Detect if symlinks are supported on target file system

This detects symlink suppot on destination filesystem
and not only in Python interpreter or OS. If symlinks
are not supported, falls back to copy behavior.

Author: techtonik

virtualenv.py

@@ -1100,6 +1100,27 @@ def install_python(home_dir, lib_dir, inc_dir, bin_dir, site_packages, clear, sy
         ## Maybe it should delete everything with #!/path/to/venv/python in it
         logger.notify('Not deleting %s', bin_dir)
 
+    # Check if symlinks are really supported by destination filesystem
+    if symlink:
+        if not hasattr(os, 'symlink'):
+            symlink = False
+            logger.info('Symlinks are not supported in this Python version')
+        else:
+            name = join(home_dir + '.symlink.test.file')
+            open(name, 'w').close()
+            try:
+                os.symlink(name, name + '.link')
+            except OSError as exc:
+                if exc.errno == 71: # [Errno 71] Protocol error
+                    symlink = False
+                    logger.notify('Symlinks are not supported on target file system')
+                else:
+                    raise
+            else:
+                os.remove(name + '.link')
+            finally:
+                os.remove(name)
+
     if hasattr(sys, 'real_prefix'):
         logger.notify('Using real prefix %r' % sys.real_prefix)
         prefix = sys.real_prefix
@@ -1115,7 +1136,7 @@ def install_python(home_dir, lib_dir, inc_dir, bin_dir, site_packages, clear, sy
         stdlib_dirs.append(join(os.path.dirname(stdlib_dirs[0]), 'DLLs'))
     elif is_darwin:
         stdlib_dirs.append(join(stdlib_dirs[0], 'site-packages'))
-    if hasattr(os, 'symlink'):
+    if symlink:
         logger.info('Symlinking Python bootstrap modules')
     else:
         logger.info('Copying Python bootstrap modules')