gh-142447: Fix cast warning in pycore_backoff.h

[vstinner]

MAKE_VALUE_AND_BACKOFF() macro casts its result to uint16_t.

Add pycore_backoff.h header to test_cppext tests.

• Issue: greenlet does not compile with main branch with clang #142447

[colesbury]

LGTM, but consider comment about test includes.

[colesbury]

Maybe better to include the headers greenlet actually uses:

internal/pycore_interpframe_structs.h
internal/pycore_interpframe.h
internal/pycore_tstate.h

https://github.com/python-greenlet/greenlet/blob/dc04a9fb7711611960066ec786b2185fa1e0f497/src/greenlet/TGreenlet.hpp#L30-L40

pycore_backoff.h isn't included directly in greenlet.

[vstinner]

The problem is that including pycore_interpframe.h (for example) produces tons of compiler warnings from mimalloc, and I have no idea how to fix them. Examples of warning:

In file included from /usr/include/features.h:524,
                 from /usr/include/assert.h:35,
                 from /home/vstinner/python/main/Include/Python.h:20,
                 from extension.cpp:13:
/home/vstinner/python/main/Include/internal/mimalloc/mimalloc/types.h:73:33: error: expected unqualified-id before ‘noexcept’
   73 | #define mi_decl_throw           __THROW
      |                                 ^~~~~~~
/home/vstinner/python/main/Include/internal/mimalloc/mimalloc/types.h:611:31: note: in expansion of macro ‘mi_decl_throw’
  611 | mi_decl_noreturn mi_decl_cold mi_decl_throw
      |                               ^~~~~~~~~~~~~


In file included from /home/vstinner/python/main/Include/internal/pycore_mimalloc.h:44,
                 from /home/vstinner/python/main/Include/internal/pycore_tstate.h:14,
                 from /home/vstinner/python/main/Include/internal/pycore_code.h:13,
                 from /home/vstinner/python/main/Include/internal/pycore_interpframe.h:8,
                 from extension.cpp:18:
/home/vstinner/python/main/Include/internal/mimalloc/mimalloc/internal.h: In function ‘bool _mi_is_aligned(void*, size_t)’:
/home/vstinner/python/main/Include/internal/mimalloc/mimalloc/types.h:613:49: error: ‘_mi_assert_fail’ was not declared in this scope; did you mean ‘__assert_fail’?
  613 | #define mi_assert(expr)     ((expr) ? (void)0 : _mi_assert_fail(#expr,__FILE__,__LINE__,__func__))
      |                                                 ^~~~~~~~~~~~~~~
/home/vstinner/python/main/Include/internal/mimalloc/mimalloc/types.h:619:31: note: in expansion of macro ‘mi_assert’
  619 | #define mi_assert_internal    mi_assert
      |                               ^~~~~~~~~
/home/vstinner/python/main/Include/internal/mimalloc/mimalloc/internal.h:273:3: note: in expansion of macro ‘mi_assert_internal’
  273 |   mi_assert_internal(alignment != 0);
      |   ^~~~~~~~~~~~~~~~~~

[colesbury]

That's going to be a problem in greenlet.

[vstinner]

@tacaswell: Do you get compiler warnings, or even compiler errors, from Python internal/mimalloc/ headers in greenlet?

[tacaswell]

I only see one warning with cpython main after this PR was merged:

  running build_ext
  building 'greenlet._greenlet' extension
  creating build/temp.linux-x86_64-cpython-315/src/greenlet
  clang++ -fno-strict-overflow -Wsign-compare -Wunreachable-code -DNDEBUG -g -O3 -Wall -fPIC -I/home/tcaswell/.virtualenvs/cp315-clang/include -I/home/tcaswell/.pybuild/cp315-clang/include/python3.15 -c src/greenlet/greenlet.cpp -o build/temp.linux-x86_64-cpython-315/src/greenlet/greenlet.o
  In file included from src/greenlet/greenlet.cpp:43:
  src/greenlet/PyGreenlet.cpp:62:23: warning: unused variable 'c' [-Wunused-variable]
     62 |         UserGreenlet* c = new UserGreenlet(o, GET_THREAD_STATE().state().borrow_current());
        |                       ^
  1 warning generated.
  clang++ -fno-strict-overflow -Wsign-compare -Wunreachable-code -DNDEBUG -g -O3 -Wall -shared -Wl,-rpath,/home/tcaswell/.pybuild/cp315-clang/lib build/temp.linux-x86_64-cpython-315/src/greenlet/greenlet.o -L/home/tcaswell/.pybuild/cp315-clang/lib -o build/lib.linux-x86_64-cpython-315/greenlet/_greenlet.cpython-315-x86_64-linux-gnu.so
  building 'greenlet.tests._test_extension' extension
  creating build/temp.linux-x86_64-cpython-315/src/greenlet/tests
  clang -fno-strict-overflow -Wsign-compare -Wunreachable-code -DNDEBUG -g -O3 -Wall -fPIC -Isrc/greenlet/ -I/home/tcaswell/.virtualenvs/cp315-clang/include -I/home/tcaswell/.pybuild/cp315-clang/include/python3.15 -c src/greenlet/tests/_test_extension.c -o build/temp.linux-x86_64-cpython-315/src/greenlet/tests/_test_extension.o
  clang -shared -Wl,-rpath,/home/tcaswell/.pybuild/cp315-clang/lib build/temp.linux-x86_64-cpython-315/src/greenlet/tests/_test_extension.o -L/home/tcaswell/.pybuild/cp315-clang/lib -o build/lib.linux-x86_64-cpython-315/greenlet/tests/_test_extension.cpython-315-x86_64-linux-gnu.so
  building 'greenlet.tests._test_extension_cpp' extension
  clang++ -fno-strict-overflow -Wsign-compare -Wunreachable-code -DNDEBUG -g -O3 -Wall -fPIC -Isrc/greenlet/ -I/home/tcaswell/.virtualenvs/cp315-clang/include -I/home/tcaswell/.pybuild/cp315-clang/include/python3.15 -c src/greenlet/tests/_test_extension_cpp.cpp -o build/temp.linux-x86_64-cpython-315/src/greenlet/tests/_test_extension_cpp.o
  clang++ -fno-strict-overflow -Wsign-compare -Wunreachable-code -DNDEBUG -g -O3 -Wall -shared -Wl,-rpath,/home/tcaswell/.pybuild/cp315-clang/lib build/temp.linux-x86_64-cpython-315/src/greenlet/tests/_test_extension_cpp.o -L/home/tcaswell/.pybuild/cp315-clang/lib -o build/lib.linux-x86_64-cpython-315/greenlet/tests/_test_extension_cpp.cpython-315-x86_64-linux-gnu.so
  installing to build/bdist.linux-x86_64/wheel

greenlet also includes internal/pycore_frame.h (the GREENLET_PYXXX guards are "that version or newer"), not sure if that is relevant.

[vstinner]

Ok, thanks for checking!

greenlet also includes internal/pycore_frame.h (the GREENLET_PYXXX guards are "that version or newer"), not sure if that is relevant.

test_cppext already checks that pycore_frame.h doesn't emit compiler warnings.

[tacaswell]

greenlets imports pycore_farem.h first.