Re: Darcs and git: plan of action

Re: Darcs and git: plan of action

[David Roundy]

Linus and gittish people,

I'm cc'ing you on this email, since Juliusz had some interesting ideas as
to how darcs could interact with git, which then gave me an idea concerning
which I'd like feedback from you.  In particular, it would make life (that
is, life interacting back and forth with git) easier if we were to embed
darcs patches in their entirety in the git comment block.  It's a bit of an
ugly idea, but would greatly simplify the two-way interaction between git
and darcs, since no information would be lost when a darcs patch was merged
into git.  See below for the discussion.

As I say, it's a bit ugly, and before we explore the idea further, it would
be nice to know if this would cause Linus to vomit in disgust and/or refuse
patches from darcs users.  Another slightly less noxious possibility would
be to store the darcs patch as a "hidden" file, if git were given the
concept of commit-specific files.  So then we could include in the commit
log something like "Darcs-patch:
780c057447d4feef015a905aaf6c87db894ff58c".  We could do this silently,
except that I wonder if fsck would delete these files, since they aren't
pointed to by any trees.

On Mon, Apr 18, 2005 at 12:02:15AM +0200, Juliusz Chroboczek wrote:
> David,
> 
> I've read git over the week-end.  I think I can see where it's coming
> from.
> 
> Git is basically a (userspace) filesystem with support for efficiently
> finding identical objects.  It's both simple and generic enough to be
> usable by us.

Right.

> You mentioned that you'd like to use git as a cache for Darcs; and I
> don't think I agree.  Caches are tricky -- they need to be kept in
> synch -- and they might result in unexpected performance (you need to
> update both the native and the cached data structures on every
> modification).

It's true that we'd need to keep the cache in sync, which would mean making
sure it gets updated with every repository-modifying darcs command, but
we've already got a cache that has those properties, and it seems like
modifying the interface to deal with a more complex cache would be
relatively straightforward, and would likely have other advantages, such as
if we wanted to implement a per-file cache to speed up annotate (since the
speed of annotate seems to be a relatively common concern).

Basically, I'm imagining that we'd have to replace writePristine and
write_dirty_Pristine with the applyPristine that Ian implemented for
efficiency reasons.  So we'd write to pristine by throwing patches at it,
and letting it do what it pleases with them.  Then we'd read from Pristine
as usual--but we might want to add interfaces for reading slurpies of older
versions from the pristine cache.  This would again be a helpful interface
anyways, since it might allow us, for example, to use checkpoints when
reading older versions.

> I'd rather remodularise Darcs so that the on-disk patch representation
> is decoupled from the in-memory representation, so that we can use
> various backends in the same way as we use the native repository
> format.

The problem I have with this is that "other" repository formats (e.g. git)
store "tree versions", not "changes", and I think it would be fragile to
try to store "changes" (in the darcs sense) in them.

> As you seem motivated by git (my motivation is slightly different -- I
> want to be able to pull from Arch and other widespread systems with
> dysfunctional user interfaces), I suggest that we start with that.

I see.  You're thinking of using darcs as a client for other SCMs.  That's
sort of how I'm thinking of darcs interacting with git, so we aren't so far
off in terms of goals.  My hope would tend to be that people would coalesce
around git--since Linus will be using git.  If everyone can interoperate
with git, we'd be able to interoperate with everyone, in a sense, anyways.

> I suggest we do the following:
> 
>  1. remove the assumption that patch IDs have a fixed format.  Patch
>     IDs should be opaque blobs of binary data that Darcs only compares
>     for equality.

I'm not really comfortable with this, although I can see that there is an
appeal to it, and that something like it may turn out to be necesary for
interacting with systems for which we can't create a simple mapping of
patch IDs.

>  2. get Darcs to pull from git.  By restricting ourselves to a fairly
>     simple command, this should be doable in finite time.

Okay, this is definitely a good goal.  See below for thoughts on how this
should be accomplished.

>  3. allow a patch to have multiple IDs; if the IDs associated to two
>     patches are not disjoint, then the patches are the same patch.

This I find a bit confusing.  So a patch can have two IDs, presumably
something like a "darcs ID" and a "git ID"? I can see that this might
simplify some things, but am not sure how it would work.  The IDs would
have to have a hierarchy, so that you wouldn't ever end up with the "same"
patch having disjoint IDs in two cases.

>  4. allow applying to git repos of non-merger patches.

Here's where I think I'd differ.  I think when dealing with git (and
probably also with *any* other SCM (arch being a possible exception), we
need to consider the exchange medium to be not a patch, but a tag.  Git
only knows about "versions" of the tree, which in darcs terminology is a
tag.  It *does* know about the (possibly multiple) parents of a given
version, so we have a "context" for the patch--provided those two (or
one...) parents are treated as tags.

So in pulling from git, I'd treat each git change as a patch followed by a
tag.  When pulling from git, unfortunately, the contents of that patch will
be determined by our diff algorithm, so if we want long-term stability we
might need to mummify a variant of the diff algorithm that we agree not to
change, and to always use when computing patches from a git archive.  This
tagging (and I imagine the tags will look something like
"git:0c16636264037e8b5ccd38b28ecd191aebc67389") will mean that we can
create a single-patch darcs "patch bundle" for any given git commit.  Which
is to say, that we'll be able to "see" a git repository as an odd-looking
darcs repository.

This means that getting a fresh darcs repository from git would potentially
involve a whole lot of merging...

Putting darcs patches *into* git is more complicated, since we'll want to
get them back again without modification.  Normal "hunk" patches would be
no problem, provided we never change our diff algorithm (which has been
discussed recently, in the context of making hunks better align with blocks
of code).  We could perhaps tell users not to use "replace" patches.  But
avoiding "mv" patches would be downright silly.  So we're somehow going to
have to either sneak this sort of metadata into the git repository, or
we're going to have to store just the darcs "patch ID" in the git
repository, and require that darcs users get the actual patch from
somewhere else.  I had been imagining the latter, but now I'm wondering if
the former is a reasonable possibility.

