Select hangs after some reads

[alsmeirelles at gmail.com]

Steve Holden wrote:
> alsmeirelles at gmail.com wrote:
> > Hi,
> >
> > I'm building a multithreaded application and I encountered a tiny and
> > annoying problem. I use a select to wait for data to be read from a
> > socket, after some reads, the select simply blocks and stays that way
> > until I close the connection on the other side of the socket. When the
> > socket is closed on the writer end the select releases and then I get
> > only empty strings from the socket.
>
> That's to be expected: the first return of a zero-length string from
> socket.read() indicates end of file.
>
> > My question is this: Why did it block? The reading has never ended,
> > every test I make I write 50 requests (wich are strings) to the socket
> > and I have read the maximum of 34 requests. I'm using winPdb to take a
> > closer look on what's happening and I see the threads blocked on this
> > same select. If I send anything more through the socket, the select
> > releases for a thread, despite the other data that is still unread.
> >
> Are you sure that the received data is presenting in block the same size
> as are being sent? There's no guarantee this will be so on a TCP socket,
> and it may be that multiple sends are being coalesced into a single
> read. The important thing to focus on is the total number of bytes sent
> and received.

Well, actually I´m using a very simple protocol wich sends only
strings ended by newline. I need to send 3 chunks of information and a
newline after them. On the reader side I make 3 readline(), this way I
wouldn´t have to care about this problem, but maybe that´s where I´m
falling. If that´s the case, I´ll have to use a more complex
protocol.

>
> > This is the select:
> >  rd,w,e = select.select([self.rfd],[],[])
> >
> > self.rfd is the fileno of the file object returned by the makefile
> > method from the socket object.
> >
> It would be simpler to use the result of the socket's .fileno() method
> directly.
>
Yes, you are right, this is something remanescent from old ideas.

> > I know that's some buffer behavior that I'm missing but I don't know
> > what it is.
> >
> > anything is helpfull, If I'm beeing stupid you can say it.
> > thanks
> >
> Without being able to see all your code it's hard to say whether or not
> you are doing something daft, but you give a general impression of
> competence.
>

Thanks, the code has a significant size now, if I don´t solve it
quickly I´ll post the most important parts.

thanks again,

Andre LS Meirelles
> regards
>   Steve
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