[gdal-dev] Sort files in static library to make the build reproducible

[Sebastiaan Couwenberg]

[I'm sending this to the list because I'm unable to successfully create
a ticket with description in Trac at the moment. As reported in #6520
[0] adding a ticket description causes an internal server errors on
submit, it looks like the spam filter cannot handle the content.]


As part of the effort to support Reproducible Builds [1] Alexis
Bienvenüe submitted two patches in Debian Bug #824808 [2] to sort the
object files in the static library: sort-files-1 [3] & sort-files-2 [4]

These patches resolve the random_order_in_static_libraries issue [5],
which is illustrated by the diffoscope output [6].

The attached patch is modified version of the sort-files-2 patch doing
do wildcard sorting in the `$(LIBGDAL)` target instead of a separate
`$(LIBGDAL).buildit` target. Alexis chose that approach because:

> In some situations some of the target dependencies do not exist
> before the "make target" call. When this arises, as $(wildcard) is
> expanded before building the dependencies, it can miss some files…
> I was not sure of the situation here, so I added a transitional
> target to be safe.

I prefer the single line change in the attached patch, but you may want
to consider the separate target as done in the sort-files-2 patch too.

[0] https://trac.osgeo.org/gdal/ticket/6520
[1] https://wiki.debian.org/ReproducibleBuilds
[2] https://bugs.debian.org/824808
[3]
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=824808;msg=5;att=1;filename=sort-files-1
[4]
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=824808;msg=5;att=2;filename=sort-files-2
[5]
https://tests.reproducible-builds.org/issues/unstable/random_order_in_static_libraries_issue.html
[6]
https://tests.reproducible-builds.org/dbd/unstable/amd64/gdal_2.1.0+dfsg-2.diffoscope.html


Kind Regards,

Bas

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