[tarantool-patches] [PATCH] replication: do not skip master's rows in case of an error

[Serge Petrenko <sergepetrenko@tarantool.org>]

Applier used to promote vclock prior to applying the row. This lead to a
situation when master's row would be skipped forever in case there is an
error trying to apply it. However, some errors are transient, and we
might be able to successfully apply the same row later.

So only advance vclock if a corresponding row is successfully written to
WAL. This makes sure that rows are not skipped.
replication_skip_conflict works as before: if it is set an a conflict
occurs while applying the row, it is silently ignored and the vclock is
advanced, so the row is never applied.

While we're at it, make wal writer the only one responsible for
advancing replicaset vclock. It was already doing it for rows coming from
the local instance, besides, it makes the code cleaner since now we want
to advance vclock only after a successful write and lets us get rid of
unnecessary checks whether applier or wal has already advanced the
vclock.

Closes #2283
---
https://github.com/tarantool/tarantool/issues/2283
https://github.com/tarantool/tarantool/tree/sp/gh-2283-dont-skip-rows

 src/box/applier.cc                          |  49 +++++-----
 src/box/wal.c                               |  59 ++++++-----
 src/errinj.h                                |   1 +
 test/box/errinj.result                      |   2 +
 test/replication/skip_conflict_row.result   |  64 ++++++++++++
 test/replication/skip_conflict_row.test.lua |  19 ++++
 test/xlog/errinj.result                     |  17 +++-
 test/xlog/errinj.test.lua                   |   4 +
 test/xlog/panic_on_lsn_gap.result           | 103 +++++++++++---------
 test/xlog/panic_on_lsn_gap.test.lua         |  46 +++++----
 10 files changed, 241 insertions(+), 123 deletions(-)

diff --git a/src/box/applier.cc b/src/box/applier.cc
index ff4af95e5..bdc303a7d 100644
--- a/src/box/applier.cc
+++ b/src/box/applier.cc
@@ -355,6 +355,10 @@ applier_join(struct applier *applier)
 		coio_read_xrow(coio, ibuf, &row);
 		applier->last_row_time = ev_monotonic_now(loop());
 		if (iproto_type_is_dml(row.type)) {
+			/*
+			 * Manually advance vclock, since no WAL writes
+			 * are done during this stage of recovery.
+			 */
 			vclock_follow_xrow(&replicaset.vclock, &row);
 			xstream_write_xc(applier->subscribe_stream, &row);
 			if (++row_count % 100000 == 0)
@@ -513,31 +517,21 @@ applier_subscribe(struct applier *applier)
 		applier->lag = ev_now(loop()) - row.tm;
 		applier->last_row_time = ev_monotonic_now(loop());
 
