[Buildroot] Build reproducibility

[Thomas Petazzoni <thomas.petazzoni@free-electrons.com>]

Dear Peter Korsgaard,

On Tue, 03 Sep 2013 08:26:07 +0200, Peter Korsgaard wrote:

>  Arnout>  Note that doing more randomized build order in the autobuilder also
>  Arnout> will not capture the latter scenario. You would have to compare the
>  Arnout> build result - but binary differences are likely because of changing
>  Arnout> timestamps or changing optimizations depending on memory randomness.
> 
> Exactly. I don't have any good ideas about how to detect this (besides
> building all packages in clean staging dirs, E.G. only populated with
> its explicit dependencies like afaik OE lite can do, but that would
> require quite some work), anyone?

Doing a per-package sysroot that is generated for each package before
it gets built, with only the explicitly listed dependencies, is indeed
the only way to ensure that a package is not seeing/using something
that isn't declared as a dependency.

This would certainly be nice to have (as it also helps top-level
parallel build, as was discussed with Fabio Porcedda some time ago),
but:

 1) I'm worried about the additional complexity inside Buildroot.

 2) I'm worried about the additional build time required to generate a
    per-package sysroot for each package. When building large stacks
    like X.org that has many small packages, but each have a lot of
    dependencies, the cost of creating a sysroot before building each
    package could be huge.

 3) We still need to provide the user a global sysroot with all
    libraries installed, so that he can use the toolchain generated by
    Buildroot to build his own libraries/applications. This would mean
    we would need to have two sysroots: the global sysroot, that gets
    incrementally populated with what all packages are installing, but
    that isn't used for building packages inside Buildroot, and a
    separate temporary sysroot, used when building the current package.
    Since the compiler would default to the 'global sysroot', we would
    have to pass --sysroot $(TMPSYSROOT) all the time, or have a
    separate wrapper, or something. Not impossible, but see (1).

Best regards,

Thomas
-- 
Thomas Petazzoni, Free Electrons
Kernel, drivers, real-time and embedded Linux
development, consulting, training and support.
http://free-electrons.com