Merge branch 'ps/reftable-iteration-perf'

The code to iterate over refs with the reftable backend has seen
some optimization.

* ps/reftable-iteration-perf:
  reftable/reader: add comments to `table_iter_next()`
  reftable/record: don't try to reallocate ref record name
  reftable/block: swap buffers instead of copying
  reftable/pq: allocation-less comparison of entry keys
  reftable/merged: skip comparison for records of the same subiter
  reftable/merged: allocation-less dropping of shadowed records
  reftable/record: introduce function to compare records by key

reftable/block.c

@@ -339,8 +339,7 @@ int block_iter_next(struct block_iter *it, struct reftable_record *rec)
 		return -1;
 	string_view_consume(&in, n);
 
-	strbuf_reset(&it->last_key);
+	strbuf_swap(&it->last_key, &it->key);
-	strbuf_addbuf(&it->last_key, &it->key);
 	it->next_off += start.len - in.len;
 	return 0;
 }

reftable/merged.c

@@ -49,8 +49,6 @@ static void merged_iter_close(void *p)
 	for (size_t i = 0; i < mi->stack_len; i++)
 		reftable_iterator_destroy(&mi->stack[i]);
 	reftable_free(mi->stack);
-	strbuf_release(&mi->key);
-	strbuf_release(&mi->entry_key);
 }
 
 static int merged_iter_advance_nonnull_subiter(struct merged_iter *mi,
@@ -105,14 +103,19 @@ static int merged_iter_next_entry(struct merged_iter *mi,
 	  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, &mi->entry_key);
 	while (!merged_iter_pqueue_is_empty(mi->pq)) {
 		struct pq_entry top = merged_iter_pqueue_top(mi->pq);
-		int cmp = 0;
+		int cmp;
 
-		reftable_record_key(&top.rec, &mi->key);
+		/*
+		 * When the next entry comes from the same queue as the current
+		 * entry then it must by definition be larger. This avoids a
+		 * comparison in the most common case.
+		 */
+		if (top.index == entry.index)
+			break;
 
-		cmp = strbuf_cmp(&mi->key, &mi->entry_key);
+		cmp = reftable_record_cmp(&top.rec, &entry.rec);
 		if (cmp > 0)
 			break;
 
@@ -243,8 +246,6 @@ static int merged_table_seek_record(struct reftable_merged_table *mt,
 		.typ = reftable_record_type(rec),
 		.hash_id = mt->hash_id,
 		.suppress_deletions = mt->suppress_deletions,
-		.key = STRBUF_INIT,
-		.entry_key = STRBUF_INIT,
 	};
 	struct merged_iter *p;
 	int err;

reftable/merged.h

@@ -31,8 +31,6 @@ struct merged_iter {
 	uint8_t typ;
 	int suppress_deletions;
 	struct merged_iter_pqueue pq;
-	struct strbuf key;
-	struct strbuf entry_key;
 };
 
 void merged_table_release(struct reftable_merged_table *mt);

reftable/pq.c

@@ -14,20 +14,9 @@ https://developers.google.com/open-source/licenses/bsd
 
 int pq_less(struct pq_entry *a, struct pq_entry *b)
 {
-	struct strbuf ak = STRBUF_INIT;
+	int cmp = reftable_record_cmp(&a->rec, &b->rec);
-	struct strbuf bk = STRBUF_INIT;
-	int cmp = 0;
-	reftable_record_key(&a->rec, &ak);
-	reftable_record_key(&b->rec, &bk);
-
-	cmp = strbuf_cmp(&ak, &bk);
-
-	strbuf_release(&ak);
-	strbuf_release(&bk);
-
 	if (cmp == 0)
 		return a->index > b->index;
-
 	return cmp < 0;
 }
 

reftable/reader.c

@@ -357,24 +357,32 @@ static int table_iter_next(struct table_iter *ti, struct reftable_record *rec)
 
 	while (1) {
 		struct table_iter next = TABLE_ITER_INIT;
-		int err = 0;
+		int err;
-		if (ti->is_finished) {
+
+		if (ti->is_finished)
 			return 1;
-		}
 
