[systemd-devel] [RFC 4/6] proxy-discoveryd: Execute the PAC based proxy in a thread

[Marcel Holtmann]

Hi Lennart,

>>> sd-bus is not thread-safe. You cannot send messages from different
>>> threads at the same time...
>>> 
>>> Why precisely do you need threads here? Because the PAC programs might
>>> be slow?
>> 
>> the Javascript execution is normally blocking and it can do whatever
>> it wants. If it wants to draw a smiley face, while looking up the
>> proxy, it can do that.
>> 
>> I think the PAC execution needs to be done in a thread, but not the
>> D-Bus message handling.
>> 
>>> WHat about unbounded loops in PAC programs? We need to protect from
>>> that, which kinda suggests we muight want to run the PAC stuff out of
>>> process so that we can neatly kill the program when it runs for too
>>> long?
>> 
>> Remember that in theory for every HTTP request a browser makes it is
>> suppose to ask the PAC file first. Meaning it will run this a
>> lot. So spawning processes might be not really useful. Even if we
>> start one PAC processing process for each interface, you still want
>> threads since you might have multiple requests from different
>> application. And if one can be answered from a cache, then you do
>> not want to wait until the Javascript execution finished.
> 
> Hmm, this really feels like we should still do this out-of-process
> with a worker process logic. Reuse processes to keep the cost down,
> and run up to a fixed number of them in parallel to allow multiple
> sumultaneous requests. THis would have have the major added benefit
> that we could lock the worker threads down with seccomp and whatnot,
> which is certainly a good thing given that we'll execute foreign,
> untrusted code we downloaded from the network with this. And we can
> easily kill the execution with a time limit if we desir to do so...

I get the feeling that this needs to be measured and someone has to write some performance tool around this. So while you have only seen tiny PAC files, I am seen ours. And that one is massive. I bet that other corporation have similar large ones.

Regards

Marcel