builtin/receive-pack: introduce option to disable connectivity checks

builtin/receive-pack: introduce option to skip connectivity checks

Greetings,

At GitLab, we are interested in introducing an optional means to bypass the connectivity checks performed through git-receive-pack(1). This series implements a --skip-connectivity-check option to facilitate this. I'm interested in collecting some thoughts before pursuing further though.

For some background, we have a transaction management system that runs in our Git RPC service and wraps all repository operations. Operations that write to a repository are first recorded and staged outside of the repository. When committing a transaction, the connectivity of newly written objects is checked by walking the object graph containing only the new objects from the updated tips and identifying the missing objects which represent the boundary between the new objects and the repository. The boundary objects are then checked in the canonical repository to ensure the new objects will connect as expected. All repository operations are run in a transaction and conflict checked before being applied serially to the canonical repository. This ensures that operations that would break the repository are not applied.

For our specific use case, the conflict checks performed by git-receive-pack(1) are redundant and thus we would like to introduce an option that allows connectivity checks to be skipped.

One concern I see could be the --receive-pack flag for git-push(1) or the remote.<name>.receivepack option which configures the receive-pack program that gets executed on the remote side for some protocols. This could provide a way for users to enable such a flag if the remote doesn't protect against arbitrary arguments being added. I don't think git-shell(1) would protect against this, so maybe instead of a flag a config option could be used or an ENV var?

Thanks for taking a look.

-Justin

Related: #163 (closed)