diff mbox series

[09/13] reftable/generic: move seeking of records into the iterator

Message ID 859b399e71e92dd7aa0fa7fc9f2d9ca6f389d34a.1715166175.git.ps@pks.im (mailing list archive)
State Superseded
Headers show
Series reftable: prepare for re-seekable iterators | expand

Commit Message

Patrick Steinhardt May 8, 2024, 11:04 a.m. UTC 
Reftable iterators are created by seeking on the parent structure of a
corresponding record. For example, to create an iterator for the merged
table you would call `reftable_merged_table_seek_ref()`. Most notably,
it is not posible to create an iterator and then seek it afterwards.

While this may be a bit easier to reason about, it comes with two
significant downsides. The first downside is that the logic to find
records is split up between the parent data structure and the iterator
itself. Conceptually, it is more straight forward if all that logic was
contained in a single place, which should be the iterator.

The second and more significant downside is that it is impossible to
reuse iterators for multiple seeks. Whenever you want to look up a
record, you need to re-create the whole infrastructure again, which is
quite a waste of time. Furthermore, it is impossible to for example
optimize seeks, for example when seeking the same record multiple times.

To address this, we essentially split up the concerns properly such that
the parent data structure is responsible for setting up the iterator via
a new `init_iter()` callback, whereas the iterator handles seeks via a
new `seek()` callback. This will eventually allow us to call `seek()` on
the iterator multiple times, where every iterator can potentially
optimize for certain cases.

Note that at this point in time we are not yet ready to reuse the
iterators. This will be left for a future patch series.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
---
 reftable/generic.c |  47 +++++++++++++----
 reftable/generic.h |   9 +++-
 reftable/iter.c    |  15 ++++++
 reftable/merged.c  |  97 +++++++++++++++++-----------------
 reftable/reader.c  | 126 +++++++++++++++++++++++++--------------------
 5 files changed, 177 insertions(+), 117 deletions(-)

Comments

Justin Tobler May 10, 2024, 9:44 p.m. UTC | #1 
On 24/05/08 01:04PM, Patrick Steinhardt wrote:
> Reftable iterators are created by seeking on the parent structure of a
> corresponding record. For example, to create an iterator for the merged
> table you would call `reftable_merged_table_seek_ref()`. Most notably,
> it is not posible to create an iterator and then seek it afterwards.
> 
> While this may be a bit easier to reason about, it comes with two
> significant downsides. The first downside is that the logic to find
> records is split up between the parent data structure and the iterator
> itself. Conceptually, it is more straight forward if all that logic was
> contained in a single place, which should be the iterator.
> 
> The second and more significant downside is that it is impossible to
> reuse iterators for multiple seeks. Whenever you want to look up a
> record, you need to re-create the whole infrastructure again, which is
> quite a waste of time. Furthermore, it is impossible to for example
> optimize seeks, for example when seeking the same record multiple times.

The last setence could use some rewording.

"Furthermore, it is impossible to optimize seeks, such as when seeking
the same record multiple times."

> 
> To address this, we essentially split up the concerns properly such that
> the parent data structure is responsible for setting up the iterator via
> a new `init_iter()` callback, whereas the iterator handles seeks via a
> new `seek()` callback. This will eventually allow us to call `seek()` on
> the iterator multiple times, where every iterator can potentially
> optimize for certain cases.
> 
> Note that at this point in time we are not yet ready to reuse the
> iterators. This will be left for a future patch series.
> 
> Signed-off-by: Patrick Steinhardt <ps@pks.im>
> ---
>  reftable/generic.c |  47 +++++++++++++----
>  reftable/generic.h |   9 +++-
>  reftable/iter.c    |  15 ++++++
>  reftable/merged.c  |  97 +++++++++++++++++-----------------
>  reftable/reader.c  | 126 +++++++++++++++++++++++++--------------------
>  5 files changed, 177 insertions(+), 117 deletions(-)
> 
> diff --git a/reftable/generic.c b/reftable/generic.c
> index b9f1c7c18a..1cf68fe124 100644
> --- a/reftable/generic.c
> +++ b/reftable/generic.c
> @@ -12,25 +12,39 @@ license that can be found in the LICENSE file or at
>  #include "reftable-iterator.h"
>  #include "reftable-generic.h"
>  
> +void table_init_iter(struct reftable_table *tab,

The following table related functions are prefixed with `reftable_`. Do
we want to do the same here?

