xtrabackup --prepare may hang with 'Waiting for 1 active transactions to finish'

/usr/bin/innobackupex version 2.4.7 based on MySQL server 5.7.13 Linux (x86_64) (revision id: 6f7a799)

xtrabackup: cd to /dev/shm/__var0/var1/backup/

xtrabackup: This target seems to be not prepared yet.

InnoDB: Number of pools: 1

xtrabackup: xtrabackup_logfile detected: size=10092544, start_lsn=(1504807)

xtrabackup: using the following InnoDB configuration for recovery:

xtrabackup:   innodb_data_home_dir = .

xtrabackup:   innodb_data_file_path = ibdata1:12M:autoextend

xtrabackup:   innodb_log_group_home_dir = .

xtrabackup:   innodb_log_files_in_group = 1

xtrabackup:   innodb_log_file_size = 10092544

xtrabackup: using the following InnoDB configuration for recovery:

xtrabackup:   innodb_data_home_dir = .

xtrabackup:   innodb_data_file_path = ibdata1:12M:autoextend

xtrabackup:   innodb_log_group_home_dir = .

xtrabackup:   innodb_log_files_in_group = 1

xtrabackup:   innodb_log_file_size = 10092544

xtrabackup: Starting InnoDB instance for recovery.

xtrabackup: Using 104857600 bytes for buffer pool (set by --use-memory parameter)

InnoDB: PUNCH HOLE support available

InnoDB: Mutexes and rw_locks use GCC atomic builtins

InnoDB: Uses event mutexes

InnoDB: GCC builtin __atomic_thread_fence() is used for memory barrier

InnoDB: Compressed tables use zlib 1.2.8

InnoDB: Number of pools: 1

InnoDB: Using CPU crc32 instructions

InnoDB: Initializing buffer pool, total size = 100M, instances = 1, chunk size = 100M

InnoDB: Completed initialization of buffer pool

InnoDB: If the mysqld execution user is authorized, page cleaner thread priority can be changed. See the man page of setpriority().

InnoDB: Opened 4 undo tablespaces

InnoDB: 4 undo tablespaces made active

InnoDB: Highest supported file format is Barracuda.

InnoDB: Log scan progressed past the checkpoint lsn 1504807

InnoDB: Doing recovery: scanned up to log sequence number 6682112 (57%)

InnoDB: Doing recovery: scanned up to log sequence number 10416535 (99%)

InnoDB: Database was not shutdown normally!

InnoDB: Starting crash recovery.

InnoDB: 1 transaction(s) which must be rolled back or cleaned up in total 16049 row operations to undo

InnoDB: Trx id counter is 1792

InnoDB: Starting an apply batch of log records to the database...

InnoDB: Progress in percent: 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 

InnoDB: Apply batch completed

InnoDB: xtrabackup: Last MySQL binlog file position 3503344, file name ./mysql-bin.000002

InnoDB: Creating shared tablespace for temporary commit

InnoDB: Creating shared tablespace for temporary commit

InnoDB: Rolling back trx with id 1466, 16049 rows to undo

InnoDB: Progress in percents: 1InnoDB: Setting file './ibtmp1' size to 12 MB. Physically writing the file full; Please wait ...

 2 3InnoDB: File './ibtmp1' size is now 12 MB.

InnoDB: 96 redo rollback segment(s) found. 1 redo rollback segment(s) are active.

InnoDB: 32 non-redo rollback segment(s) are active.

 4InnoDB: 5.7.13 started; log sequence number 10416535

InnoDB: xtrabackup: Last MySQL binlog file position 3503344, file name ./mysql-bin.000002

xtrabackup: starting shutdown with innodb_fast_shutdown = 1

InnoDB: FTS optimize thread exiting.

InnoDB: Starting shutdown...

InnoDB: Waiting for 1 active transactions to finish

InnoDB: Waiting for 1 active transactions to finish