reftable: add merged table view

This adds an abstract, read-only interface to the ref database.

This primitive is used to construct the read view of the ref database
(the read view is constructed by merging several *.ref files). It also
provides the mechanism to provide a unified view of the refs in the main
repository and the per-worktree refs.

Signed-off-by: Han-Wen Nienhuys <hanwen@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

6 files changed, 943 insertions, 0 deletions

diff --git a/Makefile b/Makefile
index e80f253ecc..6c8545b9eb 100644
--- a/Makefile
+++ b/Makefile
@@ -2462,6 +2462,7 @@ REFTABLE_OBJS += reftable/block.o
 REFTABLE_OBJS += reftable/blocksource.o
 REFTABLE_OBJS += reftable/iter.o
 REFTABLE_OBJS += reftable/publicbasics.o
+REFTABLE_OBJS += reftable/merged.o
 REFTABLE_OBJS += reftable/pq.o
 REFTABLE_OBJS += reftable/reader.o
 REFTABLE_OBJS += reftable/record.o
@@ -2473,6 +2474,7 @@ REFTABLE_OBJS += reftable/writer.o
 
 REFTABLE_TEST_OBJS += reftable/basics_test.o
 REFTABLE_TEST_OBJS += reftable/block_test.o
+REFTABLE_TEST_OBJS += reftable/merged_test.o
 REFTABLE_TEST_OBJS += reftable/pq_test.o
 REFTABLE_TEST_OBJS += reftable/record_test.o
 REFTABLE_TEST_OBJS += reftable/readwrite_test.o
diff --git a/reftable/merged.c b/reftable/merged.c
new file mode 100644
index 0000000000..e5b53da6db
--- /dev/null
+++ b/reftable/merged.c
@@ -0,0 +1,362 @@
+/*
+Copyright 2020 Google LLC
+
+Use of this source code is governed by a BSD-style
+license that can be found in the LICENSE file or at
+https://developers.google.com/open-source/licenses/bsd
+*/
+
+#include "merged.h"
+
+#include "constants.h"
+#include "iter.h"
+#include "pq.h"
+#include "reader.h"
+#include "record.h"
+#include "generic.h"
+#include "reftable-merged.h"
+#include "reftable-error.h"
+#include "system.h"
+
+static int merged_iter_init(struct merged_iter *mi)
+{
+	int i = 0;
+	for (i = 0; i < mi->stack_len; i++) {
+		struct reftable_record rec = reftable_new_record(mi->typ);
+		int err = iterator_next(&mi->stack[i], &rec);
+		if (err < 0) {
+			return err;
+		}
+
+		if (err > 0) {
+			reftable_iterator_destroy(&mi->stack[i]);
+			reftable_record_destroy(&rec);
+		} else {
+			struct pq_entry e = {
+				.rec = rec,
+				.index = i,
+			};
+			merged_iter_pqueue_add(&mi->pq, e);
+		}
+	}
+
+	return 0;
+}
+
+static void merged_iter_close(void *p)
+{
+	struct merged_iter *mi = p;
+	int i = 0;
+	merged_iter_pqueue_release(&mi->pq);
+	for (i = 0; i < mi->stack_len; i++) {
+		reftable_iterator_destroy(&mi->stack[i]);
+	}
+	reftable_free(mi->stack);
+}
+
+static int merged_iter_advance_nonnull_subiter(struct merged_iter *mi,
+					       size_t idx)
+{
+	struct reftable_record rec = reftable_new_record(mi->typ);
+	struct pq_entry e = {
+		.rec = rec,
+		.index = idx,
+	};
+	int err = iterator_next(&mi->stack[idx], &rec);
+	if (err < 0)
+		return err;
+
+	if (err > 0) {
+		reftable_iterator_destroy(&mi->stack[idx]);
+		reftable_record_destroy(&rec);
+		return 0;
+	}
+
+	merged_iter_pqueue_add(&mi->pq, e);
+	return 0;
+}
+
+static int merged_iter_advance_subiter(struct merged_iter *mi, size_t idx)
+{
+	if (iterator_is_null(&mi->stack[idx]))
+		return 0;
+	return merged_iter_advance_nonnull_subiter(mi, idx);
+}
+
+static int merged_iter_next_entry(struct merged_iter *mi,
+				  struct reftable_record *rec)
+{
+	struct strbuf entry_key = STRBUF_INIT;
+	struct pq_entry entry = { 0 };
+	int err = 0;
+
+	if (merged_iter_pqueue_is_empty(mi->pq))
+		return 1;
+
+	entry = merged_iter_pqueue_remove(&mi->pq);
+	err = merged_iter_advance_subiter(mi, entry.index);
+	if (err < 0)
+		return err;
+
+	/*
+	  One can also use reftable as datacenter-local storage, where the ref
+	  database is maintained in globally consistent database (eg.