> +		     struct reftable_iterator *it,
> +		     uint8_t typ)
> +{
> +
> +	tab->ops->init_iter(tab->table_arg, it, typ);
> +}
> +
>  int reftable_table_seek_ref(struct reftable_table *tab,
>  			    struct reftable_iterator *it, const char *name)
>  {
> -	struct reftable_record rec = { .type = BLOCK_TYPE_REF,
> -				       .u.ref = {
> -					       .refname = (char *)name,
> -				       } };
> -	return tab->ops->seek_record(tab->table_arg, it, &rec);
> +	struct reftable_record want = {
> +		.type = BLOCK_TYPE_REF,
> +		.u.ref = {
> +			.refname = (char *)name,
> +		},
> +	};
> +	table_init_iter(tab, it, BLOCK_TYPE_REF);
> +	return it->ops->seek(it->iter_arg, &want);
>  }
>  
>  int reftable_table_seek_log(struct reftable_table *tab,
>  			    struct reftable_iterator *it, const char *name)
>  {
> -	struct reftable_record rec = { .type = BLOCK_TYPE_LOG,
> -				       .u.log = {
> -					       .refname = (char *)name,
> -					       .update_index = ~((uint64_t)0),
> -				       } };
> -	return tab->ops->seek_record(tab->table_arg, it, &rec);
> +	struct reftable_record want = {
> +		.type = BLOCK_TYPE_LOG,
> +		.u.log = {
> +			.refname = (char *)name,
> +			.update_index = ~((uint64_t)0),
> +		},
> +	};
> +	table_init_iter(tab, it, BLOCK_TYPE_LOG);
> +	return it->ops->seek(it->iter_arg, &want);
>  }
>  
>  int reftable_table_read_ref(struct reftable_table *tab, const char *name,
...
> @@ -23,6 +23,13 @@ static void filtering_ref_iterator_close(void *iter_arg)
>  	reftable_iterator_destroy(&fri->it);
>  }
>  
> +static int filtering_ref_iterator_seek(void *iter_arg,
> +				       struct reftable_record *want)
> +{
> +	struct filtering_ref_iterator *fri = iter_arg;
> +	return iterator_seek(&fri->it, want);
> +}

I've found the `filtering_ref_iterator_seek()` here to be a little
confusing. At first, I assumed that the `filtering_ref_iterator` would
have referenced `filtering_ref_iterator_vtable` thus resulting in a
cycle, but on closer inspection this does not seem to be the case and is
in face always set to some other iterator operation.

Am I understanding this correctly?

-Justin
Patrick Steinhardt May 13, 2024, 8:36 a.m. UTC | #2 
On Fri, May 10, 2024 at 04:44:54PM -0500, Justin Tobler wrote:
> On 24/05/08 01:04PM, Patrick Steinhardt wrote:
> > Reftable iterators are created by seeking on the parent structure of a
> > corresponding record. For example, to create an iterator for the merged
> > table you would call `reftable_merged_table_seek_ref()`. Most notably,
> > it is not posible to create an iterator and then seek it afterwards.
> > 
> > While this may be a bit easier to reason about, it comes with two
> > significant downsides. The first downside is that the logic to find
> > records is split up between the parent data structure and the iterator
> > itself. Conceptually, it is more straight forward if all that logic was
> > contained in a single place, which should be the iterator.
> > 
> > The second and more significant downside is that it is impossible to
> > reuse iterators for multiple seeks. Whenever you want to look up a
> > record, you need to re-create the whole infrastructure again, which is
> > quite a waste of time. Furthermore, it is impossible to for example
> > optimize seeks, for example when seeking the same record multiple times.
> 
> The last setence could use some rewording.
> 
> "Furthermore, it is impossible to optimize seeks, such as when seeking
> the same record multiple times."

Done.

