Running

Run these in the command line:

$ cd dart-multipart-bug
$ dart server.dart 1337

Then connect to localhost:1337. Note that you may substitute the port. Now, pick a file (it can be anything, even a blank file demonstrates an issue). When you hit Submit, the server will hang indefinitely.

The Issue

The issue is that the MIME package does not set a handler for onListen for a _MimeMultipart object. Instead, it assumes that you will listen() to the multipart stream synchronously the moment you get it. I have a patch below which fixes the issue (although I have not tested it extensively to make sure it doesn't break anything).

Here is how I found the issue.

I am using a MIME multipart transformer to get data from a file upload. all works well if I immediately listen() to the MimeMultipart object. However, if I wait for the runloop to run before attempting to listen(), I never get any callbacks.

.listen((HttpMultipartFormData formData) {
  // if I do this, it works:
  // print('listening for data...');
  // formData.listen((x) => print('got data of length ${x.length}'));
  
  // this does NOT work
  Duration duration = new Duration(seconds: 1);
  new Timer(duration, () {
    print('listening for data...');
    formData.listen((x) => print('got data of length ${x.length}'));
  });
}

Patch

The problem is in the mime/bound_multipart_stream.dart file. This patch will fix it (in the current version, 0.9.0+3):

--- /Users/alex/.pub-cache/hosted/pub.dartlang.org/mime-0.9.0+3/lib/src/bound_multipart_stream.dart	2014-05-30 12:47:51.000000000 -0400
+++ /Users/alex/Desktop/bound_multipart_stream.dart	2014-07-21 22:36:41.000000000 -0400
@@ -173,7 +173,9 @@ class BoundMultipartStream {
      boundaryPrefix = _boundaryIndex;

      while ((_index < _buffer.length) && _state != _FAIL && _state != _DONE) {
-       if (_multipartController != null && _multipartController.isPaused) {
+       if (_multipartController != null &&
+           (_multipartController.isPaused ||
+           !_multipartController.hasListener)) {
          return;
        }
        int byte;
@@ -292,6 +294,7 @@ class BoundMultipartStream {
            break;

          case _HEADER_ENDING:
+           var isDoneAdding = false; // set to true after add() call
            _expectByteValue(byte, CharCode.LF);
            _multipartController = new StreamController(
                sync: true,
@@ -301,9 +304,16 @@ class BoundMultipartStream {
                onResume: () {
                  _resumeStream();
                  _parse();
+               },
+               onListen: () {
+                 if (isDoneAdding) {
+                   _resumeStream();
+                   _parse();
+                 }
                });
            _controller.add(
                new _MimeMultipart(_headers, _multipartController.stream));
+           isDoneAdding = true;
            _headers = null;
            _state = _CONTENT;
            contentStartIndex = _index + 1;

Workaround

As a temporary workaround, I have found that I can start listening, pause() the subscription, and then resume() it when I need the data:

.listen((MimeMultipart formData) {
  var a = formData.listen((x) => print('got data of length ${x.length}'));
  a.pause();
  Duration duration = new Duration(seconds: 1);
  new Timer(duration, () {
    print('listening for data...');
    a.resume();
  });
}

However, this will not allow me to use nice utilities like pipe() in the future.