[YEthYuan]

When another process claims the heartbeat's candidate port before the heartbeat thread binds it, the existing retry logic selects a replacement port. However, IPKernelApp has already copied the original port, so it can write a connection file that points clients at the wrong endpoint.

Wait for heartbeat binding to complete before copying and logging the port. Heartbeat.start() now waits for a future completed after successful binding. The ZeroMQ socket is still created, bound, and used on the heartbeat thread; Heartbeat.addr is updated after successful binding. Socket setup and exception handling retain their existing implementation.

Port conflict before and after this change

P is the initial candidate port; Q is the replacement selected after a bind conflict. The temporary socket releases P before the actual heartbeat socket binds it.

sequenceDiagram
    participant App as Kernel startup
    participant OS as Operating system
    participant Other as Another process
    participant HB as Heartbeat thread

    App->>OS: Bind temporary TCP socket to port 0
    OS-->>App: Assign port P
    App->>OS: Close temporary socket
    Note over App,OS: P is available again
    Other->>OS: Bind socket with automatic port selection
    OS-->>Other: Assign and bind P

    alt Before this change
        App->>App: Copy candidate P into hb_port
        App->>HB: Start heartbeat thread
        HB->>OS: Bind heartbeat socket to P
        OS-->>HB: Address already in use
        HB->>OS: Retry binding on port Q
        OS-->>HB: Binding succeeds
        App->>App: Write connection file with hb_port P
        Note over App,HB: Clients receive P but heartbeat listens on Q
    else With this change
        App->>HB: Start heartbeat thread and wait for binding
        HB->>OS: Bind heartbeat socket to P
        OS-->>HB: Address already in use
        HB->>OS: Retry binding on port Q
        OS-->>HB: Binding succeeds
        HB-->>App: Signal successful binding
        App->>App: Copy bound port Q into hb_port
        App->>App: Write connection file with hb_port Q
        Note over App,HB: Published port matches the bound heartbeat socket
    end

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