facebook/flow · error

Cannot get hostname

Error message

Cannot get hostname

What it means

Rollout bucketing in flowconfig parsing seeds a hasher with hostname + uid + rollout name via `hostname::get().expect("Cannot get hostname")`. `hostname::get()` performs an OS gethostname call; it errors when the syscall fails (rare; broken container/namespace setup) or when the resulting name is not usable as configured, and the expect kills every flow command during config parsing since the rollout percentage is computed eagerly.

Source

Thrown at rust_port/crates/flow_config/src/flowconfig.rs:3124

fn parse_strict(config: &mut FlowConfig, lines: &[(u32, String)]) -> Result<(), Error> {
    let lines = trim_numbered_lines(lines);
    let strict_mode =
        StrictModeSettings::of_lines(&lines).map_err(|(line, msg)| Error(line, msg))?;
    config.strict_mode = strict_mode;
    Ok(())
}

// Rollouts are based on randomness, but we want it to be stable from run to run. So we seed our
// pseudo random number generator with
//
// 1. The hostname
// 2. The user
// 3. The name of the rollout
fn calculate_rollout_percentage(rollout_name: &str) -> u32 {
    use std::hash::Hash;
    use std::hash::Hasher;
    let host_name = hostname::get().expect("Cannot get hostname");
    let mut hasher = DefaultHasher::new();
    host_name.hash(&mut hasher);
    #[cfg(unix)]
    {
        unsafe { libc::getuid() }.hash(&mut hasher);
    }
    rollout_name.hash(&mut hasher);
    (hasher.finish() % 100) as u32
}

// The optional rollout section has 0 or more lines. Each line defines a single rollout. For example
//
// [rollouts]
//
// testA=40% on, 60% off
// testB=50% blue, 20% yellow, 30% pink
//
// The first line defines a rollout named "testA" with two groups.

View on GitHub (pinned to 5c86586199)

Solutions

1. Set a valid hostname and retry: `hostnamectl set-hostname myhost` (or docker `--hostname`, k8s hostname field), or `sudo sysctl kernel.hostname=myhost`.
2. Verify the current hostname is sane: `cat /proc/sys/kernel/hostname` should print valid UTF-8.
3. Remove or rename the `[rollout]` entries in .flowconfig if rollout gating is not needed, so this code path is skipped.
4. As a code fix, use `to_string_lossy()` or a fixed fallback ("unknown-host") instead of expect.

Example fix

// before
let host_name = hostname::get().expect("Cannot get hostname");

// after
let host_name = hostname::get()
    .map(|h| h.to_string_lossy().into_owned())
    .unwrap_or_else(|_| "unknown-host".to_string());

// Check the hostname is retrievable and UTF-8 before running flow commands
fn hostname_ok() -> bool {
    hostname::get().map(|h| h.to_str().is_some()).unwrap_or(false)
}

let host_name = hostname::get()
    .map(|h| h.to_string_lossy().into_owned())
    .unwrap_or_else(|_| "unknown-host".to_string());

When it happens

Trigger: A hostname containing invalid bytes (provisioning tools setting raw bytes via sysctl); a container with a broken/uninitialized hostname; hosts where gethostname fails under a restrictive sandbox.

Common situations: Containers with unusual hostname setups; VMs/images where /etc/hostname or the kernel hostname was set to invalid data; minimal environments after botched provisioning.

Related errors

• init failed: {:?}
• max_workers should be positive
• Unsupported .flowconfig option `log.file`. The VS Code exten
• flow-dot-js wasm requires crypto.getRandomValues
• Unable to determine executable path: {}

AI-assisted analysis of facebook/flow@5c86586199 (2026-08-20). Data as JSON: /api/errors/2c898fba1e60918c. Report an issue: GitHub.