+		/*
+		 * In a full mesh topology, the same set
+		 * of changes may arrive via two
+		 * concurrently running appliers.
+		 * Thus the rows may execute out of order,
+		 * when the following xstream_write()
+		 * yields on WAL. Hence we need a latch to
+		 * strictly order all changes which belong
+		 * to the same server id.
+		 */
+		struct replica *replica = replica_by_id(row.replica_id);
+		struct latch *latch = (replica ? &replica->order_latch :
+				       &replicaset.applier.order_latch);
+		latch_lock(latch);
 		if (vclock_get(&replicaset.vclock, row.replica_id) < row.lsn) {
-			/**
-			 * Promote the replica set vclock before
-			 * applying the row. If there is an
-			 * exception (conflict) applying the row,
-			 * the row is skipped when the replication
-			 * is resumed.
-			 */
-			vclock_follow_xrow(&replicaset.vclock, &row);
-			struct replica *replica = replica_by_id(row.replica_id);
-			struct latch *latch = (replica ? &replica->order_latch :
-					       &replicaset.applier.order_latch);
-			/*
-			 * In a full mesh topology, the same set
-			 * of changes may arrive via two
-			 * concurrently running appliers. Thanks
-			 * to vclock_follow() above, the first row
-			 * in the set will be skipped - but the
-			 * remaining may execute out of order,
-			 * when the following xstream_write()
-			 * yields on WAL. Hence we need a latch to
-			 * strictly order all changes which belong
-			 * to the same server id.
-			 */
-			latch_lock(latch);
 			int res = xstream_write(applier->subscribe_stream, &row);
 			latch_unlock(latch);
 			if (res != 0) {
@@ -548,12 +542,13 @@ applier_subscribe(struct applier *applier)
 				 */
 				if (e->type == &type_ClientError &&
 				    box_error_code(e) == ER_TUPLE_FOUND &&
-				    replication_skip_conflict)
+				    replication_skip_conflict) {
 					diag_clear(diag_get());
-				else
+					vclock_follow_xrow(&replicaset.vclock, &row);
+				} else
 					diag_raise();
 			}
-		}
+		} else latch_unlock(latch);
 		if (applier->state == APPLIER_SYNC ||
 		    applier->state == APPLIER_FOLLOW)
 			fiber_cond_signal(&applier->writer_cond);
diff --git a/src/box/wal.c b/src/box/wal.c
index 3b50d3629..c6c3c4ee5 100644
--- a/src/box/wal.c
+++ b/src/box/wal.c
@@ -121,6 +121,11 @@ struct wal_writer
 	 * with this LSN and LSN becomes "real".
 	 */
 	struct vclock vclock;
+	/*
+	 * Uncommitted vclock of the latest entry to be written,
+	 * which will become "real" on a successful WAL write.
+	 */
+	struct vclock req_vclock;
 	/**
 	 * VClock of the most recent successfully created checkpoint.
 	 * The WAL writer must not delete WAL files that are needed to
@@ -171,6 +176,11 @@ struct wal_msg {
 	 * be rolled back.
 	 */
 	struct stailq rollback;
+	/**
+	 * Vclock of latest successfully written request in batch
+	 * used to advance replicaset vclock.
+	 */
+	struct vclock vclock;
 };
 
 /**
@@ -284,6 +294,7 @@ tx_schedule_commit(struct cmsg *msg)
 		/* Closes the input valve. */
 		stailq_concat(&writer->rollback, &batch->rollback);
 	}
+	vclock_copy(&replicaset.vclock, &batch->vclock);
 	tx_schedule_queue(&batch->commit);
 }
 
@@ -368,6 +379,7 @@ wal_writer_create(struct wal_writer *writer, enum wal_mode wal_mode,
 	writer->checkpoint_triggered = false;
 
 	vclock_copy(&writer->vclock, vclock);
+	vclock_copy(&writer->req_vclock, vclock);
 	vclock_copy(&writer->checkpoint_vclock, checkpoint_vclock);
 	rlist_create(&writer->watchers);
 
@@ -907,10 +919,15 @@ wal_assign_lsn(struct wal_writer *writer, struct xrow_header **row,
 	/** Assign LSN to all local rows. */
 	for ( ; row < end; row++) {
 		if ((*row)->replica_id == 0) {
-			(*row)->lsn = vclock_inc(&writer->vclock, instance_id);
+			(*row)->lsn = vclock_inc(&writer->req_vclock, instance_id);
 			(*row)->replica_id = instance_id;
 		} else {
-			vclock_follow_xrow(&writer->vclock, *row);
+			vclock_follow_xrow(&writer->req_vclock, *row);
+		}
+		struct errinj *inj = errinj(ERRINJ_WAL_LSN_GAP, ERRINJ_INT);
+		if (inj != NULL && inj->iparam > 0) {
+			(*row)->lsn += inj->iparam;
+			vclock_follow_xrow(&writer->req_vclock, *row);
 		}
 	}
 }
@@ -975,13 +992,14 @@ wal_write_to_disk(struct cmsg *msg)
 	struct stailq_entry *last_committed = NULL;
 	stailq_foreach_entry(entry, &wal_msg->commit, fifo) {
 		wal_assign_lsn(writer, entry->rows, entry->rows + entry->n_rows);
-		entry->res = vclock_sum(&writer->vclock);
-		rc = xlog_write_entry(l, entry);
+		entry->res = vclock_sum(&writer->req_vclock);
+		int rc = xlog_write_entry(l, entry);
 		if (rc < 0)
 			goto done;
 		if (rc > 0) {
 			writer->checkpoint_wal_size += rc;
 			last_committed = &entry->fifo;
+			vclock_copy(&writer->vclock, &writer->req_vclock);
 		}
 		/* rc == 0: the write is buffered in xlog_tx */
 	}
@@ -991,6 +1009,7 @@ wal_write_to_disk(struct cmsg *msg)
 
 	writer->checkpoint_wal_size += rc;
 	last_committed = stailq_last(&wal_msg->commit);
+	vclock_copy(&writer->vclock, &writer->req_vclock);
 
