Problem with socket

[Thomas Hervé]

My problem is not really python specific but as I do my implementation
in python I hope someone here can help me.

I have two programs that talk through a socket. Here is the code :

<server>
sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
sock.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)

sock.settimeout(5.0)

sock.bind(("", port)
# We only handle one connection
sock.listen(1)

while 1:
newsock, address = sock.accept()
handle(newsock, address)

def handle(sock, address) :
print "Connection from", address
dataReceived = newsock.recv(1024)
while 1:
try:
newsock.send(data)
except socket.timeout, err:
print "Connection timeout %s" % err
break
except socket.error, err:
print "Connection broken %s" % err
break

</server>

<client>
sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
sock.settimeout(5.0)
sock.connect((host, port))
sock.send("Gimme a piece of information, please\r\n")

while 1:
r, w, e = select.select([s], [], [], 1.0)
if r != []:
handle_connection(s)
def handle_connection(sock):
try:
response_data = sock.recv(1024)
except socket.timeout:
print "Socket timeout"
return
manage(response_data)

</client>

Ok I hope it's clear. My problem is that "select" only tests if there's
data on the socket, and not the state of the socket. I want to be able
to know if socket is "alive" or something like that. But I don't want
to make "send" in the client (stay passive). Then, if I know that my
socket is closed or my link down, I can try to reconnect periodically.

I would rewrite the while in the client like that :
<client>
while 1:
if not test_connect(s) :
reconnect(s)
# else continue normally
r, w, e = select.select([s], [], [], 1.0)
if r != []:
handle_connection(s)
</client

My env: python2.3, linux/win32.

Thanks in advance,

--
Thomas Herve <sorry for my english>

[Donn Cave]

In article <40***********************@news.free.fr>,
Thomas Herve <th****@neocles.com> wrote:
....

Ok I hope it's clear. My problem is that "select" only tests if there's
data on the socket, and not the state of the socket. I want to be able
to know if socket is "alive" or something like that. But I don't want
to make "send" in the client (stay passive). Then, if I know that my
socket is closed or my link down, I can try to reconnect periodically.

Try a SO_KEEPALIVE option on the socket (setsockopt). That
should (I would sort of expect anyway) raise an exception when
the keepalive negotiation fails. The reaction won't be immediate,
it may take hours to notice a dead connection.

Donn Cave, do**@u.washington.edu

[Grant Edwards]

On 2004-04-28, Thomas Hervé <th****@neocles.com> wrote:

Ok I hope it's clear. My problem is that "select" only tests
if there's data on the socket, and not the state of the
socket.
That's not true. If the socket is closed, it will return from
select as readable.
I want to be able to know if socket is "alive" or something
like that. But I don't want to make "send" in the client (stay
passive). Then, if I know that my socket is closed or my link
down, I can try to reconnect periodically.

You'll know if the socket is closed because select will mark it
as readable, and you'll get 0 bytes when you read it.

If you want to know if the network or the other host has "gone
away", set the SO_KEEPALIVE option on the socket. That will
generate an error if the link is down. IIRC, it takes 75
minutes (or is it 150?) to time out after the other end goes
away.

--
Grant Edwards grante Yow! I wonder if there's
at anything GOOD on tonight?
visi.com

[Thomas Hervé]

Grant Edwards wrote:

On 2004-04-28, Thomas Hervé <th****@neocles.com> wrote:

Ok I hope it's clear. My problem is that "select" only tests
if there's data on the socket, and not the state of the
socket.
That's not true. If the socket is closed, it will return from
select as readable.

Exact.

I want to be able to know if socket is "alive" or something
like that. But I don't want to make "send" in the client (stay
passive). Then, if I know that my socket is closed or my link
down, I can try to reconnect periodically.

You'll know if the socket is closed because select will mark it
as readable, and you'll get 0 bytes when you read it.

It seems that it's not true : when I read it I have a "Connection reset
by peer" error.
If you want to know if the network or the other host has "gone
away", set the SO_KEEPALIVE option on the socket. That will
generate an error if the link is down. IIRC, it takes 75
minutes (or is it 150?) to time out after the other end goes
away.

Yes, I've seen this option, but the delay is a bit too large (2 hours ?
depending on OS it seems) and not configurable. So I made it by send
some hearbeat regulary, even if I didn't want to I haven't found another
way. And it's better for server side, from which I can make some recv
that make the break found earlier.

--
Thomas Herve