Understanding last ~28 bytes of index file

Understanding last ~28 bytes of index file

[Farhan Khan]

Hi all,

I am trying to understand how the index (.git/index) file works. When looking through extensions, it loops until it reaches the last 28 bytes (SHA size + 8). This is referenced here. https://github.com/git/git/blob/master/read-cache.c#L1933
What do the last 28 bytes consist of? Where in the source are bytes written?

I am trying to understand how to reproduce the index file and am not sure how this part works.

Thank you,
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Farhan Khan
PGP Fingerprint: 1312 89CE 663E 1EB2 179C  1C83 C41D 2281 F8DA C0DE

Re: Understanding last ~28 bytes of index file

[Jeff King]

On Thu, Jun 13, 2019 at 06:32:03AM +0000, Farhan Khan wrote:

> I am trying to understand how the index (.git/index) file works. When
> looking through extensions, it loops until it reaches the last 28
> bytes (SHA size + 8). This is referenced here.
> https://github.com/git/git/blob/master/read-cache.c#L1933
> What do the last 28 bytes consist of? Where in the source are bytes written?

It is not that the last 28 bytes are magical. It is that you must have
at least 28 bytes remaining in the file for there to be any extension.
20 of those are the trailing sha1 hash (which are written as part of the
hashfile code in csum-file.c).

And then for there to be an extension, it must have 8 bytes of name and
length, which will then be followed by however many bytes the length
indicated. After parsing that, we check to see if there are enough bytes
for another extension, and so on.

To find where the index extension blocks themselves are written, perhaps
grep for write_index_ext_header(), which should be called before writing
out the data for each.

-Peff