Merge branch 'ps/reftable-exclude'

The reftable backend learned to more efficiently handle exclude
patterns while enumerating the refs.

* ps/reftable-exclude:
  refs/reftable: wire up support for exclude patterns
  reftable/reader: make table iterator reseekable
  t/unit-tests: introduce reftable library
  Makefile: stop listing test library objects twice
  builtin/receive-pack: fix exclude patterns when announcing refs
  refs: properly apply exclude patterns to namespaced refs

16 files changed, 594 insertions, 201 deletions

diff --git a/Makefile b/Makefile
index 9cf2be070f..7344a7f725 100644
--- a/Makefile
+++ b/Makefile
@@ -1356,6 +1356,7 @@ UNIT_TEST_PROGRAMS += t-reftable-basics
 UNIT_TEST_PROGRAMS += t-reftable-block
 UNIT_TEST_PROGRAMS += t-reftable-merged
 UNIT_TEST_PROGRAMS += t-reftable-pq
+UNIT_TEST_PROGRAMS += t-reftable-reader
 UNIT_TEST_PROGRAMS += t-reftable-readwrite
 UNIT_TEST_PROGRAMS += t-reftable-record
 UNIT_TEST_PROGRAMS += t-reftable-stack
@@ -1365,9 +1366,9 @@ UNIT_TEST_PROGRAMS += t-strcmp-offset
 UNIT_TEST_PROGRAMS += t-trailer
 UNIT_TEST_PROGRAMS += t-urlmatch-normalization
 UNIT_TEST_PROGS = $(patsubst %,$(UNIT_TEST_BIN)/%$X,$(UNIT_TEST_PROGRAMS))
-UNIT_TEST_OBJS = $(patsubst %,$(UNIT_TEST_DIR)/%.o,$(UNIT_TEST_PROGRAMS))
 UNIT_TEST_OBJS += $(UNIT_TEST_DIR)/test-lib.o
 UNIT_TEST_OBJS += $(UNIT_TEST_DIR)/lib-oid.o
+UNIT_TEST_OBJS += $(UNIT_TEST_DIR)/lib-reftable.o
 
 # xdiff and reftable libs may in turn depend on what is in libgit.a
 GITLIBS = common-main.o $(LIB_FILE) $(XDIFF_LIB) $(REFTABLE_LIB) $(LIB_FILE)
@@ -2725,6 +2726,7 @@ OBJECTS += $(FUZZ_OBJS)
 OBJECTS += $(REFTABLE_OBJS) $(REFTABLE_TEST_OBJS)
 OBJECTS += $(UNIT_TEST_OBJS)
 OBJECTS += $(CLAR_TEST_OBJS)
+OBJECTS += $(patsubst %,$(UNIT_TEST_DIR)/%.o,$(UNIT_TEST_PROGRAMS))
 
 ifndef NO_CURL
 	OBJECTS += http.o http-walker.o remote-curl.o
@@ -3864,9 +3866,7 @@ $(FUZZ_PROGRAMS): %: %.o oss-fuzz/dummy-cmd-main.o $(GITLIBS) GIT-LDFLAGS
 		-Wl,--allow-multiple-definition \
 		$(filter %.o,$^) $(filter %.a,$^) $(LIBS) $(LIB_FUZZING_ENGINE)
 
-$(UNIT_TEST_PROGS): $(UNIT_TEST_BIN)/%$X: $(UNIT_TEST_DIR)/%.o \
-	$(UNIT_TEST_DIR)/test-lib.o \
-	$(UNIT_TEST_DIR)/lib-oid.o \
+$(UNIT_TEST_PROGS): $(UNIT_TEST_BIN)/%$X: $(UNIT_TEST_DIR)/%.o $(UNIT_TEST_OBJS) \
 	$(GITLIBS) GIT-LDFLAGS
 	$(call mkdir_p_parent_template)
 	$(QUIET_LINK)$(CC) $(ALL_CFLAGS) -o $@ $(ALL_LDFLAGS) \
diff --git a/builtin/receive-pack.c b/builtin/receive-pack.c
index c4b766c613..ca9ee7f59f 100644
--- a/builtin/receive-pack.c
+++ b/builtin/receive-pack.c
@@ -340,12 +340,26 @@ static void show_one_alternate_ref(const struct object_id *oid,
 static void write_head_info(void)
 {
 	static struct oidset seen = OIDSET_INIT;
+	struct strvec excludes_vector = STRVEC_INIT;
+	const char **exclude_patterns;
+
+	/*
+	 * We need access to the reference names both with and without their
+	 * namespace and thus cannot use `refs_for_each_namespaced_ref()`. We
+	 * thus have to adapt exclude patterns to carry the namespace prefix
+	 * ourselves.
+	 */
+	exclude_patterns = get_namespaced_exclude_patterns(
+		hidden_refs_to_excludes(&hidden_refs),
+		get_git_namespace(), &excludes_vector);
 
 	refs_for_each_fullref_in(get_main_ref_store(the_repository), "",
-				 hidden_refs_to_excludes(&hidden_refs),
-				 show_ref_cb, &seen);
+				 exclude_patterns, show_ref_cb, &seen);
 	for_each_alternate_ref(show_one_alternate_ref, &seen);
+
 	oidset_clear(&seen);
+	strvec_clear(&excludes_vector);
+
 	if (!sent_capabilities)
 		show_ref("capabilities^{}", null_oid());
 
diff --git a/refs.c b/refs.c
index 3bee3e7829..5f729ed412 100644
--- a/refs.c
+++ b/refs.c
@@ -1518,6 +1518,19 @@ const char **hidden_refs_to_excludes(const struct strvec *hide_refs)
 	return hide_refs->v;
 }
 
+const char **get_namespaced_exclude_patterns(const char **exclude_patterns,
+					     const char *namespace,
+					     struct strvec *out)
+{
+	if (!namespace || !*namespace || !exclude_patterns || !*exclude_patterns)
+		return exclude_patterns;
+
+	for (size_t i = 0; exclude_patterns[i]; i++)
+		strvec_pushf(out, "%s%s", namespace, exclude_patterns[i]);
+
+	return out->v;
+}
+
 const char *find_descendant_ref(const char *dirname,
 				const struct string_list *extras,
 				const struct string_list *skip)
@@ -1635,11 +1648,19 @@ int refs_for_each_namespaced_ref(struct ref_store *refs,
 				 const char **exclude_patterns,
 				 each_ref_fn fn, void *cb_data)
 {
-	struct strbuf buf = STRBUF_INIT;
+	struct strvec namespaced_exclude_patterns = STRVEC_INIT;
+	struct strbuf prefix = STRBUF_INIT;
 	int ret;
-	strbuf_addf(&buf, "%srefs/", get_git_namespace());
-	ret = do_for_each_ref(refs, buf.buf, exclude_patterns, fn, 0, 0, cb_data);
-	strbuf_release(&buf);
+
+	exclude_patterns = get_namespaced_exclude_patterns(exclude_patterns,
+							   get_git_namespace(),
+							   &namespaced_exclude_patterns);
+
+	strbuf_addf(&prefix, "%srefs/", get_git_namespace());
+	ret = do_for_each_ref(refs, prefix.buf, exclude_patterns, fn, 0, 0, cb_data);
+
+	strvec_clear(&namespaced_exclude_patterns);
+	strbuf_release(&prefix);
 	return ret;
 }
 