+	  CockroachDB or Spanner). In this scenario, replication delays together
+	  with compaction may cause newer tables to contain older entries. In
+	  such a deployment, the loop below must be changed to collect all
+	  entries for the same key, and return new the newest one.
+	*/
+	reftable_record_key(&entry.rec, &entry_key);
+	while (!merged_iter_pqueue_is_empty(mi->pq)) {
+		struct pq_entry top = merged_iter_pqueue_top(mi->pq);
+		struct strbuf k = STRBUF_INIT;
+		int err = 0, cmp = 0;
+
+		reftable_record_key(&top.rec, &k);
+
+		cmp = strbuf_cmp(&k, &entry_key);
+		strbuf_release(&k);
+
+		if (cmp > 0) {
+			break;
+		}
+
+		merged_iter_pqueue_remove(&mi->pq);
+		err = merged_iter_advance_subiter(mi, top.index);
+		if (err < 0) {
+			return err;
+		}
+		reftable_record_destroy(&top.rec);
+	}
+
+	reftable_record_copy_from(rec, &entry.rec, hash_size(mi->hash_id));
+	reftable_record_destroy(&entry.rec);
+	strbuf_release(&entry_key);
+	return 0;
+}
+
+static int merged_iter_next(struct merged_iter *mi, struct reftable_record *rec)
+{
+	while (1) {
+		int err = merged_iter_next_entry(mi, rec);
+		if (err == 0 && mi->suppress_deletions &&
+		    reftable_record_is_deletion(rec)) {
+			continue;
+		}
+
+		return err;
+	}
+}
+
+static int merged_iter_next_void(void *p, struct reftable_record *rec)
+{
+	struct merged_iter *mi = p;
+	if (merged_iter_pqueue_is_empty(mi->pq))
+		return 1;
+
+	return merged_iter_next(mi, rec);
+}
+
+static struct reftable_iterator_vtable merged_iter_vtable = {
+	.next = &merged_iter_next_void,
+	.close = &merged_iter_close,
+};
+
+static void iterator_from_merged_iter(struct reftable_iterator *it,
+				      struct merged_iter *mi)
+{
+	assert(!it->ops);
+	it->iter_arg = mi;
+	it->ops = &merged_iter_vtable;
+}
+
+int reftable_new_merged_table(struct reftable_merged_table **dest,
+			      struct reftable_table *stack, int n,
+			      uint32_t hash_id)
+{
+	struct reftable_merged_table *m = NULL;
+	uint64_t last_max = 0;
+	uint64_t first_min = 0;
+	int i = 0;
+	for (i = 0; i < n; i++) {
+		uint64_t min = reftable_table_min_update_index(&stack[i]);
+		uint64_t max = reftable_table_max_update_index(&stack[i]);
+
+		if (reftable_table_hash_id(&stack[i]) != hash_id) {
+			return REFTABLE_FORMAT_ERROR;
+		}
+		if (i == 0 || min < first_min) {
+			first_min = min;
+		}
+		if (i == 0 || max > last_max) {
+			last_max = max;
+		}
+	}
+
+	m = reftable_calloc(sizeof(struct reftable_merged_table));
+	m->stack = stack;
+	m->stack_len = n;
+	m->min = first_min;
+	m->max = last_max;
+	m->hash_id = hash_id;
+	*dest = m;
+	return 0;
+}
+
+/* clears the list of subtable, without affecting the readers themselves. */
+void merged_table_release(struct reftable_merged_table *mt)
+{
+	FREE_AND_NULL(mt->stack);
+	mt->stack_len = 0;
+}
+
+void reftable_merged_table_free(struct reftable_merged_table *mt)
+{
+	if (!mt) {
+		return;
+	}
+	merged_table_release(mt);
+	reftable_free(mt);
+}
+
+uint64_t
+reftable_merged_table_max_update_index(struct reftable_merged_table *mt)
+{
+	return mt->max;
+}
+
+uint64_t
+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)
+{
+	struct reftable_iterator *iters = reftable_calloc(
