[PATCH 07/13] memtx: use ref counting to pin indexes for snapshot

[Vladimir Davydov <vdavydov.dev@gmail.com>]

Currently, to prevent an index from going away while it is being
written to a snapshot, we postpone memtx_gc_task's free() invocation
until checkpointing is complete, see commit 94de0a081b3a ("Don't take
schema lock for checkpointing"). This works fine, but makes it rather
difficult to reuse snapshot iterators for other purposes, e.g. feeding
a consistent read view to a newly joined replica.

Let's instead use index reference counting for pinning indexes for
checkpointing. A reference is taken in a snapshot iterator constructor
and released when the snapshot iterator is destroyed.
---
 src/box/memtx_engine.c | 26 +-------------------------
 src/box/memtx_engine.h |  5 -----
 src/box/memtx_hash.c   | 15 +++++++++------
 src/box/memtx_tree.c   | 16 +++++++++-------
 4 files changed, 19 insertions(+), 43 deletions(-)

diff --git a/src/box/memtx_engine.c b/src/box/memtx_engine.c
index 59ad1682..ec667e7a 100644
--- a/src/box/memtx_engine.c
+++ b/src/box/memtx_engine.c
@@ -651,19 +651,6 @@ memtx_engine_wait_checkpoint(struct engine *engine,
 	return result;
 }
 
-/**
- * Called after checkpointing is complete to free indexes dropped
- * while checkpointing was in progress, see memtx_engine_run_gc().
- */
-static void
-memtx_engine_gc_after_checkpoint(struct memtx_engine *memtx)
-{
-	struct memtx_gc_task *task, *next;
-	stailq_foreach_entry_safe(task, next, &memtx->gc_to_free, link)
-		task->vtab->free(task);
-	stailq_create(&memtx->gc_to_free);
-}
-
 static void
 memtx_engine_commit_checkpoint(struct engine *engine,
 			       const struct vclock *vclock)
@@ -701,8 +688,6 @@ memtx_engine_commit_checkpoint(struct engine *engine,
 
 	checkpoint_delete(memtx->checkpoint);
 	memtx->checkpoint = NULL;
-
-	memtx_engine_gc_after_checkpoint(memtx);
 }
 
 static void
@@ -887,15 +872,7 @@ memtx_engine_run_gc(struct memtx_engine *memtx, bool *stop)
 	task->vtab->run(task, &task_done);
 	if (task_done) {
 		stailq_shift(&memtx->gc_queue);
-		/*
-		 * If checkpointing is in progress, the index may be
-		 * used by the checkpoint thread so we postpone freeing
-		 * until checkpointing is complete.
-		 */
-		if (memtx->checkpoint == NULL)
-			task->vtab->free(task);
-		else
-			stailq_add_entry(&memtx->gc_to_free, task, link);
+		task->vtab->free(task);
 	}
 }
 
@@ -967,7 +944,6 @@ memtx_engine_new(const char *snap_dirname, bool force_recovery,
 	}
 
 	stailq_create(&memtx->gc_queue);
-	stailq_create(&memtx->gc_to_free);
 	memtx->gc_fiber = fiber_new("memtx.gc", memtx_engine_gc_f);
 	if (memtx->gc_fiber == NULL)
 		goto fail;
diff --git a/src/box/memtx_engine.h b/src/box/memtx_engine.h
index fcf595e7..ccb51678 100644
--- a/src/box/memtx_engine.h
+++ b/src/box/memtx_engine.h
@@ -155,11 +155,6 @@ struct memtx_engine {
 	 * memtx_gc_task::link.
 	 */
 	struct stailq gc_queue;
-	/**
-	 * List of tasks awaiting to be freed once checkpointing
-	 * is complete, linked by memtx_gc_task::link.
-	 */
-	struct stailq gc_to_free;
 };
 
 struct memtx_gc_task;
diff --git a/src/box/memtx_hash.c b/src/box/memtx_hash.c
index e30170d1..2762d973 100644
--- a/src/box/memtx_hash.c
+++ b/src/box/memtx_hash.c
@@ -399,7 +399,7 @@ memtx_hash_index_create_iterator(struct index *base, enum iterator_type type,
 
 struct hash_snapshot_iterator {
 	struct snapshot_iterator base;
-	struct light_index_core *hash_table;
+	struct memtx_hash_index *index;
 	struct light_index_iterator iterator;
 };
 
