using git directory cache code in darcs?

using git directory cache code in darcs?

[David Roundy]

Hello Linus and various git developers (ccing darcs developers),

I've been thinking about the possibility of using the git "current
directory cache" code in darcs.  Darcs already has an abstraction layer
over its pristine directory cache, so this shouldn't be too hard--provided
the git code is understandable.  The default in darcs is currently to use
an actual directory ("_darcs/current/") as the cache, and we synchronize
the file modification times in the cache with those of identical files in
the working directory to speed up compares.  We (the darcs developers) have
talked for some time about introducing a single-file directory cache, but
noone ever got around to it, partly because there wasn't a particularly
compelling need.

It seems that the git directory cache is precisely what we want.  Also, if
we switch to (optionally) using the git directory cache, I imagine it'll
make interfacing with git a lot easier.  And, of course, it would
significantly speed up a number of darcs commands, which are limited by the
slowness of the readdir-related code.  We haven't tracked this down why
this is, but a recursive directory compare in which we readdir only one of
the directories (since we don't care about new files in the other one)
takes half the time of a compare in which we readdir both directories.

So my questions are:

1) Would this actually be a good idea? It seems good to me, but there may
be other considerations that I haven't thought of.

2) Will a license be chosen soon for git? Or has one been chosen, and I
missed it? I can't really include git code in darcs without a license.  I'd
prefer GPLv2 or later (since that's how darcs is licensed), but as long as
it's at least compabible with GPLv2, I'll be all right.

3) Is it likely that git will switch to not using global variables for
active_cache, active_nr and active_alloc? I'd be more comfortable passing
these things around, since it would make the haskell interface easier and
safer.  e.g. I'd like

struct git_cache {
        struct cache_entry **cache;
        unsigned int nr, alloc;
};

git_cache *read_cache(char *path_to_index);

or alternatively

int read_cache(char *path_to_index, git_cache *);

Would anyone on the git side be interested in making such changes? If not,
would they be likely to be accepted if a darcs person submitted patches?

4) Would there be interest in creating a libgit? I've been imagining taking
git source files and including them directly in darcs' code, but in the
long run it would be easier if there were a standard git API we could use.

I guess that's about all.
-- 
David Roundy
http://www.darcs.net

Re: using git directory cache code in darcs?

[Ingo Molnar]

* David Roundy <droundy@abridgegame.org> wrote:

> 2) Will a license be chosen soon for git? Or has one been chosen, and 
> I missed it? I can't really include git code in darcs without a 
> license.  I'd prefer GPLv2 or later (since that's how darcs is 
> licensed), but as long as it's at least compabible with GPLv2, I'll be 
> all right.

there's a license in the latest code, it's GPLv2. Here's the COPYING 
file:

---------------
 Note that the only valid version of the GPL as far as this project
 is concerned is _this_ particular version of the license (ie v2, not
 v2.2 or v3.x or whatever), unless explicitly otherwise stated.

 HOWEVER, in order to allow a migration to GPLv3 if that seems like
 a good idea, I also ask that people involved with the project make
 their preferences known. In particular, if you trust me to make that
 decision, you might note so in your copyright message, ie something
 like

	This file is licensed under the GPL v2, or a later version
	at the discretion of Linus.

  might avoid issues. But we can also just decide to synchronize and
  contact all copyright holders on record if/when the occasion arises.

			Linus Torvalds

----------------------------------------

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Re: using git directory cache code in darcs?

[Junio C Hamano]

>>>>> "DR" == David Roundy <droundy@abridgegame.org> writes:

DR> 1) Would this actually be a good idea?

I think it is sensible, especially if you are doing a lot of
comparison between the working area and the pristine.

DR> 3) Is it likely that git will switch to not using global
DR> variables for active_cache, active_nr and active_alloc?

DR> 4) Would there be interest in creating a libgit?

These are related.  I have seen some people interested in
libifying it, and encapsulating those globals would naturally
fall out of it.  My impression from the list however is that a
lot more people are interested in the upper SCM layer than the
git layer right now.  And git layer, although solid enough to
host itself, is still slushy.  A couple of days ago dircache
format was changed from host to network endian.  Last night
Linus made another change to dircache format, which fortunately
is upward compatible if you stay within pathnames shorter than
2^12 bytes ;-).  Another problem I see for somebody to pick up
and start libifying things right now is that, although there is
one central person on the SCM side (Petr Baudis), git layer is
still fractured between Linus and Petr.