Linus has said that he figures an SCM needs to be built on top of git, and
that SCM--rather than git itself--would be the one that would know about
things like file renames, probably by storing some sort of rename metadata.
I wonder if we could perhaps store the entire darcs patch in the git
commit? It seems a bit abusive, but would certainly be the easiest way to
interface losslessly with git.  So when we pull from git, we'd look in the
commit log for the magic words indicating that this is really a darcs
patch.  If so, we could handle it natively.  If not, we'd know it was
actually a "gittish" entity, and requires that we diff a couple of trees to
find the actual patch to be used with darcs' patch theory.

The ugliness of this idea is that it involves storing redundant
information.  And I think we'll have a bit of an excercise in commutation
and merging when we get the patches from a git repository, since in git
they'll be stored in tree form, but that's something we'll have to do even
to create a read-only darcs mirror of a git repository.

We could perhaps alleviate the pain by perhaps not including the actual
contents of new or deleted files in the patch, but instead retrieve those
from git directly.  But that might be more trouble than it's worth, at
least for the first sketch.

>  5. think about mergers.

Since git stores a branched history rather than a linear history, I'm not
sure that we'll ever need to store mergers in git.  Instead, we could just
commute until the mergers disappear (which might be a bit scary), and then
store *that* in git.  On the other hand, if this is to inefficient, and if
we store actual darcs patches in git, then we wouldn't perhaps need to
worry about mergers as a special case.

> Whether we end up with a useful implementation of Darcs/git or not,
> this will result in a more modular Darcs, and hence one that will be
> easier to optimise.
> 
> What do you think?

Err, I think I've answered that one... :)
-- 
David Roundy
http://www.darcs.net

Re: Darcs and git: plan of action

[Linus Torvalds]

On Mon, 18 Apr 2005, David Roundy wrote:
> 
> I'm cc'ing you on this email, since Juliusz had some interesting ideas as
> to how darcs could interact with git, which then gave me an idea concerning
> which I'd like feedback from you.  In particular, it would make life (that
> is, life interacting back and forth with git) easier if we were to embed
> darcs patches in their entirety in the git comment block.

Hell no.

The commit _does_ specify the patch uniquely and exactly, so I really 
don't see the point. You can always get the patch by just doing a

	git diff $parent_tree $thistree

so putting the patch in the comment is not an option.

Then you can use the patch to index to whatever extra "darcs index" 
information you want to.

> As I say, it's a bit ugly, and before we explore the idea further, it would
> be nice to know if this would cause Linus to vomit in disgust and/or refuse
> patches from darcs users.

That's definitely the case. I will _not_ be taking random files etc just 
to keep other peoples stuff straightened up.

If you want to add a "log ID", you can certainly do that, but the data the 
ID refers to is _you_ data, and will not go into the git archive. So:

> Another slightly less noxious possibility would
> be to store the darcs patch as a "hidden" file, if git were given the
> concept of commit-specific files.

No, git will not track commit-specific files. There's the comment section,
and that _is_ the commit-specific file. But I will refuse to take any
comments that aren't just human-readable explanations, together with maybe 
one extra line of

	# Darcs ID: 780c057447d4feef015a905aaf6c87db894ff58c

(others will want to track _their_ PR numbers etc) and that's it. The 
actual darcs data that that ID refers to can obviously be maintained in 
_another_ git archive, but it's not one I'm going to carry about.

			Linus

Re: [darcs-devel] Darcs and git: plan of action

[Ray Lee]

On Mon, 2005-04-18 at 08:20 -0400, David Roundy wrote:
> Putting darcs patches *into* git is more complicated, since we'll want to
> get them back again without modification.  Normal "hunk" patches would be
> no problem, provided we never change our diff algorithm (which has been
> discussed recently, in the context of making hunks better align with blocks
> of code).  We could perhaps tell users not to use "replace" patches.  But
> avoiding "mv" patches would be downright silly.

Okay, I still haven't used git yet (and have only toyed around with
darcs for a bit), so take what I'm saying with a grain of salt.
Regardless, I think you may be asking the wrong question. The tracking
of renames was bandied about pretty thoroughly on-list from Wednesday
through Friday (for far better commentary and insight, see Linus'
messages with subject: Merge with git-pasky II.)

git does track changesets that describe the parent tree(s) and the
result. The trees track filenames and hashes. So, doing a fairly
straightforward compare on two trees will let you immediately discover
renames that have occurred, as the filename in the tree changed while
the hash didn't.

So, the question then becomes, can an outside tool cheaply derive all
the information that darcs would need to perform it's work? The renames
should be easy, as long as no content changed during the rename. As for
token replacement (and whitespace changes, etc.), that could be
discovered via domain-specific parsers (something specific per language,
for example). Linus tossed a link to one such tool (hmm, where was it.
Sheesh. You sure right a lot, dude :-).)

	http://minnie.tuhs.org/Programs   (see Ctcompare)

...which should be viewed more as a proof-of-concept than a mergeable
code-set. It does show that diff's vocabulary is sadly lacking in
expressiveness, and improving that, I think, would be a useful area to
expend effort. 

Again, I may be off here, especially considering I've a backlog of a
couple hundred messages to read since the weekend. (You guys need to go
outside more often.)

Ray

Re: Darcs and git: plan of action

[Juliusz Chroboczek]

[Using git as a backend for Darcs.]

> The problem I have with this is that "other" repository formats (e.g. git)
> store "tree versions", not "changes", and I think it would be fragile to
> try to store "changes" (in the darcs sense) in them.

