makefiles: select default prng _after_ recursive dependency resolution

[mguetschow]

Contribution description

The PRNG is supposed to be set only if there is none selected at the end of the recursive dependency resolution. However, this is currently not the case: a transitive dependency setting the PRNG (encountered with psa_crypto which selects prng_sha256prng) will lead to a user-facing dependency clash.

The first commit extends tests/build_system/external_module to showcase the issue and the second one fixes it by moving the automatic prng selection to defaultmodules_deps.inc.mk.

Disclaimer: I'm not sure if this is the right approach to follow, but it seems to work. Any suggestions from people with more insights on the build system are welcome.

Testing procedure

with only the first commit:

$ make -C tests/build_system/external_module_dirs info-modules
make: Entering directory '/home/mikolai/TUD/Code/RIOT-build/tests/build_system/external_module_dirs'
Currently the following prng modules are used: prng_musl_lcg prng_xorshift
/home/mikolai/TUD/Code/RIOT-build/sys/random/Makefile.include:14: *** Only one implementation of PRNG should be used..  Stop.
make: Leaving directory '/home/mikolai/TUD/Code/RIOT-build/tests/build_system/external_module_dirs'

with both commits:

$ make -C tests/build_system/external_module_dirs info-modules
make: Entering directory '/home/mikolai/TUD/Code/RIOT-build/tests/build_system/external_module_dirs'
auto_init
auto_init_random
board
board_common_init
core
core_idle_thread
core_init
core_lib
core_msg
core_panic
core_thread
cpu
external_module
libc
luid
native_drivers
periph
periph_common
periph_cpuid
periph_gpio
periph_gpio_linux
periph_hwrng
periph_init
periph_init_cpuid
periph_init_gpio
periph_init_gpio_linux
periph_init_hwrng
periph_init_led0
periph_init_led1
periph_init_led2
periph_init_led3
periph_init_led4
periph_init_led5
periph_init_led6
periph_init_led7
periph_init_leds
periph_init_pm
periph_init_uart
periph_pm
periph_uart
preprocessor
preprocessor_successor
prng
prng_xorshift
random
stdio_native
sys
transitive
make: Leaving directory '/home/mikolai/TUD/Code/RIOT-build/tests/build_system/external_module_dirs'

[maribu]

lgtm.

If you would reorder the commits (first the fix than the test) every commit would paas the CI, which is nice for bisecting. But that is purely optional.

[maribu]

lgtm.

If you would reorder the commits (first the fix than the test) every commit would paas the CI, which is nice for bisecting. But that is purely optional.

[mguetschow]

Reordered the commits as proposed :)

[riot-ci]

Murdock results

✔️ PASSED

9f2755a tests/build_system/external_module: custom prng in transitive dep

Success	Failures	Total	Runtime
10249	0	10249	17m:58s

Artifacts

• Documentation preview

[mguetschow]

Thank you!