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Re: [Fastcgipp-users] SIGPIPE causes SEGFAULT

From: Volker Schreiner
Subject: Re: [Fastcgipp-users] SIGPIPE causes SEGFAULT
Date: Mon, 10 Jan 2011 09:13:51 +0100
User-agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv: Gecko/20101208 Lightning/1.0b2 Thunderbird/3.1.7

After i downloaded
and installed the library it now seems to work.
The same test with apache benchmark causes the MySql Example to cause an
timeout. Thats the
behaviour i expected if the Webserver is not able to handle all
connections. After the connections timed out
and the system cooled down i can access the application via the browser.
So i think the problem is solved.

Am 08.01.2011 21:07, schrieb Eddie Carle:
> On Wed, 2010-11-10 at 00:51 +0100, Volker Schreiner wrote:
>> The system i am working with is a Core2Duo with 2 Gbytes of ram
>> running a Ubuntu 10.04 LTS - Lucid Lynx with the installed
>> prerequisites of boost 1.40 and libmysqlclient16. I am using nginx
>> version 0.7.65 that connects the fastcgi application through a TCPIP
>> socket on the
>> IP address (localhost). Furthermore nginx starts 10
>> worker_processes with up to 1024 concurrent worker_connections per
>> process (10240 concurrent connections). The test i
>> was running on localhost with Apache Benchmark counted 10000 Requests
>> with 1000 concurrent requests. I think the 1000 concurrent requests in
>> combination with the busy system caused the nginx to create a large
>> number of SIGPIPE signals that needs to be handled
>> by the busy webapplication and causes a confusion in the transceiver
>> that leads to the segmentation fault.
> Alrighty, I've finally gotten around to messing around with this and
> I've discovered some interesting issues. I can't reproduce this so after
> perusing through the code and your data I've come up with a hypothesis.
> First off I'll mention that I'm pretty sure this ironically has nothing
> to do with signals. The following is a description of how fastcgi++
> handles opening/closing file descriptors.
>      1. The FastCGI protocol specifies that the server should tell the
>         FastCGI application whether or not to handle closing the pipe/fd
>         upon request completion when the request is first set up (see
>         Protocol::BeginRequest in the documentation). Most servers do
>         one Request per fd (which really annoys me because it is so
>         stupidly inefficient) and therebye leave management of the pipes
>         to the applications. It looks like when Nginx gets a lot on it's
>         plate it starts closing pipes despite this.
>      2. fastcgi++ takes this value and sets it into the Request objects
>         at manager.hpp:264. 
>      3. When the Fastcgipp::Request has data to send back to the server,
>         it writes it into a ring buffer and when the request is complete
>         it writes a "request complete" record into the ring buffer at
>         request.cpp:160. With this line we also embed a boolean value
>         (the value from step 1) into the chunk of data in the ring
>         buffer telling Transceiver to close the pipe/fd when said chunk
>         is transmitted. 
>      4. Once the data chunk is transmitted,
>         Transceiver::Buffer::freeRead() is called to free the data in
>         the ring buffer. Notice at transceiver.cpp:175 it tests whether
>         or not the pipe should be closed or not. 
>      5. transceiver.cpp:177 searches the Transceiver::pollFds container
>         for the appropriate fd and immediately calls erase on said fd. 
>      6. This is all fine and dandy except that transceiver.cpp:36 has
>         already called an erase on that fd should we run into an EPIPE
>         error while trying to write to said fd. 
>      7. What is segfaulting? Well, my guess is that we are calling
>         pollFds.erase() on pollFds.end() and that is doing it.
> So to fix this I guess a check to see if the iterator is pollFds.end()
> before calling erase on it is the best solution. I've pushed an
> attempted fix to the git repo. Please test it out as apache doesn't have
> the same issue.


Volker Schreiner

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