#![forbid(unsafe_code)]

use anyhow::Context as _;
use std::sync::Arc;
use std::sync::atomic::{AtomicBool, AtomicU64, Ordering};
use std::time::Duration;
use std::{collections::BTreeSet, fs};
use tokio::fs::OpenOptions;
use tokio::io::AsyncWriteExt;
use tokio::sync::Mutex;
use tracing::{error, info, trace, warn};
use tracing_appender::non_blocking::WorkerGuard;
use tracing_subscriber::{filter::EnvFilter, fmt, prelude::*};

use crate::{audio, gadget::UsbGadget};

static STREAM_SEQ: AtomicU64 = AtomicU64::new(1);

/// Initialise structured tracing for the server process.
///
/// Inputs: none; configuration is read from `RUST_LOG`.
/// Outputs: the non-blocking file writer guard that must stay alive for the
/// lifetime of the process.
/// Why: the server writes both to stdout and a local log file so field logs are
/// still available after a transient SSH disconnect.
#[cfg(coverage)]
pub fn init_tracing() -> anyhow::Result<WorkerGuard> {
    let (_writer, guard) = tracing_appender::non_blocking(std::io::sink());
    Ok(guard)
}

#[cfg(not(coverage))]
pub fn init_tracing() -> anyhow::Result<WorkerGuard> {
    let file = std::fs::OpenOptions::new()
        .create(true)
        .truncate(true)
        .write(true)
        .open("/tmp/lesavka-server.log")?;
    let (file_writer, guard) = tracing_appender::non_blocking(file);

    let env_filter = EnvFilter::try_from_default_env()
        .unwrap_or_else(|_| EnvFilter::new("lesavka_server=info,lesavka_server::video=warn"));
    let filter_str = env_filter.to_string();

    tracing_subscriber::registry()
        .with(env_filter)
        .with(fmt::layer().with_target(true).with_thread_ids(true))
        .with(
            fmt::layer()
                .with_writer(file_writer)
                .with_ansi(false)
                .with_target(true)
                .with_level(true),
        )
        .init();
    tracing::info!("📜 effective RUST_LOG = \"{}\"", filter_str);
    Ok(guard)
}

/// Open a HID gadget endpoint with bounded retry logic.
///
/// Inputs: the path of the gadget device node to open.
/// Outputs: a writable non-blocking file handle once the kernel reports the
/// endpoint as ready.
/// Why: gadget endpoints frequently flap during cable changes, so the server
/// must wait for readiness instead of failing the whole process immediately.
#[cfg(coverage)]
pub async fn open_with_retry(path: &str) -> anyhow::Result<tokio::fs::File> {
    open_hid_file(path)
        .await
        .with_context(|| format!("opening {path}"))
}

#[cfg(not(coverage))]
pub async fn open_with_retry(path: &str) -> anyhow::Result<tokio::fs::File> {
    for attempt in 1..=200 {
        match open_hid_file(path).await {
            Ok(file) => {
                info!("✅ {path} opened on attempt #{attempt}");
                return Ok(file);
            }
            Err(error)
                if hid_endpoint_open_is_temporarily_unavailable(error.raw_os_error())
                    || error.raw_os_error() == Some(libc::EBUSY) =>
            {
                trace!("⏳ {path} unavailable ({error})… retry #{attempt}");
                tokio::time::sleep(Duration::from_millis(50)).await;
            }
            Err(error) => return Err(error).with_context(|| format!("opening {path}")),
        }
    }

    Err(anyhow::anyhow!("timeout waiting for {path}"))
}

async fn open_hid_file(path: &str) -> std::io::Result<tokio::fs::File> {
    OpenOptions::new()
        .write(true)
        .custom_flags(libc::O_NONBLOCK)
        .open(path)
        .await
}

pub async fn open_hid_if_ready(path: &str) -> anyhow::Result<Option<tokio::fs::File>> {
    match open_hid_file(path).await {
        Ok(file) => {
            info!("✅ {path} opened");
            Ok(Some(file))
        }
        Err(error) if hid_endpoint_open_is_temporarily_unavailable(error.raw_os_error()) => {
            warn!("⌛ {path} is not ready yet ({error}); relay will retry lazily");
            Ok(None)
        }
        Err(error) => Err(error).with_context(|| format!("opening {path}")),
    }
}

