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Re: [PATCH] Add -p to show C function names was Re: [PATCH] Extra options for libsvn_diff

From: Julian Foad <julianfoad_at_btopenworld.com>
Date: Sun, 27 Jan 2008 01:28:08 +0000

Justin Erenkrantz wrote:
> On Dec 27, 2007 2:44 PM, C. Michael Pilato <cmpilato_at_collab.net> wrote:
>>Justin Erenkrantz wrote:
>>
>>>I'd like to resurrect this discussion - IOW, I want Subversion to
>>>support -p out-of-the-box to print C-style function names.
>>[...]
>>
>>>Patch against trunk below. I plan to commit this unless someone
>>>intends to veto it.
>>
>>No veto here. We now have precedent in the diff option space that we didn't
>>have before. This won't look like a weird wart now.
>
> Thanks, C-Mike. I realize that Julian might still veto this when he
> ostensibly comes back from vacation, but enough other committers
> stated that they are in favor of the concept now that I went ahead and
> committed it in r28700.
>
> Enjoy! -- justin

Heh! I'm back. And fear not, for I can't really veto it in good conscience now
that so many have agreed to it. Anyway, I'll want to use it.

In fact, I use "diff-cmd = /usr/bin/diff" and "svn diff -x -p" all the time:
it's really useful for me because I'm a C programmer. Having it built in is
fractionally more convenient, especially for those who don't have a suitable
external diff program already installed.

But, AARGH! There goes another blow to the spirit of free software being a
force for bringing together the best tools for each job.

Understanding C syntax is not Subversion's job. (I'm sure you realised your
comment about how this being a specialist feature is in some way comparable to
merge tracking being a specialist feature was completely off-base. Merging is
in the domain of version control; understanding programming languages is not.)

The GNU diff implementation of "-p" is rather poor. It's enough to give a
useful clue most of the time, and it's conveniently available to most people,
and there isn't a better tool so readily to hand, so we like to use it. But I
use it for automatically constructing Subversion log message templates (file
names and function names formatted as required) and I find that a large
proportion of its output is wrong. One big cause is that it stops looking for a
function name when it starts emitting pre-context lines, which is usually too
soon to pick up any change to a function prototype in a header file. There are
many other problems with it which I could start to list if you want.

One thing that's galling is that we have now diminished people's incentive to
improve that tool that is best suited to that job, by adding another mediocre
implementation to the world. (Sorry if that sounds rude, but without having
tried it I can guarantee it's mediocre.) I want to improve GNU diff's
show-function support. Do I want to improve Subversion's implementation as well
or instead? No, not really; I want it to be improved, but I don't want me or
anyone else to have to consider two separate implementations.

I don't like having mediocre features in Subversion. It gives a poor impression
to users. The argument that it's no worse than the common alternative is not a
good one. That argument only works for you and those of us who were previously
using the alternative external tool. New users will see it differently.

Another thing is that this focuses on one particular domain of usage in *the
core* of Subversion client libraries, and if this trend continues it *will*
over a long time lead to other groups of users gradually finding Subversion
less and less suitable for their needs.

The correct solution to the genuine desire (or even need) for this feature is
to recognise that it is a feature specific to certain file types and certain
users, and implement it as part of the pluggable diff mechanism, in a way that
is unbiased and equally able to support InterWeb-9.9 file formats and
everything else that other groups of (especially non-programmer) users will want.

By "unbiased" I include the desire for similar features for other file formats
to be supportable in a way that is at least as easy as invoking "svn diff -x -p".

Let us enjoy it now we've got it, but let's also bear in mind that we should be
untangling it from the core code, making it an auxiliary feature via a
domain-neutral extension mechanism. This could be the incentive we need to get
the "pluggable diff" support moving forward. In the short term, don't you think
it might be a good idea to move the C code for this out of the central "diff"
functions in such a way that other such features can be added alongside it
without cluttering the basic diff functionality?

Well, you knew I'd have to comment!

- Julian

p.s. I know some might see this as a bit of an extreme angle on it. Yes, it is
useful for Subversion to support some commonly wanted domain-specific options.
Yes, any harm it may do long term isn't measurable yet. Yes, I could argue the
same way about the "diff" feature as a whole.

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Received on 2008-01-27 02:28:26 CET

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