it's already been published. Google used Perforce for at least a decade and then cloned the perforce wire protocol when it was clear perforce wouldn't keep scaling (there's a great presentation online about google using RAMSANs to store the perforce index), but backed the repo in something like bigtable or spanner.

I'll give google credit for one thing: it can change backends a lot without too much user visible pain.

It is, like most of the tools in Google/Facebook/..., a custom solution. These companies need their custom solution because of their scale.