I don't believe Google has (had?) any objections to using open source or open sourcing things but you have to remember two things:
1. Most companies weaponize open source. They use it to "commoditize their product's complements" [1]; and
2. Google3 are so deeply integrated in a way that you can't really separate some of the tech because of the dependencies on other tech. More on that below.
> Open sourcing Stubby could certainly have been done. You just open source the dependencies too
Yeah, I don't think it's always that simple. You may not own the rights to something to be able to open source it. Releasing something may trigger more viral licenses (ie GPL) to force you to open source things you don't want to or can't.
I actually went through the process of trying to import a few open source packages into Google's third party repo and there are a lot of "no nos". Like a project had to have a definite license (that was white listed by legal). Some projects liked to do silly things like having a license like "do whatever you want" or "this is public domain". That's not how public domain works BTW. And if you contacted them, they would refuse to change it even something like an MIT license, which basically emans the same thing, because they didn't understand what they were doing.
> And then Borg wasn't open sourced
This actually makes sense. Later on your suggest you were a Google SRE so you should be aware of this but to whoever else reads this: Google's traffic management was deeply integrated into the entire software stack. Load balancing, DDoS defense, inter-service routing, service deployment onto particular data centers, cells and racks and so on.
It just doesn't make sense to open source Borg without everything from global traffic management down to software network switching.
> I think only Blaze/Bazel is core infrastructure in which the open source version is actually genuinely the same codebase
I don't know the specifics but I believe that Bazel too was "Blaze inspired". I suspect it's still possible to do things in Blaze that you can't do in Bazel even though the days of Blaze BUILD files being Python rather than "Python syntax like" are long gone.
Also, Blaze itself has to integrate with various other systems than Bazel doesn' eg ObjFS, SrcFS, Forge, Perforce/Piper, MPM and various continuous build systems.
[1]: https://www.joelonsoftware.com/2002/06/12/strategy-letter-v/
