Get content type of an HTTP request

Question

I am developing a Firefox extension on the Addon SDK (V1.9).

Objective

I need to examine HTTP requests for a specific content type before they are loaded (before the request is sent).

Problem

When adding an observer through the 'observer-service' module, I use "http-on-modify-request" to examine all HTTP requests. This provides the headers. However, when checking for content-type, it is never set on request.

Content-type is indeed set on response, which I examine using "http-on-examine-response". But this means that the request has already loaded, defeating the purpose of my extension.

Edit

@Wladimir Palant Thanks for pointing out that content-type is not set when the request headers are being sent.

I am trying to test HTTP requests' urls on a bunch of regexes that are relevant for me and block them. However, I wanted to be able to skip certain content-types (such as images) since they are not relevant for my application.

I'm not sure if that's what you mean - I think what you really mean is that you want to interrupt the response and examine the headers? — therealjeffg
The content type isn't known when the request is sent - it is coming from the server. Maybe you want to explain the problem you are trying to solve rather than what you consider to be its solution. Are you trying to distinguish images from scripts or something like that? — Wladimir Palant
@WladimirPalant Thanks for pointing that out. I just added a better explanation of why I wanted to know the content-type before the HTTP request is sent. — josesigna
@josesigna: Apparently you already figured out that using content policies is the answer - they have extended context information unlike HTTP observers. Meaning that I don't need to write an extended answer :) — Wladimir Palant

josesigna josesigna · Accepted Answer · 2012-08-24T19:09:34

So, to wrap up on the issue. The best way to have full control of resources loading into the browser seems to be using nsIContentPolicy.

@WladimirPalant wrote a great example here. And this is how his code would look adapted to the Addon SDK (v1.9):

const { Cc, Ci, Cu, Cm, components } = require('chrome');
const { XPCOMUtils } = Cu.import("resource://gre/modules/XPCOMUtils.jsm");


let policy =
{
    classDescription: "Test content policy",
    classID: components.ID("{12345678-1234-1234-1234-123456789abc}"),
    contractID: "@adblockplus.org/test-policy;1",
    xpcom_categories: ["content-policy"],

    init: function()
    {
        let registrar = Cm.QueryInterface(Ci.nsIComponentRegistrar);
        registrar.registerFactory(this.classID, this.classDescription, this.contractID, this);

        let catMan = Cc["@mozilla.org/categorymanager;1"].getService(Ci.nsICategoryManager);
        for each (let category in this.xpcom_categories)
            catMan.addCategoryEntry(category, this.contractID, this.contractID, false, true);
},

    // nsIContentPolicy interface implementation
    shouldLoad: function(contentType, contentLocation, requestOrigin, node, mimeTypeGuess, extra)
    {
        dump("shouldLoad: " + contentType + " " +
                      (contentLocation ? contentLocation.spec : "null") + " " +
                      (requestOrigin ? requestOrigin.spec : "null") + " " +
                      node + " " +
                      mimeTypeGuess + "\n");
        return Ci.nsIContentPolicy.ACCEPT;
    },

    shouldProcess: function(contentType, contentLocation, requestOrigin, node, mimeTypeGuess, extra)
    {
        dump("shouldProcess: " + contentType + " " +
                        (contentLocation ? contentLocation.spec : "null") + " " +
                        (requestOrigin ? requestOrigin.spec : "null") + " " +
                        node + " " +
                        mimeTypeGuess + "\n");
        return Ci.nsIContentPolicy.ACCEPT;
    },

    // nsIFactory interface implementation
    createInstance: function(outer, iid)
    {
        if (outer)
            throw Cr.NS_ERROR_NO_AGGREGATION;
        return this.QueryInterface(iid);
    },

    // nsISupports interface implementation
    QueryInterface: XPCOMUtils.generateQI([Ci.nsIContentPolicy, Ci.nsIFactory])
};

policy.init();

Get content type of an HTTP request

1 Answers