cpu/esp32: activate cpp feature

[gschorcht]

Contribution description

This PR activates the C++ feature for ESP32 boards. It simply adds the pthread module and libstdc++ to linked libraries to solve the problem of unresolved symbols when feature cpp is required.

Testing procedure

Compile and flash examples/riot_and_cpp, for example

make BOARD=esp32-wroom-32 -C examples/riot_and_cpp flash

to test the device with a C++ application.

Issues/PRs references

[gschorcht]

@cladmi, @kaspar030 Since someone was asking for using cpp feature with ESP32, I tried to activate it. Principally, it is working when pthread module is used and libstdc++ is linked.

However, to avoid that module pthread and libstdc++ are used always even if they aren't needed, I tried to use them only, if feature cpp is required. Since there is no pseudomodule cpp, I just used FEATURE_REQUIRED in cpu/esp32/Makefile.dep:

ifneq (,$(filter cpp,$(FEATURES_REQUIRED)))
    USEMODULE += pthread
    BASELIBS += -lstdc++
endif

Unfortunatly, static tests fail with

Running './dist/tools/buildsystem_sanity_check/check.sh' x
Command output:
	Invalid build system patterns found by ./dist/tools/buildsystem_sanity_check/check.sh:
	Modules should not check the content of FEATURES_PROVIDED/_REQUIRED/OPTIONAL
		cpu/esp32/Makefile.dep:ifneq (,$(filter cpp,$(FEATURES_REQUIRED)))

How should I realize this dependency? Or should I add module pthread and link libstdc++ always, even if they are not needed?

[gschorcht]

@cladmi @kaspar030 If it isn't allowed to evaluate FEATURES_REQUIRED to determine dependencies, shouldn't the activation of FEATURE_REQUIRED=cpp not lead to enabling a pseudomodule cpp?

[cladmi]

@gschorcht you should filter with FEATURES_USED. I put the info for this in the check.sh script, but it is for sure not the easiest place to find it.
At least the issue was detected automatically :)

RIOT/dist/tools/buildsystem_sanity_check/check.sh

Lines 38 to 41 in a9ab9d3

	# Modules should not check the content of FEATURES_PROVIDED/_REQUIRED/OPTIONAL
	# Handling specific behaviors/dependencies should by checking the content of:
	# * `USEMODULE`
	# * maybe `FEATURES_USED` if it is not a module (== not a periph_)

The issue with using FEATURES_REQUIRED, is that if cpp is enabled by an application with FEATURES_OPTIONAL, it would not match the condition.

FEATURES_USED has both.

RIOT/Makefile.features

Line 35 in a9ab9d3

FEATURES_USED = $(sort $(FEATURES_REQUIRED) $(FEATURES_OPTIONAL_USED))

[gschorcht]

@cladmi Ah, thanks, I will try.

[gschorcht]

@cladmi Works in that way, thanks again.

[leandrolanzieri]

Tested with the CPP example application, and also the cpp11_* tests are working, on a esp32-wroom-32 board. ACK.

[gschorcht]

@leandrolanzieri Thanks for testing and merging