[snip]
> > diff --git a/reftable/generic.c b/reftable/generic.c
> > index b9f1c7c18a..1cf68fe124 100644
> > --- a/reftable/generic.c
> > +++ b/reftable/generic.c
> > @@ -12,25 +12,39 @@ license that can be found in the LICENSE file or at
> >  #include "reftable-iterator.h"
> >  #include "reftable-generic.h"
> >  
> > +void table_init_iter(struct reftable_table *tab,
> 
> The following table related functions are prefixed with `reftable_`. Do
> we want to do the same here?

Functions with the `reftable_` prefix are supposed to be public, whereas
functions without them are private. So this is intentionally missing the
prefix.

[snip]
> > @@ -23,6 +23,13 @@ static void filtering_ref_iterator_close(void *iter_arg)
> >  	reftable_iterator_destroy(&fri->it);
> >  }
> >  
> > +static int filtering_ref_iterator_seek(void *iter_arg,
> > +				       struct reftable_record *want)
> > +{
> > +	struct filtering_ref_iterator *fri = iter_arg;
> > +	return iterator_seek(&fri->it, want);
> > +}
> 
> I've found the `filtering_ref_iterator_seek()` here to be a little
> confusing. At first, I assumed that the `filtering_ref_iterator` would
> have referenced `filtering_ref_iterator_vtable` thus resulting in a
> cycle, but on closer inspection this does not seem to be the case and is
> in face always set to some other iterator operation.
> 
> Am I understanding this correctly?

Yes. The filtering ref iterator wraps a _different_ iterator, which is
`fri->it` in the above case, and only returns a subset of the records of
that wrapped iterator. So we eventually end up calling the callbacks of
the wrapped iterator, which are likely not a filtering ref iterator
themselves (even though that would in theory be possible).

Patrick
diff mbox series

Patch

diff --git a/reftable/generic.c b/reftable/generic.c
index b9f1c7c18a..1cf68fe124 100644
--- a/reftable/generic.c
+++ b/reftable/generic.c
@@ -12,25 +12,39 @@  license that can be found in the LICENSE file or at
 #include "reftable-iterator.h"
 #include "reftable-generic.h"
 
+void table_init_iter(struct reftable_table *tab,
+		     struct reftable_iterator *it,
+		     uint8_t typ)
+{
+
+	tab->ops->init_iter(tab->table_arg, it, typ);
+}
+
 int reftable_table_seek_ref(struct reftable_table *tab,
 			    struct reftable_iterator *it, const char *name)
 {
-	struct reftable_record rec = { .type = BLOCK_TYPE_REF,
-				       .u.ref = {
-					       .refname = (char *)name,
-				       } };
-	return tab->ops->seek_record(tab->table_arg, it, &rec);
+	struct reftable_record want = {
+		.type = BLOCK_TYPE_REF,
+		.u.ref = {
+			.refname = (char *)name,
+		},
+	};
+	table_init_iter(tab, it, BLOCK_TYPE_REF);
+	return it->ops->seek(it->iter_arg, &want);
 }
 
 int reftable_table_seek_log(struct reftable_table *tab,
 			    struct reftable_iterator *it, const char *name)
 {
-	struct reftable_record rec = { .type = BLOCK_TYPE_LOG,
-				       .u.log = {
-					       .refname = (char *)name,
-					       .update_index = ~((uint64_t)0),
-				       } };
-	return tab->ops->seek_record(tab->table_arg, it, &rec);
+	struct reftable_record want = {
+		.type = BLOCK_TYPE_LOG,
+		.u.log = {
+			.refname = (char *)name,
+			.update_index = ~((uint64_t)0),
+		},
+	};
+	table_init_iter(tab, it, BLOCK_TYPE_LOG);
+	return it->ops->seek(it->iter_arg, &want);
 }
 
 int reftable_table_read_ref(struct reftable_table *tab, const char *name,
@@ -152,11 +166,21 @@  int reftable_iterator_next_log(struct reftable_iterator *it,
 	return err;
 }
 
+int iterator_seek(struct reftable_iterator *it, struct reftable_record *want)
+{
+	return it->ops->seek(it->iter_arg, want);
+}
+
 int iterator_next(struct reftable_iterator *it, struct reftable_record *rec)
 {
 	return it->ops->next(it->iter_arg, rec);
 }
 
