Support more libvmod_tinykvm functionality in simplekvm

[lrowe]

For debugging programs intended to be run by libvmod_tinykvm it would be helpful if simplekvm or a similar program supported a larger subset of libvmod_tinykvm's functionality.

Specifically I'd like to be able to provide a configuration json file or string that let me set environment variables, allowed files, and memory limits so I can easily run my guest program under simplekvm and attach gdb.

[fwsGonzo]

I agree. I'll be porting the system calls implemented in the VMOD to the main library at some point, just not right now.

That said, there is scaffolding to debug requests directly in Varnish with GDB. So maybe an easier way forward is to expose that.

[fwsGonzo]

I've gotten basic debugging working again now. The JSON to enable is "allow_debug": true. Note that there's still a timeout when debugging, probably 60 seconds, so if you lose control the timeout may have been reached.

I also added a breakpoint() function to the Rust API. The Rust API does not cover even half the normal API, largely because I don't write much Rust. If you look at kvm_api.h you can see there's quite a bit available.

[fwsGonzo]

It's also possible to debug from the start now. Slightly useless because sys_breakpoint() also works during startup, but at least it allows you to debug the language run-time startup. An astute reader might also notice the absence of --jitless.

[fwsGonzo]

#31 is currently good enough to hopefully load Rust dynamic executables in the simple example.

[fwsGonzo]

The basic Rust program boots now:

Info: Child (354151) said Program 'rust2' is loaded (local, not cached, vm=2 ephemeral, nodes=1, huge=0/0, ready=7.77ms)

[fwsGonzo]

I remember that the custom main arguments came up at some point. I'm going to make a breaking change in two steps. I will be adding the main arguments as environment variables first, and then after a week or so I will remove the custom main arguments.

I imagine the environment variables will be:

KVMNAME=my_program
KVMTYPE=request/storage
KVMSTATE=state

The state file could also come from a constant string in the API. I think it's a worthwhile goal to be able to at least debug the initialization stage in TinyKVM's simplekvm.

[fwsGonzo]

varnish/libvmod-tinykvm@b167c0b

This is what I've done so far. It's nice that with env vars it's easy to add new ones without breaking anything.

[fwsGonzo]

Just found a grave mistake in the RSP client. It had a very small buffer for reading memory, which is halved because bytes are "packed" into 2 bytes on the wire. So, I was struggling to print a struct pthread_t *. Making the buffer 4k suddenly made the debugging experience overall better, because apparently GDB can now see more stuff, print more variables etc. The problem was "masked" because GDB would just say "cannot read memory at xxx".