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

[Antoine Pitrou]

Antoine Pitrou <pitrou at free.fr> added the comment:

> """Generally there is no guarantee that a buffered object works
> "properly" when the raw IO object raises some exception
> intermittently"""
> 
> I disagree.  EINTR is a classic case of this and is something that
> buffering IO layers deal with all the time.  (readline is just one
> example of a buffering io layer)

EINTR is a different matter. To handle EINTR in Python, it is enough to
call the signal handlers and then retry the system call (that's what is
done in SocketIO.readinto, although FileIO doesn't have such logic).
Only if the signal handler raises an exception (which it probably
shouldn't do, since asynchronous exceptions are very bad) do you abort
the operation.

You can't apply the same logic to a socket timeout; the timeout is
really an error condition and you certainly shouldn't retry the system
call (that would defeat the point of using a timeout). So, to handle it
in an entirely correct way, you need to add some out-of-band buffering
logic where you store the pending raw reads which have been done but
could not be returned to the user. That complicates things quite a bit,
especially given that it has to be grafted on at least two layers of the
IO stack (the raw IO layer, and the buffered IO layer). Ross' patch does
it, but incompletely (it lets the user handle the out-of-band data) and
only for readline() (while buffered read() would probably need it too).

> The normal behavior for code calling readline() on a socket with a
> timeout is likely going to be to close it.  Anything else does not
> make much sense.  (someone may try, but really they're writing their
> I/O code wrong if they are using a socket timeout a poor form of task
> switching ;)

That's my opinion too. So, instead, of doing the above surgery inside
the IO stack, the SocketIO layer could detect the timeout and disallow
further access. What do you think?

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