+static int empty_iterator_seek(void *arg, struct reftable_record *want)
+{
+	return 0;
+}
+
 static int empty_iterator_next(void *arg, struct reftable_record *rec)
 {
 	return 1;
@@ -167,6 +191,7 @@  static void empty_iterator_close(void *arg)
 }
 
 static struct reftable_iterator_vtable empty_vtable = {
+	.seek = &empty_iterator_seek,
 	.next = &empty_iterator_next,
 	.close = &empty_iterator_close,
 };
diff --git a/reftable/generic.h b/reftable/generic.h
index 98886a0640..8341fa570e 100644
--- a/reftable/generic.h
+++ b/reftable/generic.h
@@ -14,19 +14,24 @@  license that can be found in the LICENSE file or at
 
 /* generic interface to reftables */
 struct reftable_table_vtable {
-	int (*seek_record)(void *tab, struct reftable_iterator *it,
-			   struct reftable_record *);
+	void (*init_iter)(void *tab, struct reftable_iterator *it, uint8_t typ);
 	uint32_t (*hash_id)(void *tab);
 	uint64_t (*min_update_index)(void *tab);
 	uint64_t (*max_update_index)(void *tab);
 };
 
+void table_init_iter(struct reftable_table *tab,
+		     struct reftable_iterator *it,
+		     uint8_t typ);
+
 struct reftable_iterator_vtable {
+	int (*seek)(void *iter_arg, struct reftable_record *want);
 	int (*next)(void *iter_arg, struct reftable_record *rec);
 	void (*close)(void *iter_arg);
 };
 
 void iterator_set_empty(struct reftable_iterator *it);
+int iterator_seek(struct reftable_iterator *it, struct reftable_record *want);
 int iterator_next(struct reftable_iterator *it, struct reftable_record *rec);
 
 #endif
diff --git a/reftable/iter.c b/reftable/iter.c
index aa9ac199b1..b4528fab47 100644
--- a/reftable/iter.c
+++ b/reftable/iter.c
@@ -23,6 +23,13 @@  static void filtering_ref_iterator_close(void *iter_arg)
 	reftable_iterator_destroy(&fri->it);
 }
 
+static int filtering_ref_iterator_seek(void *iter_arg,
+				       struct reftable_record *want)
+{
+	struct filtering_ref_iterator *fri = iter_arg;
+	return iterator_seek(&fri->it, want);
+}
+
 static int filtering_ref_iterator_next(void *iter_arg,
 				       struct reftable_record *rec)
 {
@@ -73,6 +80,7 @@  static int filtering_ref_iterator_next(void *iter_arg,
 }
 
 static struct reftable_iterator_vtable filtering_ref_iterator_vtable = {
+	.seek = &filtering_ref_iterator_seek,
 	.next = &filtering_ref_iterator_next,
 	.close = &filtering_ref_iterator_close,
 };
@@ -119,6 +127,12 @@  static int indexed_table_ref_iter_next_block(struct indexed_table_ref_iter *it)
 	return 0;
 }
 
+static int indexed_table_ref_iter_seek(void *p, struct reftable_record *want)
+{
+	BUG("seeking indexed table is not supported");
+	return -1;
+}
+
 static int indexed_table_ref_iter_next(void *p, struct reftable_record *rec)
 {
 	struct indexed_table_ref_iter *it = p;
@@ -175,6 +189,7 @@  int new_indexed_table_ref_iter(struct indexed_table_ref_iter **dest,
 }
 
 static struct reftable_iterator_vtable indexed_table_ref_iter_vtable = {
+	.seek = &indexed_table_ref_iter_seek,
 	.next = &indexed_table_ref_iter_next,
 	.close = &indexed_table_ref_iter_close,
 };
diff --git a/reftable/merged.c b/reftable/merged.c
index 18a2a6f09b..fc7946d90d 100644
--- a/reftable/merged.c
+++ b/reftable/merged.c
@@ -31,12 +31,18 @@  struct merged_iter {
 };
 
 static void merged_iter_init(struct merged_iter *mi,
-			     struct reftable_merged_table *mt)
+			     struct reftable_merged_table *mt,
+			     uint8_t typ)
 {
 	memset(mi, 0, sizeof(*mi));
 	mi->advance_index = -1;
 	mi->suppress_deletions = mt->suppress_deletions;
+
 	REFTABLE_CALLOC_ARRAY(mi->subiters, mt->stack_len);
+	for (size_t i = 0; i < mt->stack_len; i++) {
+		reftable_record_init(&mi->subiters[i].rec, typ);
+		table_init_iter(&mt->stack[i], &mi->subiters[i].iter, typ);
+	}
 	mi->stack_len = mt->stack_len;
 }
 