Not really; a Darcs patch is just a pair of two git versions (from and
to).  Which is why Darcs needs to support arbitrarily formatted patch
ids -- a patch originating from git will be identified by a pair of
git hashes.

Obviously, we'll need to think harder when pushing from darcs into git
(we'll need to preserve the Darcs patch id somehow), but it's premature
to worry about that right now.

>>  1. remove the assumption that patch IDs have a fixed format.  Patch
>>     IDs should be opaque blobs of binary data that Darcs only compares
>>     for equality.

> I'm not really comfortable with this,

Why?

>>  3. allow a patch to have multiple IDs; if the IDs associated to two
>>     patches are not disjoint, then the patches are the same patch.
>
> This I find a bit confusing.  So a patch can have two IDs, presumably
> something like a "darcs ID" and a "git ID"? I can see that this might
> simplify some things, but am not sure how it would work.  The IDs would
> have to have a hierarchy, so that you wouldn't ever end up with the "same"
> patch having disjoint IDs in two cases.

It's a case of ``don't do that''.

Suppose I record a patch in Darcs; it gets a Darcs id.  I push it into
git, at which point it gets a git id, whether we want it to or not.
What do we do when we pull that patch back into darcs?

Either we arbitrarily discard one of the ids (which one?), or we keep
both.  If there's more pulling/pushing going on on the git side, we
definitely need to keep both.

> Here's where I think I'd differ.

Same to you ;-)

> I think when dealing with git (and probably also with *any* other
> SCM (arch being a possible exception), we need to consider the
> exchange medium to be not a patch, but a tag.

We're thinking in opposite directions -- you're thinking of the alien
versions as integrals of Darcs patches, I'm thinking of Darcs patches
as derivatives of alien versions.

  You:  alien version = Darcs tag

  Me:   Darcs patch = pair of successive alien versions

My gut instinct is that the second model can be made to work almost
seamlessly, unlike the first one.  But that's just a guess.

> if we want long-term stability we might need to mummify a variant of
> the diff algorithm that we agree not to change,

Good point, noted.

> But avoiding "mv" patches would be downright silly.

Aye, that will require some metadata on the git side (the hack,
suggested by Linus, of using git hashes to notice moves won't work).
Happily, it's premature to worry about that, too.

                                        Juliusz

Re: [darcs-devel] Darcs and git: plan of action

[Ray Lee]

On Tue, 2005-04-19 at 02:55 +0200, Juliusz Chroboczek wrote:
> > But avoiding "mv" patches would be downright silly.
> 
> Aye, that will require some metadata on the git side (the hack,
> suggested by Linus, of using git hashes to notice moves won't work).

Okay, I'm coming to believe I missed something. So, why won't it work?

Ray

Re: [darcs-devel] Darcs and git: plan of action

[Juliusz Chroboczek]

> > Aye, that will require some metadata on the git side (the hack,
> > suggested by Linus, of using git hashes to notice moves won't work).

> So, why won't it work?

Because two files can legitimately have identical contents without
being ``the same'' file from the VC system's point of view.

In other words, two files may happen to have the same contents but
have distinct histories.

                                        Juliusz

Re: [darcs-devel] Darcs and git: plan of action

[David Roundy]

On Mon, Apr 18, 2005 at 08:38:25AM -0700, Linus Torvalds wrote:
> On Mon, 18 Apr 2005, David Roundy wrote:
> > .... In particular, it would make life (that is, life interacting back
> > and forth with git) easier if we were to embed darcs patches in their
> > entirety in the git comment block.
> 
> Hell no.

I was afraid that would be the response...

> The commit _does_ specify the patch uniquely and exactly, so I really 
> don't see the point. You can always get the patch by just doing a
> 
> 	git diff $parent_tree $thistree
> 
> so putting the patch in the comment is not an option.

The issue is that in darcs the parent and child trees *don't* uniquely or
exactly specify the patch.  In fact, even the output of git diff will
depend on what version of diff you're using (e.g. if someone were to use
BSD diff rather than GNU diff).

> > As I say, it's a bit ugly, and before we explore the idea further, it would
> > be nice to know if this would cause Linus to vomit in disgust and/or refuse
> > patches from darcs users.
> 
> That's definitely the case. I will _not_ be taking random files etc just 
> to keep other peoples stuff straightened up.

Okay.

> > Another slightly less noxious possibility would be to store the darcs
> > patch as a "hidden" file, if git were given the concept of
> > commit-specific files.
> 
> No, git will not track commit-specific files. There's the comment
> section, and that _is_ the commit-specific file. But I will refuse to
> take any comments that aren't just human-readable explanations, together
> with maybe one extra line of
> 
> 	# Darcs ID: 780c057447d4feef015a905aaf6c87db894ff58c
> 
> (others will want to track _their_ PR numbers etc) and that's it. The 
> actual darcs data that that ID refers to can obviously be maintained in 
> _another_ git archive, but it's not one I'm going to carry about.

The trouble is that the philosophy of darcs and git are about as orthogonal
as one can come.  Git treats the content as fundamental, where in darcs the
changes are fundamental.  Since in darcs there can be different changes
that lead from the same parent to the same child--and these differences are
meaningful when merges happen---when interacting with git, we either need
to restrict darcs to only describe changes in a way that can be uniquely
determined by a parent and child, or we need to have extra metadata
somewhere.

For bidirectional functionality, we either need to avoid the use of
advanced darcs features, or we need to include that information in git
somehow, or we need to keep a parallel darcs archive holding that
information.

