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.

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!(

View on GitHub (pinned to f88ac94bcf)

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.