+		sizeof(struct reftable_iterator) * mt->stack_len);
+	struct merged_iter merged = {
+		.stack = iters,
+		.typ = reftable_record_type(rec),
+		.hash_id = mt->hash_id,
+		.suppress_deletions = mt->suppress_deletions,
+	};
+	int n = 0;
+	int err = 0;
+	int i = 0;
+	for (i = 0; i < mt->stack_len && err == 0; i++) {
+		int e = reftable_table_seek_record(&mt->stack[i], &iters[n],
+						   rec);
+		if (e < 0) {
+			err = e;
+		}
+		if (e == 0) {
+			n++;
+		}
+	}
+	if (err < 0) {
+		int i = 0;
+		for (i = 0; i < n; i++) {
+			reftable_iterator_destroy(&iters[i]);
+		}
+		reftable_free(iters);
+		return err;
+	}
+
+	merged.stack_len = n;
+	err = merged_iter_init(&merged);
+	if (err < 0) {
+		merged_iter_close(&merged);
+		return err;
+	} else {
+		struct merged_iter *p =
+			reftable_malloc(sizeof(struct merged_iter));
+		*p = merged;
+		iterator_from_merged_iter(it, p);
+	}
+	return 0;
+}
+
+int reftable_merged_table_seek_ref(struct reftable_merged_table *mt,
+				   struct reftable_iterator *it,
+				   const char *name)
+{
+	struct reftable_ref_record ref = {
+		.refname = (char *)name,
+	};
+	struct reftable_record rec = { NULL };
+	reftable_record_from_ref(&rec, &ref);
+	return merged_table_seek_record(mt, it, &rec);
+}
+
+int reftable_merged_table_seek_log_at(struct reftable_merged_table *mt,
+				      struct reftable_iterator *it,
+				      const char *name, uint64_t update_index)
+{
+	struct reftable_log_record log = {
+		.refname = (char *)name,
+		.update_index = update_index,
+	};
+	struct reftable_record rec = { NULL };
+	reftable_record_from_log(&rec, &log);
+	return merged_table_seek_record(mt, it, &rec);
+}
+
+int reftable_merged_table_seek_log(struct reftable_merged_table *mt,
+				   struct reftable_iterator *it,
+				   const char *name)
+{
+	uint64_t max = ~((uint64_t)0);
+	return reftable_merged_table_seek_log_at(mt, it, name, max);
+}
+
+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)
+{
+	return merged_table_seek_record(tab, it, rec);
+}
+
+static uint32_t reftable_merged_table_hash_id_void(void *tab)
+{
+	return reftable_merged_table_hash_id(tab);
+}
+
+static uint64_t reftable_merged_table_min_update_index_void(void *tab)
+{
+	return reftable_merged_table_min_update_index(tab);
+}
+
+static uint64_t reftable_merged_table_max_update_index_void(void *tab)
+{
+	return reftable_merged_table_max_update_index(tab);
+}
+
+static struct reftable_table_vtable merged_table_vtable = {
+	.seek_record = reftable_merged_table_seek_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,
+};
+
+void reftable_table_from_merged_table(struct reftable_table *tab,
+				      struct reftable_merged_table *merged)
+{
+	assert(!tab->ops);
+	tab->ops = &merged_table_vtable;
+	tab->table_arg = merged;
+}
diff --git a/reftable/merged.h b/reftable/merged.h
new file mode 100644
index 0000000000..7d9f95d27e
--- /dev/null
+++ b/reftable/merged.h
@@ -0,0 +1,38 @@
+/*
+Copyright 2020 Google LLC
+
+Use of this source code is governed by a BSD-style
+license that can be found in the LICENSE file or at
+https://developers.google.com/open-source/licenses/bsd
+*/
+
+#ifndef MERGED_H
+#define MERGED_H
+
+#include "pq.h"
