
 
   /// Creates a new [Message] by copying existing values and applying specified
   /// changes.
   Message change({Map<String, String> headers, Map<String, Object> context});
 }