@@ -68,6 +74,27 @@  static int merged_iter_advance_subiter(struct merged_iter *mi, size_t idx)
 	return 0;
 }
 
+static int merged_iter_seek(struct merged_iter *mi, struct reftable_record *want)
+{
+	int err;
+
+	mi->advance_index = -1;
+
+	for (size_t i = 0; i < mi->stack_len; i++) {
+		err = iterator_seek(&mi->subiters[i].iter, want);
+		if (err < 0)
+			return err;
+		if (err > 0)
+			continue;
+
+		err = merged_iter_advance_subiter(mi, i);
+		if (err < 0)
+			return err;
+	}
+
+	return 0;
+}
+
 static int merged_iter_next_entry(struct merged_iter *mi,
 				  struct reftable_record *rec)
 {
@@ -133,6 +160,11 @@  static int merged_iter_next_entry(struct merged_iter *mi,
 	return 0;
 }
 
+static int merged_iter_seek_void(void *it, struct reftable_record *want)
+{
+	return merged_iter_seek(it, want);
+}
+
 static int merged_iter_next_void(void *p, struct reftable_record *rec)
 {
 	struct merged_iter *mi = p;
@@ -147,6 +179,7 @@  static int merged_iter_next_void(void *p, struct reftable_record *rec)
 }
 
 static struct reftable_iterator_vtable merged_iter_vtable = {
+	.seek = merged_iter_seek_void,
 	.next = &merged_iter_next_void,
 	.close = &merged_iter_close,
 };
@@ -220,47 +253,13 @@  reftable_merged_table_min_update_index(struct reftable_merged_table *mt)
 	return mt->min;
 }
 
-static int reftable_table_seek_record(struct reftable_table *tab,
-				      struct reftable_iterator *it,
-				      struct reftable_record *rec)
-{
-	return tab->ops->seek_record(tab->table_arg, it, rec);
-}
-
-static int merged_table_seek_record(struct reftable_merged_table *mt,
-				    struct reftable_iterator *it,
-				    struct reftable_record *rec)
+static void merged_table_init_iter(struct reftable_merged_table *mt,
+				   struct reftable_iterator *it,
+				   uint8_t typ)
 {
-	struct merged_iter merged, *p;
-	int err;
-
-	merged_iter_init(&merged, mt);
-
-	for (size_t i = 0; i < mt->stack_len; i++) {
-		reftable_record_init(&merged.subiters[i].rec,
-				     reftable_record_type(rec));
-
-		err = reftable_table_seek_record(&mt->stack[i],
-						 &merged.subiters[i].iter, rec);
-		if (err < 0)
-			goto out;
-		if (err > 0)
-			continue;
-
-		err = merged_iter_advance_subiter(&merged, i);
-		if (err < 0)
-			goto out;
-	}
-
-	p = reftable_malloc(sizeof(*p));
-	*p = merged;
-	iterator_from_merged_iter(it, p);
-	err = 0;
-
-out:
-	if (err < 0)
-		merged_iter_close(&merged);
-	return err;
+	struct merged_iter *mi = reftable_malloc(sizeof(*mi));
+	merged_iter_init(mi, mt, typ);
+	iterator_from_merged_iter(it, mi);
 }
 
 int reftable_merged_table_seek_ref(struct reftable_merged_table *mt,
@@ -273,7 +272,8 @@  int reftable_merged_table_seek_ref(struct reftable_merged_table *mt,
 			.refname = (char *)name,
 		},
 	};
-	return merged_table_seek_record(mt, it, &rec);
+	merged_table_init_iter(mt, it, BLOCK_TYPE_REF);
+	return iterator_seek(it, &rec);
 }
 
