Handle slow TCP client handshake

[jfallows]

Description

Selector and OP_CONNECT

• In Selector, OP_CONNECT means “the socket is ready to finish the connection”, not necessarily that the handshake is complete.
• This readiness is based on low-level OS signals:
• On Linux/BSD/macOS: the kernel sets socket writable once the handshake progresses enough for connect() to be completed or fails immediately.
• The selector interprets a writable socket in non-blocking connect as OP_CONNECT ready.
• Key point: ready-to-finish does not guarantee finishConnect() will succeed immediately.

Why finishConnect() can still return false

Even after the OS signals writable / OP_CONNECT ready:

• The TCP handshake may still be in progress (delays, packet loss, retransmission).
• The channel must call finishConnect() to:
  • Complete the handshake in Java’s SocketChannel API.
  • Update the internal channel state.
  • Assign local/remote addresses.
• If handshake is still incomplete, finishConnect() returns false — the Java API is just reporting the true channel state.