+		/*
+		 * Check whether the current block still has more records. If
+		 * so, return it. If the iterator returns positive then the
+		 * current block has been exhausted.
+		 */
 		err = table_iter_next_in_block(ti, rec);
-		if (err <= 0) {
+		if (err <= 0)
+			return err;
+
+		/*
+		 * Otherwise, we need to continue to the next block in the
+		 * table and retry. If there are no more blocks then the
+		 * iterator is drained.
+		 */
+		err = table_iter_next_block(&next, ti);
+		table_iter_block_done(ti);
+		if (err) {
+			ti->is_finished = 1;
 			return err;
 		}
 
-		err = table_iter_next_block(&next, ti);
-		if (err != 0) {
-			ti->is_finished = 1;
-		}
-		table_iter_block_done(ti);
-		if (err != 0) {
-			return err;
-		}
 		table_iter_copy_from(ti, &next);
 		block_iter_close(&next.bi);
 	}

reftable/record.c

@@ -377,10 +377,11 @@ static int reftable_ref_record_decode(void *rec, struct strbuf key,
 
 	assert(hash_size > 0);
 
-	r->refname = reftable_realloc(r->refname, key.len + 1);
+	r->refname = reftable_malloc(key.len + 1);
 	memcpy(r->refname, key.buf, key.len);
-	r->update_index = update_index;
 	r->refname[key.len] = 0;
+
+	r->update_index = update_index;
 	r->value_type = val_type;
 	switch (val_type) {
 	case REFTABLE_REF_VAL1:
@@ -430,7 +431,6 @@ static int reftable_ref_record_is_deletion_void(const void *p)
 		(const struct reftable_ref_record *)p);
 }
 
-
 static int reftable_ref_record_equal_void(const void *a,
 					  const void *b, int hash_size)
 {
@@ -439,6 +439,13 @@ static int reftable_ref_record_equal_void(const void *a,
 	return reftable_ref_record_equal(ra, rb, hash_size);
 }
 
+static int reftable_ref_record_cmp_void(const void *_a, const void *_b)
+{
+	const struct reftable_ref_record *a = _a;
+	const struct reftable_ref_record *b = _b;
+	return strcmp(a->refname, b->refname);
+}
+
 static void reftable_ref_record_print_void(const void *rec,
 					   int hash_size)
 {
@@ -455,6 +462,7 @@ static struct reftable_record_vtable reftable_ref_record_vtable = {
 	.release = &reftable_ref_record_release_void,
 	.is_deletion = &reftable_ref_record_is_deletion_void,
 	.equal = &reftable_ref_record_equal_void,
+	.cmp = &reftable_ref_record_cmp_void,
 	.print = &reftable_ref_record_print_void,
 };
 
@@ -627,6 +635,25 @@ static int reftable_obj_record_equal_void(const void *a, const void *b, int hash
 	return 1;
 }
 
+static int reftable_obj_record_cmp_void(const void *_a, const void *_b)
+{
+	const struct reftable_obj_record *a = _a;
+	const struct reftable_obj_record *b = _b;
+	int cmp;
+
+	cmp = memcmp(a->hash_prefix, b->hash_prefix,
+		     a->hash_prefix_len > b->hash_prefix_len ?
+		     a->hash_prefix_len : b->hash_prefix_len);
+	if (cmp)
+		return cmp;
+
+	/*
+	 * When the prefix is the same then the object record that is longer is
+	 * considered to be bigger.
+	 */
+	return a->hash_prefix_len - b->hash_prefix_len;
+}
+
 static struct reftable_record_vtable reftable_obj_record_vtable = {
 	.key = &reftable_obj_record_key,
 	.type = BLOCK_TYPE_OBJ,
@@ -637,6 +664,7 @@ static struct reftable_record_vtable reftable_obj_record_vtable = {
 	.release = &reftable_obj_record_release,
 	.is_deletion = &not_a_deletion,
 	.equal = &reftable_obj_record_equal_void,
+	.cmp = &reftable_obj_record_cmp_void,
 	.print = &reftable_obj_record_print,
 };
 