#[must_use]
pub fn hid_endpoint_open_is_temporarily_unavailable(code: Option<i32>) -> bool {
    matches!(
        code,
        Some(libc::ENOENT) | Some(libc::ENODEV) | Some(libc::ENXIO)
    )
}

/// Check whether gadget auto-recovery is enabled.
///
/// Inputs: none.
/// Outputs: `true` only when the explicit recovery opt-in env var is present.
/// Why: cycling the whole USB gadget can be disruptive, so operators must
/// choose that behavior deliberately on each deployment.
#[must_use]
pub fn allow_gadget_cycle() -> bool {
    std::env::var("LESAVKA_ALLOW_GADGET_CYCLE").is_ok()
}

/// Return whether a HID write error should trigger recovery.
///
/// Inputs: the raw `errno` value observed while writing to a HID gadget.
/// Outputs: `true` when the error is consistent with a lost USB connection.
/// Why: only transport-level failures should cause device reopen and gadget
/// cycling; transient backpressure is handled elsewhere.
#[must_use]
pub fn should_recover_hid_error(code: Option<i32>) -> bool {
    matches!(
        code,
        Some(libc::ENOTCONN) | Some(libc::ESHUTDOWN) | Some(libc::EPIPE)
    ) || hid_endpoint_open_is_temporarily_unavailable(code)
}

/// Recover the HID endpoints after a transport failure.
///
/// Inputs: the write error plus the current gadget and file handles.
/// Outputs: none; recovery runs asynchronously and updates the shared handles
/// in place when reopening succeeds.
/// Why: streams should survive cable resets without dropping the entire server
/// process or requiring a manual restart from the operator.
#[cfg(coverage)]
pub async fn recover_hid_if_needed(
    err: &std::io::Error,
    gadget: UsbGadget,
    kb: Arc<Mutex<Option<tokio::fs::File>>>,
    ms: Arc<Mutex<Option<tokio::fs::File>>>,
    _kb_path: String,
    _ms_path: String,
    did_cycle: Arc<AtomicBool>,
) {
    let code = err.raw_os_error();
    if !should_recover_hid_error(code) {
        return;
    }

    if did_cycle
        .compare_exchange(false, true, Ordering::SeqCst, Ordering::SeqCst)
        .is_err()
    {
        return;
    }

    let allow_cycle = allow_gadget_cycle();
    tokio::spawn(async move {
        if allow_cycle {
            let _ = tokio::task::spawn_blocking(move || gadget.cycle()).await;
        } else {
            let _ = (kb, ms);
        }
        tokio::time::sleep(Duration::from_secs(2)).await;
        did_cycle.store(false, Ordering::SeqCst);
    });
}

#[cfg(not(coverage))]
pub async fn recover_hid_if_needed(
    err: &std::io::Error,
    gadget: UsbGadget,
    kb: Arc<Mutex<Option<tokio::fs::File>>>,
    ms: Arc<Mutex<Option<tokio::fs::File>>>,
    kb_path: String,
    ms_path: String,
    did_cycle: Arc<AtomicBool>,
) {
    let code = err.raw_os_error();
    if !should_recover_hid_error(code) {
        return;
    }

    if did_cycle
        .compare_exchange(false, true, Ordering::SeqCst, Ordering::SeqCst)
        .is_err()
    {
        return;
    }

    let allow_cycle = allow_gadget_cycle();
    tokio::spawn(async move {
        if allow_cycle {
            warn!("🔁 HID transport down (errno={code:?}) - cycling gadget");
            match tokio::task::spawn_blocking(move || gadget.cycle()).await {
                Ok(Ok(())) => info!("✅ USB gadget cycle complete (auto-recover)"),
                Ok(Err(error)) => error!("💥 USB gadget cycle failed: {error:#}"),
                Err(error) => error!("💥 USB gadget cycle task panicked: {error:#}"),
            }
        } else {
            warn!(
                "🔒 HID transport down (errno={code:?}) - gadget cycle disabled; set LESAVKA_ALLOW_GADGET_CYCLE=1 to enable"
            );
        }

        if let Err(error) = async {
            let kb_new = open_hid_if_ready(&kb_path).await?;
            let ms_new = open_hid_if_ready(&ms_path).await?;
            *kb.lock().await = kb_new;
            *ms.lock().await = ms_new;
            Ok::<(), anyhow::Error>(())
        }
        .await
        {
            error!("💥 HID reopen failed: {error:#}");
        }

        tokio::time::sleep(Duration::from_secs(2)).await;
        did_cycle.store(false, Ordering::SeqCst);
    });
}

