PilotUUID non-uniqueness can break Dirac and DiracX

[Robin-Van-de-Merghel]

I think it is a known issue, but if we can add to the fixes of V9..

Currently, Dirac PilotAgentDB (as well as DiracX) has a PilotID primary key, an Integer. Most of the time, we use PilotJobReferences instead of PilotID, and we even translate the PilotJobReference into PilotID:

def deletePilot(self, pilotRef, conn=False):
        """Delete Pilot with the given reference from the PilotAgentsDB"""

        if isinstance(pilotRef, str):
            pilotID = self.__getPilotID(pilotRef)
        else:
            pilotID = pilotRef

        return self.deletePilots([pilotID], conn=conn)

With the example above, we will delete a pilot with a pilotReference. If for whatever reason there is a duplicate (two pilots with the same reference), we would delete one of them without being sure of which one.

This is an issue later on with DiracX with secrets, because:

• if there is a duplicate by error, one of the pilots won't have a secret
• we could have a race condition where the original pilot doesn't have a secret, and the "malicious" one could obtain a secret
• currently a pilot only knows its reference, not its ID (it's own identity is based on a non-unique string, it is not an identity anymore)

Changes:

-- From
`PilotJobReference` VARCHAR(255) NOT NULL DEFAULT 'Unknown',
-- To
`PilotJobReference` VARCHAR(255) UNIQUE NOT NULL DEFAULT 'Unknown',