Petr syncs with Linus often and he seems to be doing a good job
at keeping track of public patches, but the git layer Linus
works on does not have some patches Petr collected or wrote
himself.  In time, a better coordination would emerge, of
course, but the project is still young at this moment.  Stay
tuned ;-).

Re: using git directory cache code in darcs?

[Linus Torvalds]

On Sat, 16 Apr 2005, David Roundy wrote:
> 
> I've been thinking about the possibility of using the git "current
> directory cache" code in darcs.

Go wild. The license is GPLv2, with the limitation that I really do want
to see v3 before I re-license anything at all, so if you take it into
darcs, you'd need to add that as a per-file comment (I just doing it in
the LICENSE file - I hate cluttering up individual files with tons of
commentary).

> So my questions are:
> 
> 1) Would this actually be a good idea? It seems good to me, but there may
> be other considerations that I haven't thought of.

I really don't know how well the git index file will work with darcs, and 
the main issue is that the index file names the "stable copy" using the 
sha1 hash. If darcs uses something else (and I imagine it does) you'd need 
to do a fair amount of surgery, and I suspect merging changes won't be 
very easy.

So it might well make sense to wait a bit, until the git thing has calmed
down some more. For example, I made some rather large changes
(conceptually, if not in layout of the physical file) to the index file
just yesterday, since git now uses it for merging too.

In git, the index file isn't just a speedup, it's the "work" file _and_
the merge entity. It's not just a floor wax, it's a dessert topping too!

> 2) Will a license be chosen soon for git? Or has one been chosen, and I
> missed it? I can't really include git code in darcs without a license.  I'd
> prefer GPLv2 or later (since that's how darcs is licensed), but as long as
> it's at least compabible with GPLv2, I'll be all right.

Yup, GPL, with the same "v2 by default" that the kernel uses).

> 3) Is it likely that git will switch to not using global variables for
> active_cache, active_nr and active_alloc?

I wouldn't hate it, although for the intent of git, the global approach 
actually makes sense (dammit, I want the basic plumbing to be so small 
that trying to abstract it out more is a waste of time). There's simply 
not a lot of code that should even work at that level.

But if you wait a while, and bide your time, and then spring a clean patch 
on me, I don't see any reason to be difficult about it either.

> 4) Would there be interest in creating a libgit? I've been imagining taking
> git source files and including them directly in darcs' code, but in the
> long run it would be easier if there were a standard git API we could use.

I think libgit might make sense, but again, not quite yet. Maybe the new
merge model was my last smart thought even on the subject of SCM's (I kind
of hope so), but maybe it's not.

My gut _feel_ is that the basic git low-level architecture is done, and
you can certainly start looking around and see if it matches darcs at all. 

			Linus

Re: using git directory cache code in darcs?

[David Roundy]

On Sat, Apr 16, 2005 at 03:43:02PM -0700, Linus Torvalds wrote:
> On Sat, 16 Apr 2005, David Roundy wrote:
> > 1) Would this actually be a good idea? It seems good to me, but there may
> > be other considerations that I haven't thought of.
> 
> I really don't know how well the git index file will work with darcs, and
> the main issue is that the index file names the "stable copy" using the
> sha1 hash. If darcs uses something else (and I imagine it does) you'd
> need to do a fair amount of surgery, and I suspect merging changes won't
> be very easy.

Oh, I'm starting to see (having just browsed the git code for another half
hour or so)... I had been under the (false) impression that the index file
stored the contents of the files themselves, which in retrospect doesn't
make any sense.  So when you run update-cache --add, the file data itself
immediately goes into its final hashed location, and only the sha1 info
goes into the index.

That's all right.  Darcs would only access the cached data through a
git-caching layer, and we've already got an abstraction layer over the
pristine cache.  As long as the git layer can quickly retrieve the contents
of a given file, we should be fine.

