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It was reported in bug #399 that the DNS sometimes enters endless loops after hours working fine. The issue is caused by a lack of error processing in the DNS's recv() path combined with an exclusive recv OR send in the UDP layer, resulting in some errors causing CPU loops that will never stop until the process is restarted. The basic cause is that the FD_POLL_ERR and FD_POLL_HUP flags are sticky on the FD, and contrary to a stream socket, receiving an error on a datagram socket doesn't indicate that this socket cannot be used anymore. Thus the Rx code must at least handle this situation and flush the error otherwise it will constantly be reported. In theory this should not be a big issue but in practise it is due to another bug in the UDP datagram handler which prevents the send() callback from being called when Rx readiness was reported, so the situation cannot go away. It happens way more easily with threads enabled, so that there is no dead time between the moment the FD is disabled and another recv() is called, such as in the example below where the request was sent to a closed port on the loopback provoking an ICMP unreachable to be sent back: [pid 20888] 18:26:57.826408 sendto(29, ";\340\1\0\0\1\0\0\0\0\0\1\0031wt\2eu\0\0\34\0\1\0\0)\2\0\0\0\0\0\0\0", 35, 0, NULL, > [pid 20893] 18:26:57.826566 recvfrom(29, 0x7f97c54ef2f0, 513, 0, NULL, NULL) = -1 ECONNREFUSED (Connection refused) [pid 20889] 18:26:57.826601 recvfrom(29, 0x7f97c76182f0, 513, 0, NULL, NULL) = -1 EAGAIN (Resource temporarily unavailable) [pid 20892] 18:26:57.826630 recvfrom(29, 0x7f97c5cf02f0, 513, 0, NULL, NULL) = -1 EAGAIN (Resource temporarily unavailable) [pid 20891] 18:26:57.826684 recvfrom(29, 0x7f97c66162f0, 513, 0, NULL, NULL) = -1 EAGAIN (Resource temporarily unavailable) [pid 20895] 18:26:57.826716 recvfrom(29, 0x7f97bffda2f0, 513, 0, NULL, NULL) = -1 EAGAIN (Resource temporarily unavailable) [pid 20894] 18:26:57.826747 recvfrom(29, 0x7f97c4cee2f0, 513, 0, NULL, NULL) = -1 EAGAIN (Resource temporarily unavailable) [pid 20888] 18:26:58.419838 recvfrom(29, 0x7ffcc8712c20, 513, 0, NULL, NULL) = -1 EAGAIN (Resource temporarily unavailable) [pid 20893] 18:26:58.419900 recvfrom(29, 0x7f97c54ef2f0, 513, 0, NULL, NULL) = -1 EAGAIN (Resource temporarily unavailable) (... hundreds before next sendto() ...) This situation was handled by clearing HUP and ERR when recv() returns <0. A second case was handled, there was a control for a missing dgram handler, but it does nothing, causing the FD to ring again if this situation ever happens. After looking at the rest of the code, it doesn't seem possible to face such a situation because these handlers are registered during startup, but at least we need to handle it properly. A third case was handled, that's mainly a small optimization. With threads and massive responses, due to the large lock around the loop, it's likely that some threads will have seen fd_recv_ready() and will wait at the lock(). But if they wait here, chances are that other threads will have eliminated pending data and issued fd_cant_recv(). In this case, better re-check fd_recv_ready() before performing the recv() call to avoid the huge amounts of syscalls that happen on massively threaded setups. This patch must be backported as far as 1.6 (the atomic AND just needs to be turned to a regular AND).
The HAProxy documentation has been split into a number of different files for ease of use. Please refer to the following files depending on what you're looking for : - INSTALL for instructions on how to build and install HAProxy - BRANCHES to understand the project's life cycle and what version to use - LICENSE for the project's license - CONTRIBUTING for the process to follow to submit contributions The more detailed documentation is located into the doc/ directory : - doc/intro.txt for a quick introduction on HAProxy - doc/configuration.txt for the configuration's reference manual - doc/lua.txt for the Lua's reference manual - doc/SPOE.txt for how to use the SPOE engine - doc/network-namespaces.txt for how to use network namespaces under Linux - doc/management.txt for the management guide - doc/regression-testing.txt for how to use the regression testing suite - doc/peers.txt for the peers protocol reference - doc/coding-style.txt for how to adopt HAProxy's coding style - doc/internals for developer-specific documentation (not all up to date)