 	/*
 	 * Notify TX if the checkpoint threshold has been exceeded.
@@ -1021,6 +1040,10 @@ done:
 		error_log(error);
 		diag_clear(diag_get());
 	}
+	/* Latest successfully written row vclock. */
+	vclock_copy(&wal_msg->vclock, &writer->vclock);
+	/* Rollback request vclock to the latest committed. */
+	vclock_copy(&writer->req_vclock, &writer->vclock);
 	/*
 	 * We need to start rollback from the first request
 	 * following the last committed request. If
@@ -1152,31 +1175,6 @@ wal_write(struct journal *journal, struct journal_entry *entry)
 	bool cancellable = fiber_set_cancellable(false);
 	fiber_yield(); /* Request was inserted. */
 	fiber_set_cancellable(cancellable);
-	if (entry->res > 0) {
-		struct xrow_header **last = entry->rows + entry->n_rows - 1;
-		while (last >= entry->rows) {
-			/*
-			 * Find last row from local instance id
-			 * and promote vclock.
-			 */
-			if ((*last)->replica_id == instance_id) {
-				/*
-				 * In master-master configuration, during sudden
-				 * power-loss, if the data have not been written
-				 * to WAL but have already been sent to others,
-				 * they will send the data back. In this case
-				 * vclock has already been promoted by applier.
-				 */
-				if (vclock_get(&replicaset.vclock,
-					       instance_id) < (*last)->lsn) {
-					vclock_follow_xrow(&replicaset.vclock,
-							   *last);
-				}
-				break;
-			}
-			--last;
-		}
-	}
 	return entry->res;
 }
 
@@ -1187,11 +1185,12 @@ wal_write_in_wal_mode_none(struct journal *journal,
 	struct wal_writer *writer = (struct wal_writer *) journal;
 	wal_assign_lsn(writer, entry->rows, entry->rows + entry->n_rows);
 	int64_t old_lsn = vclock_get(&replicaset.vclock, instance_id);
-	int64_t new_lsn = vclock_get(&writer->vclock, instance_id);
+	int64_t new_lsn = vclock_get(&writer->req_vclock, instance_id);
 	if (new_lsn > old_lsn) {
 		/* There were local writes, promote vclock. */
 		vclock_follow(&replicaset.vclock, instance_id, new_lsn);
 	}
+	vclock_copy(&writer->vclock, &writer->req_vclock);
 	return vclock_sum(&writer->vclock);
 }
 
diff --git a/src/errinj.h b/src/errinj.h
index 39de63d19..a425d809e 100644
--- a/src/errinj.h
+++ b/src/errinj.h
@@ -122,6 +122,7 @@ struct errinj {
 	_(ERRINJ_VY_INDEX_FILE_RENAME, ERRINJ_BOOL, {.bparam = false}) \
 	_(ERRINJ_RELAY_BREAK_LSN, ERRINJ_INT, {.iparam = -1}) \
 	_(ERRINJ_WAL_BREAK_LSN, ERRINJ_INT, {.iparam = -1}) \
+	_(ERRINJ_WAL_LSN_GAP, ERRINJ_INT, {.iparam = 0}) \
 	_(ERRINJ_VY_COMPACTION_DELAY, ERRINJ_BOOL, {.bparam = false}) \
 
