Connection.setsavepoint() / releaseSavepoint() is not thread-safe

Hi everyone,

I have already posted this bug as #10847 to pqsql-bugs(at)postgresql(dot)org, but was redirected here:

The following test code…
import org.junit.Test;

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.SQLException;
import java.sql.Savepoint;
import java.util.concurrent.CountDownLatch;

/** @author Christian Schlichtherle */
public class Ticket251IT {

private static final String CONNECTION_STRING = "jdbc:postgresql:postgres";
private static final int NUM_THREADS = 2;

@Test public void foo() throws SQLException, InterruptedException {
try (Connection c = DriverManager.getConnection(CONNECTION_STRING)) {
c.setAutoCommit(false);
final Runnable task = new Runnable() {
final CountDownLatch startSignal = new CountDownLatch(NUM_THREADS);

@Override public void run() {
try {
startSignal.countDown();
startSignal.await();
// FIXME: This idiom doesn't work on a shared connection!
Savepoint sp = c.setSavepoint();
try {
// Insert transaction script here...
} finally {
c.releaseSavepoint(sp);
}
} catch (SQLException | InterruptedException e) {
e.printStackTrace();
}
}
};
final Thread[] threads = new Thread[NUM_THREADS];
for (int i = 0; i < threads.length; i++)
(threads[i] = new Thread(task)).start();
for (Thread thread : threads)
thread.join();
},
}
}
…frequently produces the following output…
org.postgresql.util.PSQLException: ERROR: no such savepoint
at org.postgresql.core.v3.QueryExecutorImpl.receiveErrorResponse(QueryExecutorImpl.java:2161)
at org.postgresql.core.v3.QueryExecutorImpl.processResults(QueryExecutorImpl.java:1890)
at org.postgresql.core.v3.QueryExecutorImpl.execute(QueryExecutorImpl.java:255)
at org.postgresql.jdbc2.AbstractJdbc2Statement.execute(AbstractJdbc2Statement.java:559)
at org.postgresql.jdbc2.AbstractJdbc2Statement.executeWithFlags(AbstractJdbc2Statement.java:403)
at org.postgresql.jdbc2.AbstractJdbc2Connection.execSQLUpdate(AbstractJdbc2Connection.java:376)
at org.postgresql.jdbc3.AbstractJdbc3Connection.releaseSavepoint(AbstractJdbc3Connection.java:192)
at cpssd.db.Ticket251IT$1.run(Ticket251IT.java:32)
at java.lang.Thread.run(Thread.java:745)
The obvious workaround is to put all calls on the connection into a synchronized (whatever) { … } block. However, although the workaround works fine when applied to this isolated test case, it fails in my production code. I have yet to find out why that is. Nevertheless, this seems to be a bug in the JDBC driver.

The real use case behind this simplified test case is to distribute work to multiple threads. A parent thread creates the connection and starts a transaction, then spawns a number of child threads. Each child thread creates a save point, does its work and releases or rolls back the save point again. If all child threads succeed, the parent thread then commits the entire transaction. I figure from this document that this is indeed „fair use“ of a Connection. Thus, it would be superb if PostgreSQL could support it.

Regards,
Christian Schlichtherle