+
+struct reftable_merged_table {
+	struct reftable_table *stack;
+	size_t stack_len;
+	uint32_t hash_id;
+
+	/* If unset, produce deletions. This is useful for compaction. For the
+	 * full stack, deletions should be produced. */
+	int suppress_deletions;
+
+	uint64_t min;
+	uint64_t max;
+};
+
+struct merged_iter {
+	struct reftable_iterator *stack;
+	uint32_t hash_id;
+	size_t stack_len;
+	uint8_t typ;
+	int suppress_deletions;
+	struct merged_iter_pqueue pq;
+};
+
+void merged_table_release(struct reftable_merged_table *mt);
+
+#endif
diff --git a/reftable/merged_test.c b/reftable/merged_test.c
new file mode 100644
index 0000000000..24461e8a80
--- /dev/null
+++ b/reftable/merged_test.c
@@ -0,0 +1,468 @@
+/*
+Copyright 2020 Google LLC
+
+Use of this source code is governed by a BSD-style
+license that can be found in the LICENSE file or at
+https://developers.google.com/open-source/licenses/bsd
+*/
+
+#include "merged.h"
+
+#include "system.h"
+
+#include "basics.h"
+#include "blocksource.h"
+#include "constants.h"
+#include "reader.h"
+#include "record.h"
+#include "test_framework.h"
+#include "reftable-merged.h"
+#include "reftable-tests.h"
+#include "reftable-generic.h"
+#include "reftable-writer.h"
+
+static void write_test_table(struct strbuf *buf,
+			     struct reftable_ref_record refs[], int n)
+{
+	int min = 0xffffffff;
+	int max = 0;
+	int i = 0;
+	int err;
+
+	struct reftable_write_options opts = {
+		.block_size = 256,
+	};
+	struct reftable_writer *w = NULL;
+	for (i = 0; i < n; i++) {
+		uint64_t ui = refs[i].update_index;
+		if (ui > max) {
+			max = ui;
+		}
+		if (ui < min) {
+			min = ui;
+		}
+	}
+
+	w = reftable_new_writer(&strbuf_add_void, buf, &opts);
+	reftable_writer_set_limits(w, min, max);
+
+	for (i = 0; i < n; i++) {
+		uint64_t before = refs[i].update_index;
+		int n = reftable_writer_add_ref(w, &refs[i]);
+		EXPECT(n == 0);
+		EXPECT(before == refs[i].update_index);
+	}
+
+	err = reftable_writer_close(w);
+	EXPECT_ERR(err);
+
+	reftable_writer_free(w);
+}
+
+static void write_test_log_table(struct strbuf *buf,
+				 struct reftable_log_record logs[], int n,
+				 uint64_t update_index)
+{
+	int i = 0;
+	int err;
+
+	struct reftable_write_options opts = {
+		.block_size = 256,
+		.exact_log_message = 1,
+	};
+	struct reftable_writer *w = NULL;
+	w = reftable_new_writer(&strbuf_add_void, buf, &opts);
+	reftable_writer_set_limits(w, update_index, update_index);
+
+	for (i = 0; i < n; i++) {
+		int err = reftable_writer_add_log(w, &logs[i]);
+		EXPECT_ERR(err);
+	}
+
+	err = reftable_writer_close(w);
+	EXPECT_ERR(err);
+
+	reftable_writer_free(w);
+}
+
+static struct reftable_merged_table *
+merged_table_from_records(struct reftable_ref_record **refs,
+			  struct reftable_block_source **source,
+			  struct reftable_reader ***readers, int *sizes,
+			  struct strbuf *buf, int n)
+{
+	int i = 0;
+	struct reftable_merged_table *mt = NULL;
+	int err;
+	struct reftable_table *tabs =
+		reftable_calloc(n * sizeof(struct reftable_table));
+	*readers = reftable_calloc(n * sizeof(struct reftable_reader *));
+	*source = reftable_calloc(n * sizeof(**source));
+	for (i = 0; i < n; i++) {
