[zenios]

The problem

With no port set, Builder.start() picks a free port with new ServerSocket(0), closes it, runs initdb and only then starts the server. Anything can take the port in that window, which lasts as long as initdb. When tests run in parallel, another test's socket sometimes does. The server then logs:

LOG:  could not bind IPv4 address "127.0.0.1": Address already in use
FATAL:  could not create any TCP/IP sockets

and start() fails with Gave up waiting for server to start after ..., once the whole startup wait has passed. Nothing is wrong with the test, and it passes when run again.

The change

• Builder.start() tries up to 3 times, with a new port and a new temporary directory each time, if the failed port really is taken.
• It does not retry when the caller set the port or the data directory, or when the port is still free, as that failure has another cause and would only wait again.
• A failed start now closes the instance: it releases the lock and removes the data directory. Without this, each failed attempt also left its lock open and its directory behind, and a shutdown hook that tries to stop a server that never started.

Tests

• testDetectedPortTakenBeforeTheServerBindsItStartsOnAnother holds the first detected port and expects a working server on another. A package-private nextPort() makes the clash deterministic.
• testAPortAskedForAndTakenFailsWithoutAnotherAttempt keeps today's behaviour when the caller chose the port.

Known limits

• A failed attempt still waits out pgStartupWait before retrying, as today. The server has already exited by then, so failing fast could be a separate change.
• Closing a failed instance logs Could not stop postmaster at ERROR, as pg_ctl stop finds nothing running.