MySQL semi-synchronous replication--handle_slave_io waiting for master to send event

The Handle_slave_io function calls the Read_event function:

The Read_event function reads events from the connection, and if no events are blocked here, wait for master to send the event.

#0   0x00898402 in __kernel_vsyscall  () #1   0x00c28133 in poll   ()  from /lib/libc.so.6#2  0x08a90315 in vio_io_wait  (VIO=0XA82FAA8,  event=vio_io_event_read, timeout=3600000)     at /home/mysql26/mysql-5.6.26 /vio/viosocket.c:771#3  0x08a8eb78 in vio_socket_io_wait  (vio=0xa82faa8, event= Vio_io_event_read)     at /home/mysql26/mysql-5.6.26/vio/viosocket.c:68#4   0x08a8ec6a in vio_read  (vio=0xa82faa8, buf=0xa882c90  "P",  size=16384)      at /home/mysql26/mysql-5.6.26/vio/viosocket.c:123#5  0x08a8ef05 in  vio_read_buff  (vio=0xa82faa8, buf=0xa89a530  "\031",  size=4)     at  /home/mysql26/mysql-5.6.26/vio/viosocket.c:157#6  0x0830e010 in net_read_raw_loop   (Net=0xa7591b8, count=4)  at /home/mysql26/mysql-5.6.26/sql/net_serv.cc:669#7  0x0830e1f2 in net_read_ packet_header  (NET=0XA7591B8)  at /home/mysql26/mysql-5.6.26/sql/net_serv.cc:757#8   0x0830ef83 in net_read_packet  (net=0xa7591b8, complen=0xa34fc168)      at /home/mysql26/mysql-5.6.26/sql/net_serv.cc:816#9  0x0830f0ea in my_net_read  ( NET=0XA7591B8)  at /home/mysql26/mysql-5.6.26/sql/net_serv.cc:894#10 0x084af8a0 in cli _safe_read  (MYSQL=0XA7591B8)  at /home/mysql26/mysql-5.6.26/sql-common/client.c:620#11  0x0866f2da in read_event  (mysql=0xa7591b8, mi=0xa709368, suppress_warnings=0xa34fc31b )     at /home/mysql26/mysql-5.6.26/sql/rpl_slave.cc:3533#12 0x086799f6 in  handle_slave_io  (arg=0xa709368)  at /home/mysql26/mysql-5.6.26/sql/rpl_slave.cc:4730#13  0x089b3141 in pfs_spawn_thread  (arg=0xa8302b0)  at /home/mysql26/mysql-5.6.26/storage/perfschema/ pfs.cc:1860#14 0x00cdc832 in start_thread  ()  from /lib/libpthread.so.0#15  0x00c31e0e in clone  ()  from /lib/libc.so.6

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MySQL semi-synchronous replication--handle_slave_io waiting for master to send event