Always retry with polyfill on Windows

The polyfill is actually nicer than the native function because
it does not require admin rights, so even if the system supports
symlink creation natively, we try the polyfill if native fails.

Author: gonzalocasas

src/compas/_os.py

@@ -5,6 +5,7 @@
 """
 import os
 import sys
+import subprocess
 
 PY3 = sys.version_info[0] == 3
 system = sys.platform
@@ -65,6 +66,15 @@ def select_python(python_executable):
 def absjoin(*parts):
     return os.path.abspath(os.path.join(*parts))
 
+# Cache whatever symlink function works (native or polyfill)
+_os_symlink = None
+
+def create_symlink_polyfill():
+    def symlink_ms(source, link_name):
+        subprocess.check_output(
+            ['mklink', '/D', link_name, source], stderr=subprocess.STDOUT, shell=True)
+
+    return symlink_ms
 
 def create_symlink(source, link_name):
     """Create a symbolic link pointing to source named link_name.
@@ -77,19 +87,26 @@ def create_symlink(source, link_name):
         This function is a polyfill of the native ``os.symlink``
         for Python 2.x on Windows platforms.
     """
-    os_symlink = getattr(os, 'symlink', None)
-
-    if not callable(os_symlink) and os.name == 'nt':
-        import subprocess
+    global _os_symlink
+    enable_retry_with_polyfill = False
 
-        def symlink_ms(source, link_name):
-            subprocess.check_output(
-                ['mklink', '/D', link_name, source], stderr=subprocess.STDOUT, shell=True)
+    if not _os_symlink:
+        _os_symlink = getattr(os, 'symlink', None)
 
-        os_symlink = symlink_ms
+        if os.name == 'nt':
+            if not callable(_os_symlink):
+                _os_symlink = create_symlink_polyfill()
+            else:
+                enable_retry_with_polyfill = True
 
-    os_symlink(source, link_name)
+    try:
+        _os_symlink(source, link_name)
+    except OSError:
+        if not enable_retry_with_polyfill:
+            raise
 
+        _os_symlink = create_symlink_polyfill()
+        _os_symlink(source, link_name)
 
 def remove_symlink(link):
     if os.path.isdir(link):