Re: Why is "git fetch --prune" so much slower than "git remote prune"?

[Junio C Hamano <gitster@pobox.com>]

Michael Haggerty <mhagger@alum.mit.edu> writes:

>> Now that we have ref_transaction_*, I think if git-fetch fed all of the
>> deletes (along with the updates) into a single transaction, we would get
>> the same optimization for free. Maybe that is even part of some of the
>> pending ref_transaction work from Stefan or Michael (both cc'd). I
>> haven't kept up very well with what is cooking in pu.
>
> I am looking into this now.
>
> For pruning, we can't use a ref_transaction as it is currently
> implemented because it would fail if any of the reference deletions
> failed. But in this case I think if any deletions fail, we would prefer
> to emit a warning but keep going.

I am not quite sure what you mean here.  I agree with you if you
meant "we shouldn't fail the fetch only because 'fetch --prune'
failed to remove only one of the remote-tracking refs that are no
longer there" but that can easily be solved by the pruning phase
into a separate transaction.  If you meant "we would rather remove
origin/{a,b} non-atomically when we noticed that origin/{a,b,c} are
all gone than leaving all three intact only because we failed to
remove origin/c for whatever reason", my knee-jerk reaction is "does
it make practical difference to the end user between these two?"

What are the plausible cause of failing to prune unused
remote-tracking refs?