 ENUM0(errinj_id, ERRINJ_LIST);
diff --git a/test/box/errinj.result b/test/box/errinj.result
index 12303670e..6bcfaabdb 100644
--- a/test/box/errinj.result
+++ b/test/box/errinj.result
@@ -48,6 +48,8 @@ errinj.info()
     state: 0
   ERRINJ_SNAP_COMMIT_DELAY:
     state: false
+  ERRINJ_WAL_LSN_GAP:
+    state: 0
   ERRINJ_TUPLE_ALLOC:
     state: false
   ERRINJ_VY_RUN_WRITE_DELAY:
diff --git a/test/replication/skip_conflict_row.result b/test/replication/skip_conflict_row.result
index 6ca13b472..6ca5ad5ae 100644
--- a/test/replication/skip_conflict_row.result
+++ b/test/replication/skip_conflict_row.result
@@ -4,6 +4,7 @@ env = require('test_run')
 test_run = env.new()
 ---
 ...
+test_run:cmd("restart server default with cleanup=1")
 engine = test_run:get_cfg('engine')
 ---
 ...
@@ -82,6 +83,69 @@ box.info.status
 ---
 - running
 ...
+-- test that if replication_skip_conflict is off vclock
+-- is not advanced on errors.
+test_run:cmd("restart server replica")
+---
+- true
+...
+test_run:cmd("switch replica")
+---
+- true
+...
+box.cfg{replication_skip_conflict=false}
+---
+...
+box.space.test:insert{3}
+---
+- [3]
+...
+box.info.vclock
+---
+- {2: 2, 1: 7}
+...
+test_run:cmd("switch default")
+---
+- true
+...
+box.space.test:insert{3, 3}
+---
+- [3, 3]
+...
+box.space.test:insert{4}
+---
+- [4]
+...
+box.info.vclock
+---
+- {1: 9}
+...
+test_run:cmd("switch replica")
+---
+- true
+...
+box.info.vclock
+---
+- {2: 2, 1: 7}
+...
+box.info.replication[1].upstream.message
+---
+- Duplicate key exists in unique index 'primary' in space 'test'
+...
+box.info.replication[1].upstream.status
+---
+- stopped
+...
+box.space.test:select()
+---
+- - [1]
+  - [2]
+  - [3]
+...
+test_run:cmd("switch default")
+---
+- true
+...
 -- cleanup
 test_run:cmd("stop server replica")
 ---
diff --git a/test/replication/skip_conflict_row.test.lua b/test/replication/skip_conflict_row.test.lua
index 4406ced95..c60999b9b 100644
--- a/test/replication/skip_conflict_row.test.lua
+++ b/test/replication/skip_conflict_row.test.lua
@@ -1,5 +1,6 @@
 env = require('test_run')
 test_run = env.new()
+test_run:cmd("restart server default with cleanup=1")
 engine = test_run:get_cfg('engine')
 
 box.schema.user.grant('guest', 'replication')
@@ -28,6 +29,24 @@ box.space.test:select()
 test_run:cmd("switch default")
 box.info.status
 
+-- test that if replication_skip_conflict is off vclock
+-- is not advanced on errors.
+test_run:cmd("restart server replica")
+test_run:cmd("switch replica")
+box.cfg{replication_skip_conflict=false}
+box.space.test:insert{3}
+box.info.vclock
+test_run:cmd("switch default")
+box.space.test:insert{3, 3}
+box.space.test:insert{4}
+box.info.vclock
+test_run:cmd("switch replica")
+box.info.vclock
+box.info.replication[1].upstream.message
+box.info.replication[1].upstream.status
+box.space.test:select()
+test_run:cmd("switch default")
+
 -- cleanup
 test_run:cmd("stop server replica")
 test_run:cmd("cleanup server replica")
diff --git a/test/xlog/errinj.result b/test/xlog/errinj.result
index 6243ac701..f23bcf2bc 100644
--- a/test/xlog/errinj.result
+++ b/test/xlog/errinj.result
@@ -21,12 +21,28 @@ box.space._schema:insert{"key"}
 ---
 - error: Failed to write to disk
 ...
+box.info.lsn
+---
+- 0
+...
 test_run:cmd('restart server default')
+box.info.lsn
+---
+- 0
+...
 box.space._schema:insert{"key"}
 ---
 - ['key']
 ...
+box.info.lsn
+---
+- 1
+...
 test_run:cmd('restart server default')
+box.info.lsn
+---
+- 1
+...
 box.space._schema:get{"key"}
 ---
 - ['key']
@@ -43,7 +59,6 @@ require('fio').glob(name .. "/*.xlog")
 ---
 - - xlog/00000000000000000000.xlog
   - xlog/00000000000000000001.xlog
-  - xlog/00000000000000000002.xlog
 ...
 test_run:cmd('restart server default with cleanup=1')
 -- gh-881 iproto request with wal IO error
diff --git a/test/xlog/errinj.test.lua b/test/xlog/errinj.test.lua
index 7a5a29cb6..a359f7145 100644
--- a/test/xlog/errinj.test.lua
+++ b/test/xlog/errinj.test.lua
@@ -12,9 +12,13 @@ test_run:cmd('restart server default with cleanup=1')
 