@@ -955,6 +983,22 @@ static int reftable_log_record_equal_void(const void *a,
 					 hash_size);
 }
 
+static int reftable_log_record_cmp_void(const void *_a, const void *_b)
+{
+	const struct reftable_log_record *a = _a;
+	const struct reftable_log_record *b = _b;
+	int cmp = strcmp(a->refname, b->refname);
+	if (cmp)
+		return cmp;
+
+	/*
+	 * Note that the comparison here is reversed. This is because the
+	 * update index is reversed when comparing keys. For reference, see how
+	 * we handle this in reftable_log_record_key()`.
+	 */
+	return b->update_index - a->update_index;
+}
+
 int reftable_log_record_equal(const struct reftable_log_record *a,
 			      const struct reftable_log_record *b, int hash_size)
 {
@@ -1004,6 +1048,7 @@ static struct reftable_record_vtable reftable_log_record_vtable = {
 	.release = &reftable_log_record_release_void,
 	.is_deletion = &reftable_log_record_is_deletion_void,
 	.equal = &reftable_log_record_equal_void,
+	.cmp = &reftable_log_record_cmp_void,
 	.print = &reftable_log_record_print_void,
 };
 
@@ -1079,6 +1124,13 @@ static int reftable_index_record_equal(const void *a, const void *b, int hash_si
 	return ia->offset == ib->offset && !strbuf_cmp(&ia->last_key, &ib->last_key);
 }
 
+static int reftable_index_record_cmp(const void *_a, const void *_b)
+{
+	const struct reftable_index_record *a = _a;
+	const struct reftable_index_record *b = _b;
+	return strbuf_cmp(&a->last_key, &b->last_key);
+}
+
 static void reftable_index_record_print(const void *rec, int hash_size)
 {
 	const struct reftable_index_record *idx = rec;
@@ -1096,6 +1148,7 @@ static struct reftable_record_vtable reftable_index_record_vtable = {
 	.release = &reftable_index_record_release,
 	.is_deletion = &not_a_deletion,
 	.equal = &reftable_index_record_equal,
+	.cmp = &reftable_index_record_cmp,
 	.print = &reftable_index_record_print,
 };
 
@@ -1149,6 +1202,14 @@ int reftable_record_is_deletion(struct reftable_record *rec)
 		reftable_record_data(rec));
 }
 
+int reftable_record_cmp(struct reftable_record *a, struct reftable_record *b)
+{
+	if (a->type != b->type)
+		BUG("cannot compare reftable records of different type");
+	return reftable_record_vtable(a)->cmp(
+		reftable_record_data(a), reftable_record_data(b));
+}
+
 int reftable_record_equal(struct reftable_record *a, struct reftable_record *b, int hash_size)
 {
 	if (a->type != b->type)

reftable/record.h

@@ -62,6 +62,12 @@ struct reftable_record_vtable {
 	/* Are two records equal? This assumes they have the same type. Returns 0 for non-equal. */
 	int (*equal)(const void *a, const void *b, int hash_size);
 
+	/*
+	 * Compare keys of two records with each other. The records must have
+	 * the same type.
+	 */
+	int (*cmp)(const void *a, const void *b);
+
 	/* Print on stdout, for debugging. */
 	void (*print)(const void *rec, int hash_size);
 };
@@ -114,6 +120,7 @@ struct reftable_record {
 void reftable_record_init(struct reftable_record *rec, uint8_t typ);
 
 /* see struct record_vtable */
+int reftable_record_cmp(struct reftable_record *a, struct reftable_record *b);
 int reftable_record_equal(struct reftable_record *a, struct reftable_record *b, int hash_size);
 void reftable_record_print(struct reftable_record *rec, int hash_size);
 void reftable_record_key(struct reftable_record *rec, struct strbuf *dest);