Description:
There is a bug in function minAck in semisync_source.h in semisync plugin. The function doesn't find the minimum ack but the maximum index ack which is smaller than given position. And this will lead to unexpected behavior when rpl_semi_sync_master_wait_slaves is set to more than 3.
```
/**
Find the minimum ack which is smaller than given position. When more than
one slots are minimum acks, it returns the one has smallest index.
@param[in] log_file_name binlog file name
@param[in] log_file_pos binlog file position
@return NULL if no ack is smaller than given position, otherwise
return its pointer.
*/
AckInfo *minAck(const char *log_file_name, my_off_t log_file_pos) {
unsigned int i;
AckInfo *ackinfo = nullptr;
for (i = 0; i < m_size; i++) {
if (m_ack_array[i].less_than(log_file_name, log_file_pos))
ackinfo = m_ack_array + i;
}
return ackinfo;
}
```
How to repeat:
Suppose rpl_semi_sync_master_wait_slaves = 4.
coming acks slot 1 slot 2 slot 3
=========== ========== ========== ==========
1:log1:120 1:log1:120
2:log1:120 1:log1:120 2:log1:120
1:log1:150 1:log1:150 2:log1:120
1:log1:170 1:log1:170 2:log1:120
3:log1:150 1:log1:170 2:log1:120 3:log1:150
4:log1:170
The minAck will return '3:log1:150' and m_greatest_ack will asign to '3:log1:150' too. Then the array will look like:
coming acks slot 1 slot 2 slot 3
=========== ========== ========== ==========
1:log1:170 2:log1:120 log1:170
Slot 2 in this case will never become empty again.
Suggested fix:
Just fix the bug in minAck. Like:
```
AckInfo *minAck(const char *log_file_name, my_off_t log_file_pos) {
unsigned int i;
AckInfo *ackinfo = nullptr;
const char *min_log_file_name = log_file_name;
my_off_t min_log_file_pos = log_file_pos;
for (i = 0; i < m_size; i++) {
if (m_ack_array[i].less_than(min_log_file_name, min_log_file_pos)) {
ackinfo = m_ack_array + i;
min_log_file_name = m_ack_array[i].binlog_name;
min_log_file_pos = m_ack_array[i].binlog_pos;
}
}
```
return ackinfo;
}
Description: There is a bug in function minAck in semisync_source.h in semisync plugin. The function doesn't find the minimum ack but the maximum index ack which is smaller than given position. And this will lead to unexpected behavior when rpl_semi_sync_master_wait_slaves is set to more than 3. ``` /** Find the minimum ack which is smaller than given position. When more than one slots are minimum acks, it returns the one has smallest index. @param[in] log_file_name binlog file name @param[in] log_file_pos binlog file position @return NULL if no ack is smaller than given position, otherwise return its pointer. */ AckInfo *minAck(const char *log_file_name, my_off_t log_file_pos) { unsigned int i; AckInfo *ackinfo = nullptr; for (i = 0; i < m_size; i++) { if (m_ack_array[i].less_than(log_file_name, log_file_pos)) ackinfo = m_ack_array + i; } return ackinfo; } ``` How to repeat: Suppose rpl_semi_sync_master_wait_slaves = 4. coming acks slot 1 slot 2 slot 3 =========== ========== ========== ========== 1:log1:120 1:log1:120 2:log1:120 1:log1:120 2:log1:120 1:log1:150 1:log1:150 2:log1:120 1:log1:170 1:log1:170 2:log1:120 3:log1:150 1:log1:170 2:log1:120 3:log1:150 4:log1:170 The minAck will return '3:log1:150' and m_greatest_ack will asign to '3:log1:150' too. Then the array will look like: coming acks slot 1 slot 2 slot 3 =========== ========== ========== ========== 1:log1:170 2:log1:120 log1:170 Slot 2 in this case will never become empty again. Suggested fix: Just fix the bug in minAck. Like: ``` AckInfo *minAck(const char *log_file_name, my_off_t log_file_pos) { unsigned int i; AckInfo *ackinfo = nullptr; const char *min_log_file_name = log_file_name; my_off_t min_log_file_pos = log_file_pos; for (i = 0; i < m_size; i++) { if (m_ack_array[i].less_than(min_log_file_name, min_log_file_pos)) { ackinfo = m_ack_array + i; min_log_file_name = m_ack_array[i].binlog_name; min_log_file_pos = m_ack_array[i].binlog_pos; } } ``` return ackinfo; }