Daemon child: failed to set TCP_NODELAY on parent out-socket

Error message

Daemon child: failed to set TCP_NODELAY on parent out-socket: {}

What it means

Immediately after connecting to the parent's out-socket, the daemon child enables TCP_NODELAY for low-latency writes. set_nodelay failing at this point is an OS-level setsockopt error, which in practice only happens when the socket is already invalid/errored (parent closed it) or the platform refuses the option in the current network context.

Solutions

Retry the command (the race window is tiny and rarely repeats)
Run flow stop first to reset any half-spawned daemon state
If persistent in a sandbox, verify TCP socket options are supported and report to the sandbox/network layer owners

Defensive patterns

Strategy: retry

Prevention

Treat one-off handshake failures as transient; retry before debugging
In sandboxed runtimes, confirm TCP socket options like TCP_NODELAY are supported
Avoid killing the parent during daemon spawn windows

When it happens

Trigger: The parent closes the accepted socket in the microsecond window between the child's connect and setsockopt; exotic network stacks/sandboxes rejecting TCP_NODELAY on loopback sockets.

Common situations: Racing parent exit during daemon spawn; minimal kernel/sandbox network implementations; extremely rare on stock Linux/macOS/Windows.

Related errors

AI-assisted analysis of facebook/flow@f88ac94bcf (2026-08-20). Data as JSON: /api/errors/9f095bdcbc83368e. Report an issue: GitHub.

Appendix: source

Thrown at rust_port/crates/flow_daemon/src/daemon.rs:542

    let Some((entry_name, context)) = entry::get_context() else {
        return;
    };
    entry::clear_context();
    let entry::Context {
        parent_in_addr,
        parent_out_addr,
        token,
        param_bytes,
    } = context;

    let mut child_in_sock = TcpStream::connect(parent_out_addr).unwrap_or_else(|e| {
        panic!(
            "Daemon child: failed to connect to parent out-socket (child read end): {}",
            e
        )
    });
    child_in_sock.set_nodelay(true).unwrap_or_else(|e| {
        panic!(
            "Daemon child: failed to set TCP_NODELAY on parent out-socket: {}",
            e
        )
    });
    child_in_sock.write_all(&token).unwrap_or_else(|e| {
        panic!(
            "Daemon child: failed to write token to parent out-socket: {}",
            e
        )
    });
    let mut child_out_sock = TcpStream::connect(parent_in_addr).unwrap_or_else(|e| {
        panic!(
            "Daemon child: failed to connect to parent in-socket (child write end): {}",
            e
        )
    });
    child_out_sock.set_nodelay(true).unwrap_or_else(|e| {
        panic!(

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