 box.error.injection.set("ERRINJ_WAL_WRITE", true)
 box.space._schema:insert{"key"}
+box.info.lsn
 test_run:cmd('restart server default')
+box.info.lsn
 box.space._schema:insert{"key"}
+box.info.lsn
 test_run:cmd('restart server default')
+box.info.lsn
 box.space._schema:get{"key"}
 box.space._schema:delete{"key"}
 -- list all the logs
diff --git a/test/xlog/panic_on_lsn_gap.result b/test/xlog/panic_on_lsn_gap.result
index 4dd1291f8..1bab21c98 100644
--- a/test/xlog/panic_on_lsn_gap.result
+++ b/test/xlog/panic_on_lsn_gap.result
@@ -35,13 +35,22 @@ s = box.space._schema
 -- xlog otherwise the server believes that there
 -- is an lsn gap during recovery.
 --
+-- The first WAL will start with lsn 11.
+box.error.injection.set("ERRINJ_WAL_LSN_GAP", 10)
+---
+- ok
+...
 s:replace{"key", 'test 1'}
 ---
 - ['key', 'test 1']
 ...
+box.error.injection.set("ERRINJ_WAL_LSN_GAP", 0)
+---
+- ok
+...
 box.info.vclock
 ---
-- {1: 1}
+- {1: 11}
 ...
 box.error.injection.set("ERRINJ_WAL_WRITE", true)
 ---
@@ -82,17 +91,14 @@ t
   - Failed to write to disk
   - Failed to write to disk
 ...
---
--- Before restart: our LSN is 1, because
--- LSN is promoted in tx only on successful
--- WAL write.
---
 name = string.match(arg[0], "([^,]+)%.lua")
 ---
 ...
+-- Vclock is promoted on a successful WAL write.
+-- Last successfully written row {"key", "test 1"} has LSN 11.
 box.info.vclock
 ---
-- {1: 1}
+- {1: 11}
 ...
 require('fio').glob(name .. "/*.xlog")
 ---
@@ -165,10 +171,18 @@ box.error.injection.set("ERRINJ_WAL_WRITE", false)
 -- but it's *inside* a single WAL, so doesn't
 -- affect WAL search in recover_remaining_wals()
 --
+box.error.injection.set("ERRINJ_WAL_LSN_GAP", 10)
+---
+- ok
+...
 s:replace{'key', 'test 2'}
 ---
 - ['key', 'test 2']
 ...
+box.error.injection.set("ERRINJ_WAL_LSN_GAP", 0)
+---
+- ok
+...
 --
 -- notice that vclock before and after
 -- server stop is the same -- because it's
@@ -259,11 +273,19 @@ box.error.injection.set("ERRINJ_WAL_WRITE", false)
 ---
 - ok
 ...
--- then a success
+-- Now write a row after a gap.
+box.error.injection.set("ERRINJ_WAL_LSN_GAP", 2)
+---
+- ok
+...
 box.space._schema:replace{"key", 'test 4'}
 ---
 - ['key', 'test 4']
 ...
+box.error.injection.set("ERRINJ_WAL_LSN_GAP", 0)
+---
+- ok
+...
 box.info.vclock
 ---
 - {1: 35}
@@ -286,45 +308,35 @@ box.space._schema:select{'key'}
 -- that appeared due to a disk error and no files is
 -- actually missing, we won't panic on recovery.
 --
-box.space._schema:replace{'key', 'test 4'} -- creates new WAL
----
-- ['key', 'test 4']
-...
-box.error.injection.set("ERRINJ_WAL_WRITE_DISK", true)
----
-- ok
-...
-box.space._schema:replace{'key', 'test 5'} -- fails, makes gap
----
-- error: Failed to write to disk
-...
-box.snapshot() -- fails, rotates WAL
+test_run:cmd("setopt delimiter ';'")
 ---
-- error: Error injection 'xlog write injection'
+- true
 ...
-box.error.injection.set("ERRINJ_WAL_WRITE_DISK", false)