@@ -414,7 +414,8 @@ hash_snapshot_iterator_free(struct snapshot_iterator *iterator)
 	assert(iterator->free == hash_snapshot_iterator_free);
 	struct hash_snapshot_iterator *it =
 		(struct hash_snapshot_iterator *) iterator;
-	light_index_iterator_destroy(it->hash_table, &it->iterator);
+	light_index_iterator_destroy(&it->index->hash_table, &it->iterator);
+	index_unref(&it->index->base);
 	free(iterator);
 }
 
@@ -429,7 +430,8 @@ hash_snapshot_iterator_next(struct snapshot_iterator *iterator, uint32_t *size)
 	assert(iterator->free == hash_snapshot_iterator_free);
 	struct hash_snapshot_iterator *it =
 		(struct hash_snapshot_iterator *) iterator;
-	struct tuple **res = light_index_iterator_get_and_next(it->hash_table,
+	struct light_index_core *hash_table = &it->index->hash_table;
+	struct tuple **res = light_index_iterator_get_and_next(hash_table,
 							       &it->iterator);
 	if (res == NULL)
 		return NULL;
@@ -455,9 +457,10 @@ memtx_hash_index_create_snapshot_iterator(struct index *base)
 
 	it->base.next = hash_snapshot_iterator_next;
 	it->base.free = hash_snapshot_iterator_free;
-	it->hash_table = &index->hash_table;
-	light_index_iterator_begin(it->hash_table, &it->iterator);
-	light_index_iterator_freeze(it->hash_table, &it->iterator);
+	it->index = index;
+	index_ref(base);
+	light_index_iterator_begin(&index->hash_table, &it->iterator);
+	light_index_iterator_freeze(&index->hash_table, &it->iterator);
 	return (struct snapshot_iterator *) it;
 }
 
diff --git a/src/box/memtx_tree.c b/src/box/memtx_tree.c
index aeba2ba3..77223a6d 100644
--- a/src/box/memtx_tree.c
+++ b/src/box/memtx_tree.c
@@ -1205,7 +1205,7 @@ memtx_tree_index_end_build(struct index *base)
 
 struct tree_snapshot_iterator {
 	struct snapshot_iterator base;
-	struct memtx_tree *tree;
+	struct memtx_tree_index *index;
 	struct memtx_tree_iterator tree_iterator;
 };
 
@@ -1215,8 +1215,8 @@ tree_snapshot_iterator_free(struct snapshot_iterator *iterator)
 	assert(iterator->free == tree_snapshot_iterator_free);
 	struct tree_snapshot_iterator *it =
 		(struct tree_snapshot_iterator *)iterator;
-	struct memtx_tree *tree = (struct memtx_tree *)it->tree;
-	memtx_tree_iterator_destroy(tree, &it->tree_iterator);
+	memtx_tree_iterator_destroy(&it->index->tree, &it->tree_iterator);
+	index_unref(&it->index->base);
 	free(iterator);
 }
 
@@ -1226,11 +1226,12 @@ tree_snapshot_iterator_next(struct snapshot_iterator *iterator, uint32_t *size)
 	assert(iterator->free == tree_snapshot_iterator_free);
 	struct tree_snapshot_iterator *it =
 		(struct tree_snapshot_iterator *)iterator;
-	struct memtx_tree_data *res =
-		memtx_tree_iterator_get_elem(it->tree, &it->tree_iterator);
+	struct memtx_tree *tree = &it->index->tree;
+	struct memtx_tree_data *res = memtx_tree_iterator_get_elem(tree,
+							&it->tree_iterator);
 	if (res == NULL)
 		return NULL;
-	memtx_tree_iterator_next(it->tree, &it->tree_iterator);
+	memtx_tree_iterator_next(tree, &it->tree_iterator);
 	return tuple_data_range(res->tuple, size);
 }
 
@@ -1253,7 +1254,8 @@ memtx_tree_index_create_snapshot_iterator(struct index *base)
 
 	it->base.free = tree_snapshot_iterator_free;
 	it->base.next = tree_snapshot_iterator_next;
-	it->tree = &index->tree;
+	it->index = index;
+	index_ref(base);
 	it->tree_iterator = memtx_tree_iterator_first(&index->tree);
 	memtx_tree_iterator_freeze(&index->tree, &it->tree_iterator);
 	return (struct snapshot_iterator *) it;
-- 
2.20.1