+		write_test_table(&buf[i], refs[i], sizes[i]);
+		block_source_from_strbuf(&(*source)[i], &buf[i]);
+
+		err = reftable_new_reader(&(*readers)[i], &(*source)[i],
+					  "name");
+		EXPECT_ERR(err);
+		reftable_table_from_reader(&tabs[i], (*readers)[i]);
+	}
+
+	err = reftable_new_merged_table(&mt, tabs, n, GIT_SHA1_FORMAT_ID);
+	EXPECT_ERR(err);
+	return mt;
+}
+
+static void readers_destroy(struct reftable_reader **readers, size_t n)
+{
+	int i = 0;
+	for (; i < n; i++)
+		reftable_reader_free(readers[i]);
+	reftable_free(readers);
+}
+
+static void test_merged_between(void)
+{
+	uint8_t hash1[GIT_SHA1_RAWSZ] = { 1, 2, 3, 0 };
+
+	struct reftable_ref_record r1[] = { {
+		.refname = "b",
+		.update_index = 1,
+		.value_type = REFTABLE_REF_VAL1,
+		.value.val1 = hash1,
+	} };
+	struct reftable_ref_record r2[] = { {
+		.refname = "a",
+		.update_index = 2,
+		.value_type = REFTABLE_REF_DELETION,
+	} };
+
+	struct reftable_ref_record *refs[] = { r1, r2 };
+	int sizes[] = { 1, 1 };
+	struct strbuf bufs[2] = { STRBUF_INIT, STRBUF_INIT };
+	struct reftable_block_source *bs = NULL;
+	struct reftable_reader **readers = NULL;
+	struct reftable_merged_table *mt =
+		merged_table_from_records(refs, &bs, &readers, sizes, bufs, 2);
+	int i;
+	struct reftable_ref_record ref = { NULL };
+	struct reftable_iterator it = { NULL };
+	int err = reftable_merged_table_seek_ref(mt, &it, "a");
+	EXPECT_ERR(err);
+
+	err = reftable_iterator_next_ref(&it, &ref);
+	EXPECT_ERR(err);
+	EXPECT(ref.update_index == 2);
+	reftable_ref_record_release(&ref);
+	reftable_iterator_destroy(&it);
+	readers_destroy(readers, 2);
+	reftable_merged_table_free(mt);
+	for (i = 0; i < ARRAY_SIZE(bufs); i++) {
+		strbuf_release(&bufs[i]);
+	}
+	reftable_free(bs);
+}
+
+static void test_merged(void)
+{
+	uint8_t hash1[GIT_SHA1_RAWSZ] = { 1 };
+	uint8_t hash2[GIT_SHA1_RAWSZ] = { 2 };
+	struct reftable_ref_record r1[] = {
+		{
+			.refname = "a",
+			.update_index = 1,
+			.value_type = REFTABLE_REF_VAL1,
+			.value.val1 = hash1,
+		},
+		{
+			.refname = "b",
+			.update_index = 1,
+			.value_type = REFTABLE_REF_VAL1,
+			.value.val1 = hash1,
+		},
+		{
+			.refname = "c",
+			.update_index = 1,
+			.value_type = REFTABLE_REF_VAL1,
+			.value.val1 = hash1,
+		}
+	};
+	struct reftable_ref_record r2[] = { {
+		.refname = "a",
+		.update_index = 2,
+		.value_type = REFTABLE_REF_DELETION,
+	} };
+	struct reftable_ref_record r3[] = {
+		{
+			.refname = "c",
+			.update_index = 3,
+			.value_type = REFTABLE_REF_VAL1,
+			.value.val1 = hash2,
+		},
+		{
+			.refname = "d",
+			.update_index = 3,
+			.value_type = REFTABLE_REF_VAL1,
+			.value.val1 = hash1,
+		},
+	};
+
+	struct reftable_ref_record want[] = {
+		r2[0],
+		r1[1],
+		r3[0],
+		r3[1],
+	};
+
+	struct reftable_ref_record *refs[] = { r1, r2, r3 };
+	int sizes[3] = { 3, 1, 2 };
+	struct strbuf bufs[3] = { STRBUF_INIT, STRBUF_INIT, STRBUF_INIT };
+	struct reftable_block_source *bs = NULL;
+	struct reftable_reader **readers = NULL;
+	struct reftable_merged_table *mt =
+		merged_table_from_records(refs, &bs, &readers, sizes, bufs, 3);
+
+	struct reftable_iterator it = { NULL };
