gh-146636: abi3t: Test that Python.h avoids checking Py_GIL_DISABLED

Add a test that Python headers themselves don't use
Py_GIL_DISABLED in abi3t: abi3 and abi3t ought to be the
same except the _Py_OPAQUE_PYOBJECT differences.
This is done using the GCC-only poison pragma.

Author: encukou

Include/pyport.h

@@ -83,6 +83,17 @@
 // locking needed in for free-threaded interpreters builds.
 #    define Py_GIL_DISABLED
 #  endif
+#  if defined(_Py_IS_TESTCEXT)
+// When compiling for  abi3t, contents of Python.h should not depend
+// on Py_GIL_DISABLED.
+// We ask GCC to error if it sees the macro from this point on.
+// Since users are free to the macro, and there's no way to undo the poisoning
+// at the end of Python.h, we only do this in a test module.
+#    ifdef __GNUC__
+#      undef Py_GIL_DISABLED
+#      pragma GCC poison Py_GIL_DISABLED
+#    endif
+#  endif
 #endif
 
 

Lib/test/test_cext/setup.py

@@ -18,6 +18,11 @@
         # The purpose of test_cext extension is to check that building a C
         # extension using the Python C API does not emit C compiler warnings.
         '-Werror',
+        # Enable extra checks for header files, which:
+        #  - need to be enabled somewhere inside Python headers (rather than
+        #    before including Python.h)
+        #  - should not be checked for user code
+        '-D_Py_IS_TESTCEXT',
     ]
 
     # C compiler flags for GCC and clang