Fix headers in testrand_impl.h

[hebasto]

While reviewing #1734 and suggesting keeping the new test miniframework as a separate translation unit, I noticed a couple of issues with the headers included in testrand_impl.h. In my approach, these lead to compiler warnings and linker errors. This PR fixes them.

These changes are reasonable on their own.

[real-or-random]

That one is also in #1724. (Want to review it? :) )

[hebasto]

Want to review it? :)

It is next one in my list after #1734 :)

[real-or-random]

Why do we need this?

[hebasto]

As I pointed out in the PR description, when building the new test miniframework (from #1734) as a separate translation unit, it fails:

$ cmake --build build
[1/20] Building C object src/CMakeFiles/unit_test.dir/unit_test.c.o
In file included from /home/hebasto/dev/secp256k1/secp256k1/src/testrand_impl.h:17,
                 from /home/hebasto/dev/secp256k1/secp256k1/src/unit_test.c:31:
/home/hebasto/dev/secp256k1/secp256k1/src/hash.h:19:13: warning: ‘secp256k1_sha256_initialize’ used but never defined
   19 | static void secp256k1_sha256_initialize(secp256k1_sha256 *hash);
      |             ^~~~~~~~~~~~~~~~~~~~~~~~~~~
/home/hebasto/dev/secp256k1/secp256k1/src/hash.h:20:13: warning: ‘secp256k1_sha256_write’ used but never defined
   20 | static void secp256k1_sha256_write(secp256k1_sha256 *hash, const unsigned char *data, size_t size);
      |             ^~~~~~~~~~~~~~~~~~~~~~
/home/hebasto/dev/secp256k1/secp256k1/src/hash.h:21:13: warning: ‘secp256k1_sha256_finalize’ used but never defined
   21 | static void secp256k1_sha256_finalize(secp256k1_sha256 *hash, unsigned char *out32);
      |             ^~~~~~~~~~~~~~~~~~~~~~~~~
[18/20] Linking C executable bin/noverify_tests
FAILED: bin/noverify_tests 
: && /usr/bin/cc -O2 -g -Wl,--dependency-file=src/CMakeFiles/noverify_tests.dir/link.d src/CMakeFiles/unit_test.dir/unit_test.c.o src/CMakeFiles/secp256k1_precomputed.dir/precomputed_ecmult.c.o src/CMakeFiles/secp256k1_precomputed.dir/precomputed_ecmult_gen.c.o src/CMakeFiles/noverify_tests.dir/tests.c.o -o bin/noverify_tests   && :
/usr/bin/ld: src/CMakeFiles/unit_test.dir/unit_test.c.o: in function `testrand_seed':
/home/hebasto/dev/secp256k1/secp256k1/src/testrand_impl.h:29:(.text+0x6c2): undefined reference to `secp256k1_sha256_initialize'
/usr/bin/ld: /home/hebasto/dev/secp256k1/secp256k1/src/testrand_impl.h:30:(.text+0x6d6): undefined reference to `secp256k1_sha256_write'
/usr/bin/ld: /home/hebasto/dev/secp256k1/secp256k1/src/testrand_impl.h:31:(.text+0x6e6): undefined reference to `secp256k1_sha256_write'
/usr/bin/ld: /home/hebasto/dev/secp256k1/secp256k1/src/testrand_impl.h:32:(.text+0x6f1): undefined reference to `secp256k1_sha256_finalize'
collect2: error: ld returned 1 exit status
[19/20] Building C object src/CMakeFiles/tests.dir/tests.c.o
ninja: build stopped: subcommand failed.

[furszy]

As I pointed out in the PR description, when building the new test miniframework (from #1734) as a separate translation unit, it fails

In that case, you should add the *_impl.h include in the binary’s top-level file, not here.
I think it helps to think about the *_impl.h files as if they were .cpp files: you would never include them directly in any file. You would only include the headers with the public functions/structs and then link the object files to get their implementations. But since we have a single translation unit here, we include them only in the top-level binary.

Side note: we could also remove this include from the unit test side if needed. Could just add a pointer to the RNG init function and done.

[real-or-random]

As I pointed out in the PR description, when building the new test miniframework (from #1734) as a separate translation unit, it fails:

That may be true, but without #1734, including just hash.h is perfectly correct. Exactly because hash_impl.h is included in tests.c. That's our "unity-build" convention, as correctly described by @furszy.

[hebasto]

Right. Including hash_impl.h in unit_tests.c fixes my branch.