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Date: Mon, 24 Aug 2015 16:58:33 +0200
From: =?UTF-8?B?SmFuIEthbHXFvmE=?= <jkaluza@redhat.com>
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To: dev@httpd.apache.org
Subject: Re: PR 58267: Regression in 2.2.31 caused by r1680920
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On 08/24/2015 04:47 PM, Jan Kaluža wrote:
> Hi,
>
> unfortunately, the r1680920 brought undesired behavior described in PR
> 58267 to 2.2.x. The bug is well described in the PR, so I won't describe
> it in this email.
>
> I have tried to debug it and I think the problem is that we use also
> server->server_hostname to compute the hash in the
> ap_proxy_set_scoreboard_lb. This hash is used to find out proper
> ap_scoreboard field.
>
> It all happens in mod_proxy.c:child_init's scope.
>
> If the "<Proxy Balancer://foobar>" has been defined, all the
> BalancerMembers are initialized with the hash computed with usage of
> global server->server_hostname.
>
> Later, if the "ProxyPass /foobar/ Balancer://foobar/" has been used in
> the VirtualHost, ap_proxy_initialize_worker_share is called again with
> server->server_hostname set to the VirtualHost's one.
>
> Now, the root of the error is that the scoreboard size is static (set to
> proxy_lb_workers + PROXY_DYNAMIC_BALANCER_LIMIT), but it is not
> incremented when ProxyPass with balancer is used in the virtualhost.
> This leads to lack of space in scoreboard when Balancers are used in
> multiple virtualhosts.
>
> I think there are two possible fixes:
>
> 1) Do not use server->server_hostname when computing hash which is used
> to determine right scoreboard field. I think this would fix this bug,
> but I'm not sure what would happen in situations when you define 2
> balancers with the same name in different virtualhosts...
>
> On the other-side, when there is global Proxy balancer, it make sense to
> use the same worker->s for all the ProxyPass in virtualhosts.

It seems that 2.4.x uses just the name of the worker to determine slot 
for shared memory, so maybe it wouldn't be a problem for 2.2.x too...

Jan Kaluza

> 2) Increment proxy_lb_workers according to number of workers in balancer
> when using "ProxyPass /foobar/ Balancer://foobar/" in the VirtualHost.
> The scoreboard would have right size and ap_proxy_set_scoreboard_lb
> would not fail then.
>
>
> Since it's 2.2.x which should be probably stable without big changes,
> I'm asking the list for more opinions... I will try to implement patch
> for option 2) tomorrow and see if this really fixes the issue.
>
> Regards,
> Jan Kaluza