@@ -1720,6 +1741,7 @@ int refs_for_each_fullref_in_prefixes(struct ref_store *ref_store,
 				      const char **exclude_patterns,
 				      each_ref_fn fn, void *cb_data)
 {
+	struct strvec namespaced_exclude_patterns = STRVEC_INIT;
 	struct string_list prefixes = STRING_LIST_INIT_DUP;
 	struct string_list_item *prefix;
 	struct strbuf buf = STRBUF_INIT;
@@ -1731,6 +1753,10 @@ int refs_for_each_fullref_in_prefixes(struct ref_store *ref_store,
 		strbuf_addstr(&buf, namespace);
 	namespace_len = buf.len;
 
+	exclude_patterns = get_namespaced_exclude_patterns(exclude_patterns,
+							   namespace,
+							   &namespaced_exclude_patterns);
+
 	for_each_string_list_item(prefix, &prefixes) {
 		strbuf_addstr(&buf, prefix->string);
 		ret = refs_for_each_fullref_in(ref_store, buf.buf,
@@ -1740,6 +1766,7 @@ int refs_for_each_fullref_in_prefixes(struct ref_store *ref_store,
 		strbuf_setlen(&buf, namespace_len);
 	}
 
+	strvec_clear(&namespaced_exclude_patterns);
 	string_list_clear(&prefixes, 0);
 	strbuf_release(&buf);
 	return ret;
diff --git a/refs.h b/refs.h
index f2c4ccde61..be03da6d3b 100644
--- a/refs.h
+++ b/refs.h
@@ -861,6 +861,15 @@ int ref_is_hidden(const char *, const char *, const struct strvec *);
  */
 const char **hidden_refs_to_excludes(const struct strvec *hide_refs);
 
+/*
+ * Prefix all exclude patterns with the namespace, if any. This is required
+ * because exclude patterns apply to the stripped reference name, not the full
+ * reference name with the namespace.
+ */
+const char **get_namespaced_exclude_patterns(const char **exclude_patterns,
+					     const char *namespace,
+					     struct strvec *out);
+
 /* Is this a per-worktree ref living in the refs/ namespace? */
 int is_per_worktree_ref(const char *refname);
 
diff --git a/refs/reftable-backend.c b/refs/reftable-backend.c
index 043e19439f..f5f957e6de 100644
--- a/refs/reftable-backend.c
+++ b/refs/reftable-backend.c
@@ -22,6 +22,7 @@
 #include "../repo-settings.h"
 #include "../setup.h"
 #include "../strmap.h"
+#include "../trace2.h"
 #include "parse.h"
 #include "refs-internal.h"
 
@@ -451,10 +452,81 @@ struct reftable_ref_iterator {
 
 	const char *prefix;
 	size_t prefix_len;
+	char **exclude_patterns;
+	size_t exclude_patterns_index;
+	size_t exclude_patterns_strlen;
 	unsigned int flags;
 	int err;
 };
 
+/*
+ * Handle exclude patterns. Returns either `1`, which tells the caller that the
+ * current reference shall not be shown. Or `0`, which indicates that it should
+ * be shown.
+ */
+static int should_exclude_current_ref(struct reftable_ref_iterator *iter)
+{
+	while (iter->exclude_patterns[iter->exclude_patterns_index]) {
+		const char *pattern = iter->exclude_patterns[iter->exclude_patterns_index];
+		char *ref_after_pattern;
+		int cmp;
+
+		/*
+		 * Lazily cache the pattern length so that we don't have to
+		 * recompute it every time this function is called.
+		 */
+		if (!iter->exclude_patterns_strlen)
+			iter->exclude_patterns_strlen = strlen(pattern);
+
+		/*
+		 * When the reference name is lexicographically bigger than the
+		 * current exclude pattern we know that it won't ever match any
+		 * of the following references, either. We thus advance to the
+		 * next pattern and re-check whether it matches.
+		 *
+		 * Otherwise, if it's smaller, then we do not have a match and
+		 * thus want to show the current reference.
+		 */
+		cmp = strncmp(iter->ref.refname, pattern,
+			      iter->exclude_patterns_strlen);
+		if (cmp > 0) {
+			iter->exclude_patterns_index++;
+			iter->exclude_patterns_strlen = 0;
+			continue;
+		}
+		if (cmp < 0)
+			return 0;
+
+		/*
+		 * The reference shares a prefix with the exclude pattern and
+		 * shall thus be omitted. We skip all references that match the
+		 * pattern by seeking to the first reference after the block of
+		 * matches.
+		 *
+		 * This is done by appending the highest possible character to
+		 * the pattern. Consequently, all references that have the
+		 * pattern as prefix and whose suffix starts with anything in
+		 * the range [0x00, 0xfe] are skipped. And given that 0xff is a
+		 * non-printable character that shouldn't ever be in a ref name,
+		 * we'd not yield any such record, either.
+		 *
+		 * Note that the seeked-to reference may also be excluded. This
+		 * is not handled here though, but the caller is expected to
+		 * loop and re-verify the next reference for us.
+		 */
+		ref_after_pattern = xstrfmt("%s%c", pattern, 0xff);
+		iter->err = reftable_iterator_seek_ref(&iter->iter, ref_after_pattern);
+		iter->exclude_patterns_index++;
+		iter->exclude_patterns_strlen = 0;
+		trace2_counter_add(TRACE2_COUNTER_ID_REFTABLE_RESEEKS, 1);
+
+		free(ref_after_pattern);
+		return 1;
+	}
+
+	return 0;
+}
+
 static int reftable_ref_iterator_advance(struct ref_iterator *ref_iterator)
 {
 	struct reftable_ref_iterator *iter =
@@ -485,6 +557,9 @@ static int reftable_ref_iterator_advance(struct ref_iterator *ref_iterator)
 			break;
 		}
 
+		if (iter->exclude_patterns && should_exclude_current_ref(iter))
+			continue;
+
 		if (iter->flags & DO_FOR_EACH_PER_WORKTREE_ONLY &&
 		    parse_worktree_ref(iter->ref.refname, NULL, NULL, NULL) !=
 			    REF_WORKTREE_CURRENT)
@@ -574,6 +649,11 @@ static int reftable_ref_iterator_abort(struct ref_iterator *ref_iterator)
 		(struct reftable_ref_iterator *)ref_iterator;
 	reftable_ref_record_release(&iter->ref);
 	reftable_iterator_destroy(&iter->iter);
+	if (iter->exclude_patterns) {
+		for (size_t i = 0; iter->exclude_patterns[i]; i++)
+			free(iter->exclude_patterns[i]);
+		free(iter->exclude_patterns);
+	}
 	free(iter);
 	return ITER_DONE;
 }
@@ -584,9 +664,53 @@ static struct ref_iterator_vtable reftable_ref_iterator_vtable = {
 	.abort = reftable_ref_iterator_abort
 };
 