+	int err = reftable_merged_table_seek_ref(mt, &it, "a");
+	struct reftable_ref_record *out = NULL;
+	size_t len = 0;
+	size_t cap = 0;
+	int i = 0;
+
+	EXPECT_ERR(err);
+	EXPECT(reftable_merged_table_hash_id(mt) == GIT_SHA1_FORMAT_ID);
+	EXPECT(reftable_merged_table_min_update_index(mt) == 1);
+
+	while (len < 100) { /* cap loops/recursion. */
+		struct reftable_ref_record ref = { NULL };
+		int err = reftable_iterator_next_ref(&it, &ref);
+		if (err > 0) {
+			break;
+		}
+		if (len == cap) {
+			cap = 2 * cap + 1;
+			out = reftable_realloc(
+				out, sizeof(struct reftable_ref_record) * cap);
+		}
+		out[len++] = ref;
+	}
+	reftable_iterator_destroy(&it);
+
+	EXPECT(ARRAY_SIZE(want) == len);
+	for (i = 0; i < len; i++) {
+		EXPECT(reftable_ref_record_equal(&want[i], &out[i],
+						 GIT_SHA1_RAWSZ));
+	}
+	for (i = 0; i < len; i++) {
+		reftable_ref_record_release(&out[i]);
+	}
+	reftable_free(out);
+
+	for (i = 0; i < 3; i++) {
+		strbuf_release(&bufs[i]);
+	}
+	readers_destroy(readers, 3);
+	reftable_merged_table_free(mt);
+	reftable_free(bs);
+}
+
+static struct reftable_merged_table *
+merged_table_from_log_records(struct reftable_log_record **logs,
+			      struct reftable_block_source **source,
+			      struct reftable_reader ***readers, int *sizes,
+			      struct strbuf *buf, int n)
+{
+	int i = 0;
+	struct reftable_merged_table *mt = NULL;
+	int err;
+	struct reftable_table *tabs =
+		reftable_calloc(n * sizeof(struct reftable_table));
+	*readers = reftable_calloc(n * sizeof(struct reftable_reader *));
+	*source = reftable_calloc(n * sizeof(**source));
+	for (i = 0; i < n; i++) {
+		write_test_log_table(&buf[i], logs[i], sizes[i], i + 1);
+		block_source_from_strbuf(&(*source)[i], &buf[i]);
+
+		err = reftable_new_reader(&(*readers)[i], &(*source)[i],
+					  "name");
+		EXPECT_ERR(err);
+		reftable_table_from_reader(&tabs[i], (*readers)[i]);
+	}
+
+	err = reftable_new_merged_table(&mt, tabs, n, GIT_SHA1_FORMAT_ID);
+	EXPECT_ERR(err);
+	return mt;
+}
+
+static void test_merged_logs(void)
+{
+	uint8_t hash1[GIT_SHA1_RAWSZ] = { 1 };
+	uint8_t hash2[GIT_SHA1_RAWSZ] = { 2 };
+	uint8_t hash3[GIT_SHA1_RAWSZ] = { 3 };
+	struct reftable_log_record r1[] = {
+		{
+			.refname = "a",
+			.update_index = 2,
+			.value_type = REFTABLE_LOG_UPDATE,
+			.value.update = {
+				.old_hash = hash2,
+				/* deletion */
+				.name = "jane doe",
+				.email = "jane@invalid",
+				.message = "message2",
+			}
+		},
+		{
+			.refname = "a",
+			.update_index = 1,
+			.value_type = REFTABLE_LOG_UPDATE,
+			.value.update = {
+				.old_hash = hash1,
+				.new_hash = hash2,
+				.name = "jane doe",
+				.email = "jane@invalid",
+				.message = "message1",
+			}
+		},
+	};
+	struct reftable_log_record r2[] = {
+		{
+			.refname = "a",
+			.update_index = 3,
+			.value_type = REFTABLE_LOG_UPDATE,
+			.value.update = {
+				.new_hash = hash3,
+				.name = "jane doe",
+				.email = "jane@invalid",
+				.message = "message3",
+			}
+		},
+	};
+	struct reftable_log_record r3[] = {
+		{
+			.refname = "a",
+			.update_index = 2,
+			.value_type = REFTABLE_LOG_DELETION,
+		},
+	};
+	struct reftable_log_record want[] = {
+		r2[0],
+		r3[0],
+		r1[1],
