Concurrency issue still possible with OracleJdbcIndexedSessionRepositoryCustomizer

In specific circumstances there is still a concrete chance to run into concurrency issues.

The reason lies in the use of the MERGE SQL statement which, albeit elegant, doesn't prevent a unique constraint violation in this case: 1) connection A adds attribute X, 2) connection B adds attribute X, 3) connection A commits, 4) connection B gets a unique constraint violation.

A way to reduce greatly the frequency of the problem is to replace the MERGE with an anonymous PL/SQL block of the form

BEGIN
INSERT ...
EXCEPTION
WHEN DUP_VAL_ON_INDEX THEN
UPDATE ...
END;

This statement is less compact and less elegant, but nearly eliminates the problem described above.

I will open a PR with a working implementation.

Start at OracleJdbcIndexedSessionRepositoryCustomizer and inspect the MERGE SQL used when attributes are added. Replace the concurrency-prone operation with the proposed anonymous PL/SQL insert/update flow, then verify that the concurrent duplicate-attribute scenario no longer produces a unique constraint violation.