+static int qsort_strcmp(const void *va, const void *vb)
+{
+	const char *a = *(const char **)va;
+	const char *b = *(const char **)vb;
+	return strcmp(a, b);
+}
+
+static char **filter_exclude_patterns(const char **exclude_patterns)
+{
+	size_t filtered_size = 0, filtered_alloc = 0;
+	char **filtered = NULL;
+
+	if (!exclude_patterns)
+		return NULL;
+
+	for (size_t i = 0; ; i++) {
+		const char *exclude_pattern = exclude_patterns[i];
+		int has_glob = 0;
+
+		if (!exclude_pattern)
+			break;
+
+		for (const char *p = exclude_pattern; *p; p++) {
+			has_glob = is_glob_special(*p);
+			if (has_glob)
+				break;
+		}
+		if (has_glob)
+			continue;
+
+		ALLOC_GROW(filtered, filtered_size + 1, filtered_alloc);
+		filtered[filtered_size++] = xstrdup(exclude_pattern);
+	}
+
+	if (filtered_size) {
+		QSORT(filtered, filtered_size, qsort_strcmp);
+		ALLOC_GROW(filtered, filtered_size + 1, filtered_alloc);
+		filtered[filtered_size++] = NULL;
+	}
+
+	return filtered;
+}
+
 static struct reftable_ref_iterator *ref_iterator_for_stack(struct reftable_ref_store *refs,
 							    struct reftable_stack *stack,
 							    const char *prefix,
+							    const char **exclude_patterns,
 							    int flags)
 {
 	struct reftable_ref_iterator *iter;
@@ -599,6 +723,7 @@ static struct reftable_ref_iterator *ref_iterator_for_stack(struct reftable_ref_
 	iter->base.oid = &iter->oid;
 	iter->flags = flags;
 	iter->refs = refs;
+	iter->exclude_patterns = filter_exclude_patterns(exclude_patterns);
 
 	ret = refs->err;
 	if (ret)
@@ -620,7 +745,7 @@ done:
 
 static struct ref_iterator *reftable_be_iterator_begin(struct ref_store *ref_store,
 						       const char *prefix,
-						       const char **exclude_patterns UNUSED,
+						       const char **exclude_patterns,
 						       unsigned int flags)
 {
 	struct reftable_ref_iterator *main_iter, *worktree_iter;
@@ -631,7 +756,8 @@ static struct ref_iterator *reftable_be_iterator_begin(struct ref_store *ref_sto
 		required_flags |= REF_STORE_ODB;
 	refs = reftable_be_downcast(ref_store, required_flags, "ref_iterator_begin");
 
-	main_iter = ref_iterator_for_stack(refs, refs->main_stack, prefix, flags);
+	main_iter = ref_iterator_for_stack(refs, refs->main_stack, prefix,
+					   exclude_patterns, flags);
 
 	/*
 	 * The worktree stack is only set when we're in an actual worktree
@@ -645,7 +771,8 @@ static struct ref_iterator *reftable_be_iterator_begin(struct ref_store *ref_sto
 	 * Otherwise we merge both the common and the per-worktree refs into a
 	 * single iterator.
 	 */
-	worktree_iter = ref_iterator_for_stack(refs, refs->worktree_stack, prefix, flags);
+	worktree_iter = ref_iterator_for_stack(refs, refs->worktree_stack, prefix,
+					       exclude_patterns, flags);
 	return merge_ref_iterator_begin(&worktree_iter->base, &main_iter->base,
 					ref_iterator_select, NULL);
 }
diff --git a/reftable/reader.c b/reftable/reader.c
index f877099087..6494ce2e32 100644
--- a/reftable/reader.c
+++ b/reftable/reader.c
@@ -328,6 +328,7 @@ static int table_iter_seek_to(struct table_iter *ti, uint64_t off, uint8_t typ)
 	ti->typ = block_reader_type(&ti->br);
 	ti->block_off = off;
 	block_iter_seek_start(&ti->bi, &ti->br);
+	ti->is_finished = 0;
 	return 0;
 }
 
diff --git a/t/t1419-exclude-refs.sh b/t/t1419-exclude-refs.sh
index 1359574419..3256e4462f 100755
--- a/t/t1419-exclude-refs.sh
+++ b/t/t1419-exclude-refs.sh
@@ -8,12 +8,6 @@ export GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME
 TEST_PASSES_SANITIZE_LEAK=true
 . ./test-lib.sh
 
-if test_have_prereq !REFFILES
-then
-	skip_all='skipping `git for-each-ref --exclude` tests; need files backend'
-	test_done
-fi
-
 for_each_ref__exclude () {
 	GIT_TRACE2_PERF=1 test-tool ref-store main \
 		for-each-ref--exclude "$@" >actual.raw
@@ -28,7 +22,14 @@ assert_jumps () {
 	local nr="$1"
 	local trace="$2"
 
-	grep -q "name:jumps_made value:$nr$" $trace
+	case "$GIT_DEFAULT_REF_FORMAT" in
+	files)
+		grep -q "name:jumps_made value:$nr$" $trace;;
+	reftable)
+		grep -q "name:reseeks_made value:$nr$" $trace;;
+	*)
+		BUG "unhandled ref format $GIT_DEFAULT_REF_FORMAT";;
+	esac
 }
 
 assert_no_jumps () {
@@ -89,7 +90,14 @@ test_expect_success 'adjacent, non-overlapping excluded regions' '
 	for_each_ref refs/heads/foo refs/heads/quux >expect &&
 
 	test_cmp expect actual &&
-	assert_jumps 1 perf
+	case "$GIT_DEFAULT_REF_FORMAT" in
+	files)
+		assert_jumps 1 perf;;
+	reftable)
+		assert_jumps 2 perf;;
+	*)
+		BUG "unhandled ref format $GIT_DEFAULT_REF_FORMAT";;
+	esac
 '
 
 test_expect_success 'overlapping excluded regions' '
@@ -106,7 +114,30 @@ test_expect_success 'several overlapping excluded regions' '
 	for_each_ref refs/heads/quux >expect &&
 
 	test_cmp expect actual &&
-	assert_jumps 1 perf
+	case "$GIT_DEFAULT_REF_FORMAT" in
+	files)
+		assert_jumps 1 perf;;
+	reftable)
+		assert_jumps 3 perf;;
+	*)
+		BUG "unhandled ref format $GIT_DEFAULT_REF_FORMAT";;
+	esac
+'
+
+test_expect_success 'unordered excludes' '
+	for_each_ref__exclude refs/heads \
+		refs/heads/foo refs/heads/baz >actual 2>perf &&
+	for_each_ref refs/heads/bar refs/heads/quux >expect &&
+
+	test_cmp expect actual &&
+	case "$GIT_DEFAULT_REF_FORMAT" in
+	files)
+		assert_jumps 1 perf;;
+	reftable)
+		assert_jumps 2 perf;;
+	*)
+		BUG "unhandled ref format $GIT_DEFAULT_REF_FORMAT";;
+	esac
 '
 
 test_expect_success 'non-matching excluded section' '
diff --git a/t/t5509-fetch-push-namespaces.sh b/t/t5509-fetch-push-namespaces.sh
index 05090feaf9..f029ae0d28 100755
--- a/t/t5509-fetch-push-namespaces.sh
+++ b/t/t5509-fetch-push-namespaces.sh
@@ -96,6 +96,7 @@ test_expect_success 'hide namespaced refs with transfer.hideRefs' '
 '
 
 test_expect_success 'check that transfer.hideRefs does not match unstripped refs' '
+	git -C pushee pack-refs --all &&
 	GIT_NAMESPACE=namespace \
 		git -C pushee -c transfer.hideRefs=refs/namespaces/namespace/refs/tags \
 		ls-remote "ext::git %s ." >actual &&
@@ -123,6 +124,14 @@ test_expect_success 'try to update a ref that is not hidden' '
 	git -C original push pushee-namespaced main
 '
 