+for i = 1, box.cfg.rows_per_wal do
+    box.space._schema:replace{"key", "test 5"}
+end;
 ---
-- ok
 ...
-box.space._schema:replace{'key', 'test 5'} -- creates new WAL
+test_run:cmd("setopt delimiter ''");
 ---
-- ['key', 'test 5']
+- true
 ...
-box.error.injection.set("ERRINJ_WAL_WRITE_DISK", true)
+box.error.injection.set("ERRINJ_WAL_LSN_GAP", 1)
 ---
 - ok
 ...
-box.space._schema:replace{'key', 'test 6'} -- fails, makes gap
+box.space._schema:replace{"key", "test 6"}
 ---
-- error: Failed to write to disk
+- ['key', 'test 6']
 ...
-box.snapshot() -- fails, rotates WAL
+test_run:cmd("restart server panic")
+box.space._schema:get{"key"}
 ---
-- error: Error injection 'xlog write injection'
+- ['key', 'test 6']
 ...
-box.space._schema:replace{'key', 'test 6'} -- fails, creates empty WAL
+box.info.vclock
 ---
-- error: Failed to write to disk
+- {1: 47}
 ...
 name = string.match(arg[0], "([^,]+)%.lua")
 ---
@@ -336,18 +348,19 @@ require('fio').glob(name .. "/*.xlog")
   - panic/00000000000000000022.xlog
   - panic/00000000000000000032.xlog
   - panic/00000000000000000035.xlog
-  - panic/00000000000000000037.xlog
-  - panic/00000000000000000039.xlog
+  - panic/00000000000000000045.xlog
+  - panic/00000000000000000047.xlog
 ...
-test_run:cmd("restart server panic")
-box.space._schema:select{'key'}
+-- Make sure we don't create a WAL in the gap between
+-- the last two.
+box.space._schema:replace{"key", "test 7"}
 ---
-- - ['key', 'test 5']
+- ['key', 'test 7']
 ...
--- Check that we don't create a WAL in the gap between the last two.
-box.space._schema:replace{'key', 'test 6'}
+test_run:cmd("restart server panic")
+box.info.vclock
 ---
-- ['key', 'test 6']
+- {1: 48}
 ...
 name = string.match(arg[0], "([^,]+)%.lua")
 ---
@@ -359,11 +372,11 @@ require('fio').glob(name .. "/*.xlog")
   - panic/00000000000000000022.xlog
   - panic/00000000000000000032.xlog
   - panic/00000000000000000035.xlog
-  - panic/00000000000000000037.xlog
-  - panic/00000000000000000039.xlog
-  - panic/00000000000000000040.xlog
+  - panic/00000000000000000045.xlog
+  - panic/00000000000000000047.xlog
+  - panic/00000000000000000048.xlog
 ...
-test_run:cmd('switch default')
+test_run:cmd("switch default")
 ---
 - true
 ...
diff --git a/test/xlog/panic_on_lsn_gap.test.lua b/test/xlog/panic_on_lsn_gap.test.lua
index 6221261a7..1d77292bc 100644
--- a/test/xlog/panic_on_lsn_gap.test.lua
+++ b/test/xlog/panic_on_lsn_gap.test.lua
@@ -17,7 +17,10 @@ s = box.space._schema
 -- xlog otherwise the server believes that there
 -- is an lsn gap during recovery.
 --
+-- The first WAL will start with lsn 11.
+box.error.injection.set("ERRINJ_WAL_LSN_GAP", 10)
 s:replace{"key", 'test 1'}
+box.error.injection.set("ERRINJ_WAL_LSN_GAP", 0)
 box.info.vclock
 box.error.injection.set("ERRINJ_WAL_WRITE", true)
 t = {}
@@ -33,12 +36,9 @@ for i=1,box.cfg.rows_per_wal do
 end;
 test_run:cmd("setopt delimiter ''");
 t
---
