fix hang because of infinite loop reading EOF

Read() should return empty string when EOF happen, instead of looping
forever. This is the right semantics of read(), and a wrapped version
should not change it.

If you read the read(2) system call manpage, it tells you that when EOF
is seen, return value is 0; it does not say

	``loop forever when EOF happen''.

After the EOF detection is patched you can see the
following exception:

  WARNING: ERROR attempting to copy message 344 for account Gmail:Traceback (most recent call last):
    File "/usr/lib/pymodules/python2.6/offlineimap/folder/Base.py", line 282, in copymessageto
      message = self.getmessage(uid)
    File "/usr/lib/pymodules/python2.6/offlineimap/folder/IMAP.py", line 216, in getmessage
      initialresult = imapobj.uid('fetch', '%d' % uid, '(BODY.PEEK[])')
    File "/usr/lib/python2.6/imaplib.py", line 753, in uid
      typ, dat = self._simple_command(name, command, *args)
    File "/usr/lib/python2.6/imaplib.py", line 1060, in _simple_command
      return self._command_complete(name, self._command(name, *args))
    File "/usr/lib/python2.6/imaplib.py", line 890, in _command_complete
      raise self.abort('command: %s => %s' % (name, val))
  abort: command: UID => socket error: EOF

Signed-off-by: Bao Haojun <baohaojun@gmail.com>
Signed-off-by: Nicolas Sebrecht <nicolas.s-dev@laposte.net>

offlineimap/imaplibutil.py

@@ -48,7 +48,10 @@ class IMAP4_Tunnel(IMAP4):
     def read(self, size):
         retval = ''
         while len(retval) < size:
-            retval += self.infd.read(size - len(retval))
+            buf = self.infd.read(size - len(retval))
+            if not buf:
+                break
+            retval += buf
         return retval
 
     def readline(self):
@@ -188,6 +191,8 @@ class WrappedIMAP4_SSL(IMAP4_SSL):
         read = 0
         while read < n:
             data = self._read_upto (n-read)
+            if not data:
+                break
             read += len(data)
             chunks.append(data)
 
@@ -200,6 +205,8 @@ class WrappedIMAP4_SSL(IMAP4_SSL):
         retval = ''
         while 1:
             linebuf = self._read_upto(1024)
+            if not linebuf:
+                return retval
             nlindex = linebuf.find("\n")
             if nlindex != -1:
                 retval += linebuf[:nlindex + 1]