unexpected behaviour of a mysql transaction in a specific concurrency scenario - mysql

Question

I have trouble understanding the behaviour of a mysql transaction in a specific concurrency scenario.
It involves a table with a primary key and another column with a unique key constraint.
In the Java test case give below two DB connections are used concurrently:
The first connection uses a transaction with isolation level "repeatable read", and tries to read a row which is not existing at this point, then tries to insert it using a INSERT ... ON DUPLICATE KEY statement, and tries to read it once more.
The second connection inserts a row with the same value for the unique column AFTER the first read on connection 1 and BEFORE the insert attempt on connection 2.
The output is as follows and I can reproduce it with MySQL 5.7 and 8.0
...
[conTransaction] SELECT * FROM test.tmp WHERE uniq = 'x'; => false
[conAutoCommit ] INSERT INTO test.tmp(uniq, t) VALUES('x', NOW()) ON DUPLICATE KEY UPDATE t = VALUES(t); => 1
[conTransaction] SELECT * FROM test.tmp WHERE uniq = 'x'; => false
[conTransaction] INSERT INTO test.tmp(uniq, t) VALUES('x', NOW()) ON DUPLICATE KEY UPDATE t = VALUES(t); => 1
[conTransaction] SELECT * FROM test.tmp WHERE uniq = 'x'; => false
[conTransaction] COMMIT => 0
[conTransaction] UNEXPECTED: no row found after INSERT ON DUPLICATE KEY
I consider this behaviour unexpected:
the insert in the transaction appears to have succeeded but the row still cannot be read from that transaction.
Is this really correct behaviour for a repeatable read transaction?
More over, if the timing is changed by uncommenting the sleep in the line marked by (A)
then the INSERT ... ON DUPLICATE KEY actually changes something and the row becomes visible:
...
[conTransaction] SELECT * FROM test.tmp WHERE uniq = 'x'; => false
[conAutoCommit ] INSERT INTO test.tmp(uniq, t) VALUES('x', NOW()) ON DUPLICATE KEY UPDATE t = VALUES(t); => 1
[conTransaction] SELECT * FROM test.tmp WHERE uniq = 'x'; => false
[conTransaction] INSERT INTO test.tmp(uniq, t) VALUES('x', NOW()) ON DUPLICATE KEY UPDATE t = VALUES(t); => 2
[conTransaction] SELECT * FROM test.tmp WHERE uniq = 'x'; => true
[conTransaction] COMMIT => 0
This is the sample test case in Java.
import static org.junit.Assert.assertEquals;
import static org.junit.Assert.fail;
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.SQLException;
import org.junit.Test;
public class MyTests {
Connection conTA;
Connection conAutoCommit;
#Test
public void test() throws Exception {
conAutoCommit = getConnection();
conTA = getConnection();
conTA.setAutoCommit(false);
setup(conAutoCommit);
boolean hasIt = hasRow(conTA);
assertEquals(false, hasIt);
insert(conAutoCommit);
// Thread.sleep(1200); // (A)
hasRow(conTA);
insert(conTA);
boolean found = hasRow(conTA);
execNonQuery(conTA, "COMMIT");
if (!found) {
log(conTA, "UNEXPECTED: no row found after INSERT ON DUPLICATE KEY");
fail("UNEXPECTED: no row found after INSERT ON DUPLICATE KEY");
}
}
private int execNonQuery(Connection con, String sql) throws SQLException {
try(var stmt = con.createStatement()) {
int res = stmt.executeUpdate(sql);
log(con, sql + " => " + res);
return res;
}
}
private void setup(Connection con) throws SQLException {
execNonQuery(con, "DROP TABLE IF EXISTS test.tmp");
execNonQuery(con, """
CREATE TABLE test.tmp (
id int(11) NOT NULL AUTO_INCREMENT,
uniq varchar(45) DEFAULT NULL,
t datetime DEFAULT NULL,
PRIMARY KEY (id),
UNIQUE KEY serverDomain (uniq)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
""");
}
private void insert(Connection con) throws SQLException {
String sql = "INSERT INTO test.tmp(uniq, t) VALUES('x', NOW()) ON DUPLICATE KEY UPDATE t = VALUES(t);";
execNonQuery(con, sql);
}
private boolean hasRow(Connection con) throws SQLException {
try(var stmt = con.createStatement()) {
String sql = "SELECT * FROM test.tmp WHERE uniq = 'x';";
try(var rs = stmt.executeQuery(sql)) {
boolean res = rs.next();
log(con, sql + " => " + res);
return res;
}
}
}
private void log(Connection con, String msg) {
String conName = con == conTA ? "conTransaction" : "conAutoCommit ";
System.out.println("[" + conName + "] " + msg);
}
private Connection getConnection() throws SQLException {
var con = DriverManager.getConnection("jdbc:mysql://localhost:3306?useSSL=false","root","");
con.setTransactionIsolation(Connection.TRANSACTION_REPEATABLE_READ);
return con;
}
}