fix(eppp_link): Fix const qualifier violation in eppp_link (IDFGH-17135)

[sudeep-mohanty]

Description

ESP-IDF MR 45069, 45375 enables -Wwrite-strings and -Werror=discarded-qualifiers in ESP-IDF CI builds. This PR fixes const qualifier violations in examples that use espressif/eppp_link managed component causing build failures.

Related

• heap/heap_task_info.c:640 'error: initialization discards 'const' qualifier target type' (IDFGH-17093) esp-idf#18120

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Note

Updates callback signature for const-correctness.

• Change have_some_eppp_netif second parameter from void *ctx to const void *ctx to align with esp_netif_find_if usage and eliminate discarded-qualifiers warnings.

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[david-cermak]

Thanks for the fix!

[david-cermak]

but the predicate is defined as

https://github.com/espressif/esp-idf/blob/f621cc4fffca0879df5e1a951342b56291661e91/components/esp_netif/include/esp_netif.h#L1037

just void*, not const

[sudeep-mohanty]

Hello @david-cermak, That is true. The complementary internal GitLab MR !45375 has the required update.

[david-cermak]

Then we need some #ifdefs around it, right? Is the GL-MR targeted to v6.1?

[sudeep-mohanty]

Yes, the GL MR is targeted to v6.1. I now realize that for the eppp_link update we would need some #ifdefs for it work with several IDF versions. Or perhaps, we could handle it in idf_component.yml somehow. I will explore. Setting this MR to draft state for now.

[david-cermak]

@sudeep-mohanty There a few ways to resolve this across multiple IDF versions, none of them is perfect tho

1. #ifdefs based on IDF_VERSION macro -- would work on release branches, needs IDF MR sync with this PR
2. typecast to the used callback -- defeats type correctness of C
3. C++ traits magic

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From my perspective the least evil is nr 3) -- implemented in #1000
(if we have C++ concepts the solution would be even nicer)

I'm marking this WIP for now