1. Move dependency on setuptools closer to where it's needed
2. Scope multiprocessing Pool so a pool left open doesn't hang the process
3. Move run_setup_py to its own file so the subprocess doesn't need to import much

Author: rotu

colcon_python_setup_py/package_identification/python_setup_py.py

@@ -8,10 +8,6 @@
 import os
 from pathlib import Path
 import runpy
-try:
-    import setuptools
-except ImportError:
-    pass
 import subprocess
 import sys
 from threading import Lock
@@ -25,6 +21,7 @@
     create_dependency_descriptor
 from colcon_core.plugin_system import satisfies_version
 
+from .run_setup_py import run_setup_py
 
 class PythonPackageIdentification(PackageIdentificationExtensionPoint):
     """Identify Python packages with `setup.py` files."""
@@ -98,6 +95,7 @@ def get_setup_arguments(setup_py):
         'colcon_python_setup_py.package_identification.python_setup_py.'
         'get_setup_information() instead',
         stacklevel=2)
+    import setuptools
     global cwd_lock
     if not cwd_lock:
         cwd_lock = Lock()
@@ -237,9 +235,6 @@ def get_setup_arguments_with_context(setup_py, env):
     return ast.literal_eval(output)
 
 
-_process_pool = multiprocessing.Pool()
-
-
 def get_setup_information(setup_py: Path, *, env: Mapping[str, str]):
     """
     Dry run the setup.py file and get the configuration information.
@@ -250,56 +245,18 @@ def get_setup_information(setup_py: Path, *, env: Mapping[str, str]):
     :raise: RuntimeError if the setup script encountered an error
     """
     try:
-        return _process_pool.apply(
-            run_setup_py,
-            kwds={
-                'cwd': os.path.abspath(str(setup_py.parent)),
-                # might be os.environ, which is not picklable
-                'env': dict(env),
-                'script_args': ('--dry-run',),
-                'stop_after': 'config'
-            }
-        )
+        with multiprocessing.Pool(1) as _process_pool:
+            return _process_pool.apply(
+                run_setup_py,
+                kwds={
+                    'cwd': os.path.abspath(str(setup_py.parent)),
+                    # might be os.environ, which is not picklable
+                    'env': dict(env),
+                    'script_args': ('--dry-run',),
+                    'stop_after': 'config'
+                }
+            )
     except Exception as e:
         raise RuntimeError(
             "Failed to dry run setup script '{setup_py}': "
             .format_map(locals()) + traceback.format_exc()) from e
-
-
-def run_setup_py(cwd, env, script_args=(), stop_after='run'):
-    """
-    Modify the current process and run setup.py.
-
-    This should be run in a subprocess to not affect the state of the current
-    process.
-
-    :param str cwd: absolute path to a directory containing a setup.py script
-    :param dict env: environment variables to set before running setup.py
-    :param script_args: command-line arguments to pass to setup.py
-    :param stop_after: tells setup() when to stop processing
-    :returns: the public properties of a Distribution object, minus objects
-      with are generally not picklable
-    """
-    # need to be in setup.py's parent dir to detect any setup.cfg
-    os.chdir(cwd)
-
-    os.environ.clear()
-    os.environ.update(env)
-
-    result = distutils.core.run_setup(
-        'setup.py', script_args=script_args, stop_after=stop_after)
-
-    return {
-        key: value for key, value in result.__dict__.items()
-        if (
-            # Private properties
-            not key.startswith('_') and
-            # Getter methods
-            not callable(value) and
-            # Objects that are generally not picklable
-            key not in ('cmdclass', 'distclass', 'ext_modules') and
-            # These *seem* useful but always have the value 0.
-            # Look for their values in the 'metadata' object instead.
-            key not in result.display_option_names
-        )
-    }

colcon_python_setup_py/package_identification/run_setup_py.py

@@ -0,0 +1,41 @@
+import distutils.core
+import os
+
+
+def run_setup_py(cwd, env, script_args=(), stop_after='run'):
+    """
+    Modify the current process and run setup.py.
+
+    This should be run in a subprocess to not affect the state of the current
+    process.
+
+    :param str cwd: absolute path to a directory containing a setup.py script
+    :param dict env: environment variables to set before running setup.py
+    :param script_args: command-line arguments to pass to setup.py
+    :param stop_after: tells setup() when to stop processing
+    :returns: the public properties of a Distribution object, minus objects
+      with are generally not picklable
+    """
+    # need to be in setup.py's parent dir to detect any setup.cfg
+    os.chdir(cwd)
+
+    os.environ.clear()
+    os.environ.update(env)
+
+    result = distutils.core.run_setup(
+        'setup.py', script_args=script_args, stop_after=stop_after)
+
+    return {
+        key: value for key, value in result.__dict__.items()
+        if (
+            # Private properties
+            not key.startswith('_') and
+            # Getter methods
+            not callable(value) and
+            # Objects that are generally not picklable
+            key not in ('cmdclass', 'distclass', 'ext_modules') and
+            # These *seem* useful but always have the value 0.
+            # Look for their values in the 'metadata' object instead.
+            key not in result.display_option_names
+        )
+    }