Would a small amount of human-readable change information be acceptable in
the free-form comment area? In the rename thread I got the impression this
would be okay for renames.  For example,

rename foo bar

or (this is less important, but you might consider it to be a useful
human-readable comment)

replace [_a-zA-Z0-9] old_variable new_variable file/path

Currently these two patch types account for almost the sum total of the
cases where different patches lead to the same resulting trees.
-- 
David Roundy

Re: [darcs-devel] Darcs and git: plan of action

[David Roundy]

On Tue, Apr 19, 2005 at 02:55:05AM +0200, Juliusz Chroboczek wrote:
> [Using git as a backend for Darcs.]
...
> >>  1. remove the assumption that patch IDs have a fixed format.  Patch
> >>  IDs should be opaque blobs of binary data that Darcs only compares
> >>  for equality.
> 
> > I'm not really comfortable with this,
> 
> Why?

I'm not clear why it would be necesary, and it takes the only immutable
piece of information regarding a patch, and makes it variable.  Just seems
dangerous and complicated, and I'm not sure why we'd need to do it.

> Suppose I record a patch in Darcs; it gets a Darcs id.  I push it into
> git, at which point it gets a git id, whether we want it to or not.
> What do we do when we pull that patch back into darcs?
> 
> Either we arbitrarily discard one of the ids (which one?), or we keep
> both.  If there's more pulling/pushing going on on the git side, we
> definitely need to keep both.

Or alternatively, we could have a one-to-one mapping between git IDs and
darcs IDs, which is what I'd do.

> > I think when dealing with git (and probably also with *any* other SCM
> > (arch being a possible exception), we need to consider the exchange
> > medium to be not a patch, but a tag.
> 
> We're thinking in opposite directions -- you're thinking of the alien
> versions as integrals of Darcs patches, I'm thinking of Darcs patches
> as derivatives of alien versions.
> 
>   You:  alien version = Darcs tag
> 
>   Me:   Darcs patch = pair of successive alien versions
> 
> My gut instinct is that the second model can be made to work almost
> seamlessly, unlike the first one.  But that's just a guess.

The problem is that there is no sequence of alien versions that one can
differentiate.  Git has a branched history, with each version that follows
a merge having multiple parents.  How do you define that change?  It's easy
enough to do if we tag each git version in darcs, since we know what the
two parents are, and we know what the final state is, but there *is* no
translation from a single git ID either to a single patch(1) patch, or to a
single darcs patch--unless you treat its parents as tags.

The key is that we can't make git work like darcs, so we'll have to make
darcs work like git.  If we do it right (automatically tagging like crazy
people), darcs users between themselves can cherry-pick all they like,
without introducing inconsistencies or losing interoperability with git.

To summarize how I'd see the mapping between git information and darcs, a
git commit would be composed of one darcs patch and one darcs tag.  With
this mapping, I don't believe we lose any information, and I believe we'll
be able to (except that patches would have to be uniquely determined by a
pair of trees) simply translate the darcs system right back again, since
it's a one-to-one correspondence of information.

My proposed mapping:

tree 6ff0e9f3d131bd110d32829f0b14f07da8313c45
# This is a darcs tag ID
parent abd62b9caee377595a9bf75f363328c82a38f86e
# This is the context of both a patch and tag.
author James Bottomley <James.Bottomley@SteelEye.com> 1113879319 -0700
# This is the author and date of the patch
committer Linus Torvalds <torvalds@ppc970.osdl.org.(none)> 1113879319 -0700
# This is the author and date of the tag
# Everything below would be the name and long comment of the patch

[PATCH] SCSI trees, merges and git status

Doing the latest SCSI merge exposed two bugs in your merge script:

1) It doesn't like a completely new directory (the misc tree contains a
   new drivers/scsi/lpfc)
2) the merge testing logic is wrong.  You only want to exit 1 if the
   merge fails. 


-- 
David Roundy
http://www.darcs.net

Re: Darcs and git: plan of action

[Juliusz Chroboczek]

[Removing Linus from CC, keeping the Git list -- or should we remove it?]

> I'm not clear why it would be necesary, and it takes the only immutable
> piece of information regarding a patch, and makes it variable.

Er... I'm not suggesting to make it variable, just to make it an
opaque blob of bytes (still immutable).  I see from the examples you
give below that you agree that the format needs extending, so I
suspect we're actually agreeing here, just failing to communicate.

about having multiple ids per patch:

> Or alternatively, we could have a one-to-one mapping between git IDs and
> darcs IDs, which is what I'd do.

Okay, you've convinced me.  It's much simpler that way, we'll see how
well it works.

> The problem is that there is no sequence of alien versions that one can
> differentiate.  Git has a branched history, with each version that follows
> a merge having multiple parents.

Yep.  I've just realised that this morning.  Is there some notion of
``primary parent'' as in Arch?  Can a changeset have 0 parents?

> If we do it right (automatically tagging like crazy people), darcs
> users between themselves can cherry-pick all they like, without
> introducing inconsistencies or losing interoperability with git.

You've lost me here.  How can you cherry-pick if every tag depends on
the preceding patches?  Or are you thinking of pulling just the patch
and not the tag -- in that case, what happens when you push to git a
Darcs patch that depends on a patch that originated with git?

I've started interfacing Haskell with git this week-end, that's
something we'll need whichever model we choose.  We should be able to
start playing with actually modifying Darcs after next week-end.

                                        Juliusz

Re: [darcs-devel] Darcs and git: plan of action

[Petr Baudis]

Dear diary, on Tue, Apr 19, 2005 at 02:20:55PM CEST, I got a letter
where Juliusz Chroboczek <Juliusz.Chroboczek@pps.jussieu.fr> told me that...
> > The problem is that there is no sequence of alien versions that one can
> > differentiate.  Git has a branched history, with each version that follows
> > a merge having multiple parents.
> 
> Yep.  I've just realised that this morning.  Is there some notion of
> ``primary parent'' as in Arch?  Can a changeset have 0 parents?