+	};
+
+	struct reftable_log_record *logs[] = { r1, r2, r3 };
+	int sizes[3] = { 2, 1, 1 };
+	struct strbuf bufs[3] = { STRBUF_INIT, STRBUF_INIT, STRBUF_INIT };
+	struct reftable_block_source *bs = NULL;
+	struct reftable_reader **readers = NULL;
+	struct reftable_merged_table *mt = merged_table_from_log_records(
+		logs, &bs, &readers, sizes, bufs, 3);
+
+	struct reftable_iterator it = { NULL };
+	int err = reftable_merged_table_seek_log(mt, &it, "a");
+	struct reftable_log_record *out = NULL;
+	size_t len = 0;
+	size_t cap = 0;
+	int i = 0;
+
+	EXPECT_ERR(err);
+	EXPECT(reftable_merged_table_hash_id(mt) == GIT_SHA1_FORMAT_ID);
+	EXPECT(reftable_merged_table_min_update_index(mt) == 1);
+
+	while (len < 100) { /* cap loops/recursion. */
+		struct reftable_log_record log = { NULL };
+		int err = reftable_iterator_next_log(&it, &log);
+		if (err > 0) {
+			break;
+		}
+		if (len == cap) {
+			cap = 2 * cap + 1;
+			out = reftable_realloc(
+				out, sizeof(struct reftable_log_record) * cap);
+		}
+		out[len++] = log;
+	}
+	reftable_iterator_destroy(&it);
+
+	EXPECT(ARRAY_SIZE(want) == len);
+	for (i = 0; i < len; i++) {
+		EXPECT(reftable_log_record_equal(&want[i], &out[i],
+						 GIT_SHA1_RAWSZ));
+	}
+
+	err = reftable_merged_table_seek_log_at(mt, &it, "a", 2);
+	EXPECT_ERR(err);
+	reftable_log_record_release(&out[0]);
+	err = reftable_iterator_next_log(&it, &out[0]);
+	EXPECT_ERR(err);
+	EXPECT(reftable_log_record_equal(&out[0], &r3[0], GIT_SHA1_RAWSZ));
+	reftable_iterator_destroy(&it);
+
+	for (i = 0; i < len; i++) {
+		reftable_log_record_release(&out[i]);
+	}
+	reftable_free(out);
+
+	for (i = 0; i < 3; i++) {
+		strbuf_release(&bufs[i]);
+	}
+	readers_destroy(readers, 3);
+	reftable_merged_table_free(mt);
+	reftable_free(bs);
+}
+
+static void test_default_write_opts(void)
+{
+	struct reftable_write_options opts = { 0 };
+	struct strbuf buf = STRBUF_INIT;
+	struct reftable_writer *w =
+		reftable_new_writer(&strbuf_add_void, &buf, &opts);
+
+	struct reftable_ref_record rec = {
+		.refname = "master",
+		.update_index = 1,
+	};
+	int err;
+	struct reftable_block_source source = { NULL };
+	struct reftable_table *tab = reftable_calloc(sizeof(*tab) * 1);
+	uint32_t hash_id;
+	struct reftable_reader *rd = NULL;
+	struct reftable_merged_table *merged = NULL;
+
+	reftable_writer_set_limits(w, 1, 1);
+
+	err = reftable_writer_add_ref(w, &rec);
+	EXPECT_ERR(err);
+
+	err = reftable_writer_close(w);
+	EXPECT_ERR(err);
+	reftable_writer_free(w);
+
+	block_source_from_strbuf(&source, &buf);
+
+	err = reftable_new_reader(&rd, &source, "filename");
+	EXPECT_ERR(err);
+
+	hash_id = reftable_reader_hash_id(rd);
+	EXPECT(hash_id == GIT_SHA1_FORMAT_ID);
+
+	reftable_table_from_reader(&tab[0], rd);
+	err = reftable_new_merged_table(&merged, tab, 1, GIT_SHA1_FORMAT_ID);
+	EXPECT_ERR(err);
+
+	reftable_reader_free(rd);
+	reftable_merged_table_free(merged);
+	strbuf_release(&buf);
+}
+
+/* XXX test refs_for(oid) */
+
+int merged_test_main(int argc, const char *argv[])
+{
+	RUN_TEST(test_merged_logs);
+	RUN_TEST(test_merged_between);
+	RUN_TEST(test_merged);
+	RUN_TEST(test_default_write_opts);
+	return 0;
+}
