got-reload

The goal

Be able to hot-reload code at function/method-level granularity in a running process.

The roadmap in broad strokes

Use the -toolexec flag of go build to install a source filter into the compilation process.

From go help build:

-toolexec 'cmd args'
	a program to use to invoke toolchain programs like vet and asm.
	For example, instead of running asm, the go command will run
	'cmd args /path/to/asm <arguments for asm>'.

The filter will transparently change functions from this

func Foo(... args ...) (...return values...) {
  // body
}

into this

func Foo(... args ...) (...return values...) {
  return YY_Foo(...args...)
}

var YY_Foo = func(...args...) (...return values...) {
   // body
}

func YY_SetFoo(f func(...Foo's signature)...) {
  YY_Foo = f
}

and similarly for methods.

A thread will watch for changes in all filtered files

When a filtered source file changes, it will be read, parsed, and changed functions will be installed with new versions of themselves via the generated YY_Set* functions, via Yaegi, a Go interpreter.

Limitations

I'm not positive, but I'm pretty sure it's not gonna be possible to change the signature of a function, change a type, declare a new type or variable, and probably several other things.

I've performed the above function replacement process by hand in a real Go program, so I know that at least that much is possible.

It's not everything, but it's not nothing, either.

Who came up with this hair-brained idea?

• Larry Clapp @theclapp
• Chris Waldon @whereswaldon

Given that Yaegi's been out for a while, and Go's parsing tools have been out since the beginning (well, a lot of them, anyway), we both wonder why nobody has done this yet, to be honest.

Can I use it now?

No. We've been working on this for, like, three days, as of this writing. We're still working on the basic translation bit.

Watch/Star/Sponsor, as the spirit moves you.

Do you have a demo?

See this video that Chris did for something similar:

https://user-images.githubusercontent.com/2324697/106301108-4e2fce80-6225-11eb-8038-1d726b3eb269.mp4