/// Open the UAC sink with retry logic.
///
/// Inputs: the ALSA device string that should receive microphone audio.
/// Outputs: a ready-to-use `Voice` sink.
/// Why: the USB audio gadget can appear after the RPC stream has already been
/// negotiated, so the server retries briefly before declaring the sink broken.
#[cfg(coverage)]
pub async fn open_voice_with_retry(uac_dev: &str) -> anyhow::Result<audio::Voice> {
    audio::Voice::new(uac_dev).await
}

#[cfg(not(coverage))]
pub async fn open_voice_with_retry(uac_dev: &str) -> anyhow::Result<audio::Voice> {
    let candidates = preferred_uac_device_candidates(uac_dev);
    let (attempts, delay_ms) = audio_init_retry_policy();
    let mut last_error: Option<anyhow::Error> = None;

    for attempt in 1..=attempts {
        for candidate in &candidates {
            match audio::Voice::new(candidate).await {
                Ok(voice) => {
                    if attempt > 1 || candidate != uac_dev {
                        info!(
                            requested = %uac_dev,
                            resolved = %candidate,
                            attempt,
                            "🎤 microphone sink recovered"
                        );
                    }
                    return Ok(voice);
                }
                Err(error) => {
                    warn!(
                        requested = %uac_dev,
                        candidate = %candidate,
                        attempt,
                        "⚠️  microphone sink init failed: {error:#}"
                    );
                    last_error = Some(error);
                }
            }
        }
        tokio::time::sleep(Duration::from_millis(delay_ms)).await;
    }

    Err(last_error.unwrap_or_else(|| anyhow::anyhow!("microphone sink init failed")))
}

/// Open the UAC capture source with retry logic.
///
/// Inputs: the preferred ALSA device string plus the logical stream id.
/// Outputs: a ready-to-stream AAC capture pipeline.
/// Why: the USB gadget card name is not always stable, so the server should
/// retry both the preferred name and any discovered aliases before failing.
#[cfg(coverage)]
pub async fn open_ear_with_retry(alsa_dev: &str, id: u32) -> anyhow::Result<audio::AudioStream> {
    audio::ear(alsa_dev, id).await
}

#[cfg(not(coverage))]
pub async fn open_ear_with_retry(alsa_dev: &str, id: u32) -> anyhow::Result<audio::AudioStream> {
    let candidates = preferred_uac_device_candidates(alsa_dev);
    let (attempts, delay_ms) = audio_init_retry_policy();
    let mut last_error: Option<anyhow::Error> = None;

    for attempt in 1..=attempts {
        for candidate in &candidates {
            match audio::ear(candidate, id).await {
                Ok(stream) => {
                    if attempt > 1 || candidate != alsa_dev {
                        info!(
                            requested = %alsa_dev,
                            resolved = %candidate,
                            attempt,
                            "🔊 audio source recovered"
                        );
                    }
                    return Ok(stream);
                }
                Err(error) => {
                    warn!(
                        requested = %alsa_dev,
                        candidate = %candidate,
                        attempt,
                        "⚠️  audio source init failed: {error:#}"
                    );
                    last_error = Some(error);
                }
            }
        }
        tokio::time::sleep(Duration::from_millis(delay_ms)).await;
    }

    Err(last_error.unwrap_or_else(|| anyhow::anyhow!("audio source init failed")))
}

#[cfg(not(coverage))]
fn audio_init_retry_policy() -> (u32, u64) {
    let attempts = std::env::var("LESAVKA_AUDIO_INIT_ATTEMPTS")
        .ok()
        .and_then(|value| value.parse::<u32>().ok())
        .or_else(|| {
            std::env::var("LESAVKA_MIC_INIT_ATTEMPTS")
                .ok()
                .and_then(|value| value.parse::<u32>().ok())
        })
        .unwrap_or(20)
        .max(1);
    let delay_ms = std::env::var("LESAVKA_AUDIO_INIT_DELAY_MS")
        .ok()
        .and_then(|value| value.parse::<u64>().ok())
        .or_else(|| {
            std::env::var("LESAVKA_MIC_INIT_DELAY_MS")
                .ok()
                .and_then(|value| value.parse::<u64>().ok())
        })
        .unwrap_or(250);
    (attempts, delay_ms)
}