The sha1 file and tree hashing isn't direcly useful for darcs, but people
will want to interoperate with git, and for that it would be nice to be
able to know what the hash of a given version is.  I imagine something like

darcs tag --git

which would tag the current version with its git hash.  Of course, to
implement that we only need to reproduce your algorithm for hashing trees,
which probably would be easier to do ourselves without using any git
code... but it would be far faster to recompute with the git backend, since
git stores the hashes of all the unmodified files, and since I also imagine

darcs record --git

which would record a change, and then tag the resulting tree with a git
hash, we might be recomputing the git hashes reasonably often, and we
certainly don't want to rehash the entire kernel each time! :)

> So it might well make sense to wait a bit, until the git thing has calmed
> down some more. For example, I made some rather large changes
> (conceptually, if not in layout of the physical file) to the index file
> just yesterday, since git now uses it for merging too.
> 
> In git, the index file isn't just a speedup, it's the "work" file _and_
> the merge entity. It's not just a floor wax, it's a dessert topping too!

I think that sounds like a pretty reasonable match.  In darcs, there are
internally two main datatypes.  One is the Patch (as you might imagine),
and the other is called a "Slurpy", which is basically a tree lazily
"slurped" into memory.

The pristine cache is then just a way of storing the tree and so we can
"slurp" it again later to retrieve the current state.  So in a sense we'd
be using only one side of the index file interface, the "working directory"
side, where you check files out and add files in--treating it as an
fast filesystem with a few extra-fancy features (like storing inodes of the
files in the working directory).

> I think libgit might make sense, but again, not quite yet. Maybe the new
> merge model was my last smart thought even on the subject of SCM's (I kind
> of hope so), but maybe it's not.
> 
> My gut _feel_ is that the basic git low-level architecture is done, and
> you can certainly start looking around and see if it matches darcs at all. 

Sounds good.  That's sort of the feel I had gotten from other people's
responses as well.  We'll definitely look into how we can use (and
interface with) git.
-- 
David Roundy
http://www.darcs.net

Re: using git directory cache code in darcs?

[Linus Torvalds]

On Sun, 17 Apr 2005, David Roundy wrote:
> 
> That's all right.  Darcs would only access the cached data through a
> git-caching layer, and we've already got an abstraction layer over the
> pristine cache.  As long as the git layer can quickly retrieve the contents
> of a given file, we should be fine.

Yes.

In fact, one of my hopes was that other SCM's could just use the git
plumbing. But then I'd really suggest that you use "git" itself, not any
"libgit". Ie you take _all_ the plumbing as real programs, and instead of
trying to link against individual routines, you'd _script_ it.

In other words, "git" would be an independent cache of the real SCM,
and/or the "old history" (ie an SCM that uses git could decide that the
git stuff is fine for archival, and really use git as the base: and then
the SCM could entirely concentrate on _only_ the "interesting" parts, ie
the actual merging etc).

That was really what I always personally saw "git" as, just the plumbing
beneath the surface. For example, something like arch, which is based on
"patches and tar-balls" (I think darcs is similar in that respect), could
use git as a _hell_ of a better "history of tar-balls".

The thing is, unless you take the git object database approach, using 
_just_ the index part doesn't really mean all that much. Sure, you could 
just keep the "current objects" in the object database, but quite 
frankly, there would probably not be a whole lot of point to that. You'd 
waste so much time pruning and synchronizing with your "real" database 
that I suspect you'd be better off not using it.

(Or you could prune nightly or something, I guess).

		Linus

Re: using git directory cache code in darcs?

[Mike Taht]

Linus Torvalds wrote:
> 
> On Sun, 17 Apr 2005, David Roundy wrote:
> 
>>That's all right.  Darcs would only access the cached data through a
>>git-caching layer, and we've already got an abstraction layer over the
>>pristine cache.  As long as the git layer can quickly retrieve the contents
>>of a given file, we should be fine.
> 
> 
> Yes.
> 
> In fact, one of my hopes was that other SCM's could just use the git
> plumbing. But then I'd really suggest that you use "git" itself, not any
> "libgit". Ie you take _all_ the plumbing as real programs, and instead of
> trying to link against individual routines, you'd _script_ it.

If you don't want it, I won't do it. Still makes sense to separate the 
plumbing from the porcelain, though.

-- 

Mike Taht


   "You can tell how far we have to go, when FORTRAN is the language of
supercomputers.
	-- Steven Feiner"

Re: using git directory cache code in darcs?

[Nomad Arton]

Linus Torvalds schrieb:
> 
> In fact, one of my hopes was that other SCM's could just use the git
> plumbing. But then I'd really suggest that you use "git" itself, not any
> "libgit". Ie you take _all_ the plumbing as real programs, and instead of
> trying to link against individual routines, you'd _script_ it.

please excuse; libgit and scripting to me arent a contradiction. many 
sripting languages are extended by C modules, while still happening to 
have all the scripting rapidity. its just a matter of how to communicate 
with the C code, isnt it?

yours,

peter

Re: using git directory cache code in darcs?

