gh-115382: Fix cross compiles when host and target use same SOABI

Lib/sysconfig/__init__.py

@@ -340,7 +340,20 @@ def _init_posix(vars):
     """Initialize the module as appropriate for POSIX systems."""
     # _sysconfigdata is generated at build time, see _generate_posix_vars()
     name = _get_sysconfigdata_name()
-    _temp = __import__(name, globals(), locals(), ['build_time_vars'], 0)
+
+    # For cross builds, the path to the target's sysconfigdata must be specified
+    # so it can be imported. It cannot be in PYTHONPATH, as foreign modules in
+    # sys.path can cause crashes when loaded by the host interpreter.
+    # Rely on truthiness as a valueless env variable is still an empty string.
+    # See OS X note in _generate_posix_vars re _sysconfigdata.
+    if (path := os.environ.get('_PYTHON_SYSCONFIGDATA_PATH')):
+        from importlib.machinery import FileFinder, SourceFileLoader, SOURCE_SUFFIXES
+        from importlib.util import module_from_spec
+        spec = FileFinder(path, (SourceFileLoader, SOURCE_SUFFIXES)).find_spec(name)
+        _temp = module_from_spec(spec)
+        spec.loader.exec_module(_temp)
+    else:
+        _temp = __import__(name, globals(), locals(), ['build_time_vars'], 0)
     build_time_vars = _temp.build_time_vars
     vars.update(build_time_vars)
 

Lib/test/libregrtest/main.py

@@ -594,6 +594,7 @@ def _add_cross_compile_opts(self, regrtest_opts):
                 '_PYTHON_PROJECT_BASE',
                 '_PYTHON_HOST_PLATFORM',
                 '_PYTHON_SYSCONFIGDATA_NAME',
+                "_PYTHON_SYSCONFIGDATA_PATH",
                 'PYTHONPATH'
             }
             old_environ = os.environ

Lib/test/pythoninfo.py

@@ -334,6 +334,7 @@ def format_groups(groups):
         "_PYTHON_HOST_PLATFORM",
         "_PYTHON_PROJECT_BASE",
         "_PYTHON_SYSCONFIGDATA_NAME",
+        "_PYTHON_SYSCONFIGDATA_PATH",
         "__PYVENV_LAUNCHER__",
 
         # Sanitizer options

Misc/NEWS.d/next/Build/2024-03-03-20-28-23.gh-issue-115382.97hJFE.rst

@@ -0,0 +1 @@
+Fix cross compile failures when the host and target SOABIs match.

configure.ac

@@ -164,7 +164,7 @@ AC_ARG_WITH([build-python],
     dnl Build Python interpreter is used for regeneration and freezing.
     ac_cv_prog_PYTHON_FOR_REGEN=$with_build_python
     PYTHON_FOR_FREEZE="$with_build_python"
-    PYTHON_FOR_BUILD='_PYTHON_PROJECT_BASE=$(abs_builddir) _PYTHON_HOST_PLATFORM=$(_PYTHON_HOST_PLATFORM) PYTHONPATH=$(shell test -f pybuilddir.txt && echo $(abs_builddir)/`cat pybuilddir.txt`:)$(srcdir)/Lib _PYTHON_SYSCONFIGDATA_NAME=_sysconfigdata_$(ABIFLAGS)_$(MACHDEP)_$(MULTIARCH) '$with_build_python
+    PYTHON_FOR_BUILD='_PYTHON_PROJECT_BASE=$(abs_builddir) _PYTHON_HOST_PLATFORM=$(_PYTHON_HOST_PLATFORM) PYTHONPATH=$(srcdir)/Lib _PYTHON_SYSCONFIGDATA_NAME=_sysconfigdata_$(ABIFLAGS)_$(MACHDEP)_$(MULTIARCH) _PYTHON_SYSCONFIGDATA_PATH=$(shell test -f pybuilddir.txt && echo $(abs_builddir)/`cat pybuilddir.txt`) '$with_build_python
     AC_MSG_RESULT([$with_build_python])
   ], [
     AS_VAR_IF([cross_compiling], [yes],