vinyl: fix recovery after aborted index creation

There's a bug in the code building index hash on recovery: we replace
a dropped index with any newer index, even incomplete one. Apparently,
this is wrong, because a dropped index may have been dropped during
final recovery and hence is still needed for initial recovery. If we
replace it with an incomplete index in the index hash, initial recovery
will fail with

  ER_INVALID_VYLOG_FILE: Invalid VYLOG file: LSM tree 512/1 not found

(see vy_lsm_recover()).

Fix this problem by checking create_lsn of the index that is going to
replace a dropped one - if it's negative, we must link it to the dropped
index via vy_lsm_recovery_info->prepared instead of inserting it into
the hash directly.

Closes #4066

src/box/vy_log.c

@@ -2141,9 +2141,10 @@ vy_recovery_build_index_id_hash(struct vy_recovery *recovery)
 		/*
 		 * If there's no LSM tree for these space_id/index_id
 		 * or it was dropped, simply replace it with the latest
-		 * LSM tree version.
+		 * committed LSM tree version.
 		 */
-		if (hashed_lsm == NULL || hashed_lsm->drop_lsn >= 0) {
+		if (hashed_lsm == NULL ||
+		    (hashed_lsm->drop_lsn >= 0 && lsm->create_lsn >= 0)) {
 			struct mh_i64ptr_node_t node;
 			node.key = vy_recovery_index_id_hash(space_id, index_id);
 			node.val = lsm;

test/vinyl/errinj_vylog.result

@@ -368,3 +368,58 @@ s.index.sk:select()
 s:drop()
 ---
 ...
+--
+-- gh-4066: recovery error if an instance is restarted while
+-- building an index and there's an index with the same id in
+-- the snapshot.
+--
+fiber = require('fiber')
+---
+...
+s = box.schema.space.create('test', {engine = 'vinyl'})
+---
+...
+_ = s:create_index('pk')
+---
+...
+_ = s:create_index('sk', {parts = {2, 'unsigned'}})
+---
+...
+s.index[1] ~= nil
+---
+- true
+...
+s:replace{1, 2}
+---
+- [1, 2]
+...
+box.snapshot()
+---
+- ok
+...
+s.index.sk:drop()
+---
+...
+-- Log index creation, but never finish building it due to an error injection.
+box.error.injection.set('ERRINJ_VY_READ_PAGE_TIMEOUT', 9000)
+---
+- ok
+...
+_ = fiber.create(function() s:create_index('sk', {parts = {2, 'unsigned'}}) end)
+---
+...
+fiber.sleep(0.01)
+---
+...
+-- Should ignore the incomplete index on recovery.
+test_run:cmd('restart server default')
+s = box.space.test
+---
+...
+s.index[1] == nil
+---
+- true
+...
+s:drop()
+---
+...

test/vinyl/errinj_vylog.test.lua

@@ -177,3 +177,31 @@ s.index.pk:select()
 s.index.sk:select()
 
 s:drop()
+
+--
+-- gh-4066: recovery error if an instance is restarted while
+-- building an index and there's an index with the same id in
+-- the snapshot.
+--
+fiber = require('fiber')
+
+s = box.schema.space.create('test', {engine = 'vinyl'})
+_ = s:create_index('pk')
+_ = s:create_index('sk', {parts = {2, 'unsigned'}})
+s.index[1] ~= nil
+s:replace{1, 2}
+box.snapshot()
+
+s.index.sk:drop()
+
+-- Log index creation, but never finish building it due to an error injection.
+box.error.injection.set('ERRINJ_VY_READ_PAGE_TIMEOUT', 9000)
+_ = fiber.create(function() s:create_index('sk', {parts = {2, 'unsigned'}}) end)
+fiber.sleep(0.01)
+
+-- Should ignore the incomplete index on recovery.
+test_run:cmd('restart server default')
+
+s = box.space.test
+s.index[1] == nil
+s:drop()