On Thu, Sep 22, 2022 at 02:19:43PM -0700, Junio C Hamano wrote: > > It is the expectation that credential helpers be liberal in what they > > accept and conservative in what they return, to allow for future growth > > and evolution of the protocol/interaction. > > That is nice in principle, and the updated code below may work well > with existing "other side of the connection" (codepaths in "git" > that asks credential API to talk to the helpers), but I am not sure > if this is always a safe thing to do. > > When we gain a new "command" in the protocol, if we just read it > without understanding it, would we open ourselves to a risk of > breaking the protocol communication, worst of which may be to > deadlock? A new command, when received by a more recent helper that > understands how to react to it, may _require_ it to write more than > "username" and "password" back to "git" from get_credential(), for > example, but the helper with this patch alone, while not complaining > about seeing such a new and unknown command, would not know how to > compute and write that third thing other than "username" and > "password"---would the other side who issued that new command get > stuck waiting for us to return the third thing? Worse yet, the new > command may expect us to read further in get_credential() > (e.g. maybe they will give us a challenge, which may need to be used > when yielding the "username" and "password" things), but because we > do not even know we need to read, their attempt to write to us may > get them stuck, and that is when we are expecting to write to them, > easily leading to a deadlock, no? This open-endedness was an intentional part of the original credential-helper design. Helpers are always "best effort", and it is OK if they ignore a request, or return a partial result. If the sending side really wants to extend the protocol in a way that the other side doesn't act at all (say, they "username" to be used _only_ if the helper also understands a new "foobar" key), then they should be adding the new fields as an atomic unit. I.e., "foobar", "foobar-userame", and so on. And then existing helpers which don't understand the new feature will just ignore it totally. In practice, I think it's not that big a deal either way, because the setup of these helpers is under the control of the user. So if you really want to use "foobar" but a helper doesn't support it, then you'd just remove that helper from your config. -Peff