+test_expect_success 'git-receive-pack(1) with transfer.hideRefs does not match unstripped refs during advertisement' '
+	git -C pushee update-ref refs/namespaces/namespace/refs/heads/foo/1 refs/namespaces/namespace/refs/heads/main &&
+	git -C pushee pack-refs --all &&
+	test_config -C pushee transfer.hideRefs refs/namespaces/namespace/refs/heads/foo &&
+	GIT_TRACE_PACKET="$(pwd)/trace" git -C original push pushee-namespaced main &&
+	test_grep refs/heads/foo/1 trace
+'
+
 test_expect_success 'try to update a hidden full ref' '
 	test_config -C pushee transfer.hideRefs "^refs/namespaces/namespace/refs/heads/main" &&
 	test_must_fail git -C original push pushee-namespaced main
diff --git a/t/unit-tests/lib-reftable.c b/t/unit-tests/lib-reftable.c
new file mode 100644
index 0000000000..ab1fa44a28
--- /dev/null
+++ b/t/unit-tests/lib-reftable.c
@@ -0,0 +1,93 @@
+#include "lib-reftable.h"
+#include "test-lib.h"
+#include "reftable/constants.h"
+#include "reftable/writer.h"
+
+void t_reftable_set_hash(uint8_t *p, int i, uint32_t id)
+{
+	memset(p, (uint8_t)i, hash_size(id));
+}
+
+static ssize_t strbuf_writer_write(void *b, const void *data, size_t sz)
+{
+	strbuf_add(b, data, sz);
+	return sz;
+}
+
+static int strbuf_writer_flush(void *arg UNUSED)
+{
+	return 0;
+}
+
+struct reftable_writer *t_reftable_strbuf_writer(struct strbuf *buf,
+						 struct reftable_write_options *opts)
+{
+	return reftable_new_writer(&strbuf_writer_write,
+				   &strbuf_writer_flush,
+				   buf, opts);
+}
+
+void t_reftable_write_to_buf(struct strbuf *buf,
+			     struct reftable_ref_record *refs,
+			     size_t nrefs,
+			     struct reftable_log_record *logs,
+			     size_t nlogs,
+			     struct reftable_write_options *_opts)
+{
+	struct reftable_write_options opts = { 0 };
+	const struct reftable_stats *stats;
+	struct reftable_writer *writer;
+	uint64_t min = 0xffffffff;
+	uint64_t max = 0;
+	int ret;
+
+	if (_opts)
+		opts = *_opts;
+
+	for (size_t i = 0; i < nrefs; i++) {
+		uint64_t ui = refs[i].update_index;
+		if (ui > max)
+			max = ui;
+		if (ui < min)
+			min = ui;
+	}
+	for (size_t i = 0; i < nlogs; i++) {
+		uint64_t ui = logs[i].update_index;
+		if (ui > max)
+			max = ui;
+		if (ui < min)
+			min = ui;
+	}
+
+	writer = t_reftable_strbuf_writer(buf, &opts);
+	reftable_writer_set_limits(writer, min, max);
+
+	if (nrefs) {
+		ret = reftable_writer_add_refs(writer, refs, nrefs);
+		check_int(ret, ==, 0);
+	}
+
+	if (nlogs) {
+		ret = reftable_writer_add_logs(writer, logs, nlogs);
+		check_int(ret, ==, 0);
+	}
+
+	ret = reftable_writer_close(writer);
+	check_int(ret, ==, 0);
+
+	stats = reftable_writer_stats(writer);
+	for (size_t i = 0; i < stats->ref_stats.blocks; i++) {
+		size_t off = i * (opts.block_size ? opts.block_size
+						  : DEFAULT_BLOCK_SIZE);
+		if (!off)
+			off = header_size(opts.hash_id == GIT_SHA256_FORMAT_ID ? 2 : 1);
+		check_char(buf->buf[off], ==, 'r');
+	}
+
+	if (nrefs)
+		check_int(stats->ref_stats.blocks, >, 0);
+	if (nlogs)
+		check_int(stats->log_stats.blocks, >, 0);
+
+	reftable_writer_free(writer);
+}
diff --git a/t/unit-tests/lib-reftable.h b/t/unit-tests/lib-reftable.h
new file mode 100644
index 0000000000..d115419084
--- /dev/null
+++ b/t/unit-tests/lib-reftable.h
@@ -0,0 +1,20 @@
+#ifndef LIB_REFTABLE_H
+#define LIB_REFTABLE_H
+
+#include "git-compat-util.h"
+#include "strbuf.h"
+#include "reftable/reftable-writer.h"
+
+void t_reftable_set_hash(uint8_t *p, int i, uint32_t id);
+
+struct reftable_writer *t_reftable_strbuf_writer(struct strbuf *buf,
+						 struct reftable_write_options *opts);
+
+void t_reftable_write_to_buf(struct strbuf *buf,
+			     struct reftable_ref_record *refs,
+			     size_t nrecords,
+			     struct reftable_log_record *logs,
+			     size_t nlogs,
+			     struct reftable_write_options *opts);
+
+#endif
diff --git a/t/unit-tests/t-reftable-merged.c b/t/unit-tests/t-reftable-merged.c
index e9d100a01e..19e54bdfb8 100644
--- a/t/unit-tests/t-reftable-merged.c
+++ b/t/unit-tests/t-reftable-merged.c
@@ -7,6 +7,7 @@ https://developers.google.com/open-source/licenses/bsd
 */
 
 #include "test-lib.h"
+#include "lib-reftable.h"
 #include "reftable/blocksource.h"
 #include "reftable/constants.h"
 #include "reftable/merged.h"
@@ -15,77 +16,6 @@ https://developers.google.com/open-source/licenses/bsd
 #include "reftable/reftable-merged.h"
 #include "reftable/reftable-writer.h"
 
