Re: [PATCH/RFC] Implemented return value for rev-list --quiet

[Junio C Hamano <gitster@pobox.com>]

Jonas Gehring <jonas.gehring@boolsoft.org> writes:

> If --quiet is given, the program will return non-zero if the traversed
> commit set was empty. This way, rev-list can be used to check commit
> ancestry as described by the documentation for --quiet.

Given two commits A and X, "rev-list --objects A..X" is a way to make sure
that everything between A and X exists.  When your ref is at A, you are
trying to fetch from a remote that wants to update you to X, and when you
happen to have X already, you run that command and see if it dies due to
disconnect in the history.  If it doesn't, you know you do not actually
have to transfer anything (this is called quickfetch test).  For the
purpose of this test, the caller is not interested in the output, so it is
perfectly OK to give --quiet to the command.

In reality, you would feed all the refs you locally have on the negative
side (i.e. "rev-list --objects --quiet X --not A B C D ...") to check if X
is connected to something you know you have a connected history behind it.
Returning non-zero to a quickfetch test when X is reachable from some of
your refs will break existing scripts.