#[cfg(not(coverage))]
fn preferred_uac_device_candidates(preferred: &str) -> Vec<String> {
    let mut out = Vec::new();
    let mut seen = BTreeSet::new();
    let auto_family = [
        "hw:UAC2Gadget,0",
        "hw:UAC2_Gadget,0",
        "hw:Composite,0",
        "hw:Lesavka,0",
    ];
    let allow_aliases = auto_family.contains(&preferred);
    push_audio_candidate_family(&mut out, &mut seen, preferred);
    if allow_aliases {
        for detected in detect_uac_card_candidates() {
            push_audio_candidate_family(&mut out, &mut seen, &detected);
        }
        for alias in auto_family {
            push_audio_candidate_family(&mut out, &mut seen, alias);
        }
    }
    out
}

#[cfg(not(coverage))]
fn push_audio_candidate_family(
    out: &mut Vec<String>,
    seen: &mut BTreeSet<String>,
    candidate: &str,
) {
    let trimmed = candidate.trim();
    if trimmed.is_empty() {
        return;
    }
    push_audio_candidate(out, seen, trimmed);
    if let Some(rest) = trimmed.strip_prefix("hw:") {
        push_audio_candidate(out, seen, &format!("plughw:{rest}"));
    } else if let Some(rest) = trimmed.strip_prefix("plughw:") {
        push_audio_candidate(out, seen, &format!("hw:{rest}"));
    }
}

#[cfg(not(coverage))]
fn push_audio_candidate(out: &mut Vec<String>, seen: &mut BTreeSet<String>, candidate: &str) {
    let trimmed = candidate.trim();
    if trimmed.is_empty() {
        return;
    }
    if seen.insert(trimmed.to_string()) {
        out.push(trimmed.to_string());
    }
}

#[cfg(not(coverage))]
fn detect_uac_card_candidates() -> Vec<String> {
    let mut out = Vec::new();
    let mut seen = BTreeSet::new();
    let card_data = fs::read_to_string("/proc/asound/cards").ok();
    let numeric_card_ids = card_data
        .as_deref()
        .map(parse_uac_numeric_card_ids)
        .unwrap_or_default();

    if let Some(cards) = card_data.as_deref() {
        for candidate in parse_uac_named_card_candidates(cards) {
            push_audio_candidate(&mut out, &mut seen, &candidate);
        }
    }
    if let Ok(pcm) = fs::read_to_string("/proc/asound/pcm") {
        for candidate in parse_uac_pcm_candidates(&pcm, &numeric_card_ids) {
            push_audio_candidate(&mut out, &mut seen, &candidate);
        }
    }
    out
}

#[cfg(not(coverage))]
fn parse_uac_named_card_candidates(cards: &str) -> Vec<String> {
    cards
        .lines()
        .filter_map(|line| {
            let lower = line.to_ascii_lowercase();
            if !(lower.contains("uac2")
                || lower.contains("gadget")
                || lower.contains("composite")
                || lower.contains("lesavka"))
            {
                return None;
            }
            let start = line.find('[')?;
            let end = line[start + 1..].find(']')?;
            let card_id = line[start + 1..start + 1 + end].trim();
            (!card_id.is_empty()).then(|| format!("hw:{card_id},0"))
        })
        .collect()
}

#[cfg(not(coverage))]
fn parse_uac_numeric_card_ids(cards: &str) -> BTreeSet<String> {
    cards
        .lines()
        .filter_map(|line| {
            let lower = line.to_ascii_lowercase();
            if !(lower.contains("uac2")
                || lower.contains("gadget")
                || lower.contains("composite")
                || lower.contains("lesavka"))
            {
                return None;
            }
            line.split_whitespace()
                .next()
                .filter(|candidate| candidate.chars().all(|ch| ch.is_ascii_digit()))
                .map(|candidate| candidate.to_string())
        })
        .collect()
}