 int reftable_merged_table_seek_log_at(struct reftable_merged_table *mt,
@@ -285,7 +285,8 @@  int reftable_merged_table_seek_log_at(struct reftable_merged_table *mt,
 					       .refname = (char *)name,
 					       .update_index = update_index,
 				       } };
-	return merged_table_seek_record(mt, it, &rec);
+	merged_table_init_iter(mt, it, BLOCK_TYPE_LOG);
+	return iterator_seek(it, &rec);
 }
 
 int reftable_merged_table_seek_log(struct reftable_merged_table *mt,
@@ -301,11 +302,11 @@  uint32_t reftable_merged_table_hash_id(struct reftable_merged_table *mt)
 	return mt->hash_id;
 }
 
-static int reftable_merged_table_seek_void(void *tab,
-					   struct reftable_iterator *it,
-					   struct reftable_record *rec)
+static void reftable_merged_table_init_iter_void(void *tab,
+						 struct reftable_iterator *it,
+						 uint8_t typ)
 {
-	return merged_table_seek_record(tab, it, rec);
+	merged_table_init_iter(tab, it, typ);
 }
 
 static uint32_t reftable_merged_table_hash_id_void(void *tab)
@@ -324,7 +325,7 @@  static uint64_t reftable_merged_table_max_update_index_void(void *tab)
 }
 
 static struct reftable_table_vtable merged_table_vtable = {
-	.seek_record = reftable_merged_table_seek_void,
+	.init_iter = reftable_merged_table_init_iter_void,
 	.hash_id = reftable_merged_table_hash_id_void,
 	.min_update_index = reftable_merged_table_min_update_index_void,
 	.max_update_index = reftable_merged_table_max_update_index_void,
diff --git a/reftable/reader.c b/reftable/reader.c
index 021608f638..a5a13cb0b9 100644
--- a/reftable/reader.c
+++ b/reftable/reader.c
@@ -369,29 +369,6 @@  static int table_iter_next(struct table_iter *ti, struct reftable_record *rec)
 	}
 }
 
-static int table_iter_next_void(void *ti, struct reftable_record *rec)
-{
-	return table_iter_next(ti, rec);
-}
-
-static void table_iter_close_void(void *ti)
-{
-	table_iter_close(ti);
-}
-
-static struct reftable_iterator_vtable table_iter_vtable = {
-	.next = &table_iter_next_void,
-	.close = &table_iter_close_void,
-};
-
-static void iterator_from_table_iter(struct reftable_iterator *it,
-				     struct table_iter *ti)
-{
-	assert(!it->ops);
-	it->iter_arg = ti;
-	it->ops = &table_iter_vtable;
-}
-
 static int table_iter_seek_to(struct table_iter *ti, uint64_t off, uint8_t typ)
 {
 	int err;
@@ -576,43 +553,74 @@  static int table_iter_seek_indexed(struct table_iter *ti,
 	return err;
 }
 
-static int reader_seek(struct reftable_reader *r, struct reftable_iterator *it,
-		       struct reftable_record *rec)
+static int table_iter_seek(struct table_iter *ti,
+			   struct reftable_record *want)
 {
-	uint8_t typ = reftable_record_type(rec);
-	struct reftable_reader_offsets *offs = reader_offsets_for(r, typ);
-	struct table_iter ti, *p;
+	uint8_t typ = reftable_record_type(want);
+	struct reftable_reader_offsets *offs = reader_offsets_for(ti->r, typ);
 	int err;
 
-	if (!offs->is_present) {
-		iterator_set_empty(it);
-		return 0;
-	}
-
-	table_iter_init(&ti, r);
-
-	err = table_iter_seek_start(&ti, reftable_record_type(rec),
+	err = table_iter_seek_start(ti, reftable_record_type(want),
 				    !!offs->index_offset);
 	if (err < 0)
 		goto out;
 
 	if (offs->index_offset)
-		err = table_iter_seek_indexed(&ti, rec);
+		err = table_iter_seek_indexed(ti, want);
 	else
-		err = table_iter_seek_linear(&ti, rec);
+		err = table_iter_seek_linear(ti, want);
 	if (err)
 		goto out;
 
