MbedTLS v3.6.0 - Compilation errors and warnings

[LuDuda]

Summary

I'm working on migrating from the legacy MbedTLS crypto API to the PSA API for the OpenThread project, as requested by @gilles-peskine-arm (openthread/openthread#10253).

I have encountered two compilation issues for version v3.6.0 using the OT build system with the -wpedantic flag.

In file included from third_party/mbedtls/repo/include/psa/crypto.h:4828,
                 from third_party/mbedtls/repo/include/mbedtls/pk.h:32,
                 from third_party/mbedtls/repo/include/mbedtls/ssl_ciphersuites.h:16,
                 from third_party/mbedtls/repo/include/mbedtls/ssl.h:20,
                 [...]
third_party/mbedtls/repo/include/psa/crypto_struct.h:229:13: error: ISO C++ forbids flexible array member ‘data’ [-Wpedantic]
  229 |     uint8_t data[];

and

third_party/mbedtls/repo/library/pk.c: In function ‘copy_from_psa’:
third_party/mbedtls/repo/library/pk.c:871:21: warning: variable ‘alg_type’ set but not used [-Wunused-but-set-variable]
  871 |     psa_algorithm_t alg_type;

For the first issue, I plan to apply a workaround, but I believe it might be better to implement variable-length structures differently (e.g., using *data or data[1] with an adjusted allocation scheme). Therefore, I will keep this GitHub issue open. @gilles-peskine-arm any thoughts here?

I intended to fix the second issue by myself, but I noticed that it has already been resolved in the development branch (#9067) - great!

[gilles-peskine-arm]

For the flexible array member, we're already tracking this in #9020. We're going to make new types and functions that split the variable-length part out of the structure and into a separate parameter. The current type and function will be skipped if defined(__cplusplus) (exact condition TBD).

For the unused-variable warnings in copy_from_psa, we already have some fixes in the development branch (390f276 and dde67bb). I'm not sure we've fixed all cases, but we have fixed the one you encountered.