On Fri, Jan 07, 2022 at 06:01:04PM -0800, Junio C Hamano wrote:
> Patrick Steinhardt <ps@xxxxxx> 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 <ps@xxxxxx>
> > ---
> > 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
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