-static ssize_t strbuf_add_void(void *b, const void *data, const size_t sz)
-{
-	strbuf_add(b, data, sz);
-	return sz;
-}
-
-static int noop_flush(void *arg UNUSED)
-{
-	return 0;
-}
-
-static void write_test_table(struct strbuf *buf,
-			     struct reftable_ref_record refs[], const size_t n)
-{
-	uint64_t min = 0xffffffff;
-	uint64_t max = 0;
-	size_t i;
-	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, &noop_flush, 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]);
-		check_int(n, ==, 0);
-		check_int(before, ==, refs[i].update_index);
-	}
-
-	err = reftable_writer_close(w);
-	check(!err);
-
-	reftable_writer_free(w);
-}
-
-static void write_test_log_table(struct strbuf *buf, struct reftable_log_record logs[],
-				 const size_t n, const uint64_t update_index)
-{
-	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, &noop_flush, buf, &opts);
-	reftable_writer_set_limits(w, update_index, update_index);
-
-	for (size_t i = 0; i < n; i++) {
-		int err = reftable_writer_add_log(w, &logs[i]);
-		check(!err);
-	}
-
-	err = reftable_writer_close(w);
-	check(!err);
-
-	reftable_writer_free(w);
-}
-
 static struct reftable_merged_table *
 merged_table_from_records(struct reftable_ref_record **refs,
 			  struct reftable_block_source **source,
@@ -93,13 +23,16 @@ merged_table_from_records(struct reftable_ref_record **refs,
 			  struct strbuf *buf, const size_t n)
 {
 	struct reftable_merged_table *mt = NULL;
+	struct reftable_write_options opts = {
+		.block_size = 256,
+	};
 	int err;
 
 	REFTABLE_CALLOC_ARRAY(*readers, n);
 	REFTABLE_CALLOC_ARRAY(*source, n);
 
 	for (size_t i = 0; i < n; i++) {
-		write_test_table(&buf[i], refs[i], sizes[i]);
+		t_reftable_write_to_buf(&buf[i], refs[i], sizes[i], NULL, 0, &opts);
 		block_source_from_strbuf(&(*source)[i], &buf[i]);
 
 		err = reftable_reader_new(&(*readers)[i], &(*source)[i],
@@ -261,6 +194,81 @@ static void t_merged_refs(void)
 	reftable_free(bs);
 }
 
+static void t_merged_seek_multiple_times(void)
+{
+	struct reftable_ref_record r1[] = {
+		{
+			.refname = (char *) "a",
+			.update_index = 1,
+			.value_type = REFTABLE_REF_VAL1,
+			.value.val1 = { 1 },
+		},
+		{
+			.refname = (char *) "c",
+			.update_index = 1,
+			.value_type = REFTABLE_REF_VAL1,
+			.value.val1 = { 2 },
+		}
+	};
+	struct reftable_ref_record r2[] = {
+		{
+			.refname = (char *) "b",
+			.update_index = 2,
+			.value_type = REFTABLE_REF_VAL1,
+			.value.val1 = { 3 },
+		},
+		{
+			.refname = (char *) "d",
+			.update_index = 2,
+			.value_type = REFTABLE_REF_VAL1,
+			.value.val1 = { 4 },
+		},
+	};
+	struct reftable_ref_record *refs[] = {
+		r1, r2,
+	};
+	size_t sizes[] = {
+		ARRAY_SIZE(r1), ARRAY_SIZE(r2),
+	};
+	struct strbuf bufs[] = {
+		STRBUF_INIT, STRBUF_INIT,
+	};
+	struct reftable_block_source *sources = NULL;
+	struct reftable_reader **readers = NULL;
+	struct reftable_ref_record rec = { 0 };
+	struct reftable_iterator it = { 0 };
+	struct reftable_merged_table *mt;
+
+	mt = merged_table_from_records(refs, &sources, &readers, sizes, bufs, 2);
+	merged_table_init_iter(mt, &it, BLOCK_TYPE_REF);
+
+	for (size_t i = 0; i < 5; i++) {
+		int err = reftable_iterator_seek_ref(&it, "c");
+		check(!err);
+
+		err = reftable_iterator_next_ref(&it, &rec);
+		check(!err);
+		err = reftable_ref_record_equal(&rec, &r1[1], GIT_SHA1_RAWSZ);
+		check(err == 1);
+
+		err = reftable_iterator_next_ref(&it, &rec);
+		check(!err);
+		err = reftable_ref_record_equal(&rec, &r2[1], GIT_SHA1_RAWSZ);
+		check(err == 1);
+
+		err = reftable_iterator_next_ref(&it, &rec);
+		check(err > 0);
+	}
+
+	for (size_t i = 0; i < ARRAY_SIZE(bufs); i++)
+		strbuf_release(&bufs[i]);
+	readers_destroy(readers, ARRAY_SIZE(refs));
+	reftable_ref_record_release(&rec);
+	reftable_iterator_destroy(&it);
+	reftable_merged_table_free(mt);
+	reftable_free(sources);
+}
+
 static struct reftable_merged_table *
 merged_table_from_log_records(struct reftable_log_record **logs,
 			      struct reftable_block_source **source,
@@ -268,13 +276,17 @@ merged_table_from_log_records(struct reftable_log_record **logs,
 			      struct strbuf *buf, const size_t n)
 {
 	struct reftable_merged_table *mt = NULL;
+	struct reftable_write_options opts = {
+		.block_size = 256,
+		.exact_log_message = 1,
+	};
 	int err;
 
 	REFTABLE_CALLOC_ARRAY(*readers, n);
 	REFTABLE_CALLOC_ARRAY(*source, n);
 
 	for (size_t i = 0; i < n; i++) {
-		write_test_log_table(&buf[i], logs[i], sizes[i], i + 1);
+		t_reftable_write_to_buf(&buf[i], NULL, 0, logs[i], sizes[i], &opts);
 		block_source_from_strbuf(&(*source)[i], &buf[i]);
 
 		err = reftable_reader_new(&(*readers)[i], &(*source)[i],
@@ -402,9 +414,7 @@ static void t_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, &noop_flush, &buf, &opts);
-
+	struct reftable_writer *w = t_reftable_strbuf_writer(&buf, &opts);
 	struct reftable_ref_record rec = {
 		.refname = (char *) "master",
 		.update_index = 1,
@@ -448,6 +458,7 @@ int cmd_main(int argc UNUSED, const char *argv[] UNUSED)
 	TEST(t_default_write_opts(), "merged table with default write opts");
 	TEST(t_merged_logs(), "merged table with multiple log updates for same ref");
 	TEST(t_merged_refs(), "merged table with multiple updates to same ref");
+	TEST(t_merged_seek_multiple_times(), "merged table can seek multiple times");
 	TEST(t_merged_single_record(), "ref ocurring in only one record can be fetched");
 
