Sensible way to see what objects are being fetched just-in-time in a partial clone?

[Tao Klerks <tao@klerks.biz>]

Hi folks,

In working with Partial / Filtered Clone repos, there are situations
where objects get fetched just-in-time - eg during a "git blame", if
you did a "blob:none" filtered clone, you can easily end up with
hundreds of fetches as git iterates backwards through the file
history.

I was trying to write a "git blame optimizer" to pre-fetch all the
suitable blobs, and it wasn't working right, so the "git blame" was
still fetching stuff - but I couldn't see what it was fetching (which
made it hard to investigate the bug in my script).

I did end up getting a list of some just-in-time fetched blobs, by
dumping a list of *all* the object IDs I had locally, before and after
a still-fetching-stuff "git blame" run, and doing a before/after
comparison of the resulting list of objects. To get the list of
objects found locally I did:

git cat-file --batch-check='%(objectname)' --batch-all-objects --unordered

(ref: a conversation with Peff last year:
https://lore.kernel.org/git/20230621064459.GA607974@coredump.intra.peff.net/
)

This was a sucky process though - and I was very surprised that I
couldn't see what was being fetched (what the stdin content to the
just-in-time fetch calls were) with any of the trace env vars that I
was able to find documented: GIT_TRACE, GIT_CURL_VERBOSE,
GIT_TRACE_PERFORMANCE, GIT_TRACE_PACK_ACCESS, GIT_TRACE_PACKET,
GIT_TRACE_PACKFILE, GIT_TRACE_SETUP, GIT_TRACE_SHALLOW

The only thing I could easily see were the *args* passed to nested git
processes.

Is there any way to see what a just-in-time fetch is fetching? Or any
way to see the content passed around on stdin in nested git processes?

Thanks,
Tao