b11495652e
As unveiled in GH issue #2711, commit 5541d4995d ("BUG/MEDIUM: queue: deal with a rare TOCTOU in assign_server_and_queue()") does have some side effects in that it can occasionally cause an endless loop. As Christopher analysed it, the problem is that process_srv_queue(), which uses a trylock in order to leave only one thread in charge of the dequeueing process, can lose the lock race against pendconn_add(). If this happens on the last served request, then there's no more thread to deal with the dequeuing, and assign_server_and_queue() will loop forever on a condition that was initially exepected to be extremely rare (and still is, except that now it can become sticky). Previously what was happening is that such queued requests would just time out and since that was very rare, nobody would notice. The root of the problem really is that trylock. It was added so that only one thread dequeues at a time but it doesn't offer only that guarantee since it also prevents a thread from dequeuing if another one is in the process of queuing. We need a different criterion. What we're doing now is to set a flag "dequeuing" in the server, which indicates that one thread is currently in the process of dequeuing requests. This one is atomically tested, and only if no thread is in this process, then the thread grabs the queue's lock and dequeues. This way it will be serialized with pendconn_add() and no request addition will be missed. It is not certain whether the original race covered by the fix above can still happen with this change, so better keep that fix for now. Thanks to @Yenya (Jan Kasprzak) for the precise and complete report allowing to spot the problem. This patch should be backported wherever the patch above was backported.
HAProxy
HAProxy is a free, very fast and reliable reverse-proxy offering high availability, load balancing, and proxying for TCP and HTTP-based applications.
Installation
The INSTALL file describes how to build HAProxy. A list of packages is also available on the wiki.
Getting help
The discourse and the mailing-list are available for questions or configuration assistance. You can also use the slack or IRC channel. Please don't use the issue tracker for these.
The issue tracker is only for bug reports or feature requests.
Documentation
The HAProxy documentation has been split into a number of different files for ease of use. It is available in text format as well as HTML. The wiki is also meant to replace the old architecture guide.
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)
License
HAProxy is licensed under GPL 2 or any later version, the headers under LGPL 2.1. See the LICENSE file for a more detailed explanation.