 	return test_done();
diff --git a/t/unit-tests/t-reftable-reader.c b/t/unit-tests/t-reftable-reader.c
new file mode 100644
index 0000000000..eea86966c0
--- /dev/null
+++ b/t/unit-tests/t-reftable-reader.c
@@ -0,0 +1,96 @@
+#include "test-lib.h"
+#include "lib-reftable.h"
+#include "reftable/blocksource.h"
+#include "reftable/reader.h"
+
+static int t_reader_seek_once(void)
+{
+	struct reftable_ref_record records[] = {
+		{
+			.refname = (char *) "refs/heads/main",
+			.value_type = REFTABLE_REF_VAL1,
+			.value.val1 = { 42 },
+		},
+	};
+	struct reftable_block_source source = { 0 };
+	struct reftable_ref_record ref = { 0 };
+	struct reftable_iterator it = { 0 };
+	struct reftable_reader *reader;
+	struct strbuf buf = STRBUF_INIT;
+	int ret;
+
+	t_reftable_write_to_buf(&buf, records, ARRAY_SIZE(records), NULL, 0, NULL);
+	block_source_from_strbuf(&source, &buf);
+
+	ret = reftable_reader_new(&reader, &source, "name");
+	check(!ret);
+
+	reftable_reader_init_ref_iterator(reader, &it);
+	ret = reftable_iterator_seek_ref(&it, "");
+	check(!ret);
+	ret = reftable_iterator_next_ref(&it, &ref);
+	check(!ret);
+
+	ret = reftable_ref_record_equal(&ref, &records[0], GIT_SHA1_RAWSZ);
+	check_int(ret, ==, 1);
+
+	ret = reftable_iterator_next_ref(&it, &ref);
+	check_int(ret, ==, 1);
+
+	reftable_ref_record_release(&ref);
+	reftable_iterator_destroy(&it);
+	reftable_reader_decref(reader);
+	strbuf_release(&buf);
+	return 0;
+}
+
+static int t_reader_reseek(void)
+{
+	struct reftable_ref_record records[] = {
+		{
+			.refname = (char *) "refs/heads/main",
+			.value_type = REFTABLE_REF_VAL1,
+			.value.val1 = { 42 },
+		},
+	};
+	struct reftable_block_source source = { 0 };
+	struct reftable_ref_record ref = { 0 };
+	struct reftable_iterator it = { 0 };
+	struct reftable_reader *reader;
+	struct strbuf buf = STRBUF_INIT;
+	int ret;
+
+	t_reftable_write_to_buf(&buf, records, ARRAY_SIZE(records), NULL, 0, NULL);
+	block_source_from_strbuf(&source, &buf);
+
+	ret = reftable_reader_new(&reader, &source, "name");
+	check(!ret);
+
+	reftable_reader_init_ref_iterator(reader, &it);
+
+	for (size_t i = 0; i < 5; i++) {
+		ret = reftable_iterator_seek_ref(&it, "");
+		check(!ret);
+		ret = reftable_iterator_next_ref(&it, &ref);
+		check(!ret);
+
+		ret = reftable_ref_record_equal(&ref, &records[0], GIT_SHA1_RAWSZ);
+		check_int(ret, ==, 1);
+
+		ret = reftable_iterator_next_ref(&it, &ref);
+		check_int(ret, ==, 1);
+	}
+
+	reftable_ref_record_release(&ref);
+	reftable_iterator_destroy(&it);
+	reftable_reader_decref(reader);
+	strbuf_release(&buf);
+	return 0;
+}
+
+int cmd_main(int argc UNUSED, const char *argv[] UNUSED)
+{
+	TEST(t_reader_seek_once(), "reader can seek once");
+	TEST(t_reader_reseek(), "reader can reseek multiple times");
+	return test_done();
+}
diff --git a/t/unit-tests/t-reftable-readwrite.c b/t/unit-tests/t-reftable-readwrite.c
index 82bfaf3287..e1b235a5f1 100644
--- a/t/unit-tests/t-reftable-readwrite.c
+++ b/t/unit-tests/t-reftable-readwrite.c
@@ -7,6 +7,7 @@ https://developers.google.com/open-source/licenses/bsd
 */
 
 #include "test-lib.h"
+#include "lib-reftable.h"
 #include "reftable/basics.h"
 #include "reftable/blocksource.h"
 #include "reftable/reader.h"
@@ -15,22 +16,6 @@ https://developers.google.com/open-source/licenses/bsd
 
 static const int update_index = 5;
 
-static void set_test_hash(uint8_t *p, int i)
-{
-	memset(p, (uint8_t)i, hash_size(GIT_SHA1_FORMAT_ID));
-}
-
-static ssize_t strbuf_add_void(void *b, const void *data, size_t sz)
-{
-	strbuf_add(b, data, sz);
-	return sz;
-}
-
-static int noop_flush(void *arg UNUSED)
-{
-	return 0;
-}
-
 static void t_buffer(void)
 {
 	struct strbuf buf = STRBUF_INIT;
@@ -62,61 +47,34 @@ static void write_table(char ***names, struct strbuf *buf, int N,
 		.block_size = block_size,
 		.hash_id = hash_id,
 	};
-	struct reftable_writer *w =
-		reftable_new_writer(&strbuf_add_void, &noop_flush, buf, &opts);
-	struct reftable_ref_record ref = { 0 };
-	int i = 0, n;
-	struct reftable_log_record log = { 0 };
-	const struct reftable_stats *stats = NULL;
+	struct reftable_ref_record *refs;
+	struct reftable_log_record *logs;
+	int i;
 
 	REFTABLE_CALLOC_ARRAY(*names, N + 1);
+	REFTABLE_CALLOC_ARRAY(refs, N);
+	REFTABLE_CALLOC_ARRAY(logs, N);
 
-	reftable_writer_set_limits(w, update_index, update_index);
 	for (i = 0; i < N; i++) {
-		char name[100];
-		int n;
-
-		snprintf(name, sizeof(name), "refs/heads/branch%02d", i);
-
-		ref.refname = name;
-		ref.update_index = update_index;
-		ref.value_type = REFTABLE_REF_VAL1;
-		set_test_hash(ref.value.val1, i);
-		(*names)[i] = xstrdup(name);
-
-		n = reftable_writer_add_ref(w, &ref);
-		check_int(n, ==, 0);
+		refs[i].refname = (*names)[i] = xstrfmt("refs/heads/branch%02d", i);
+		refs[i].update_index = update_index;
+		refs[i].value_type = REFTABLE_REF_VAL1;
+		t_reftable_set_hash(refs[i].value.val1, i, GIT_SHA1_FORMAT_ID);
 	}
 
 	for (i = 0; i < N; i++) {
-		char name[100];
-		int n;
-
-		snprintf(name, sizeof(name), "refs/heads/branch%02d", i);
-
-		log.refname = name;
-		log.update_index = update_index;
-		log.value_type = REFTABLE_LOG_UPDATE;
-		set_test_hash(log.value.update.new_hash, i);
-		log.value.update.message = (char *) "message";
-
-		n = reftable_writer_add_log(w, &log);
-		check_int(n, ==, 0);
+		logs[i].refname = (*names)[i];
+		logs[i].update_index = update_index;
+		logs[i].value_type = REFTABLE_LOG_UPDATE;
+		t_reftable_set_hash(logs[i].value.update.new_hash, i,
+				    GIT_SHA1_FORMAT_ID);
+		logs[i].value.update.message = (char *) "message";
 	}
 
-	n = reftable_writer_close(w);
-	check_int(n, ==, 0);
-
-	stats = reftable_writer_stats(w);
-	for (i = 0; i < stats->ref_stats.blocks; i++) {
-		int off = i * opts.block_size;
-		if (!off)
-			off = header_size((hash_id == GIT_SHA256_FORMAT_ID) ? 2 : 1);
-		check_char(buf->buf[off], ==, 'r');
-	}
+	t_reftable_write_to_buf(buf, refs, N, logs, N, &opts);
 
