Index: twisted/internet/test/test_process.py
===================================================================
--- twisted/internet/test/test_process.py	(revision 31396)
+++ twisted/internet/test/test_process.py	(working copy)
@@ -599,6 +599,15 @@
 
 
 
+class FakeFile(object):
+    """
+    A fake file object which pretends it can be closed.
+    """
+    def close(self):
+        pass
+
+
+
 class FakeResourceModule(object):
     """
     Fake version of L{resource} which hard-codes a particular rlimit for maximum
@@ -682,8 +691,14 @@
         """
         This is a mock for L{open}.  It just keeps track of the fact that a file
         has opened so an extra file descriptor is found in /dev/fd.
+
+        It also returns a L{FakeFile} which can be "closed". It would be more
+        realistic to make the FD list shrink when the file is "fake closed" but
+        it's not necessary for the detection algorithm we currently have in
+        process.py.
         """
         self.openedFile = True
+        return FakeFile()
 
 
     def saveResourceModule(self):
Index: twisted/internet/process.py
===================================================================
--- twisted/internet/process.py	(revision 31396)
+++ twisted/internet/process.py	(working copy)
@@ -530,6 +530,7 @@
         start = self.listdir("/dev/fd")
         fp = self.openfile("/dev/null", "r")
         end = self.listdir("/dev/fd")
+        fp.close()
         return start != end