Yes, the root commit. Usually, there is only one, but there may be
multiple of them theoretically.

-- 
				Petr "Pasky" Baudis
Stuff: http://pasky.or.cz/
C++: an octopus made by nailing extra legs onto a dog. -- Steve Taylor

Re: Darcs and git: plan of action

[Linus Torvalds]

On Tue, 19 Apr 2005, David Roundy wrote:
> 
> Would a small amount of human-readable change information be acceptable in
> the free-form comment area? In the rename thread I got the impression this
> would be okay for renames.  For example,
> 
> rename foo bar

Sure. That's human-readable and meaningful, as in "it actually makes sense 
as a commit comment regardless of any darcs issues". As does:

> replace [_a-zA-Z0-9] old_variable new_variable file/path

which is almost so (a human would have written "rename old to new", but
the above isn't _that_ different).

HOWEVER, then the requirement would be that we'd never have complex
combinations of the above. Ie having 2-5 lines of something like that is
"human-readable". Having 10+ lines of the above is not. See?

I have this suspicion that the "replace" thing often ends up being done on
dozens of files, and I don't want to have dozens of lines of stuff that
ends up really being machine-readable. But if it's ok to depend on the
content changes (you _do_ see which files changed) together with a single
line of "replace [token-def] xxx yyy", then hell yes - I consider that to
be useful information even outside of git.

(In other words: if it looks like something a careful human _could_ have
written, it's certainly ok. But if it looks like something a careful human
would have used a script to generate 40 entries of, it's bad).

		Linus

Re: [darcs-devel] Darcs and git: plan of action

[Tupshin Harper]

Linus Torvalds wrote:

>(In other words: if it looks like something a careful human _could_ have
>written, it's certainly ok. But if it looks like something a careful human
>would have used a script to generate 40 entries of, it's bad).
>
>		Linus
>  
>
This is the way that darcs would currently represent a "darcs replace 
foo bar" on 15 files, which is obviously exactly what you are objecting to:
[global foo to bar
tupshin@tupshin.com**20050419155539] {
replace ./dir1/file1 [A-Za-z_0-9] foo bar
replace ./dir1/file2 [A-Za-z_0-9] foo bar
replace ./dir1/file3 [A-Za-z_0-9] foo bar
replace ./dir1/file4 [A-Za-z_0-9] foo bar
replace ./dir1/file5 [A-Za-z_0-9] foo bar
replace ./dir2/file1 [A-Za-z_0-9] foo bar
replace ./dir2/file2 [A-Za-z_0-9] foo bar
replace ./dir2/file3 [A-Za-z_0-9] foo bar
replace ./dir2/file4 [A-Za-z_0-9] foo bar
replace ./dir2/file5 [A-Za-z_0-9] foo bar
replace ./dir3/file1 [A-Za-z_0-9] foo bar
replace ./dir3/file2 [A-Za-z_0-9] foo bar
replace ./dir3/file3 [A-Za-z_0-9] foo bar
replace ./dir3/file4 [A-Za-z_0-9] foo bar
replace ./dir3/file5 [A-Za-z_0-9] foo bar
}

I see two possible complementary ways to address this:
1) allow something akin to the above form in git free-form comments as a 
*technical* solution, while leaving it up to the individual repository 
owner whether to accept such patches on aesthetic grounds.
2) explore adding a different format to darcs that would allow a files 
affected to be represented more compactly.


I suspect that any use of wildcards in a new format would be impossible 
for darcs since it wouldn't allow darcs to construct dependencies, 
though I'll leave it to david to respond to that.

At a minimum, something like:
replace ./dir1/[file1|file2|file3|file4|file5] [A-Za-z_0-9] foo bar
replace ./dir2/[file1|file2|file3|file4|file5] [A-Za-z_0-9] foo bar
replace ./dir3/[file1|file2|file3|file4|file5] [A-Za-z_0-9] foo bar
should be pretty feasible.

I don't believe, however, that it would ever be 100% reliable to try to 
look at a one line replace description and combine it with the actual 
changes and end up with a correct darcs replace patch.

-Tupshin

Re: [darcs-devel] Darcs and git: plan of action

[Linus Torvalds]

On Tue, 19 Apr 2005, Tupshin Harper wrote:
> 
> I suspect that any use of wildcards in a new format would be impossible 
> for darcs since it wouldn't allow darcs to construct dependencies, 
> though I'll leave it to david to respond to that.

Note that git _does_ very efficiently (and I mean _very_) expose the 
changed files.

So if this kind of darcs patch is always the same pattern just repeated
over <n> files, then you really don't need to even list the files at all.  
Git gives you a very efficient file listing by just doing a "diff-tree"  
(which does not diff the _contents_ - it really just gives you a pretty
much zero-cost "which files changed" listing).

So that combination would be 100% reliable _if_ you always split up darcs 
patches to "common elements". 

And note that there does not have to be a 1:1 relationship between a git
commit and a darcs patch. For example, say that you have a darcs patch
that does a combination of "change token x to token y in 100 files" and
"rename file a into b". I don't know if you do those kind of "combination 
patches" at all, but if you do, why not just split them up into two? That 
way the list of files changed _does_ 100% determine the list of files for 
the token exchange.

		Linus

Re: Darcs and git: plan of action

[Ray Lee]

(Sorry for the delayed reply -- I'm living on tape delay for a bit.)