[Junio C Hamano]

>>>>> "NA" == Nomad Arton <lkml@lazy.shacknet.nu> writes:

NA> Linus Torvalds schrieb:
>> In fact, one of my hopes was that other SCM's could just use the git
>> plumbing. But then I'd really suggest that you use "git" itself, not any
>> "libgit". Ie you take _all_ the plumbing as real programs, and instead of
>> trying to link against individual routines, you'd _script_ it.

NA> please excuse; libgit and scripting to me arent a contradiction. many
NA> sripting languages are extended by C modules, while still happening to
NA> have all the scripting rapidity. its just a matter of how to
NA> communicate with the C code, isnt it?

You are arguing for scripting language binding like what Swig
creates.  While that would also be a worthy addition, having
language binding is not the only way to do _script_.

What Linus is saying is that he wants you to talk with git
plumbing by invoking the executables he have, via system(3),
popen(3), etc.

The C-level first has to be libified before you can start
talking about host language bindings but that just started to
happen and is not ready yet.  However, you can use and benefit
from GIT without waiting for that kind of integration, if you
use the "spawning the executables" approach.  I agree with him.

Re: using git directory cache code in darcs?

[Paul Jackson]

Junio wrote:
> What Linus is saying is that he wants you to talk with git
> plumbing by invoking the executables he have, via system(3),
> popen(3), etc.

Hopefully, Linus didn't specify system(3) or popen(3) for production
software.

They are a rich source of security holes.  Inefficient, too, since they
invoke a shell process to interpret the command.

Use execve(2), or exevl(3), execle(3), execv(3).

Or if you really enjoy the path search, use execlp or execvp, but with
your own $PATH, not trusting the one passed in via the environment any
further than you can throw it.

However, on further consideration, I think Linus is wrong to recommend
that the git executables, not a libgit library, be the 'basic user level
on which all else is based."

I will reply to a Linus post, expounding on that thought further.

-- 
                  I won't rest till it's the best ...
                  Programmer, Linux Scalability
                  Paul Jackson <pj@engr.sgi.com> 1.650.933.1373, 1.925.600.0401

Re: using git directory cache code in darcs?

[Paul Jackson]

Linus wrote:
> But then I'd really suggest that you use "git" itself, not any
> "libgit". Ie you take _all_ the plumbing as real programs, and instead of
> trying to link against individual routines, you'd _script_ it.

I think you've got this upside down, Linus.

Trying to make the executable 'git' commands the bottom layer of the
user implementation stack forces inefficiencies on higher layers
of the stack, and thus encourages stupid workarounds and cheats in
an effort to speed things up.

I'd encourage you to invite someone to provide a libgit.

Such work should _start_ with proposing and gaining acceptance on the
API - the calls, the arguments, the types, the rough idea of the
semantics. The actual coding is the easy part.  One is not slave to the
agreed API when coding.  The API will continue to evolve, but if the
originally accepted proposal was sound, the evolution will be at a
modest rate, with few incompatibilities introduced.

If several operations should be done as a unit, to preserve the
integrity of the .git data or to provide sane results, then libgit need
only provide such pre-packaged units, not the incomplete fragments from
which they are composed.  That is, the libgit calls could quite possibly
be at roughly the same semantic level as your git commands.  One could
even code up some of the libgit calls, in early versions of libgit, by
simply invoking the corresponding git command.  But, eventually, all the
git commands should be recoded on top of the libgit library, and the
libgit library become the canonical user interface to git, on which all
else is layered.

One typical way that this choice manifests itself is in the strace
output from doing some ordinary git command from a C program that is
implementing an SCM system on top of git.  Forcing every operation to be
done via a separate git command execution mushrooms the number of kernel
system calls a hundred fold, or two hundred fold if some dang fool uses
system(3S) to invoke the git command.  What might have been a handful of
calls to stat/open/read/write/close a file turns into a mini-shell
session.  That way lies insanity, or at least painful inefficiency, and
the usual parade of bugs, stupid coding tricks and painful user
interfaces that follow in the wake.

The recommended layering of such user facilities is well known, with a C
library at the bottom.  Granted, the history of source code management
tools provides few examples of this recommended layering.

