| Bug #85394 | Some confusing code in function row_vers_build_for_semi_consistent_read | ||
|---|---|---|---|
| Submitted: | 10 Mar 2017 1:59 | Modified: | 10 Mar 2017 17:54 |
| Reporter: | zhai weixiang (OCA) | Email Updates: | |
| Status: | Verified | Impact on me: | |
| Category: | MySQL Server: InnoDB storage engine | Severity: | S3 (Non-critical) |
| Version: | 5.7.17 | OS: | Any |
| Assigned to: | CPU Architecture: | Any | |
[10 Mar 2017 17:54]
MySQL Verification Team
Hi Zhai, Your analysis seems correct. Still, there are three other places where a trx can get the state TRX_STATE_COMMITTED_IN_MEMORY so I think they need to be looked at in detil and see if they affect this particular code flow. This requires a very deep look by InnoDB developers , so I am verifying this bug. Thank you.
Description: In function row_vers_build_for_semi_consistent_read, it'll extract trx_id from primary record, and then check if the corresponding transaction is active or not quoted code from function row_vers_build_for_semi_consistent_read(5.7.17): trx_sys_mutex_enter(); version_trx = (version_trx_id); /* Because version_trx is a read-write transaction, its state cannot change from or to NOT_STARTED while we are holding the trx_sys->mutex. It may change from ACTIVE to PREPARED or COMMITTED. */ if (version_trx && trx_state_eq(version_trx, TRX_STATE_COMMITTED_IN_MEMORY)) { version_trx = NULL; } trx_sys_mutex_exit(); If the trx exists in rw_trx_set, then check if the transaction state equals to TRX_STATE_COMMITTED_IN_MEMORY. But according to the code in function trx_commit_in_memory, It will firstly invoke trx_erase_lists ( which removes the trx_t from trx_sys->rw_trx_set) and then changes the trx state to TRX_STATE_COMMITTED_IN_MEMORY. The quoted code above and trx_earse_lists both acquire trx_sys->mutex. So I think if the trx exists in the set, its state shouldn't equal to TRX_STATE_COMMITTED_IN_MEMORY. And the checking is unnecessary. Just simplify the code as bellow: trx_sys_mutex_enter(); version_trx = trx_get_rw_trx_by_id(version_trx_id); trx_sys_mutex_exit(); How to repeat: code review Suggested fix: Let me know if I missed something :)