Test cleanup on Cygwin should avoid deleting any DLLs until the end of the test run.

This issue is particular to Cygwin since on Cygwin distutils.spawn uses os.fork(). Deleting DLLs can lead to Cygwin's fork() breaking, due to trying to load a DLL that no longer exists.  This means that the test run will use a little more disk space on Cygwin, but still allows full (or partial) cleanup at the end.

runtests.py

 #!/usr/bin/env python
 
+import atexit
 import os
 import sys
 import re
@@ -70,6 +71,24 @@ CY3_DIR = None
 from distutils.command.build_ext import build_ext as _build_ext
 from distutils import sysconfig
 
+_to_clean = []
+
+@atexit.register
+def _cleanup_files():
+    """
+    This is only used on Cygwin to clean up shared libraries that are unsafe
+    to delete while the test suite is running.
+    """
+
+    for filename in _to_clean:
+        if os.path.isdir(filename):
+            shutil.rmtree(filename, ignore_errors=True)
+        else:
+            try:
+                os.remove(filename)
+            except OSError:
+                pass
+
 
 def get_distutils_distro(_cache=[]):
     if _cache:
@@ -673,8 +692,10 @@ class CythonCompileTestCase(unittest.TestCase):
         cleanup = self.cleanup_failures or self.success
         cleanup_c_files = WITH_CYTHON and self.cleanup_workdir and cleanup
         cleanup_lib_files = self.cleanup_sharedlibs and cleanup
+        is_cygwin = sys.platform == 'cygwin'
+
         if os.path.exists(self.workdir):
-            if cleanup_c_files and cleanup_lib_files:
+            if cleanup_c_files and cleanup_lib_files and not is_cygwin:
                 shutil.rmtree(self.workdir, ignore_errors=True)
             else:
                 for rmfile in os.listdir(self.workdir):
@@ -683,17 +704,28 @@ class CythonCompileTestCase(unittest.TestCase):
                                 rmfile[-4:] == ".cpp" or
                                 rmfile.endswith(".html") and rmfile.startswith(self.module)):
                             continue
-                    if not cleanup_lib_files and (rmfile.endswith(".so") or rmfile.endswith(".dll")):
+
+                    is_shared_obj = rmfile.endswith(".so") or rmfile.endswith(".dll")
+
+                    if not cleanup_lib_files and is_shared_obj:
                         continue
+
                     try:
                         rmfile = os.path.join(self.workdir, rmfile)
                         if os.path.isdir(rmfile):
                             shutil.rmtree(rmfile, ignore_errors=True)
+                        elif is_cygwin and is_shared_obj:
+                            # Delete later
+                            _to_clean.append(rmfile)
                         else:
                             os.remove(rmfile)
                     except IOError:
                         pass
 
+                if cleanup_c_files and cleanup_lib_files and is_cygwin:
+                    # Finally, remove the work dir itself
+                    _to_clean.append(self.workdir)
+
     def runTest(self):
         self.success = False
         self.runCompileTest()