On top of this library go plugin modules for the fancier scripting
languages that accept such.  Swig can be used to aid this construction,
for Tcl, Python, Perl, Guile, Java, Ruby, Mzscheme, PHP, Ocaml, Pike,
C#, Chicken Allegro CL, Modula-3, Javascript and Eiffel.  Though I
personally have not worked with Swig enough to gain success with it.
The only such modules I've done were handcoded Python modules.

Also on top of this library one provides a set of command line utilities
or one multiplexed 'git foo ...' command, for use at shell prompts.  Or
the command line utilities can be coded in one of the above higher level
scripting languages, using in turn the git library plugin.  However many
of these scripting languages bring runtime requirements that are not
universally satisfied on all target systems, so are a poor choice for
this purpose.

If I am recalling correctly, from the days when I regularly used bk, one
of the things that Larry did right with bk, which RCS and SCCS did not
do right before then, was to provide a low level library to his storage
- a cleanroom recoded variant of SCCS in his case.

Implementing production source control systems on top of a set of
executable commands is a pain in the arse.  An all too familiar pain.

I'd repeat my encouragement that you invite someone to provide such a
libgit, however since I have other commitments for the next month at
least, so can't volunteer right away, if ever, it is more appropriate
that I shut up now, under the old "put up code or be quiet" rule.

-- 
                  I won't rest till it's the best ...
                  Programmer, Linux Scalability
                  Paul Jackson <pj@engr.sgi.com> 1.650.933.1373, 1.925.600.0401

Re: using git directory cache code in darcs?

[Linus Torvalds]

On Sun, 17 Apr 2005, Paul Jackson wrote:
> 
> I'd encourage you to invite someone to provide a libgit.

Not until all the data structures are really really stable.

That's the thing - we can keep the _program_ interfaces somewhat stable. 
But internally we may change stuff wildly, and anybody who depends on a 
library interface would be screwed.

Ergo: no library interfaces yet. Wait for it to stabilize. Start trying to 
just script the programs.

		Linus

Re: using git directory cache code in darcs?

[Paul Jackson]

> Not until all the data structures are really really stable.

Fine by me to wait, though perhaps not for the same reason, and perhaps
not as long.

A libgit.so can deal with data structure changes just as well as a set
of command line utilities.  So long as everything funnels through one
place, you can change by changing that one place.

However I am  willing to agree that its not libgit time yet, for two
reasons:

 1) everyone who has two clues on the subject is too busy and
    too productive on more pressing git issues, and

 2) in addition to internal data structures being not yet stable,
    I suspect that the operations (git commands, options and
    behaviour) are also not stable.

The first step of a good libgit is not coding to the internal data
structures, but rather designing the interface (the operations,
arguments, data types, and behaviour).

So, until people have time, and the interface ops are settled down, its
too early to design libgit.  Or at least too early to publish a design
and seek concensus.  If I had the time the first thing I'd be doing
right now would be designing libgit on the side, anticipating the day
when it was time to publish a draft and engage the community discussion
that leads to an adequate concensus.

===

By the way, a good libgit design, in my view, would isolate the data
structures written to files below .git from the data structures
presented at the library API, to some extent.  Changes in the file
structures must be handled without disrupting the library API.

If a libgit API didn't isolate the library caller from details of the
structures in files below .git, then yes you'd want really really stable
data structures, impossibly stable in fact.  That way leads to hacks and
workarounds in the future, because the data structures are never
perfectly stable.

-- 
                  I won't rest till it's the best ...
                  Programmer, Linux Scalability
                  Paul Jackson <pj@engr.sgi.com> 1.650.933.1373, 1.925.600.0401

git options

[Mike Taht]

Would it be useful at this point to make common and centralize some/most 
of the various options that control git? (as well as add some useful 
ones). Something like:

struct _git_opt {
         int verbose:1;
         int debug:1;
	int dry-run:1;
         int should_block:1;
         int remove_lock:1;
         int allow_add:1;
         int allow_remove:1;
         int null_termination:1;
         int show_cached:1;
         int show_deleted:1;
         int show_others:1;
         int show_ignored:1;
         int show_stage:1;
         int show_unmerged:1;
         int show_edges:1;
         int show_unreachable:1;
         int basemask:1;
         int recursive:1;
         int force_filename:1;
         int force:1;
         int quiet:1;
         int stage:1;
};


-- 

Mike Taht


   "Avoid letting temper block progress; keep cool.
	-- William Feather"