On Mon, 2005-04-18 at 22:05 -0400, Kevin Smith wrote:
> >>>>The other is "replace very instace of identifier `foo` with identifier`bar`".
> >>>
> >>>That could be derived, however, by a particularly smart parser [1].
> >>
> >>No, it can't. Seriously. A darcs replace patch is encoded as rules, not
> >>effects, and it is impossible to derive the rules just by looking at the
> >>results. Not difficult. Impossible.
> >  
> > If I do a token replace in an editor (say one of those fancy new-fangled
> > refactoring thangs, or good ol' vi), a token-level comparator can
> > discover what I did. That link I sent is an example of one such beast.
> 
> The big feature of a darcs replace patch is that it works forward and
> backward in time.

That's *not* a feature of the token replace patch, however. That's a
feature of the darcs commutation machinery, correct? (With the obvious
caveat that darcs can only *do* the commutation if it has correctly
nuanced darcs-style token replace patches, rather than mere ASCII
textual diffs.)

> Let me try to come up with an example that can help
> explain it. Hopefully I'll get it right. Let's start with a file like
> this that exists in a project for which both you and I have darcs repos:
> 
> cat
> dog
> fish
> 
> Now, you change it to:
> 
> cat dog
> dog
> fish
> 
> while I simultaneously do a replace of "dog" with "plant", resulting in:
> 
> cat
> plant
> fish
> 
> We merge. The final result in both of our trees is:
> 
> cat plant
> plant
> fish

Okay, that all makes sense.

> Notice that just by looking at my diffs, you can't tell that I used a
> replace operation.

Here's where we disagree. If you checkpoint your tree before the
replace, and immediately after, the only differences in the
source-controlled files would be due to the replace. And since the
language of the file is known (and thereby the tokenization -- it *is*
well-defined), then a tokenizer that compares the before and after trees
(for just the files that changed, obviously), can discover what you did,
and promote the mere ASCII diff into a token-replace diff. (The same
sort of idea could be done for reindention, I'd hope.)

> I didn't just replace the instances of "dog" that
> were in my file at that moment. I conceptually replaced all instances,
> including ones that aren't there yet.

Well yes, that's exactly what we want. And the key point of all of this
is that there's no magic here. The darcs machinery does all the
commutations such that the patches can wiggle together without
conflicts. To do it's job, of course, it needs nuanced patches, rather
than the quite literal ones generated by diff.

We agree on everything except that it's provable that one can discover a
replace operation, given a before and after tree.

> Now, I should mention here that I personally dislike the replace
> operation, and I think it is more dangerous than helpful. However, other
> darcs users are quite happy with it, and it certainly is a creative and
> powerful feature.

It's creative alright, though I had the same misgivings. In my common
code workflow, I almost never have global tokens -- all my replaces
would be per function, so I never saw an opportunity to use it when I
was screwing around with darcs.

> Other creative patch types have also been dreamed of. For example, a
> powerful language-specific refactoring operation has been discussed as a
> far-future possibility. That would be safe, and cool.

<subliminal> indention patch type, indention patch type... </subliminal>

> > > Automated refactoring tools, for example, perform the
> > > rename+modify as an atomic operation.
> > [...]
> Although there are no such nifty refactoring tools available today, they
> will exist at some point.

Yeah, I spent some time drooling over the refactoring editors before
slapping myself and deciding I'd wait for others to live on that
bleeding edge for a while. I've had to clean up too much code from other
people.

> Even without tools, many shops have policies against checking in code
> that won't compile. If you rename a java class, you must simultaneously
> perform the rename and modify the class name inside. If you commit
> between those steps, it's broken.

I'm trying hard to find a nice way to say that's silly. I'm failing. My
suggestion in that case would be that the local coder commit many
patches to a local repository, one of which is the rename. Then upon
completion of the refactoring, the set of patches is committed to the
group repository. Tags before and after preserve the repository's
precondition that it always compiles.

> [I do realize that the kernel doesn't have java code, by the way.]

Don't worry, I didn't think that you did :-).

Ray

Re: Darcs and git: plan of action

[Tupshin Harper]

Ray Lee wrote:

>Here's where we disagree. If you checkpoint your tree before the
>replace, and immediately after, the only differences in the
>source-controlled files would be due to the replace.
>
This is assuming that you only have one replace and no other operations 
recorded in the patch. If you have multiple replaces or a replace and a 
traditional diff  recorded in the same patch, then this is not true.

> And since the
>language of the file is known (and thereby the tokenization -- it *is*
>well-defined), then a tokenizer that compares the before and after trees
>(for just the files that changed, obviously), can discover what you did,
>and promote the mere ASCII diff into a token-replace diff. (The same
>sort of idea could be done for reindention, I'd hope.)
>  
>
See above for one set of limitations on this. A more fundamental problem 
comes back to intent. If I have a file "foo" before:
a1
a2
and after:
b1
b2
is that a "replace [_a-zA-Z0-9] a b foo" patch, or is that a
-a1
-a2
+b1
+b2
patch? Note that this comes down to heuristics, and no matter what you 
use, you will be wrong sometimes,  *and* the choice that is made can 
substantively affect the contents of the repository after additional 
patches are applied.

>We agree on everything except that it's provable that one can discover a
>replace operation, given a before and after tree.
>  
>
It's provable that you can not.

-Tupshin

Re: Darcs and git: plan of action

[Ray Lee]

On Tue, 2005-04-19 at 16:00 -0700, Tupshin Harper wrote:
> Ray Lee wrote:
> 
> >Here's where we disagree. If you checkpoint your tree before the
> >replace, and immediately after, the only differences in the
> >source-controlled files would be due to the replace.
> >
> This is assuming that you only have one replace and no other operations 
> recorded in the patch. If you have multiple replaces or a replace and a 
> traditional diff  recorded in the same patch, then this is not true.

I had a precondition on my argument (not quoted), that the code was
checkpointed before and after. Obviously, a large set of changes in one
patch is a problem. However, a darcs replace is (effectively) a commit
on its own, so I was limiting myself to the same situation under a
different system.

