Why can it be NULL? Why would you call this function if you did not want the answer?

Since I did the extraction in two pass, it behaved like git_repository_is_shallow() (which used it internally). But it doesn't make sense anymore. I'll make it mandatory.

It seems like there's not a lot of error checking here... It looks like it would handle a very malformed line if it was 80 characters of things that looked like hex characters...? Also, walking every character yourself seems a bit unnecessary and I think that you could solve both problems with some minor refactoring.

I would rather see you use some existing standard library function (git__strtok may be a good choice) to find the terminating newline, validate that it's GIT_OID_HEXSZ away from your start, and then try to parse it...

Does git handle blank linkes? Comments? Windows style newlines? I assume not, but I wanted to ask.

The documentation about the shallow file format is pretty terse (what I know of it is what's in Documentation/technical/shallow.txt), and I wanted to know if it's acceptable to look at the source (since licensing).

I rebased and pushed a version that uses git__strsep, which really feels cleaner. I do have a point to make : our implementation is "non-conforming" (for some definition of it 😉). Here's the relevant tidbit from my OS X man pages (which I was referring to when I wrote it) :

The location of the next character after the delimiter character (or NULL, if the end of the string was reached) is stored in *stringp. The original value of *stringp is returned.

Ours just leave *stringp as it is and returns NULL in that case, which can complicate parsing files where newlines-at-EOF are not guaranteed.

If that's expected, allow me to either 1) fix it or 2) add a comment on top of that function so you don't blindly trust your manpages (as I did).

For the record, the relevant changes are :

while ((line = git_strsep(&buffer, "\n")) != NULL) {
    /* stuff with line */
}
if (*buffer) {
    /* missing EOL */
}

to

while ((line = git_strsep(&buffer, "\n")) != NULL) {
    if (buffer == NULL)
        /*missing EOL */;
    /* stuff with line */
}