Re: [PATCH v3 1/2] diff: add --line-prefix option for passing in a prefix

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

Jacob Keller <jacob.e.keller@intel.com> writes:

> As suggested by Junio, I implemented --line-prefix to enable the graph
> display correctly. This works by a neat trick of adding to the msgbuf,
> so no code needs to be altered. I presumed that the line prefix should
> go *after* the graphs own prefix.

I do not understand the last sentence.

The motivation I suggested the --line-prefix for is for a scenario
in which "git log -p --graph" that recurses into a submodule causes
"git diff A B" between the two commits in a submodule to run; the
internal diff machinery driven by "log -p --graph" for the
superproject knows what the graph lines that depict the lineages of
history in the superproject should look like, but the "git diff A B"
in the submodule of course does not, so while showing e.g. "| | |"
(three lineages) for the history in the superproject, you would run
"git diff --line-prefix='| | | ' A B" and by showing them before
anything that "git diff A B" without the new option would have
produced, you could mimick and match the graph output in the
superproject.

In that scenario, the line prefix _is_ the graph's prefix in the
superproject.

You might be envisioning a future enhancement where the recursive
one uses not "-Submodule commit A"/"-Submodule commit B", and not
"diff A B", but "log -p A...B" in the submodule, and in such a case,
it might make sense to run "log -p --graph A...B" instead when the
command the end user run in the superproject asked for "--graph".
You would use the same "--line-prefix='| | | '" when running the
"log -p --graph A...B" command in the submodule, so that the output
for the submodule will go _after_ the graph of the superproject, but
in that case, the output for the submodule would also include its
own graph to show the relationship between A and B.  The line-prefix
must come before that graph part (and also if it is "git log" run in
the submodule, the same line-prefix must come before each line of
the log message output).

With that understanding/assumption, "line prefix should go after the
graph's own prefix" sounds like a wrong choice to me.  Shouldn't the
prefix go before everything else?