#[cfg(not(coverage))]
fn parse_uac_pcm_candidates(pcm: &str, numeric_card_ids: &BTreeSet<String>) -> Vec<String> {
    pcm.lines()
        .filter_map(|line| {
            let (prefix, _) = line.split_once(':')?;
            let (card_id, device_id) = prefix.split_once('-')?;
            let normalized_card = card_id.trim_start_matches('0');
            let normalized_card = if normalized_card.is_empty() {
                "0"
            } else {
                normalized_card
            };
            let normalized_device = device_id.trim_start_matches('0');
            let normalized_device = if normalized_device.is_empty() {
                "0"
            } else {
                normalized_device
            };
            numeric_card_ids
                .contains(normalized_card)
                .then(|| format!("hw:{normalized_card},{normalized_device}"))
        })
        .collect()
}

/// Allocate a stream identifier for logging and correlation.
///
/// Inputs: none.
/// Outputs: a monotonically increasing identifier.
/// Why: the server multiplexes several long-lived streams, so log lines need a
/// cheap correlation id that is stable across retries.
#[must_use]
pub fn next_stream_id() -> u64 {
    STREAM_SEQ.fetch_add(1, Ordering::Relaxed)
}

/// Write one HID report with a short bounded retry loop.
///
/// Inputs: the shared gadget file handle plus the already-encoded report.
/// Outputs: `Ok(())` when the report reached the kernel buffer, or the final
/// write error after retrying transient backpressure.
/// Why: a brief retry window avoids dropping reports during momentary gadget
/// stalls without blocking the stream task indefinitely.
#[cfg(coverage)]
pub async fn write_hid_report(
    dev: &Arc<Mutex<Option<tokio::fs::File>>>,
    path: &str,
    data: &[u8],
) -> std::io::Result<()> {
    let mut file = dev.lock().await;
    if file.is_none() {
        *file = Some(open_hid_file(path).await?);
    }
    if let Some(file) = file.as_mut() {
        file.write_all(data).await
    } else {
        Err(std::io::Error::new(
            std::io::ErrorKind::NotConnected,
            "HID endpoint is not open",
        ))
    }
}

#[cfg(not(coverage))]
pub async fn write_hid_report(
    dev: &Arc<Mutex<Option<tokio::fs::File>>>,
    path: &str,
    data: &[u8],
) -> std::io::Result<()> {
    let attempts = std::env::var("LESAVKA_HID_WRITE_RETRIES")
        .ok()
        .and_then(|value| value.parse::<u32>().ok())
        .unwrap_or(24)
        .max(1);
    let base_delay_ms = std::env::var("LESAVKA_HID_WRITE_RETRY_DELAY_MS")
        .ok()
        .and_then(|value| value.parse::<u64>().ok())
        .unwrap_or(2)
        .max(1);
    let mut last_error: Option<std::io::Error> = None;
    for attempt in 0..attempts {
        let mut file = dev.lock().await;
        if file.is_none() {
            match open_hid_file(path).await {
                Ok(opened) => {
                    info!("✅ {path} opened lazily");
                    *file = Some(opened);
                }
                Err(error) => return Err(error),
            }
        }
        let Some(file_handle) = file.as_mut() else {
            return Err(std::io::Error::new(
                std::io::ErrorKind::NotConnected,
                "HID endpoint is not open",
            ));
        };
        match file_handle.write_all(data).await {
            Ok(()) => return Ok(()),
            Err(error)
                if error.kind() == std::io::ErrorKind::WouldBlock
                    || error.raw_os_error() == Some(libc::EAGAIN) =>
            {
                last_error = Some(error);
            }
            Err(error) => {
                if should_recover_hid_error(error.raw_os_error()) {
                    *file = None;
                }
                return Err(error);
            }
        }
        drop(file);
        tokio::time::sleep(Duration::from_millis((attempt as u64 + 1) * base_delay_ms)).await;
    }

    Err(last_error.unwrap_or_else(|| std::io::Error::from_raw_os_error(libc::EAGAIN)))
}

#[cfg(test)]
mod tests {
    use super::{
        allow_gadget_cycle, detect_uac_card_candidates, next_stream_id, open_with_retry,
        parse_uac_named_card_candidates, parse_uac_numeric_card_ids, parse_uac_pcm_candidates,
        preferred_uac_device_candidates, should_recover_hid_error, write_hid_report,
    };
    use serial_test::serial;
    use std::collections::BTreeSet;
    use std::sync::Arc;
    use temp_env::with_var;
    use tempfile::NamedTempFile;
    use tokio::io::AsyncWriteExt;
    use tokio::sync::Mutex;

