Move #includes out of extern "C" blocks

[Victor-C-Zhang]

Do some include shuffling for **.h files within lib, programs, tests, and zlibWrapper. lib/legacy and lib/deprecated are untouched.
#includes within extern "C" blocks in .cpp files are untouched.

todo: shuffling for xxhash.h

I have no way of exhaustively testing this... CI to the rescue?

[felixhandte]

You could build the library before and after and see that the symbol list given by nm doesn't change? (I.e., no symbols become mangled.)

[Victor-C-Zhang]

@Cyan4973 what should we do about copied deps like fse and xxhash? Does this warrant a PR in their respective repos too?

[Cyan4973]

How did you check that this PR fixed #4187 ?

[Victor-C-Zhang]

How did you check that this PR fixed #4187 ?

I haven't. I will try to repro #4187 with the limited information provided and do a before-and-after. From reading forum posts of people with similar issues, this may be an issue specific to an old version of apple clang from an old XCode, but I'll need to double-check to make sure...

[Victor-C-Zhang]

Update: I was unable to repro the build failure.
Here's what I did:

1. Generate zstd.c from create_single_file_library.sh
2. Copy zstd.c and lib/zstd.h, lib/zdict.h, lib/zstd_error.h, lib/module.modulemap to a fresh directory (outside zstd repo) ~/module_test
3. In the directory ~/module_test use the following main.cpp:

#include "zzz.h"
#include <iostream>
#include <string>

int main() {
  std::cout << "Hello, world!" << std::endl;
  std::string input = "abcdabcdabcdabcdabcdabcdabcdabcd";
  std::string output('\0', input.size() * 100);
  auto res = ZSTD_compress(output.data(), output.size(),
                           input.data(), input.size(), 3);
  std::cout << res << std::endl;

  return 0;
}

4. For safety, rename the module libzstd -> zzz, and rename zstd.h -> zzz.h, zstd.c -> zzz.c. Also do this in the module.modulemap.
5. Build with module

clang -shared -o libzzz.dylib zzz.c
clang++ -std=c++20 main.cpp -fmodules -fcxx-modules -L. -lzzz

6. It runs:

% ./a.out
Hello, world!
18446744073709551546

Apple clang version 15.0.0 (clang-1500.3.9.4)
Target: arm64-apple-darwin24.1.0
Thread model: posix