Re: [PATCHv2 6/6] t5543-atomic-push.sh: add basic tests for atomic pushes

> Stefan Beller <sbeller@google.com> writes:
>
>>     > What's the value of this test?  Isn't it a non-fast-forward check
>>     > you already tested in the previous one?
>>
>>     I messed up there. Originally I wanted to test the 2 different
>>     stages of rejection. A non-fast-forward check is done locally and
>>     we don't even try pushing. But I also want to test if we locally
>>     thing all is good, but the server refuses a ref to update.
>
> It is tricky to arrange a test to fail a fast-forward check on the
> receiver side, because the local test is done by reading from the
> other side, not relying on your remote tracking branches.  The usual
> flow is:
>
>     pusher says to the receiver "I want to push"
>
>     receiver says to the pusher "Here are the tips of my refs"
>
>     pusher thinks: "Ah, I was about to update their master branch
>                 with this commit, but what they have is (1) not
>                 known to me so by definition I cannot fast-forward,
>                 or (2) known to me and I can definitely tell I am
>                 not fast-forwarding it, so I'd locally fail this
>                 push".
>
> You need to invent a way to successfully race with an ongoing push.
> After receiver gives the tips of its refs (all of which are
> ancestors of what is going to be pushed) but before the pusher
> finishes the "thinking" above, you would somehow update the refs at
> the receiver so that the push will not fast-forward.
>
> Such a raced flow would look like:
>
>     pusher says to the receiver "I want to push"
>
>     receiver says to the pusher "Here are the tips of my refs"
>
>     pusher thinks: "OK, everything I'll send will fast-forward"
>     pusher thinks: "Let's start generating a packfile"
>
>     you intervene and update receiver's 'master' at this point.
>
>     pusher send a pack and tells the receiver "I want to update your
>                 master from OLD to NEW".
>
>     receiver thinks: "Huh, that OLD is not where my 'master' is"
>
>     recevier says to the pusher "No fast-forward".
>
> But I do not think it is practical to run such a test.
>
> Rejecting on the receiver's end using pre-receive or update hook
> should be testable and should be tested.
>
> Thanks.
>