Prevent object file collisions in parallel extension builds

Parallel builds of extensions that share source files may write to the same
object file paths under a common build directory, resulting in race conditions
and non-deterministic build outputs.

Use a per-extension build directory to isolate object files and ensure
deterministic, parallel-safe builds.

Bug: #3942

Author: e-luks

newsfragments/3942.bugfix.rst

@@ -0,0 +1 @@
+Fixed non-deterministic parallel builds of extensions that share source files.

setuptools/_distutils/command/build_ext.py

@@ -562,9 +562,12 @@ def build_extension(self, ext) -> None:
         for undef in ext.undef_macros:
             macros.append((undef,))
 
+        # Per-extension build dir to avoid conflicts in parallel builds.
+        ext_build_temp = os.path.join(self.build_temp, ext.name)
+
         objects = self.compiler.compile(
             sources,
-            output_dir=self.build_temp,
+            output_dir=ext_build_temp,
             macros=macros,
             include_dirs=ext.include_dirs,
             debug=self.debug,
@@ -595,7 +598,7 @@ def build_extension(self, ext) -> None:
             extra_postargs=extra_args,
             export_symbols=self.get_export_symbols(ext),
             debug=self.debug,
-            build_temp=self.build_temp,
+            build_temp=ext_build_temp,
             target_lang=language,
         )