-	check_int(stats->log_stats.blocks, >, 0);
-	reftable_writer_free(w);
+	free(refs);
+	free(logs);
 }
 
 static void t_log_buffer_size(void)
@@ -138,8 +96,7 @@ static void t_log_buffer_size(void)
 					   .time = 0x5e430672,
 					   .message = (char *) "commit: 9\n",
 				   } } };
-	struct reftable_writer *w =
-		reftable_new_writer(&strbuf_add_void, &noop_flush, &buf, &opts);
+	struct reftable_writer *w = t_reftable_strbuf_writer(&buf, &opts);
 
 	/* This tests buffer extension for log compression. Must use a random
 	   hash, to ensure that the compressed part is larger than the original.
@@ -181,8 +138,7 @@ static void t_log_overflow(void)
 			},
 		},
 	};
-	struct reftable_writer *w =
-		reftable_new_writer(&strbuf_add_void, &noop_flush, &buf, &opts);
+	struct reftable_writer *w = t_reftable_strbuf_writer(&buf, &opts);
 
 	memset(msg, 'x', sizeof(msg) - 1);
 	reftable_writer_set_limits(w, update_index, update_index);
@@ -208,8 +164,7 @@ static void t_log_write_read(void)
 	struct reftable_reader *reader;
 	struct reftable_block_source source = { 0 };
 	struct strbuf buf = STRBUF_INIT;
-	struct reftable_writer *w =
-		reftable_new_writer(&strbuf_add_void, &noop_flush, &buf, &opts);
+	struct reftable_writer *w = t_reftable_strbuf_writer(&buf, &opts);
 	const struct reftable_stats *stats = NULL;
 	reftable_writer_set_limits(w, 0, N);
 	for (i = 0; i < N; i++) {
@@ -229,8 +184,10 @@ static void t_log_write_read(void)
 		log.refname = names[i];
 		log.update_index = i;
 		log.value_type = REFTABLE_LOG_UPDATE;
-		set_test_hash(log.value.update.old_hash, i);
-		set_test_hash(log.value.update.new_hash, i + 1);
+		t_reftable_set_hash(log.value.update.old_hash, i,
+				    GIT_SHA1_FORMAT_ID);
+		t_reftable_set_hash(log.value.update.new_hash, i + 1,
+				    GIT_SHA1_FORMAT_ID);
 
 		err = reftable_writer_add_log(w, &log);
 		check(!err);
@@ -297,8 +254,7 @@ static void t_log_zlib_corruption(void)
 	struct reftable_reader *reader;
 	struct reftable_block_source source = { 0 };
 	struct strbuf buf = STRBUF_INIT;
-	struct reftable_writer *w =
-		reftable_new_writer(&strbuf_add_void, &noop_flush, &buf, &opts);
+	struct reftable_writer *w = t_reftable_strbuf_writer(&buf, &opts);
 	const struct reftable_stats *stats = NULL;
 	char message[100] = { 0 };
 	int err, i, n;
@@ -528,15 +484,12 @@ static void t_table_refs_for(int indexed)
 	int err;
 	struct reftable_reader *reader;
 	struct reftable_block_source source = { 0 };
-
 	struct strbuf buf = STRBUF_INIT;
-	struct reftable_writer *w =
-		reftable_new_writer(&strbuf_add_void, &noop_flush, &buf, &opts);
-
+	struct reftable_writer *w = t_reftable_strbuf_writer(&buf, &opts);
 	struct reftable_iterator it = { 0 };
 	int j;
 
-	set_test_hash(want_hash, 4);
+	t_reftable_set_hash(want_hash, 4, GIT_SHA1_FORMAT_ID);
 
 	for (i = 0; i < N; i++) {
 		uint8_t hash[GIT_SHA1_RAWSZ];
@@ -552,8 +505,10 @@ static void t_table_refs_for(int indexed)
 		ref.refname = name;
 
 		ref.value_type = REFTABLE_REF_VAL2;
-		set_test_hash(ref.value.val2.value, i / 4);
-		set_test_hash(ref.value.val2.target_value, 3 + i / 4);
+		t_reftable_set_hash(ref.value.val2.value, i / 4,
+				    GIT_SHA1_FORMAT_ID);
+		t_reftable_set_hash(ref.value.val2.target_value, 3 + i / 4,
+				    GIT_SHA1_FORMAT_ID);
 
 		/* 80 bytes / entry, so 3 entries per block. Yields 17
 		 */
@@ -618,8 +573,7 @@ static void t_write_empty_table(void)
 {
 	struct reftable_write_options opts = { 0 };
 	struct strbuf buf = STRBUF_INIT;
-	struct reftable_writer *w =
-		reftable_new_writer(&strbuf_add_void, &noop_flush, &buf, &opts);
+	struct reftable_writer *w = t_reftable_strbuf_writer(&buf, &opts);
 	struct reftable_block_source source = { 0 };
 	struct reftable_reader *rd = NULL;
 	struct reftable_ref_record rec = { 0 };
@@ -657,8 +611,7 @@ static void t_write_object_id_min_length(void)
 		.block_size = 75,
 	};
 	struct strbuf buf = STRBUF_INIT;
-	struct reftable_writer *w =
-		reftable_new_writer(&strbuf_add_void, &noop_flush, &buf, &opts);
+	struct reftable_writer *w = t_reftable_strbuf_writer(&buf, &opts);
 	struct reftable_ref_record ref = {
 		.update_index = 1,
 		.value_type = REFTABLE_REF_VAL1,
@@ -692,8 +645,7 @@ static void t_write_object_id_length(void)
 		.block_size = 75,
 	};
 	struct strbuf buf = STRBUF_INIT;
-	struct reftable_writer *w =
-		reftable_new_writer(&strbuf_add_void, &noop_flush, &buf, &opts);
+	struct reftable_writer *w = t_reftable_strbuf_writer(&buf, &opts);
 	struct reftable_ref_record ref = {
 		.update_index = 1,
 		.value_type = REFTABLE_REF_VAL1,
@@ -726,8 +678,7 @@ static void t_write_empty_key(void)
 {
 	struct reftable_write_options opts = { 0 };
 	struct strbuf buf = STRBUF_INIT;
-	struct reftable_writer *w =
-		reftable_new_writer(&strbuf_add_void, &noop_flush, &buf, &opts);
+	struct reftable_writer *w = t_reftable_strbuf_writer(&buf, &opts);
 	struct reftable_ref_record ref = {
 		.refname = (char *) "",
 		.update_index = 1,
@@ -749,8 +700,7 @@ static void t_write_key_order(void)
 {
 	struct reftable_write_options opts = { 0 };
 	struct strbuf buf = STRBUF_INIT;
-	struct reftable_writer *w =
-		reftable_new_writer(&strbuf_add_void, &noop_flush, &buf, &opts);
+	struct reftable_writer *w = t_reftable_strbuf_writer(&buf, &opts);
 	struct reftable_ref_record refs[2] = {
 		{
 			.refname = (char *) "b",
@@ -798,7 +748,7 @@ static void t_write_multiple_indices(void)
 	struct reftable_reader *reader;
 	int err, i;
 