diff --git a/reftable/reftable-merged.h b/reftable/reftable-merged.h
new file mode 100644
index 0000000000..1a6d16915a
--- /dev/null
+++ b/reftable/reftable-merged.h
@@ -0,0 +1,72 @@
+/*
+Copyright 2020 Google LLC
+
+Use of this source code is governed by a BSD-style
+license that can be found in the LICENSE file or at
+https://developers.google.com/open-source/licenses/bsd
+*/
+
+#ifndef REFTABLE_MERGED_H
+#define REFTABLE_MERGED_H
+
+#include "reftable-iterator.h"
+
+/*
+ * Merged tables
+ *
+ * A ref database kept in a sequence of table files. The merged_table presents a
+ * unified view to reading (seeking, iterating) a sequence of immutable tables.
+ *
+ * The merged tables are on purpose kept disconnected from their actual storage
+ * (eg. files on disk), because it is useful to merge tables aren't files. For
+ * example, the per-workspace and global ref namespace can be implemented as a
+ * merged table of two stacks of file-backed reftables.
+ */
+
+/* A merged table is implements seeking/iterating over a stack of tables. */
+struct reftable_merged_table;
+
+/* A generic reftable; see below. */
+struct reftable_table;
+
+/* reftable_new_merged_table creates a new merged table. It takes ownership of
+   the stack array.
+*/
+int reftable_new_merged_table(struct reftable_merged_table **dest,
+			      struct reftable_table *stack, int n,
+			      uint32_t hash_id);
+
+/* returns an iterator positioned just before 'name' */
+int reftable_merged_table_seek_ref(struct reftable_merged_table *mt,
+				   struct reftable_iterator *it,
+				   const char *name);
+
+/* returns an iterator for log entry, at given update_index */
+int reftable_merged_table_seek_log_at(struct reftable_merged_table *mt,
+				      struct reftable_iterator *it,
+				      const char *name, uint64_t update_index);
+
+/* like reftable_merged_table_seek_log_at but look for the newest entry. */
+int reftable_merged_table_seek_log(struct reftable_merged_table *mt,
+				   struct reftable_iterator *it,
+				   const char *name);
+
+/* returns the max update_index covered by this merged table. */
+uint64_t
+reftable_merged_table_max_update_index(struct reftable_merged_table *mt);
+
+/* returns the min update_index covered by this merged table. */
+uint64_t
+reftable_merged_table_min_update_index(struct reftable_merged_table *mt);
+
+/* releases memory for the merged_table */
+void reftable_merged_table_free(struct reftable_merged_table *m);
+
+/* return the hash ID of the merged table. */
+uint32_t reftable_merged_table_hash_id(struct reftable_merged_table *m);
+
+/* create a generic table from reftable_merged_table */
+void reftable_table_from_merged_table(struct reftable_table *tab,
+				      struct reftable_merged_table *table);
+
+#endif
diff --git a/t/helper/test-reftable.c b/t/helper/test-reftable.c
index 0b5a1701df..8087f2da4e 100644
--- a/t/helper/test-reftable.c
+++ b/t/helper/test-reftable.c
@@ -5,6 +5,7 @@ int cmd__reftable(int argc, const char **argv)
 {
 	basics_test_main(argc, argv);
 	block_test_main(argc, argv);
+	merged_test_main(argc, argv);
 	pq_test_main(argc, argv);
 	record_test_main(argc, argv);
 	readwrite_test_main(argc, argv);