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Sugar-on-Native Client investigation
cananian

This post will describe the state of Native Client in general, based on week 2 of my original four week plan. In the next post, I'll link to my work so far, and compare the Native Client and the Android efforts. Recapping, the end goal of these explorations is a platform for the next generation of the Sugar learning environment.

To begin, the Native Client (NaCl) plugin is fairly mature in a number of areas. Version 0.2 of the NaCl SDK was recently released (a version number which substantiates the "fairly" in my previous sentence), and the NativeClient plugin is currently shipping in Chrome (versions 10 and 11), although you have to manually turn on a preference in the about:flags dialog to enable it. The NaCl toolchain is much more standard than the Android NDK toolchain I discussed previously, and the robust naclports tree shows that the patches required for NaCl ports of common packages tend not to be too evil. The Tcl interpreter and Qt tookit port demos show that fairly complex pieces of code can be deployed today on NaCl.

On the other hand, there are three main difficulties:

  1. The default NaCl toolchain uses newlib as its standard C library. This is consistent with Google's preference for BSD-licensed code in SDKs they provide to the public (see the discussion of Bionic in the Android SDK). However, there also exists a branch of the SDK which uses glibc. The glibc branch supports several additional features, like shared library support. However, it is unclear whether this will ever be a "supported" part of the SDK. If glibc does become supported, it is unlikely ever to be the only supported libc; the BSD-licensed newlib will need to remain available as an option. (Yes, the LGPL license of glibc shouldn't inspire such paranoia, but Google has elected not to undertake the education of all prospective third-party developers.)
  2. The naclports project, although fairly robust, is driven between the Scylla and Charybdis of compatibility. The goal is that all the code in naclports be buildable at all times on all three major platforms: Windows, Mac, and Linux. Further, it should support both x86_32 and x86_64 backends, and ideally ARM and pNaCl as well. It's auto-built against the latest SDK sources, but should also work on the latest released SDK. And with the addition of the glibc/newlib split discussed above, the possible build targets are multiplied further. Needless to say, keeping the tree building against such a large number of variants is not an easy task, and naclports is usually broken in some way. In practice, most developers seem to pay attention to some subset (say, x86_32/newlib/Linux host), but it's hard to push patches upstream without worrying about breaking some obscure target. It might be best to base future work on a proper package technology, like (say) dpkg-cross.
  3. In general, a lot of interesting NaCl development has occurred on branches that are not easily integrated. I've already mentioned glibc support, which is a toolchain branch; shared library support is on another branch that requires a new chromium plugin as well. At various times different means have been implemented to run NaCl binaries "natively" outside the sandbox (for example, in order to test some feature at build time, or auto-generate some piece of code via introspection). These efforts live on abandoned branches, while the "official" means to do this is incomplete. Similarly, a lot of interesting NaCl work used the now-abandoned legacy "NPAPI" plugin interface to interact with the browser. It was followed by the "Pepper" plugin interface, which was itself abandoned. Current work uses the Pepper2 browser plugin APIs, which (unfortunately) have not yet been implemented in non-Chrome browsers and continue to flux about. Many interesting browser interactions exist only in deprecated Pepper APIs, not having yet been built into Pepper2. ARM and pNaCl work also appears to be on unintegrated branches. There are a number of different gdb support strategies.

None of these difficulties is insurmountable—and in fact, some are side-effects of the desirable active development and productization of Native Client. To date I've done my work on the (more compatible) SDK v0.1 and the (more upstreamable) newlib library. So far newlib has not been a huge obstacle, and this basis allows my patches and ports to be more broadly useful. This might change in the future—certainly at some point we need to move to ARM and/or pNaCl for XO-3, which will probably require building chrome and the NaCl toolchain from scratch. At that point, it may be worth further exploring the non-mainstream branches.


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