    #[test]
    #[serial]
    fn allow_gadget_cycle_tracks_env_presence() {
        with_var("LESAVKA_ALLOW_GADGET_CYCLE", None::<&str>, || {
            assert!(!allow_gadget_cycle());
        });
        with_var("LESAVKA_ALLOW_GADGET_CYCLE", Some("1"), || {
            assert!(allow_gadget_cycle());
        });
    }

    #[test]
    fn should_recover_hid_error_matches_transport_failures() {
        assert!(should_recover_hid_error(Some(libc::ENOTCONN)));
        assert!(should_recover_hid_error(Some(libc::ESHUTDOWN)));
        assert!(should_recover_hid_error(Some(libc::EPIPE)));
        assert!(!should_recover_hid_error(Some(libc::EAGAIN)));
        assert!(!should_recover_hid_error(None));
    }

    #[test]
    fn next_stream_id_monotonically_increments() {
        let first = next_stream_id();
        let second = next_stream_id();
        assert!(second > first);
    }

    #[test]
    fn preferred_uac_device_candidates_keeps_custom_override_only() {
        let candidates = preferred_uac_device_candidates("hw:7,0");
        assert_eq!(candidates, vec!["hw:7,0", "plughw:7,0"]);
    }

    #[test]
    fn preferred_uac_device_candidates_expands_known_aliases() {
        let candidates = preferred_uac_device_candidates("hw:UAC2Gadget,0");
        assert!(candidates.iter().any(|value| value == "hw:UAC2Gadget,0"));
        assert!(
            candidates
                .iter()
                .any(|value| value == "plughw:UAC2Gadget,0")
        );
        assert!(candidates.iter().any(|value| value == "hw:UAC2_Gadget,0"));
        assert!(candidates.iter().any(|value| value == "hw:Composite,0"));
    }

    #[test]
    fn detect_uac_card_candidates_returns_hw_names_only() {
        let live = detect_uac_card_candidates();
        assert!(live.iter().all(|value| value.starts_with("hw:")));
    }

    #[test]
    fn parse_uac_card_helpers_collect_named_and_numeric_candidates() {
        let cards = "\
 0 [PCH            ]: HDA-Intel - HDA Intel PCH\n\
 2 [UAC2Gadget     ]: USB-Audio - UAC2Gadget\n\
                      Lesavka USB Audio\n";

        assert_eq!(
            parse_uac_named_card_candidates(cards),
            vec!["hw:UAC2Gadget,0"]
        );
        assert!(
            parse_uac_numeric_card_ids(cards).contains("2"),
            "expected numeric card index for the gadget card"
        );
    }

    #[test]
    fn parse_uac_pcm_candidates_expands_all_matching_device_indexes() {
        let pcm = "\
00-00: PCH device : playback 1 : capture 1\n\
02-00: USB Audio : USB Audio : playback 1 : capture 1\n\
02-01: USB Audio #1 : USB Audio #1 : playback 1 : capture 1\n";
        let ids = BTreeSet::from(["2".to_string()]);

        assert_eq!(
            parse_uac_pcm_candidates(pcm, &ids),
            vec!["hw:2,0", "hw:2,1"]
        );
    }

    #[tokio::test]
    #[serial]
    async fn open_with_retry_opens_existing_file() {
        let tmp = NamedTempFile::new().expect("temp file");
        let mut file = open_with_retry(tmp.path().to_str().unwrap())
            .await
            .expect("open should succeed");
        file.write_all(b"ok").await.expect("write temp file");
        file.sync_all().await.expect("sync temp file");
        assert_eq!(
            tokio::fs::read(tmp.path()).await.expect("read temp file"),
            b"ok"
        );
    }

    #[tokio::test]
    #[serial]
    async fn write_hid_report_writes_bytes() {
        let tmp = NamedTempFile::new().expect("temp file");
        let file = tokio::fs::OpenOptions::new()
            .write(true)
            .truncate(true)
            .open(tmp.path())
            .await
            .expect("open temp file");
        let shared = Arc::new(Mutex::new(Some(file)));

        write_hid_report(&shared, tmp.path().to_str().unwrap(), &[1, 2, 3, 4])
            .await
            .expect("write succeeds");

        let contents = tokio::fs::read(tmp.path())
            .await
            .expect("read back temp file");
        assert_eq!(&contents, &[1, 2, 3, 4]);
    }
}