Subject: Re: [PATCH v2 6/6] refs: skip hooks when deleting uncovered packed refs

From: Patrick Steinhardt <[email-masked]>
Date: Mon, 10 Jan 2022 14:18:14 +0100

On Fri, Jan 07, 2022 at 06:01:04PM -0800, Junio C Hamano wrote:
> Patrick Steinhardt <[email-masked]> writes:
> 
> > When deleting refs from the loose-files refs backend, then we need to be
> > careful to also delete the same ref from the packed refs backend, if it
> > exists. If we don't, then deleting the loose ref would "uncover" the
> > packed ref. We thus always have to queue up deletions of refs for both
> > the loose and the packed refs backend. This is done in two separate
> > transactions, where the end result is that the reference-transaction
> > hook is executed twice for the deleted refs.
> >
> > This behaviour is quite misleading: it's exposing implementation details
> > of how the files backend works to the user, in contrast to the logical
> > updates that we'd really want to expose via the hook. Worse yet, whether
> > the hook gets executed once or twice depends on how well-packed the
> > repository is: if the ref only exists as a loose ref, then we execute it
> > once, otherwise if it is also packed then we execute it twice.
> 
> If the ref only exists as a packed ref, what happens? ...
> 
> > Fix this behaviour and don't execute the reference-transaction hook at
> > all when refs in the packed-refs backend if it's driven by the files
> > backend.
> 
> ... We try to remove from the loose backend, which would say "nah,
> it did not exist in my store".  I am not sure if it should execute
> the delete hook in such a case for the ref.  But if it does not, not
> running the hook in the ref transaction for packed backend driven by
> the loose backend would mean nobody notifies the deletion of the
> ref, no?

This is shown in the test I've added, "deleting packed ref calls hook
once". We create a new reference and pack it such that it doesn't exist
as loose ref anymore, but only as a packed one. Updating that ref
would've caused us to execute the hook twice before, once via the
packed-backend and once via the loose-backend. With my fix we only
execute it once via the loose-backend, even if it doesn't currently know
it. This works because the loose-backend has to create a lock for the
nonexistent reference such that no concurrent call touches the same ref.

> To me, it seems that the only case this strategy would work
> correctly is when the files backend calls deletion hook for a
> request to delete nonexistent ref, which by itself sounds like a
> problem.

It does so only if the ref exists in either the loose or packed backend
though. If trying to update a ref which exists in neither of those, then
the reference transaction would fail with an "unable to resolve
reference" error in `lock_raw_ref()`.

So this should behave as expected: deleting a packed ref calls the hook
once, deleting a nonexistent ref fails and doesn't call the hook at all.

Patrick

> > Signed-off-by: Patrick Steinhardt <[email-masked]>
> > ---
> >  refs/files-backend.c             | 9 ++++++---
> >  t/t1416-ref-transaction-hooks.sh | 7 +------
> >  2 files changed, 7 insertions(+), 9 deletions(-)
> >
> > diff --git a/refs/files-backend.c b/refs/files-backend.c
> > index 3c0f3027fe..9a20cb8fa8 100644
> > --- a/refs/files-backend.c
> > +++ b/refs/files-backend.c
> > @@ -1261,7 +1261,8 @@ static int files_delete_refs(struct ref_store *ref_store, const char *msg,
> >  	if (packed_refs_lock(refs->packed_ref_store, 0, &err))
> >  		goto error;
> >  
> > -	transaction = ref_store_transaction_begin(refs->packed_ref_store, 0, &err);
> > +	transaction = ref_store_transaction_begin(refs->packed_ref_store,
> > +						  REF_TRANSACTION_SKIP_HOOK, &err);
> >  	if (!transaction)
> >  		goto error;
> >  
> > @@ -2775,7 +2776,8 @@ static int files_transaction_prepare(struct ref_store *ref_store,
> >  			 */
> >  			if (!packed_transaction) {
> >  				packed_transaction = ref_store_transaction_begin(
> > -						refs->packed_ref_store, 0, err);
> > +						refs->packed_ref_store,
> > +						REF_TRANSACTION_SKIP_HOOK, err);
> >  				if (!packed_transaction) {
> >  					ret = TRANSACTION_GENERIC_ERROR;
> >  					goto cleanup;
> > @@ -3046,7 +3048,8 @@ static int files_initial_transaction_commit(struct ref_store *ref_store,
> >  				 &affected_refnames))
> >  		BUG("initial ref transaction called with existing refs");
> >  
> > -	packed_transaction = ref_store_transaction_begin(refs->packed_ref_store, 0, err);
> > +	packed_transaction = ref_store_transaction_begin(refs->packed_ref_store,
> > +							 REF_TRANSACTION_SKIP_HOOK, err);
> >  	if (!packed_transaction) {
> >  		ret = TRANSACTION_GENERIC_ERROR;
> >  		goto cleanup;
> > diff --git a/t/t1416-ref-transaction-hooks.sh b/t/t1416-ref-transaction-hooks.sh
> > index f9d3d5213f..4e1e84a91f 100755
> > --- a/t/t1416-ref-transaction-hooks.sh
> > +++ b/t/t1416-ref-transaction-hooks.sh
> > @@ -175,16 +175,11 @@ test_expect_success 'deleting packed ref calls hook once' '
> >  	git update-ref -d refs/heads/to-be-deleted $POST_OID &&
> >  
> >  	# We only expect a single hook invocation, which is the logical
> > -	# deletion. But currently, we see two interleaving transactions, once
> > -	# for deleting the loose refs and once for deleting the packed ref.
> > +	# deletion.
> >  	cat >expect <<-EOF &&
> > -		prepared
> > -		$ZERO_OID $ZERO_OID refs/heads/to-be-deleted
> >  		prepared
> >  		$POST_OID $ZERO_OID refs/heads/to-be-deleted
> >  		committed
> > -		$ZERO_OID $ZERO_OID refs/heads/to-be-deleted
> > -		committed
> >  		$POST_OID $ZERO_OID refs/heads/to-be-deleted
> >  	EOF