Problem found during RQG testing of mariabackup. 
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					bb-10.4-monty, origin/bb-10.4-monty ea07b5227feeccba1429b687e731033e1a6d560b 2019-02-05T20:08:11+02:00 
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					One connection runs "chaotic" DDL. 
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					Another connection runs mariabackup. 
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					... 
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					# 2019-02-12T11:51:55 [9201] Executing backup: bld_debug/extra/mariabackup/mariabackup --host=127.0.0.1 --user=root --password=''  --innodb-use-native-aio=0  --port=19620 --backup --datadir=/dev/shm/vardir/1549968621/17/1/data --target-dir=/dev/shm/vardir/1549968621/17/1_clone/data 
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					[00] 2019-02-12 11:51:55 Connecting to MySQL server host: 127.0.0.1, user: root, password: set, port: 19620, socket: not set 
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					[00] 2019-02-12 11:51:55 Using server version 10.4.3-MariaDB-debug-log 
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					[00] 2019-02-12 11:51:55 bld_debug/extra/mariabackup/mariabackup based on MariaDB server 10.4.3-MariaDB Linux (x86_64) 
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					... 
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					[00] 2019-02-12 11:51:55 mariabackup: Generating a list of tablespaces 
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					2019-02-12 11:51:55 0 [Warning] InnoDB: Allocated tablespace ID 4 for mysql/gtid_slave_pos, old maximum was 0 
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					2019-02-12 11:51:55 0 [ERROR] InnoDB: Operating system error number 2 in a file operation. 
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					2019-02-12 11:51:55 0 [ERROR] InnoDB: The error means the system cannot find the path specified. 
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					2019-02-12 11:51:55 0 [ERROR] InnoDB: If you are installing InnoDB, remember that you must create directories yourself, InnoDB does not create them. 
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					2019-02-12 11:51:55 0 [ERROR] InnoDB: File ./test/FTS_00000000000003a6_BEING_DELETED_CACHE.ibd: 'open' returned OS error 71. 
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					2019-02-12 11:51:55 0x7f93afb92740  InnoDB: Assertion failure in file storage/innobase/fil/fil0fil.cc line 655 
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					InnoDB: Failing assertion: success 
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					InnoDB: We intentionally generate a memory trap. 
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					190212 11:51:55 [ERROR] mysqld got signal 6 ; 
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					... 
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					stack_bottom = 0x0 thread_stack 0x49000 
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					bld_debug/extra/mariabackup/mariabackup(my_print_stacktrace+0x40)[0x55d1dc82f4f9] 
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					bld_debug/extra/mariabackup/mariabackup(handle_fatal_signal+0x3e7)[0x55d1dc0ffdce] 
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					/lib/x86_64-linux-gnu/libpthread.so.0(+0x13150)[0x7f93af78f150] 
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					/lib/x86_64-linux-gnu/libc.so.6(gsignal+0xcb)[0x7f93ae0130bb] 
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					/lib/x86_64-linux-gnu/libc.so.6(abort+0x16d)[0x7f93ae014f5d] 
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					bld_debug/extra/mariabackup/mariabackup(+0x10308b9)[0x55d1dc5a08b9] 
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					bld_debug/extra/mariabackup/mariabackup(+0xda68d1)[0x55d1dc3168d1] 
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					bld_debug/extra/mariabackup/mariabackup(+0xda9b4b)[0x55d1dc319b4b] 
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					bld_debug/extra/mariabackup/mariabackup(+0x65cac8)[0x55d1dbbccac8] 
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					bld_debug/extra/mariabackup/mariabackup(+0x65ced6)[0x55d1dbbcced6] 
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					bld_debug/extra/mariabackup/mariabackup(+0x65d472)[0x55d1dbbcd472] 
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					bld_debug/extra/mariabackup/mariabackup(+0x65f258)[0x55d1dbbcf258] 
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					bld_debug/extra/mariabackup/mariabackup(+0x6667ae)[0x55d1dbbd67ae] 
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					bld_debug/extra/mariabackup/mariabackup(main+0x24d)[0x55d1dbbd5ffe] 
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					/lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0xf1)[0x7f93adffd1c1] 
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					bld_debug/extra/mariabackup/mariabackup(_start+0x2a)[0x55d1dbbc5aea]
 
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