Makefile: use HOST_DIR_SYMLINK instead of hardcoding

From: Thomas De Schampheleire <[email protected]>

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

Hello,

Quoted 38 lines · On Tue, 18 Feb 2020 16:01:00 +0100 Thomas De Schampheleire…

From: Thomas De Schampheleire <[email protected]>

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.

Quoted 1 line

[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 ?

Quoted 3 lines

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

No, the initial Buildroot build _is_ completed first. Then the application build starts according to the scenario described. In our case, the extra Buildroot 'make' is done to let Buildroot create the images, passing a custom TARGET_DIR which is an extended copy of the original output/target. This step happens multiple times for different boards, out of the same Buildroot (the boards differ a bit in our own software plus perhaps some other customization done in the rootfs). From the perspective of the application build, the fact that Buildroot 'make' is called shouldn't matter because all the files it needs will not be touched. Buildroot itself is also retained as-is, except it is asked to produce images.

From: Thomas De Schampheleire <[email protected]>

Signed-off-by Thomas De Schampheleire <[email protected]>

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 Makefile | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)