database - If a MySQL/InnoDB 'lock wait timeout' happens, is the lock given anyway when it's free? -


we have high performance django web application makes use of mysql's innodb's row level locking. @ point in our code see (in process a) if particular row in table available using select … update. if there process (b) using row, , got there first, there exclusive lock on row, , have 'lock wait timeout' exception. catch this, , move on try row (with similar select … update approach).

however, i'm not 100% sure happens when lock wait timeout happens. after b finished , releases it's locks, exclusive lock given a, though a's 'select … update' query has timed out, , has moved on lock row?

or (and think intuitively happens), after a's attempt lock fails, , moves on, , b commits , finishes: there no further attempt give lock a?

preferable reference/quote offical mysql documentation best, since it's canonical source. couldn't find in docs specifically addressing happens when attempt lock , times out.

the lock requested statement timed out never acquired:

when lock wait timeout occurs, current statement rolled back.

the below test exhibits fact:

-- "atable" innodb table  -- connection #1 begin; update atable set acol = 1 acol = 2;  -- connection #2 begin; select * atable update; -- hangs  -- connection #3 begin; select * atable update; -- hangs  -- wait until connection #2 times out -- connection #2 -- "lock wait timeout exceeded"  -- connection #1 commit;  -- connection #3 resumes execution , data displayed

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