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Re: finishing `svn diff': two ways to do it

From: Kevin Pilch-Bisson <kevin_at_pilch-bisson.net>
Date: 2001-10-11 22:48:04 CEST

My impressions was that part 2 of what you said is pretty much what was
concluded during that last thread. :)

There is another slight hurdle though. The option parsing code in main.c
currently won't support the same option twice, so -r 4 -r 9 won't work without
at least a little re-write. I know our code won't handle it, I'm not sure
the APR functions used for options.

On Thu, Oct 11, 2001 at 03:58:24PM -0500, kfogel@collab.net wrote:
> I believe there was a long thread about this before, but it didn't
> seem to resolve conclusively, and I'd like to restate the issue and
> get some feedback on possible implementations.
> Soon, as soon as issues 382 and 414 are resolved, we will be finishing
> the rest of `svn diff'. Right now, `svn diff' only shows local
> modifications -- it can show the diff between your base revision and
> your working file, but it can't do
> a) diff between your working-or-base-file and some other rev, nor
> b) diff between two arbitrary revisions in the repository
> First, let's talk about interface. Here are some examples of diff
> commands, examples being the easiest way to express what I'm thinking.
> These are rather extensive, please feel free to skim them and go to
> where I talk about implementation of (b), which is actually the main
> point of this mail. :-)
> $ svn st -u foo.c
> _ * 17 ./foo.c
> Head revision: 20
> $ svn diff foo.c
> ... shows diff from .svn/text-base/foo.c to ./foo.c, works now.
> $ svn diff -r 20 foo.c
> ... shows diff from local foo.c to revision 20 of foo.c in repos.
> $ svn diff -r 17 foo.c
> ... same as "svn diff foo.c" :-).
> $ svn diff -r 5 foo.c
> ... shows diff from working ./foo.c to rev 5 of foo.c; even though
> rev 5 is earlier than the base of the working version, we still
> show the diff in this order for consistency. In other words,
> running "svn diff -r REV FILE(S)" always produces a patch that,
> when applied to your working file, results in rev REV of that
> file.
> $ svn diff -r 5 -r WORKING foo.c
> ... same as above, but reverses the direction of the diff. The
> keyword "WORKING" as a revision means "use the working file",
> as opposed to the pristine base.
> $ svn diff -r 5 -r BASE foo.c
> ... obviously, shows changes from rev 5 to BASE (which is 17),
> ignores local mods.
> $ svn diff -r BASE -r 5 foo.c
> ... same as above, but reverses direction of diff.
> $ svn diff -r HEAD -r 5 foo.c
> ... I think we're getting the idea here...
> $ svn diff -r 17 -r HEAD foo.c
> ... yes, it's certainly clear what this does...
> $ svn diff -r HEAD -r BASE foo.c
> ... you can even do this, totally ignores local mods...
> $ svn diff -r 3 -r 19 foo.c
> ... a more familiar way of requesting a diff.
> Fine, I think we get the idea. The three special rev keywords are
> "BASE", "HEAD", and "WORKING", and of course they should work when
> given in lowercase too. By combining these, and using one or two -r
> flags, you can get any diff in any direction you need.
> Implementing (a)-style diffs is pretty straightforward. You already
> have one of the necessary files locally, either as text-base or as
> working file. So if you run
> $ svn diff -r REV foo.c
> or any of the similar commands, the server can just send the
> difference (as svndiff) between BASE and REV, allowing the client to
> create REV from that and run diff locally; the client would also take
> care of the logic about whether or not to include local changes in the
> diff. We avoid running `diff' on the server, which is good, since
> it's the centralized bottleneck.
> But in implementing of (b)-style diffs, we have a tougher choice. If
> you run:
> $ svn diff -r REV1 -r REV2 foo.c
> there are two ways svn can do it:
> 1. The server produces the diff, however it wants, and sends the
> diff back to the client. In practical terms, this is going to
> mean the server creates both files and runs `diff', which for
> now is an external program but could conceivably be librarized
> into the server someday. (In this case, librarization of diff
> would be a bigger win for the server than for the client, since
> saving server overhead is usually worth more than saving client
> overhead.)
> 2. The server sends over the BASE->REV1 svndiff and the BASE->REV2
> svndiff, the client creates both revisions locally, and runs
> diff locally.
> CVS uses method (1). Until recently, I thought Subversion should do
> the same thing. Now I'm not so sure; maybe it's better to only burden
> the server with sending svndiff data to the client, and let the client
> generate the human-readable diffs. This should result in no more
> network usage than plan (1), and transfers at least some of the work
> to the client, which seems a Good Thing.
> I guess I'm leaning toward (2) now. Thoughts?
> -Karl
> P.S. As an optimization, we can one day detect when a requested
> revision number results in the same data as the local base revision
> (i.e., if the file didn't change between revs 3 and 10, and its base
> rev claims 6, then using rev 8 could -- with sufficient bookkeeping --
> result in no network usage at all). But that's for later.
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Kevin Pilch-Bisson                    http://www.pilch-bisson.net
     "Historically speaking, the presences of wheels in Unix
     has never precluded their reinvention." - Larry Wall

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