Trying to get HTTP POST request in Firefox extension

Question

I'm building an extension to get the POST request in Firefox. I read through the documentation for intercepting page loads and HTTP observers, but still couldn't manage to get the specific POST data on a page load (ex: data1=50&sdata2=0&data3=50).

I looked into TamperData's code, and found that they used stream.available() and stream.read(1). However, I couldn't get these commands to work with my code.

Currently my code looks like this:

var ObserverTest = {


    observe: function(subject, topic, data) {
        if (topic == 'http-on-modify-request') {
            var httpChannel = subject.QueryInterface(Components.interfaces.nsIHttpChannel);
        }
        if (topic == "http-on-examine-response") {
            var newListener = new TracingListener();
            subject.QueryInterface(Ci.nsITraceableChannel);
            newListener.originalListener = subject.setNewListener(newListener);
        }
    },

    register: function() {
        var observerService = Components.classes["@mozilla.org/observer-service;1"].getService(Components.interfaces.nsIObserverService);
        observerService.addObserver(ObserverTest, "http-on-modify-request", false);
        observerService.addObserver(ObserverTest, "http-on-examine-response", false);
    },
    unregister: function() {
        var observerService = Components.classes["@mozilla.org/observer-service;1"].getService(Components.interfaces.nsIObserverService);
        observerService.removeObserver(ObserverTest, "http-on-modify-request");
        observerService.removeObserver(ObserverTest,"http-on-examine-response");
    }
}

window.addEventListener("load", ObserverTest.register, false);
window.addEventListener("unload", ObserverTest.unregister, false);


//Helper function for XPCOM instanciation (from Firebug)
function CCIN(cName, ifaceName) {
    return Cc[cName].createInstance(Ci[ifaceName]);
}
// Copy response listener implementation.
function TracingListener() {
    this.originalListener = null;
    this.receivedData = [];   // array for incoming data.
}

TracingListener.prototype = {
    onDataAvailable: function(request, context, inputStream, offset, count) {
        var binaryInputStream = CCIN("@mozilla.org/binaryinputstream;1", "nsIBinaryInputStream");
        var storageStream = CCIN("@mozilla.org/storagestream;1", "nsIStorageStream");
        var binaryOutputStream = CCIN("@mozilla.org/binaryoutputstream;1", "nsIBinaryOutputStream");


        var stream = Components.classes["@mozilla.org/scriptableinputstream;1"].createInstance(Components.interfaces.nsIScriptableInputStream);
        stream.init(binaryInputStream);

        binaryInputStream.setInputStream(inputStream);
        storageStream.init(8192, count, null);
        binaryOutputStream.setOutputStream(storageStream.getOutputStream(0));

        // Copy received data as they come.
        var data = binaryInputStream.readBytes(count);
        this.receivedData.push(data);

        binaryOutputStream.writeBytes(data, count);

        this.originalListener.onDataAvailable(request, context, storageStream.newInputStream(0), offset, count);
    },

    onStartRequest: function(request, context) {
        this.originalListener.onStartRequest(request, context);
    },

    onStopRequest: function(request, context, statusCode)
    {
        // Get entire response
        var responseSource = this.receivedData.join();
        this.originalListener.onStopRequest(request, context, statusCode);
    },

    QueryInterface: function (aIID) {
        if (aIID.equals(Ci.nsIStreamListener) || aIID.equals(Ci.nsISupports)) {
            return this;
        }
        throw Components.results.NS_NOINTERFACE;
    }
}

Answer 1

First of all, the "http-on-examine-response" and TracingListener isn't required at all. This stuff would have merit if you wanted to do something with the response, but you're after data in the request, so topic == 'http-on-modify-request' it is.

The following function (untested, but copied from one of my extensions and cleaned up a bit) demonstrates how to get post data. The function is assumed to be called from http-on-modify-request.

const ScriptableInputStream = Components.Constructor(
  "@mozilla.org/scriptableinputstream;1",
  "nsIScriptableInputStream",
  "init");

function observeRequest(channel, topic, data) {
  let post = null;

  if (!(channel instanceof Ci.nsIHttpChannel) ||
    !(channel instanceof Ci.nsIUploadChannel)) {
    return post;
  }
  if (channel.requestMethod !== 'POST') {
    return post;
  }

  try {
    let us = channel.uploadStream;
    if (!us) {
      return post;
    }
    if (us instanceof Ci.nsIMultiplexInputStream) {
      // Seeking in a nsIMultiplexInputStream effectively breaks the stream.
      return post;
    }
    if (!(us instanceof Ci.nsISeekableStream)) {
      // Cannot seek within the stream :(
      return post;
    }

    let oldpos = us.tell();
    us.seek(0, 0);

    try {
      let is = new ScriptableInputStream(us);

      // we'll read max 64k
      let available = Math.min(is.available(), 1 << 16);
      if (available) {
        post = is.read(available);
      }
    }
    finally {
      // Always restore the stream position!
      us.seek(0, oldpos);
    }
  }
  catch (ex) {
    Cu.reportError(ex);
  }
  return post;
}

Depending on your use case, you might want to check if the us instanceof e.g. nsIMIMEInputStream or nsIStringInputStream for special handling or fast-paths...

You'd call it from your observer like:

  observe: function(subject, topic, data) {
    if (topic == 'http-on-modify-request') {
      observeRequest(subject, topic, data);
    }
  },