Re: Manually decoding a git object

From: Thomas Rast <trast@inf.ethz.ch>
Date: Feb 20, 2012, 10:56 UTC
Message-ID: <871uppbwnu.fsf@thomas.inf.ethz.ch>
In-Reply-To: <5D290031F60941368FB1A36464201DCF@PhilipOakley>

Philip Oakley <philipoakley@iee.org> writes:

Show 28 quoted lines

> From: "Thomas Rast" <trast@inf.ethz.ch> Sent: Monday, February 20,
> 2012 8:29 AM
>>
>> The SHA1 is over the decompressed object contents.  The file simply
>> holds a zlib-compressed stream of those contents.  (It's pretty much
>> like gzip without the file header.)
>>
>> You can use any bindings to zlib and something that does sha1, e.g. in
>> python:
>>
>>  $ cd g/.git/objects/aa/  # my git.git
>>  $ ls
>>  592bda986a8380b64acd8cbb3d5bdfcbc0834d
>> 6322a757bee31919f54edcc127608a3d724c99
>>  $ python
>>  Python 2.7.2 (default, Aug 19 2011, 20:41:43) [GCC] on linux2
>>  Type "help", "copyright", "credits" or "license" for more information.
>>  >>> import hashlib
>>  >>>
>> hashlib.sha1(open('592bda986a8380b64acd8cbb3d5bdfcbc0834d').read().decode('zlib')).digest().encode('hex')
>>  'aa592bda986a8380b64acd8cbb3d5bdfcbc0834d'
>>
>> Notice that the first byte of the hash goes into the directory name.
>>
>
> At the moment I'm in a Catch 22 situation where I can't make the first
> step of examining the deflated contents, so I can't do all those next
> steps to get the sha1 etc.. Have I misunderstood your suggestions?

Huh? The method I showed does not rely on knowing the SHA1. The fact that I used it on a properly filed away (by its SHA1) object file is immaterial, if perhaps confusing.

I can untangle that python expression for you:

hashlib.sha1(foo).digest() gives the SHA1 digest of the string foo, as a (binary) string foo.encode('hex') turns foo from (binary) string into its hex representation open('filename').read() opens the file called filename, and returns its whole contents foo.decode('zlib') applies the zlib decompressor to foo, and returns the resulting data

So that trick works for any file[*], and you can then use its results to file it back where it needs to go.

[*] that is sufficiently small for Python to hold it in memory, but git shares the same problems in that department.

-- 
Thomas Rast
trast@{inf,student}.ethz.ch