[issue7322] Socket timeout can cause file-like readline() method to lose data

[David M. Beazley]

New submission from David M. Beazley <beazley at users.sourceforge.net>:

Consider a socket that has had a file-like wrapper placed around it 
using makefile()

# s is a socket created previously
f = s.makefile()

Now, suppose that this socket has had a timeout placed on it.

s.settimeout(15)

If you try to read data from f, but nothing is available. You'll 
eventually get a timeout. For example:

f.readline()   # Now, just wait
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File 
"/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/socket.
py", line 406, in readline
    data = self._sock.recv(self._rbufsize)
socket.timeout: timed out

However, now consider the case where you're reading a line of data, but 
the receiver has only received a partial line and it's waiting for the 
rest of the data to arrive.   For example, type this:

f.readline()

Now, go to the other end of the socket connection and send a buffer with 
no newline character.  For example, send the message "Hello".

Since no newline character has been received, the readline() method will 
eventually fail with a timeout as before.   However, if you now retry 
the read operation f.readline() and send more data such as the message 
"World\n", you'll find that the "Hello" message gets lost.  In other 
words, the repeated readline() operation discards any buffers 
corresponding to previously received line data and just returns the new 
data.

Admittedly this is a corner case, but you probably don't want data to be 
discarded on a TCP connection even if a timeout occurs.

Hope that makes some sense :-).  (It helps to try it out).

----------
components: Library (Lib)
messages: 95245
nosy: beazley
severity: normal
status: open
title: Socket timeout can cause file-like readline() method to lose data
type: behavior
versions: Python 2.6

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue7322>
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