> A more fundamental problem comes back to intent. If I have a file
> "foo" before:
> a1
> a2
> and after:
> b1
> b2
> is that a "replace [_a-zA-Z0-9] a b foo" patch, or is that a
> -a1
> -a2
> +b1
> +b2
> patch?

Okay, so in reading the online darcs manual (yet) again, I now see that
it allows regular expressions for the match and replace, which means
multiple unique tokens could change atomically. (Does anyone actually
*use* regexes? Sounds like a cannon that'd be hard to aim.)

Regardless, I only care about code, not free text. If it's in a language
that doesn't do some use-'em-as-you-need-'em duck typing spiel
(<cough>python</cough), then the context of your patch (namely, the
file) already has those tokens somewhere in them. And I bet that if
*you* looked at that file, you could tell if it was a replace or a mere
textual diff. Am I wrong?

> Note that this comes down to heuristics, and no matter what you 
> use, you will be wrong sometimes,  *and* the choice that is made can 
> substantively affect the contents of the repository after additional 
> patches are applied.

Unless I'm missing something, the darcs replace patch can already do the
wrong thing. If I do a replace patch on a variable introduced in a local
tree, then do a darcs replace on it before committing it to a shared
repository, and coder B introduces a variable of the same original name
in my copy, then there's a chance that the replace patch will
incorrectly apply upon his newly introduced variable. No?

> It's provable that you can not.

I'm still not seeing the problem, at least when it comes to ANSI C.

Ray

Re: Darcs and git: plan of action

[Tupshin Harper]

Ray Lee wrote:

> I'm still not communicating well.
>
>Give me a case where assuming it's a replace will do the wrong thing,
>for C code, where it's a variable or function name.
>
>Ray
>
>-
>
I think you are communicating fine, but not fully understanding darcs.

try this:
initial patch creates hello.c
#include <stdio.h>

int main(int argc, char *argv[])
{
  printf("Hello world!\n");
  return 0;
}

second patch:
replace ./hello.c [A-Za-z_0-9] world universe

third patch, for conceptual clarity, created in another repository that 
had seen the first patch, but not the second (adds function wide_world):
hunk ./hello.c 3
+void wide_world()
+{
+  printf("Hello wide world\n");
+}
+
hunk ./hello.c 11
+  wide_world();
}

If patch2 was a replace patch, then the result of running the combined 3 
patch version would be:
Hello universe!
Hello wide universe

but if patch2 was a non-replace patch, then the result would be:
Hello universe!
Hello wide world

-Tupshin

Re: Darcs and git: plan of action

[Tupshin Harper]

Ray Lee wrote:

>it allows regular expressions for the match and replace, which means
>multiple unique tokens could change atomically. (Does anyone actually
>*use* regexes? Sounds like a cannon that'd be hard to aim.)
>  
>
Yes, and replace patches need to be used very carefully.

>Regardless, I only care about code, not free text. If it's in a language
>that doesn't do some use-'em-as-you-need-'em duck typing spiel
>(<cough>python</cough), then the context of your patch (namely, the
>file) already has those tokens somewhere in them. And I bet that if
>*you* looked at that file, you could tell if it was a replace or a mere
>textual diff. Am I wrong?
>  
>
Yes. See my hello world example from my last email.

>
>Unless I'm missing something, the darcs replace patch can already do the
>wrong thing. 
>
Yes, depending on how you define wrong. Darcs replace is fully 
predictable, and poorly chosen replaces can lead to incorrect results 
after future patches are applied.

>If I do a replace patch on a variable introduced in a local
>tree, then do a darcs replace on it before committing it to a shared
>repository, and coder B introduces a variable of the same original name
>in my copy, then there's a chance that the replace patch will
>incorrectly apply upon his newly introduced variable. No?
>  
>
Absolutely correct, and the exact reason why replace patches need to be 
used *very* selectively.

>  
>
>>It's provable that you can not.
>>    
>>
>
>I'm still not seeing the problem, at least when it comes to ANSI C.
>
>Ray
>  
>
See hello world example in my other email. You can argue that it is an 
existing problem in darcs, but really, it just points out the fact that 
a computer is *incapable* of knowing whether it is safe to use a replace 
patch based on a diff because replace patches are dangerous if not used 
intelligently.

-Tupshin

Re: [darcs-devel] Darcs and git: plan of action

[Ray Lee]

On Tue, 2005-04-19 at 10:22 +0200, Juliusz Chroboczek wrote:
> > > Aye, that will require some metadata on the git side (the hack,
> > > suggested by Linus, of using git hashes to notice moves won't work).
> 
> > So, why won't it work?
> 
> Because two files can legitimately have identical contents without
> being ``the same'' file from the VC system's point of view.
> 
> In other words, two files may happen to have the same contents but
> have distinct histories.

Eh, let's not talk using integral/summation view across all the patches
that ever could have come in against the file. We're hamstringing
ourselves if we do that, and it's not what darcs does. darcs looks at a
differential view of the changes, and for a mv, it looks at it when it
happens.

darcs does a "darcs mv" to commit a "file move patch" to whatever
logging or patch repository it keeps below the surface.

The equivalent in git would be to have a given tree, move a file via
bash's mv, and then checkpoint a new tree. (I'm sure there's details in
there, but that's plumbing, and what we have Petr for.)

A differential comparison of the two trees shows no content changed, but
a file label was modified. Ergo, a rename occurred.

QED.

~r.

