Truncated or oversized response headers received from daemon process
Turned out to be not the actual problem. The problem was lying deeper, as I changed Cairo to CairoCffi and the RSVG-Handler couldn't deal with the Context-Object coming from Cffi. No my actuall problem is having an up-to-date python lib that allows me to convert svg into png. Using svg2png from CairoSVG isn't working for me. I get an
cairo returned CAIRO_STATUS_NO_MEMORY: out of memory
Error, which I'm sure of, that it does not tell the truth again and the problem lies somewhere else. But lets see.
The code used from Apache by mod_wsgi applies a limit on the size of a single response header returned from mod_wsgi daemon mode processes. This would result in a really cryptic error message from Apache which didn't at all point to the problem. From memory the previous error was:
Premature end of script headers
The size limit was also hard coded in Apache and couldn't be changed. This has caused problems for some Python web applications such as Keystone in OpenStack which generates very large authentication headers.
In mod_wsgi 4.1+, the reliance on the Apache code has been removed and the limit is now configurable. The error message is also more specific as you have seen.
The default maximum header size for what is returned from mod_wsgi daemon mode, that is header name and value, is about 8192 bytes. You can override this by using the header-buffer-size option to WSGIDaemonProcess.
Can you indicate what application and what header was so large that the limit was reached as would like to know what other Python web applications besides Keystone are generating such large headers if is a commonly used application.
A second possibility, deriving from the 'truncated' reference in that message, is that your mod_wsgi daemon process crashed. You don't say though that you saw a 'Segmentation fault' or similar message indicating that a crash occurred. Check for that and if there are other messages in the error log at the time, then indicate what they were and can look at that as the primary problem.
We recently ran into this issue, and after days of vigorous ugoogleizing and massive headache, we discovered that we were using psycopg2-binary as our database connector dependency (I know, newbs)! It states right in their documentation not to use the package in a production environment.
We did add all the other proposed answers such as adding 'WSGIApplicationGroup %{GLOBAL}' to our settings (which we kept), but all of them alone and together didn't solve the issue.
We also found that other C libraries like numpy, cause issues.
Hope this helps someone some day.
Django Webfaction 'Timeout when reading response headers from daemon process'
http://initd.org/psycopg/docs/install.html#prerequisites