Avoid copying FieldStorage if possible. (#30)

On Python 3, cgi.FieldStorage has a __del__ method that closes the underlying file [1]. This means that if the copy made from UnicodeMultiDict._decode_value is garbage collected, the file underlying the original FieldStorage will be closed! Fix this by not copying FieldStorage if it is not required by decode_keys=False. I cannot think of a nice way to fix this problem if decode_keys=True.

[1] https://github.com/python/cpython/commit/f79126f373a9d5c9b584a8db736fe490fcbfa77a

2 files changed, 15 insertions, 10 deletions

diff --git a/paste/util/multidict.py b/paste/util/multidict.py
index 0b63ebb..0a304d3 100644
--- a/paste/util/multidict.py
+++ b/paste/util/multidict.py
@@ -265,11 +265,13 @@ class UnicodeMultiDict(DictMixin):
         """
         if isinstance(value, cgi.FieldStorage):
             # decode FieldStorage's field name and filename
-            value = copy.copy(value)
-            if self.decode_keys and isinstance(value.name, six.binary_type):
-                value.name = value.name.decode(self.encoding, self.errors)
-            if six.PY2:
-                value.filename = value.filename.decode(self.encoding, self.errors)
+            decode_name = self.decode_keys and isinstance(value.name, six.binary_type)
+            if six.PY2 or decode_name:
+                value = copy.copy(value)
+                if decode_name:
+                    value.name = value.name.decode(self.encoding, self.errors)
+                if six.PY2:
+                    value.filename = value.filename.decode(self.encoding, self.errors)
         else:
             try:
                 value = value.decode(self.encoding, self.errors)
diff --git a/tests/test_multidict.py b/tests/test_multidict.py
index a33c13b..f6619fc 100644
--- a/tests/test_multidict.py
+++ b/tests/test_multidict.py
@@ -2,10 +2,11 @@
 # (c) 2007 Ian Bicking and Philip Jenvey; written for Paste (http://pythonpaste.org)
 # Licensed under the MIT license: http://www.opensource.org/licenses/mit-license.php
 import cgi
+import gc
+import io
 
 import pytest
 import six
-from six.moves import StringIO
 
 from paste.util.multidict import MultiDict, UnicodeMultiDict
 
@@ -149,14 +150,16 @@ def _test_unicode_dict(decode_param_names=False):
     fs = cgi.FieldStorage()
     fs.name = 'thefile'
     fs.filename = 'hello.txt'
-    fs.file = StringIO('hello')
+    fs.file = io.BytesIO(b'hello')
     d[k('f')] = fs
     ufs = d[k('f')]
     assert isinstance(ufs, cgi.FieldStorage)
-    assert ufs is not fs
     assert ufs.name == fs.name
     assert isinstance(ufs.name, str if six.PY3 else key_str)
     assert ufs.filename == fs.filename
     assert isinstance(ufs.filename, six.text_type)
-    assert isinstance(ufs.value, str)
-    assert ufs.value == 'hello'
+    assert isinstance(ufs.value, bytes)
+    assert ufs.value == b'hello'
+    ufs = None
+    gc.collect()
+    assert not fs.file.closed