How to solve MySQL innodb "Waiting for table metadata lock" on TRUNCATE TABLE?
<p>Running a test suite with hundreds of application unit tests in a GitLab CI server. After ran 10 tests in, somehow it always gets stuck on Waiting for table metadata lock on TRUNCATE TABLE, which is a tearDown step. </p>

<p>I am aware of the <code>SHOW ENGINE INNODB STATUS</code> command . Here are some diagnostic logs:</p>

<pre><code>mysql&gt; \s
--------------
mysql  Ver 14.14 Distrib 5.6.30, for Linux (x86_64) using  EditLine wrapper

Connection id:      190
Current database:   
Current user:       root@localhost
SSL:            Not in use
Current pager:      stdout
Using outfile:      ''
Using delimiter:    ;
Server version:     5.6.30 MySQL Community Server (GPL)
Protocol version:   10
Connection:     Localhost via UNIX socket
Server characterset:    utf8mb4
Db     characterset:    utf8mb4
Client characterset:    utf8mb4
Conn.  characterset:    utf8mb4
UNIX socket:        /var/run/mysqld/mysqld.sock
Uptime:         51 min 28 sec

Threads: 4  Questions: 3859  Slow queries: 0  Opens: 715  Flush tables: 1  Open tables: 131  Queries per second avg: 1.249
--------------

mysql&gt; show processlist;
+-----+------+----------------+------------+---------+------+---------------------------------+-----------------------------+
| Id  | User | Host           | db         | Command | Time | State                           | Info                        |
+-----+------+----------------+------------+---------+------+---------------------------------+-----------------------------+
|   1 | root | 10.0.2.1:52773 | test_3926  | Query   | 2961 | Waiting for table metadata lock | TRUNCATE TABLE `capability` |
| 188 | root | 10.0.2.1:53658 | test_3926  | Sleep   | 2962 |                                 | NULL                        |
| 189 | root | 10.0.2.1:53660 | test_3926  | Sleep   | 2962 |                                 | NULL                        |
| 190 | root | localhost      | NULL       | Query   |    0 | init                            | show processlist            |
+-----+------+----------------+------------+---------+------+---------------------------------+-----------------------------+
4 rows in set (0.00 sec)


2016-05-18 16:10:37 7f03be9ba700 INNODB MONITOR OUTPUT
=====================================
Per second averages calculated from the last 7 seconds
-----------------
BACKGROUND THREAD
-----------------
srv_master_thread loops: 126 srv_active, 0 srv_shutdown, 3047 srv_idle
srv_master_thread log flush and writes: 3173
----------
SEMAPHORES
----------
OS WAIT ARRAY INFO: reservation count 2408
OS WAIT ARRAY INFO: signal count 2525
Mutex spin waits 988, rounds 24557, OS waits 747
RW-shared spins 1339, rounds 45580, OS waits 1518
RW-excl spins 3, rounds 5283, OS waits 113
Spin rounds per wait: 24.86 mutex, 34.04 RW-shared, 1761.00 RW-excl
------------
TRANSACTIONS
------------
Trx id counter 7574
Purge done for trx's n:o &lt; 7493 undo n:o &lt; 0 state: running but idle
History list length 778
LIST OF TRANSACTIONS FOR EACH SESSION:
---TRANSACTION 0, not started
MySQL thread id 190, OS thread handle 0x7f03be9ba700, query id 3941 localhost root init
SHOW ENGINE INNODB STATUS
---TRANSACTION 7489, not started
MySQL thread id 188, OS thread handle 0x7f03bea3c700, query id 3824 10.0.2.1 root cleaning up
---TRANSACTION 7548, not started
MySQL thread id 1, OS thread handle 0x7f03bea7d700, query id 3855 10.0.2.1 root Waiting for table metadata lock
TRUNCATE TABLE `capability`
---TRANSACTION 7490, ACTIVE 3047 sec
MySQL thread id 189, OS thread handle 0x7f03be9fb700, query id 3840 10.0.2.1 root cleaning up
Trx read view will not see trx with id &gt;= 7491, sees &lt; 7491
--------
FILE I/O
--------
I/O thread 0 state: waiting for completed aio requests (insert buffer thread)
I/O thread 1 state: waiting for completed aio requests (log thread)
I/O thread 2 state: waiting for completed aio requests (read thread)
I/O thread 3 state: waiting for completed aio requests (read thread)
I/O thread 4 state: waiting for completed aio requests (read thread)
I/O thread 5 state: waiting for completed aio requests (read thread)
I/O thread 6 state: waiting for completed aio requests (write thread)
I/O thread 7 state: waiting for completed aio requests (write thread)
I/O thread 8 state: waiting for completed aio requests (write thread)
I/O thread 9 state: waiting for completed aio requests (write thread)
Pending normal aio reads: 0 [0, 0, 0, 0] , aio writes: 0 [0, 0, 0, 0] ,
 ibuf aio reads: 0, log i/o's: 0, sync i/o's: 0
Pending flushes (fsync) log: 0; buffer pool: 0
173 OS file reads, 6858 OS file writes, 6022 OS fsyncs
0.00 reads/s, 0 avg bytes/read, 0.00 writes/s, 0.00 fsyncs/s
-------------------------------------
INSERT BUFFER AND ADAPTIVE HASH INDEX
-------------------------------------
Ibuf: size 1, free list len 0, seg size 2, 0 merges
merged operations:
 insert 0, delete mark 0, delete 0
discarded operations:
 insert 0, delete mark 0, delete 0
Hash table size 276671, node heap has 2 buffer(s)
0.00 hash searches/s, 0.00 non-hash searches/s
---
LOG
---
Log sequence number 10549488
Log flushed up to   10549488
Pages flushed up to 10549488
Last checkpoint at  10549488
0 pending log writes, 0 pending chkp writes
2555 log i/o's done, 0.00 log i/o's/second
----------------------
BUFFER POOL AND MEMORY
----------------------
Total memory allocated 137363456; in additional pool allocated 0
Dictionary memory allocated 545426
Buffer pool size   8191
Free buffers       7354
Database pages     835
Old database pages 288
Modified db pages  0
Pending reads 0
Pending writes: LRU 0, flush list 0, single page 0
Pages made young 4257, not young 0
0.00 youngs/s, 0.00 non-youngs/s
Pages read 160, created 4341, written 863
0.00 reads/s, 0.00 creates/s, 0.00 writes/s
No buffer pool page gets since the last printout
Pages read ahead 0.00/s, evicted without access 0.00/s, Random read ahead 0.00/s
LRU len: 835, unzip_LRU len: 0
I/O sum[0]:cur[0], unzip sum[0]:cur[0]
--------------
ROW OPERATIONS
--------------
0 queries inside InnoDB, 0 queries in queue
1 read views open inside InnoDB
Main thread process no. 1, id 139654053570304, state: sleeping
Number of rows inserted 1187, updated 37, deleted 0, read 650
0.00 inserts/s, 0.00 updates/s, 0.00 deletes/s, 0.00 reads/s
----------------------------
END OF INNODB MONITOR OUTPUT
</code></pre>

<p>Now my question is, why TRUNCATE table would get stuck on table metalock? And how can this be resolved?</p>