Re: Darcs and git: plan of action

[David Roundy]

On Tue, Apr 19, 2005 at 09:49:12AM -0700, Linus Torvalds wrote:
> On Tue, 19 Apr 2005, Tupshin Harper wrote:
> > I suspect that any use of wildcards in a new format would be impossible
> > for darcs since it wouldn't allow darcs to construct dependencies,
> > though I'll leave it to david to respond to that.
> 
> Note that git _does_ very efficiently (and I mean _very_) expose the 
> changed files.
> 
> So if this kind of darcs patch is always the same pattern just repeated
> over <n> files, then you really don't need to even list the files at all.
> Git gives you a very efficient file listing by just doing a "diff-tree"
> (which does not diff the _contents_ - it really just gives you a pretty
> much zero-cost "which files changed" listing).

The catch is that it's possible to have a darcs patch that doesn't change
any files, or that affects files without changing them.  If I rename
function foo to bar, I might want to do

darcs replace foo bar *.c

which would issue a replace on all files, which means that when this patch
is merged with any patches that add occurrences of foo in a file, that will
get modified to a bar, regardless of whether there was previously an
occurrence of foo in that file.

I think we might (when working with git--it would be problematic within
darcs straight) be able to work out some sort of a wildcard replace
scheme, so it could be something like

replace foo bar in: mm/*.c

The regexp bit could be left out, if we restrict the definition of "tokens"
in token replaces--which probably isn't a troublesome limitation.  By
default darcs uses two tokenizing schemes, one which allows "." in tokens
(usually relevant in Makefiles), and one which doesn't, and basically
matches C identifiers.  We could allow for both of these if we had a second
option:

replace filename foo.h bar.h in: mm/*.c

We'd just need to expand the wildcards when translating from the git
repository into darcs patches.

> So that combination would be 100% reliable _if_ you always split up darcs 
> patches to "common elements". 
> 
> And note that there does not have to be a 1:1 relationship between a git
> commit and a darcs patch. For example, say that you have a darcs patch
> that does a combination of "change token x to token y in 100 files" and
> "rename file a into b". I don't know if you do those kind of "combination 
> patches" at all, but if you do, why not just split them up into two? That 
> way the list of files changed _does_ 100% determine the list of files for 
> the token exchange.

We do allow multiple sorts of changes (in darcs terminology, multiple
"primitive patches") in a single patch.

One *could* have multiple git commits for a single darcs patch, but that
seems ugly and I'd rather avoid it.  In my view, revision control system is
more about communication than history (which is why by default, darcs
doesn't "do" history), and grouping changes together is how we express
which changes "go together".  Of course, we could still have a grouping at
a higher level, so that a single "changeset" could consist of multiple git
commits (for example by recognizing that identical commit logs mean that
it's a single change), but that adds a layer of complexity that I'd like to
avoid if possible.
-- 
David Roundy
http://www.darcs.net

Re: [darcs-devel] Darcs and git: plan of action

[David Roundy]

On Tue, Apr 19, 2005 at 02:25:18PM +0200, Petr Baudis wrote:
> Dear diary, on Tue, Apr 19, 2005 at 02:20:55PM CEST, I got a letter
> where Juliusz Chroboczek <Juliusz.Chroboczek@pps.jussieu.fr> told me that...
> > > The problem is that there is no sequence of alien versions that one
> > > can differentiate.  Git has a branched history, with each version
> > > that follows a merge having multiple parents.
> > 
> > Yep.  I've just realised that this morning.  Is there some notion of
> > ``primary parent'' as in Arch?  Can a changeset have 0 parents?
> 
> Yes, the root commit. Usually, there is only one, but there may be
> multiple of them theoretically.

Incidentally (and completely off-topic for this thread), wouldn't there be
a sha1 tree hash corresponding to a completely empty directory, and
couldn't one use that as the parent for the root? Would there be any reason
to do so? Just a silly thought...
-- 
David Roundy
http://www.darcs.net

Re: [darcs-devel] Darcs and git: plan of action

[David Roundy]

On Tue, Apr 19, 2005 at 02:20:55PM +0200, Juliusz Chroboczek wrote:
> [Removing Linus from CC, keeping the Git list -- or should we remove it?]

I think leaving much of this on git would be appropriate, since there are
issues of how to relate to git that should be relevant.

> > If we do it right (automatically tagging like crazy people), darcs
> > users between themselves can cherry-pick all they like, without
> > introducing inconsistencies or losing interoperability with git.
> 
> You've lost me here.  How can you cherry-pick if every tag depends on
> the preceding patches?  Or are you thinking of pulling just the patch
> and not the tag -- in that case, what happens when you push to git a
> Darcs patch that depends on a patch that originated with git?

Yes, I'm thinking of pulling patches from one darcs repo to another.  If we
cherry-pick in this way, we need to create a "git-tag" for each patch that
we pull without its associated tag.  To git, this would look like two
separate changes that have the same commit log, except that they have
different parents and different commiters and commit dates.

I don't think this will be a problem for git, and since darcs will
recognize the two patches as the identical darcs patch (we'll need to put
somewhere in the git commit log a magic word indicating that this patch
originated in darcs), there won't be a problem for darcs either.

In case I haven't been clear (which seems likely), the scenario is that
darcs user 1 makes the following changes to his darcs version of a
git-based repository:

changes in 1: A -> B
tags in 1:    A1   B1

Darcs user 2 wants B, but not A, and didn't do any development:

changes in 2: B
tags in 2:    B2

User 2 pushes to git, and now git has (where P is the parent of both of the
above):

git:
P -> B/B2  (where B/B2 is the commit log with B2 as "committer info" and B
            as the "author info and long comment)

User 1 pushes (everything) to git and merges the two (patch M, which has
two parents, B1 and B2:

git:

   ->B/B2---------
  /               \
P--> A/A1 -> B/B1---> M

It's a little lame, and if user 2 doesn't do any real work, the git-using
person might be annoyed, but I think it's doable.
-- 
David Roundy
http://www.darcs.net