--- Before restart: our LSN is 1, because
--- LSN is promoted in tx only on successful
--- WAL write.
---
 name = string.match(arg[0], "([^,]+)%.lua")
+-- Vclock is promoted on a successful WAL write.
+-- Last successfully written row {"key", "test 1"} has LSN 11.
 box.info.vclock
 require('fio').glob(name .. "/*.xlog")
 test_run:cmd("restart server panic")
@@ -69,7 +69,9 @@ box.error.injection.set("ERRINJ_WAL_WRITE", false)
 -- but it's *inside* a single WAL, so doesn't
 -- affect WAL search in recover_remaining_wals()
 --
+box.error.injection.set("ERRINJ_WAL_LSN_GAP", 10)
 s:replace{'key', 'test 2'}
+box.error.injection.set("ERRINJ_WAL_LSN_GAP", 0)
 --
 -- notice that vclock before and after
 -- server stop is the same -- because it's
@@ -101,8 +103,10 @@ box.space._schema:replace{"key", 'test 3'}
 box.info.vclock
 require('fio').glob(name .. "/*.xlog")
 box.error.injection.set("ERRINJ_WAL_WRITE", false)
--- then a success
+-- Now write a row after a gap.
+box.error.injection.set("ERRINJ_WAL_LSN_GAP", 2)
 box.space._schema:replace{"key", 'test 4'}
+box.error.injection.set("ERRINJ_WAL_LSN_GAP", 0)
 box.info.vclock
 require('fio').glob(name .. "/*.xlog")
 -- restart is ok
@@ -113,24 +117,26 @@ box.space._schema:select{'key'}
 -- that appeared due to a disk error and no files is
 -- actually missing, we won't panic on recovery.
 --
-box.space._schema:replace{'key', 'test 4'} -- creates new WAL
-box.error.injection.set("ERRINJ_WAL_WRITE_DISK", true)
-box.space._schema:replace{'key', 'test 5'} -- fails, makes gap
-box.snapshot() -- fails, rotates WAL
-box.error.injection.set("ERRINJ_WAL_WRITE_DISK", false)
-box.space._schema:replace{'key', 'test 5'} -- creates new WAL
-box.error.injection.set("ERRINJ_WAL_WRITE_DISK", true)
-box.space._schema:replace{'key', 'test 6'} -- fails, makes gap
-box.snapshot() -- fails, rotates WAL
-box.space._schema:replace{'key', 'test 6'} -- fails, creates empty WAL
+test_run:cmd("setopt delimiter ';'")
+for i = 1, box.cfg.rows_per_wal do
+    box.space._schema:replace{"key", "test 5"}
+end;
+test_run:cmd("setopt delimiter ''");
+box.error.injection.set("ERRINJ_WAL_LSN_GAP", 1)
+box.space._schema:replace{"key", "test 6"}
+test_run:cmd("restart server panic")
+box.space._schema:get{"key"}
+box.info.vclock
 name = string.match(arg[0], "([^,]+)%.lua")
 require('fio').glob(name .. "/*.xlog")
+-- Make sure we don't create a WAL in the gap between
+-- the last two.
+box.space._schema:replace{"key", "test 7"}
 test_run:cmd("restart server panic")
-box.space._schema:select{'key'}
--- Check that we don't create a WAL in the gap between the last two.
-box.space._schema:replace{'key', 'test 6'}
+box.info.vclock
 name = string.match(arg[0], "([^,]+)%.lua")
 require('fio').glob(name .. "/*.xlog")
-test_run:cmd('switch default')
+
+test_run:cmd("switch default")
 test_run:cmd("stop server panic")
 test_run:cmd("cleanup server panic")
-- 
2.17.2 (Apple Git-113)