So, for example I want to always drop creating snapshots and send an error to client with my custom message (NOT custom error type, but exactly custom message, I don't want to modify any libvirt code on client and adding my errors). I am operating with bare tcp traffic, so I can easily read everything in the middle.

'internal error: unexpected domain snapshot snapshot1 already exists'

and started sending it every time I see procedure : REMOTE_PROC_DOMAIN_SNAPSHOT_CREATE_XML

I thought, that this is possible to insert custom message here, but this is not true. When I change the length of the message on one symbol - it is still correct, on two -also , on three symbols - I get an error that the size of the payload is incorrect. Some other errors when I insert a message of random length.

What is strange, that if I insert 'xxxxxxxxxxxxx...' message , which has the same length as 'internal error: unexpected domain snapshot snapshot1 already exists' - everything is parsed fine.

So, how does this stuff work? Are there anywhere predefined lengths for error messages and checks against them, or are there checks against exact error strings? How all this stuff is regulated? I know now, that I can pad my message with ...say, space symbols and it will be ok, but I want to know how error parsing works in all cases.

I have looked through files, where an overall structure and logic of errors is described but didn't find any helpful information. And I don't a good way to debug libvirt to see, where this happens.