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Extracting Necko


We should extract Necko for reuse in background services and on mobile platforms.


We expect that Mozilla will need to build consistent native applications and background services across platforms, including services and applications that don’t run Gecko atop a profile directory. To do this it would be beneficial (in theory) to leverage our existing investment in our network layer, avoiding the confusing situation of having multiple ways to configure network settings, and avoiding the possibility of different behaviors in different parts of an application.

Furthermore, we expect the question of reuse to recur. Necko is sometimes characterized as being a relatively self-contained piece of mozilla-central’s compiled code, so it should be one of the easiest to reuse. Also, its interdependencies are probably applicable to other components, so it’s a good opportunity to learn.

Breaking out a component for reuse is, for our purposes, equivalent to embedding: it’s taking the component and driving it in a different context. Specifically, we are interested in those contexts being (a) Rust, (b) not necessarily desktop. An alternative approach is to build a new Gecko application on top of a slimmed-down libxul. In the case of Necko, that might be something like a cURL clone. This kind of reuse is a known quantity, so we won’t discuss it here.


Analysis: obstacles to embedding




Necko is a very feature-rich C++ networking library that is directly targeted at meeting the needs of single-user, single-main-process, single-profile, interactive applications built on a persistent profile directory in the traditional Mozilla model. It is not well-suited for use outside of the Gecko lifecycle — indeed, almost every aspect is designed for the Gecko world — and would require substantial effort in decoupling to be suited for embedded use. Even if so decoupled, it would still mandate the use of XPCOM and the NSPR.

To extract Necko for reuse outside of Gecko, by non-XPCOM-based applications, would — almost by definition — require coming up with a plan to deal with XPCOM. That plan might look a lot like continuing historical efforts on deCOMtamination.

Necko’s data structures, interface definitions, error codes, macros, etc. are XPCOM’s, and so eliminating XPCOM dependency from the module is likely to involve touching almost every line of code. Automated tools to do so have been proposed.

(Going further to eliminate NSPR and mfbt data structures, yielding a more standard C++ library, would be further work still.)

This is particularly relevant when we consider oxidation — the replacement of C++ and JS components of Firefox with Rust — and even more so when we consider the use of such components both outside and inside Firefox from Rust (rather than from existing non-Rust callers).

There is ongoing work on landing Rust components in Firefox; most obstacles there are around XPCOM integration for Rust code, which currently requires writing lots of routine boilerplate by hand.

There is also some initial scoping of work to extract and use Gecko C++ components from Servo, which is a similar problem to using Gecko C++ components from Rust background services or mobile applications. Those estimates are large (but not yet published), on the order of 3-5 engineer years for non-trivial components.

Extracting such a module is not routine work: delicate balances need to be found between maintaining useful integration with Gecko (e.g., Necko’s nsIChannel permeates the codebase) versus designing for embedding, and between reimplementing Gecko dependencies in Rust for reuse versus reworking the component to avoid them.

Our position is that extracting Necko from Gecko for reuse would be a significant amount of work. Doing so in a way that allows its use from ordinary Rust consumers, without shipping XPCOM/NSPR/etc., would be even more so.

That amount of work is certainly much more than simply using the existing well-tested and idiomatic networking libraries available to Rust code. Given that the benefits of using Necko in its entirety (instead of e.g., just using Hyper for HTTP) are relatively small, our conclusion is that we should not attempt to reuse Necko outside of Gecko.

Standalone Rust components should use Rust networking libraries of equivalent functionality, e.g., Tokio/Hyper (already vendored for Servo) or libpnet. Useful Necko functionality should be ported to Rust as needed. Those Rust reimplementations can be imported back into Necko like any other piece of oxidized code.


Alternatives are, variously:

This document was originally drafted in the “Answered questions” Google Doc.


This document was written by Richard Newman (rnewman), reviewed by Jason Duell (jduell), Rob Helmer (rhelmer), Joe Walker, and Myk Melez.

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