-	REFTABLE_ALLOC_ARRAY(p, 1);
-	*p = ti;
-	iterator_from_table_iter(it, p);
-
 out:
-	if (err)
-		table_iter_close(&ti);
 	return err;
 }
 
+static int table_iter_seek_void(void *ti, struct reftable_record *want)
+{
+	return table_iter_seek(ti, want);
+}
+
+static int table_iter_next_void(void *ti, struct reftable_record *rec)
+{
+	return table_iter_next(ti, rec);
+}
+
+static void table_iter_close_void(void *ti)
+{
+	table_iter_close(ti);
+}
+
+static struct reftable_iterator_vtable table_iter_vtable = {
+	.seek = &table_iter_seek_void,
+	.next = &table_iter_next_void,
+	.close = &table_iter_close_void,
+};
+
+static void iterator_from_table_iter(struct reftable_iterator *it,
+				     struct table_iter *ti)
+{
+	assert(!it->ops);
+	it->iter_arg = ti;
+	it->ops = &table_iter_vtable;
+}
+
+static void reader_init_iter(struct reftable_reader *r,
+			     struct reftable_iterator *it,
+			     uint8_t typ)
+{
+	struct reftable_reader_offsets *offs = reader_offsets_for(r, typ);
+
+	if (offs->is_present) {
+		struct table_iter *ti;
+		REFTABLE_ALLOC_ARRAY(ti, 1);
+		table_iter_init(ti, r);
+		iterator_from_table_iter(it, ti);
+	} else {
+		iterator_set_empty(it);
+	}
+}
+
 int reftable_reader_seek_ref(struct reftable_reader *r,
 			     struct reftable_iterator *it, const char *name)
 {
@@ -622,19 +630,23 @@  int reftable_reader_seek_ref(struct reftable_reader *r,
 			.refname = (char *)name,
 		},
 	};
-	return reader_seek(r, it, &rec);
+	reader_init_iter(r, it, BLOCK_TYPE_REF);
+	return iterator_seek(it, &rec);
 }
 
 int reftable_reader_seek_log_at(struct reftable_reader *r,
 				struct reftable_iterator *it, const char *name,
 				uint64_t update_index)
 {
-	struct reftable_record rec = { .type = BLOCK_TYPE_LOG,
-				       .u.log = {
-					       .refname = (char *)name,
-					       .update_index = update_index,
-				       } };
-	return reader_seek(r, it, &rec);
+	struct reftable_record rec = {
+		.type = BLOCK_TYPE_LOG,
+		.u.log = {
+			.refname = (char *)name,
+			.update_index = update_index,
+		},
+	};
+	reader_init_iter(r, it, BLOCK_TYPE_LOG);
+	return iterator_seek(it, &rec);
 }
 
 int reftable_reader_seek_log(struct reftable_reader *r,
@@ -692,7 +704,8 @@  static int reftable_reader_refs_for_indexed(struct reftable_reader *r,
 	struct indexed_table_ref_iter *itr = NULL;
 
 	/* Look through the reverse index. */
-	err = reader_seek(r, &oit, &want);
+	reader_init_iter(r, &oit, BLOCK_TYPE_OBJ);
+	err = iterator_seek(&oit, &want);
 	if (err != 0)
 		goto done;
 
@@ -773,10 +786,11 @@  uint64_t reftable_reader_min_update_index(struct reftable_reader *r)
 
 /* generic table interface. */
 
-static int reftable_reader_seek_void(void *tab, struct reftable_iterator *it,
-				     struct reftable_record *rec)
+static void reftable_reader_init_iter_void(void *tab,
+					   struct reftable_iterator *it,
+					   uint8_t typ)
 {
-	return reader_seek(tab, it, rec);
+	reader_init_iter(tab, it, typ);
 }
 
 static uint32_t reftable_reader_hash_id_void(void *tab)
@@ -795,7 +809,7 @@  static uint64_t reftable_reader_max_update_index_void(void *tab)
 }
 
 static struct reftable_table_vtable reader_vtable = {
-	.seek_record = reftable_reader_seek_void,
+	.init_iter = reftable_reader_init_iter_void,
 	.hash_id = reftable_reader_hash_id_void,
 	.min_update_index = reftable_reader_min_update_index_void,
 	.max_update_index = reftable_reader_max_update_index_void,