Re: Using GIT to store /etc (Or: How to make GIT store all file permission bits)

[Kyle Moffett <mrmacman_g4@mac.com>]

> On 12/10/06, Kyle Moffett <mrmacman_g4@mac.com> wrote:
>> I've recently become somewhat interested in the idea of using GIT  
>> to store the contents of various folders in /etc.  However after a  
>> bit of playing with this, I discovered that GIT doesn't actually  
>> preserve all permission bits since that would cause problems with  
>> the more traditional software development model.  I'm curious if  
>> anyone has done this before; and if so, how they went about  
>> handling the permissions and ownership issues.
>>
>> I spent a little time looking over how GIT stores and compares  
>> permission bits; trying to figure out if it's possible to patch in  
>> a new configuration variable or two; say "preserve_all_perms" and  
>> "preserve_owner", or maybe even "save_acls".  It looks like  
>> standard permission preservation is fairly basic; you would just  
>> need to patch a few routines which alter the permissions read in  
>> from disk or compare them with ones from the database.  On the  
>> other hand, it would appear that preserving ownership or full  
>> POSIX ACLs might be a bit of a challenge.

On Dec 10, 2006, at 10:06:14, Santi Béjar wrote:
> I have not used it, but you could try:
>
> http://www.isisetup.ch/
>
> that uses git as a backend.

Wow, umm, that's actually really interesting for me, given that I'm  
most interested in these sorts of things on Debian.  I can't find  
much documentation on their site; the tools look vaguely immature but  
I haven't really had much time to look at it yet.

On Dec 10, 2006, at 09:49:50, Jeff Garzik wrote:
> It's a great idea, something I would like to do, and something I've  
> suggested before.  You could dig through the mailing list archives,  
> if you're motivated.

I have been digging through the archives; I was just holding out hope  
that somebody else on the list had already halfway beat me to the  
punch.  Guess not :-D

> I actively use git to version, store and distribute an exim mail  
> configuration across six servers.  So far my solution has been a  
> 'fix perms' script, or using the file perm checking capabilities of  
> cfengine.
>
> But it would be a lot better if git natively cared about ownership  
> and permissions (presumably via an option).

I was thinking about a standard config option in the GIT config file,  
that way users could have a personal default and repositories could  
specify it locally.

I started tinkering but quickly discovered that permissions handling  
in general in GIT seems to be a mess; there's about 4 different tiers  
where permissions data is manipulated in various formats.  Some  
places use network-endian 16-bit values, there's a couple functions  
which do different truncations to 644 or 755 format.  There are 2  
functions which canonicalize the file mode based on symlink or  
directory status, each in subtly different ways.

I'm slowly sorting through things but if I could get a few pointers  
from someone intimately familiar with the code that would be most  
appreciated:  I'd like to try to add new entries to tree objects  
which older versions of GIT would ignore but which newer versions of  
GIT would use to store ACL or extended-attribute data.

The simplest solution which admittedly breaks the ability of older  
GITs to read the data from a file with attributes (ignoring the ext- 
attrs themselves) is to create a new "file-with-extended-attributes"  
object which contains a binary concatenation (with length bytes and  
attribute names and such) of the file and its extended attributes.   
That breaks the old GIT assumption that permission and security data  
is part of the directory not the file, but it's more in-line with the  
way extended attributes are attached to the inodes in the filesystem  
(although that doesn't really matter IMO).

Alternatively I might be able to add a new entry to each tree object  
with invalid extended file mods bits (IE: Neither a directory, a  
file, nor a symlink), or perhaps an entry with an empty name, which  
points to a new "extended attribute table".  That table could either  
map from (entry, attribute) => (data) or from (entry) =>  
((attribute,data),(attribute,data),[...]), depending on which would  
be more efficient.  It's essential that the overhead for non-ext-attr  
repositories is O(1) and ideally the overhead for a bunch of files  
with the same ext-attr is O(size-of-ext-attr) + O(number-of-files- 
with-that-attr), although that may vary depending on implementation.

Advice, opinions, problems, and "this-has-no-chance-of-ever-even- 
remotely-working" are all useful and welcome!

Cheers,
Kyle Moffett