[Buildroot] [PATCH 2/2] Makefile: don't recreate staging symlink if it exists

[Thomas Petazzoni <thomas.petazzoni@bootlin.com>]

Hello,

On Tue, 18 Feb 2020 16:01:00 +0100
Thomas De Schampheleire <patrickdepinguin@gmail.com> wrote:

> From: Thomas De Schampheleire <thomas.de_schampheleire@nokia.com>
> 
> Create the staging symlink the same way as the host symlink. This means
> using a make dependency rather than recreating it every time.
> 
> In coreutils versions below 8.27, re-creation of symbolic links was not
> atomic. This means that there is a period in time where the existing link is
> removed, before the new one is created. In coreutils 8.27 this was fixed,
> see [1]. Note that CentOS 7 ships with coreutils 8.22.
> 
> In the following scenario, this is a problem:
> 
> - an application is compiled using the sysroot prepared by Buildroot and
>   links against Xenomai userspace libraries, but its build process is steered
>   from outside of Buildroot
> - to know the correct flags, the application makefile uses the 'xeno-config'
>   file to request them, and passes DESTDIR=/buildroot/output/staging
> - the xeno-config responds with flags based on the path
>   '/buildroot/output/staging/...'
> - while the application build is ongoing, a 'make' happens in Buildroot,
>   causing the 'staging' symlink to be recreated (even though it already
>   existed)
> - when exactly at this time, the application calls the compiler with -I
>   flags pointing to output/staging, the build fails with:
> 
>   -I/buildroot/output/staging/usr/include/xenomai/mercury: Error:  ^ is not a directory
>   -I/buildroot/output/staging/usr/include/xenomai: Error:  ^ is not a directory
>   -I/buildroot/output/staging/usr/include/xenomai/xenomai: Error:  ^ is not a directory
>   -I/buildroot/output/staging/usr/include/xenomai/psos: Error:  ^ is not a directory
>   Failed: ** ^ *
> 
> Work around this problem by only creating the staging symlink once, similar
> to how the host symlink (if any) is created.
> 
> See also commit d0f4f95e390bcb1c953efa125f5277a8a235396e which changed the
> way these symlinks are made. The reasoning in this commit is to move away
> from the 'dirs' target.
> 
> [1] https://github.com/coreutils/coreutils/commit/376967889ed7ed561e46ff6d88a66779db62737a

Wow, thanks for the investigation and detailed commit log. Quite crazy
stuff.

The question that comes to mind is: why are you building something
against the Buildroot toolchain/sysroot, before the Buildroot build has
completed ?

Thomas
-- 
Thomas Petazzoni, CTO, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com