[Vm-dev] Rant about generated files: should they stay in repository?

[Igor Stasenko]

I am against it , because it is going to nowhere.

So let me elaborate...

In Cog, we have an image which generating an universal sources for VM
on all platforms.
These generated files then put under source control.

Pros:
 - to build VM you can just checkout and build

Cons:
 - err , wait.. which exactly VM you going to build? can you use same
sources for different builds?

Yes, I can use same generated sources for everything, but depending on
configuration and/or target operating system i can receive completely
different results.

Because, for recreating VM as an artifact to reproduce same
circumstances as they were (when some bug was triggered) a generated
sources snapshot is not enough.

Simple example:
  - VM compiled for Macs using -O3 optimization are stable.
 In contrary to linux, where using -O2 with certain files makes your
VM crash few (milli)seconds after startup.

Second ( i don't like repeating myself but still ;):
 - the only way to make sure that VMMaker loads into our latest and
greatest image(s) and producing same output(s) is to have someone to
continuously try load it,
then use it to generate sources, then compile the VM and then run
tests for that VM.
We have a generous guys in our community, who can do that for us. This
is a Mr Hudson, and Mr Jenkins that joined party lately.

I'd like to stress that generated files are just an intermediate data,
which nobody interested in.

Because as input what is relevant should be only :
 - a VMMaker
 - a platform-specific sources/libs
 - a build configuration

and as output we always have:
  - VM

what happens in the middle is really unimportant as long as you can
guarantee reproducibility of  the same output from same inputs.

VM functioning should not depend on some random noise which comes from
unrelated sources like
- generated source files line ending convention
- the date stamp when VM was built
- anything which is not listed in the inputs list above
and if it does, we should eliminate it.

Because if it does, then you don't have a stable builds. And having a
snapshot of sources which kinda works and kinda stable
worth nothing, because at any moment in ongoing development you can
meet same issue which will pop up out of nowhere.
And it will pop out exactly because you don't care to always do things
from scratch , starting from image and VMMaker.
Because it is so convenient to just pull the generated sources from
repo and you done.
And those silly stupids which trying to use VMMaker are not of your
concern.. because when they fail to build stable VM (and they will),
you can always tell them that they are using wrong sources and because
you can't reproduce it (things is always working well for you, in your
warm corner :)

So, i think that keeping a generated files works as some kind of
defensive policy which doesn't allows you to address certain
problems for real.
And sure thing, because we are lazy :)


-- 
Best regards,
Igor Stasenko AKA sig.