-	writer = reftable_new_writer(&strbuf_add_void, &noop_flush, &writer_buf, &opts);
+	writer = t_reftable_strbuf_writer(&writer_buf, &opts);
 	reftable_writer_set_limits(writer, 1, 1);
 	for (i = 0; i < 100; i++) {
 		struct reftable_ref_record ref = {
@@ -876,7 +826,7 @@ static void t_write_multi_level_index(void)
 	struct reftable_reader *reader;
 	int err;
 
-	writer = reftable_new_writer(&strbuf_add_void, &noop_flush, &writer_buf, &opts);
+	writer = t_reftable_strbuf_writer(&writer_buf, &opts);
 	reftable_writer_set_limits(writer, 1, 1);
 	for (size_t i = 0; i < 200; i++) {
 		struct reftable_ref_record ref = {
diff --git a/t/unit-tests/t-reftable-stack.c b/t/unit-tests/t-reftable-stack.c
index d62a9c1bed..65e513d5ec 100644
--- a/t/unit-tests/t-reftable-stack.c
+++ b/t/unit-tests/t-reftable-stack.c
@@ -7,17 +7,13 @@ https://developers.google.com/open-source/licenses/bsd
 */
 
 #include "test-lib.h"
+#include "lib-reftable.h"
 #include "reftable/merged.h"
 #include "reftable/reader.h"
 #include "reftable/reftable-error.h"
 #include "reftable/stack.h"
 #include <dirent.h>
 
-static void set_test_hash(uint8_t *p, int i)
-{
-	memset(p, (uint8_t)i, hash_size(GIT_SHA1_FORMAT_ID));
-}
-
 static void clear_dir(const char *dirname)
 {
 	struct strbuf path = STRBUF_INIT;
@@ -125,7 +121,7 @@ static void write_n_ref_tables(struct reftable_stack *st,
 		strbuf_reset(&buf);
 		strbuf_addf(&buf, "refs/heads/branch-%04"PRIuMAX, (uintmax_t)i);
 		ref.refname = buf.buf;
-		set_test_hash(ref.value.val1, i);
+		t_reftable_set_hash(ref.value.val1, i, GIT_SHA1_FORMAT_ID);
 
 		err = reftable_stack_add(st, &write_test_ref, &ref);
 		check(!err);
@@ -470,13 +466,13 @@ static void t_reftable_stack_add(void)
 		refs[i].refname = xstrdup(buf);
 		refs[i].update_index = i + 1;
 		refs[i].value_type = REFTABLE_REF_VAL1;
-		set_test_hash(refs[i].value.val1, i);
+		t_reftable_set_hash(refs[i].value.val1, i, GIT_SHA1_FORMAT_ID);
 
 		logs[i].refname = xstrdup(buf);
 		logs[i].update_index = N + i + 1;
 		logs[i].value_type = REFTABLE_LOG_UPDATE;
 		logs[i].value.update.email = xstrdup("identity@invalid");
-		set_test_hash(logs[i].value.update.new_hash, i);
+		t_reftable_set_hash(logs[i].value.update.new_hash, i, GIT_SHA1_FORMAT_ID);
 	}
 
 	for (i = 0; i < N; i++) {
@@ -562,14 +558,14 @@ static void t_reftable_stack_iterator(void)
 		refs[i].refname = xstrfmt("branch%02"PRIuMAX, (uintmax_t)i);
 		refs[i].update_index = i + 1;
 		refs[i].value_type = REFTABLE_REF_VAL1;
-		set_test_hash(refs[i].value.val1, i);
+		t_reftable_set_hash(refs[i].value.val1, i, GIT_SHA1_FORMAT_ID);
 
 		logs[i].refname = xstrfmt("branch%02"PRIuMAX, (uintmax_t)i);
 		logs[i].update_index = i + 1;
 		logs[i].value_type = REFTABLE_LOG_UPDATE;
 		logs[i].value.update.email = xstrdup("johndoe@invalid");
 		logs[i].value.update.message = xstrdup("commit\n");
-		set_test_hash(logs[i].value.update.new_hash, i);
+		t_reftable_set_hash(logs[i].value.update.new_hash, i, GIT_SHA1_FORMAT_ID);
 	}
 
 	for (i = 0; i < N; i++) {
@@ -704,7 +700,8 @@ static void t_reftable_stack_tombstone(void)
 		refs[i].update_index = i + 1;
 		if (i % 2 == 0) {
 			refs[i].value_type = REFTABLE_REF_VAL1;
-			set_test_hash(refs[i].value.val1, i);
+			t_reftable_set_hash(refs[i].value.val1, i,
+					    GIT_SHA1_FORMAT_ID);
 		}
 
 		logs[i].refname = xstrdup(buf);
@@ -712,7 +709,8 @@ static void t_reftable_stack_tombstone(void)
 		logs[i].update_index = 42;
 		if (i % 2 == 0) {
 			logs[i].value_type = REFTABLE_LOG_UPDATE;
-			set_test_hash(logs[i].value.update.new_hash, i);
+			t_reftable_set_hash(logs[i].value.update.new_hash, i,
+					    GIT_SHA1_FORMAT_ID);
 			logs[i].value.update.email =
 				xstrdup("identity@invalid");
 		}
@@ -844,7 +842,8 @@ static void t_reflog_expire(void)
 		logs[i].value_type = REFTABLE_LOG_UPDATE;
 		logs[i].value.update.time = i;
 		logs[i].value.update.email = xstrdup("identity@invalid");
-		set_test_hash(logs[i].value.update.new_hash, i);
+		t_reftable_set_hash(logs[i].value.update.new_hash, i,
+				    GIT_SHA1_FORMAT_ID);
 	}
 
 	for (i = 1; i <= N; i++) {
diff --git a/trace2.h b/trace2.h
index 19e04bf040..901f39253a 100644
--- a/trace2.h
+++ b/trace2.h
@@ -554,6 +554,7 @@ enum trace2_counter_id {
 	TRACE2_COUNTER_ID_TEST2,     /* emits summary and thread events */
 
 	TRACE2_COUNTER_ID_PACKED_REFS_JUMPS, /* counts number of jumps */
+	TRACE2_COUNTER_ID_REFTABLE_RESEEKS, /* counts number of re-seeks */
 
 	/* counts number of fsyncs */
 	TRACE2_COUNTER_ID_FSYNC_WRITEOUT_ONLY,
diff --git a/trace2/tr2_ctr.c b/trace2/tr2_ctr.c
index d3a33715c1..036b643578 100644
--- a/trace2/tr2_ctr.c
+++ b/trace2/tr2_ctr.c
@@ -31,6 +31,11 @@ static struct tr2_counter_metadata tr2_counter_metadata[TRACE2_NUMBER_OF_COUNTER
 		.name = "jumps_made",
 		.want_per_thread_events = 0,
 	},
+	[TRACE2_COUNTER_ID_REFTABLE_RESEEKS] = {
+		.category = "reftable",
+		.name = "reseeks_made",
+		.want_per_thread_events = 0,
+	},
 	[TRACE2_COUNTER_ID_FSYNC_WRITEOUT_ONLY] = {
 		.category = "fsync",
 		.name = "writeout-only",