Re: What does 'git blame --before <rev> -- <file>' supposed to mean?

From: Junio C Hamano <gitster@pobox.com>
Date: Aug 24, 2017, 16:35 UTC
Message-ID: <xmqqy3q8x6tk.fsf@gitster.mtv.corp.google.com>
In-Reply-To: <20170824075848.GG3839@alpha.vpn.ikke.info>

Kevin Daudt <me@ikke.info> writes:

Show 20 quoted lines

> Git blame takes options that are fed to git rev-list, to limit the
> commits being taken into account for blaming.
>
> The man page shows "[--since=<date>]", which is one such option, but
> before is valid as well.
>
> git blame -h shows:
>
>     <rev-opts> are documented in git-rev-list(1) 
>
> and man git-blame shows under specifying ranges (emphasis mine): 
>
>      When you are not interested in changes older than version v2.6.18,
>      or changes older than 3 weeks, *you can use revision range
>      specifiers similar to git rev-list*:
>
> So these options are not documented under git blame, but git rev-list.
>
> Perhaps the synopsis of man git-blame could be expanded so that that
> it's clear it accepts rev-list options.

While nothing in what you said is incorrect per-se, many options that rev-list takes are parsed but then ignored by blame, simply because they do not make much sense in the context of the command, and "--before" is one of them.

It is interesting to realize that "--since" (and its synonym "--after") does make sense, unlike "--before" (and its synonym "--until") which does not.

Let's imagine a history like this (time flows from left to right):

     --c1--c2--c4--c6--c8--c9
             \         /
             c3--c5--c7

where the tip of the history is at commit "c9", and the number in the name of each commit denotes its timestamp.

 - "git rev-list c9" starts from "c9", and follows the chain of
   parents and would produce c9, c8, c7, c6, ..., c1, ....

 - If you add "--after 2", i.e. "git rev-list --after 2 c9" does
   exactly the same traversal as the above, but stops following the
   chain of parents for commits that is earlier than the specified
   time.

 - If you add "--before 6", i.e. "git rev-list --before 6 c9" does
   exactly the same traversal as the first one, but does not show
   the commit whose timestamp is later than the specified time.  It
   would be like saying "git rev-list c5 c6" in the above topology;
   the traversal from c9 down to c5 and c6 is done only to find c5
   and c6 to start the "real" traversal from.

Now, "--after 2" will still show "c9", the tip you start your traversal from, and this is important in the context of "blame".

Unlike "git rev-list" (and "git log" family of commands), which can take more than one positive endpoints in the history (e.g. it is perfectly sensible to ask "git log -p ^c1 c5 c6 -- myfile" in the example topology above), "git blame" must be given exactly one positive endpoint, as "git blame ^c1 c5 c6 -- myfile" would not make any sense (ask: "do we want to know about myfile in c5? or myfile in c6?").

I think we also ignore "-n 4" in "blame -n 4 c9 -- myfile" (I didn't check), but that is not because the operation fundamentally does not make sense (like "--until") but because whoever wrote "blame" did not think of the need to support it---in other words, if somebody wants to add support for that option, I offhand do not see a fundamental reason to reject it.

I do think appearing to take and understand and then silently ignoring an option is bad, and it will help users if we tighten the error checking while parsing the command line.