From 53d985909e21ce536a900e5c53944bd4d706315d Mon Sep 17 00:00:00 2001
From: Alexey Milovidov <milovidov@yandex-team.ru>
Date: Tue, 16 Jun 2020 01:54:19 +0300
Subject: [PATCH 1/2] Fix race condition in SYSTEM SYNC REPLICA

---
 src/Storages/MergeTree/ReplicatedMergeTreeQueue.cpp |  4 ++--
 src/Storages/StorageReplicatedMergeTree.cpp         | 13 ++++++++++---
 2 files changed, 12 insertions(+), 5 deletions(-)

diff --git a/src/Storages/MergeTree/ReplicatedMergeTreeQueue.cpp b/src/Storages/MergeTree/ReplicatedMergeTreeQueue.cpp
index 8a9dbceba04..6325b1adca4 100644
--- a/src/Storages/MergeTree/ReplicatedMergeTreeQueue.cpp
+++ b/src/Storages/MergeTree/ReplicatedMergeTreeQueue.cpp
@@ -646,7 +646,7 @@ void ReplicatedMergeTreeQueue::updateMutations(zkutil::ZooKeeperPtr zookeeper, C
         }
     }
 
-    if (some_active_mutations_were_killed)
+    if (some_active_mutations_were_killed && storage.queue_task_handle)
         storage.queue_task_handle->signalReadyToRun();
 
     if (!entries_to_load.empty())
@@ -759,7 +759,7 @@ ReplicatedMergeTreeMutationEntryPtr ReplicatedMergeTreeQueue::removeMutation(
         LOG_DEBUG(log, "Removed mutation {} from local state.", entry->znode_name);
     }
 
-    if (mutation_was_active)
+    if (mutation_was_active && storage.queue_task_handle)
         storage.queue_task_handle->signalReadyToRun();
 
     return entry;
diff --git a/src/Storages/StorageReplicatedMergeTree.cpp b/src/Storages/StorageReplicatedMergeTree.cpp
index 5931bca17ea..885db89e5b0 100644
--- a/src/Storages/StorageReplicatedMergeTree.cpp
+++ b/src/Storages/StorageReplicatedMergeTree.cpp
@@ -5643,9 +5643,16 @@ bool StorageReplicatedMergeTree::waitForShrinkingQueueSize(size_t queue_size, UI
 
     /// Let's fetch new log entries firstly
     queue.pullLogsToQueue(getZooKeeper());
-    /// This is significant, because the execution of this task could be delayed at BackgroundPool.
-    /// And we force it to be executed.
-    queue_task_handle->signalReadyToRun();
+
+    {
+        auto lock = queue.lockQueue();
+        if (!queue_task_handle)
+            return false;
+
+        /// This is significant, because the execution of this task could be delayed at BackgroundPool.
+        /// And we force it to be executed.
+        queue_task_handle->signalReadyToRun();
+    }
 
     Poco::Event target_size_event;
     auto callback = [&target_size_event, queue_size] (size_t new_queue_size)

From 00224ee94f482573ca996cecc11a8110aba8dc15 Mon Sep 17 00:00:00 2001
From: Alexey Milovidov <milovidov@yandex-team.ru>
Date: Tue, 16 Jun 2020 02:04:12 +0300
Subject: [PATCH 2/2] Added a test

---
 ...01320_create_sync_race_condition.reference |  0
 .../01320_create_sync_race_condition.sh       | 28 +++++++++++++++++++
 2 files changed, 28 insertions(+)
 create mode 100644 tests/queries/0_stateless/01320_create_sync_race_condition.reference
 create mode 100755 tests/queries/0_stateless/01320_create_sync_race_condition.sh

diff --git a/tests/queries/0_stateless/01320_create_sync_race_condition.reference b/tests/queries/0_stateless/01320_create_sync_race_condition.reference
new file mode 100644
index 00000000000..e69de29bb2d
diff --git a/tests/queries/0_stateless/01320_create_sync_race_condition.sh b/tests/queries/0_stateless/01320_create_sync_race_condition.sh
new file mode 100755
index 00000000000..2e42033644a
--- /dev/null
+++ b/tests/queries/0_stateless/01320_create_sync_race_condition.sh
@@ -0,0 +1,28 @@
+#!/usr/bin/env bash
+
+CURDIR=$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)
+. $CURDIR/../shell_config.sh
+
+set -e
+
+$CLICKHOUSE_CLIENT --query "DROP TABLE IF EXISTS r;"
+
+function thread1()
+{
+    while true; do $CLICKHOUSE_CLIENT -n --query "CREATE TABLE r (x UInt64) ENGINE = ReplicatedMergeTree('/test/table', 'r') ORDER BY x; DROP TABLE r;"; done
+}
+
+function thread2()
+{
+    while true; do $CLICKHOUSE_CLIENT --query "SYSTEM SYNC REPLICA r" 2>/dev/null; done
+}
+
+export -f thread1
+export -f thread2
+
+timeout 10 bash -c thread1 &
+timeout 10 bash -c thread2 &
+
+wait
+
+$CLICKHOUSE_CLIENT --query "DROP TABLE IF EXISTS r;"