Statement cache deadlock
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Key: DERBY-4279
URL: https://issues.apache.org/jira/browse/DERBY-4279
Project: Derby
Issue Type: Bug
Components: SQL
Affects Versions: 10.5.1.1
Environment: Windows Vista
Reporter: Jeff Stuckman
Due to a design flaw in the statement cache, a deadlock can occur if a prepared
statement becomes out-of-date.
I will illustrate this with the following example:
The application is using the embedded Derby driver. The application has two
threads, and each thread uses its own connection.
There is a table named MYTABLE with column MYCOLUMN.
1. A thread prepares and executes the query SELECT MYCOLUMN FROM MYTABLE. The
prepared statement is stored in the statement cache (see
org.apache.derby.impl.sql.GenericStatement for this logic)
2. After some time, the prepared statement becomes invalid or out-of-date for
some reason (see org.apache.derby.impl.sql.GenericPreparedStatement)
3. Thread 1 begins a transaction and executes LOCK TABLE MYTABLE IN EXCLUSIVE
MODE
4. Thread 2 begins a transaction and executes SELECT MYCOLUMN FROM MYTABLE. The
statement is in the statement cache but it is out-of-date. The thread begins to
recompile the statement. To compile the statement, the thread needs a shared
lock on MYTABLE. Thread 1 already has an exclusive lock on MYTABLE. Thread 2
waits.
5. Thread 1 executes SELECT MYCOLUMN FROM MYTABLE. The statement is in the
statement cache but it is being compiled. Thread 1 waits on the statement's
monitor.
6. We have a deadlock. Derby eventually detects a lock timeout, but the error
message is not descriptive. The stacks at the time of the deadlock are:
This deadlock is unique because it can still occur in a properly designed
database. You are only safe if all of your transactions are very simple and
cannot be interleaved in a sequence that causes the deadlock, or if your
particular statements do not require a table lock to compile. (For the sake of
simplicity, I used LOCK TABLE in my example, but any UPDATE statement would
fit.)
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