I
have an application which uses libpq for interaction with remote
PostgreSQL 9.2.4 server. Clients and Server nodes are running Linux and
connection is established using TCPv4. The client application has some
small fault-tolerance features, which are activated when server related
problems are encountered.
One day some bad things happened with network layer hardware and, long story short, host with PSQL server got isolated. All TCP messages routed to server node were NOT delivered or acknowledged in any way. Client application got blocked in libpq code according to debugger.
I have successfully reproduced the problem in the laboratory environment. These iptables commands should be run on the server node after some period of client <-> server interaction:
# iptables -A OUTPUT -p tcp --sport 5432 -j DROP
# iptables -A INPUT -p tcp --dport 5432 -j DROP
I made a glimpse over master branch of libpq sources and some questions arose. Namely:
1. Connection to PSQL server is made without an option to specify SO_RCVTIMEO and SO_SNDTIMEO. Why is that? Is setting socket timeouts considered harmful?
2. PQexec ultimately leads to PQwait, which after some function calls "lands" in pqSocketCheck and pqSocketPoll. These 2 functions have parameter end_time. It is set (-1) for PQexec scenario, which leads to infinite poll timeout in pqSocketPoll. Is it possible to implement configurable timeout for PQexec calls? Is there some implemented features, which should be used to handle situation like this?
Currently, I have changed Linux kernel tcp4 stack counters responsible for retransmission, so OS actually closes socket after some period. This is detected by pqSocketPoll's poll and libpq handles situation correctly - error is reported to my application. But it's just a workaround.
So, this infinite poll situation looks like imperfection to me and I think it should be considered as a bug. Is it?
With regards,One day some bad things happened with network layer hardware and, long story short, host with PSQL server got isolated. All TCP messages routed to server node were NOT delivered or acknowledged in any way. Client application got blocked in libpq code according to debugger.
I have successfully reproduced the problem in the laboratory environment. These iptables commands should be run on the server node after some period of client <-> server interaction:
# iptables -A OUTPUT -p tcp --sport 5432 -j DROP
# iptables -A INPUT -p tcp --dport 5432 -j DROP
I made a glimpse over master branch of libpq sources and some questions arose. Namely:
1. Connection to PSQL server is made without an option to specify SO_RCVTIMEO and SO_SNDTIMEO. Why is that? Is setting socket timeouts considered harmful?
2. PQexec ultimately leads to PQwait, which after some function calls "lands" in pqSocketCheck and pqSocketPoll. These 2 functions have parameter end_time. It is set (-1) for PQexec scenario, which leads to infinite poll timeout in pqSocketPoll. Is it possible to implement configurable timeout for PQexec calls? Is there some implemented features, which should be used to handle situation like this?
Currently, I have changed Linux kernel tcp4 stack counters responsible for retransmission, so OS actually closes socket after some period. This is detected by pqSocketPoll's poll and libpq handles situation correctly - error is reported to my application. But it's just a workaround.
So, this infinite poll situation looks like imperfection to me and I think it should be considered as a bug. Is it?
