Add parallel compile from pybind11

.ci/requirements-mypy.txt

@@ -5,6 +5,7 @@ ipython
 numpy
 packaging
 pyarrow-stubs
+pybind11
 pytest
 sphinx
 types-atheris

docs/installation/building-from-source.rst

@@ -276,10 +276,9 @@ Build options
 
 * Config setting: ``-C parallel=n``. Can also be given
   with environment variable: ``MAX_CONCURRENCY=n``. Pillow can use
-  multiprocessing to build the extension. Setting ``-C parallel=n``
+  multiprocessing to build the extensions. Setting ``-C parallel=n``
   sets the number of CPUs to use to ``n``, or can disable parallel building by
-  using a setting of 1. By default, it uses 4 CPUs, or if 4 are not
-  available, as many as are present.
+  using a setting of 1. By default, it uses as many CPUs as are present.
 
 * Config settings: ``-C zlib=disable``, ``-C jpeg=disable``,
   ``-C tiff=disable``, ``-C freetype=disable``, ``-C raqm=disable``,

pyproject.toml

@@ -1,6 +1,7 @@
 [build-system]
 build-backend = "backend"
 requires = [
+  "pybind11",
   "setuptools>=77",
 ]
 backend-path = [

setup.py

@@ -17,19 +17,27 @@
 import warnings
 from collections.abc import Iterator
 
+from pybind11.setup_helpers import ParallelCompile
 from setuptools import Extension, setup
 from setuptools.command.build_ext import build_ext
 
+configuration: dict[str, list[str]] = {}
+
+# parse configuration from _custom_build/backend.py
+while sys.argv[-1].startswith("--pillow-configuration="):
+    _, key, value = sys.argv.pop().split("=", 2)
+    configuration.setdefault(key, []).append(value)
+
+default = int(configuration.get("parallel", ["0"])[-1])
+ParallelCompile("MAX_CONCURRENCY", default).install()
+
 
 def get_version() -> str:
     version_file = "src/PIL/_version.py"
     with open(version_file, encoding="utf-8") as f:
         return f.read().split('"')[1]
 
 
-configuration: dict[str, list[str]] = {}
-
-
 PILLOW_VERSION = get_version()
 AVIF_ROOT = None
 FREETYPE_ROOT = None
@@ -386,9 +394,7 @@ def finalize_options(self) -> None:
             cpu_count = os.cpu_count()
             if cpu_count is not None:
                 try:
-                    self.parallel = int(
-                        os.environ.get("MAX_CONCURRENCY", min(4, cpu_count))
-                    )
+                    self.parallel = int(os.environ.get("MAX_CONCURRENCY", cpu_count))
                 except TypeError:
                     pass
         for x in self.feature:
@@ -1044,11 +1050,6 @@ def debug_build() -> bool:
 ]
 
 
-# parse configuration from _custom_build/backend.py
-while sys.argv[-1].startswith("--pillow-configuration="):
-    _, key, value = sys.argv.pop().split("=", 2)
-    configuration.setdefault(key, []).append(value)
-
 try:
     setup(
         cmdclass={"build_ext": pil_build_ext},