lesavka/server/src/video_sinks/mjpeg_spool.rs

use std::fs::{self, OpenOptions};
use std::io::Write;
use std::path::{Path, PathBuf};
use std::sync::atomic::{AtomicU64, Ordering};
use std::time::{SystemTime, UNIX_EPOCH};

use gstreamer as gst;
use gstreamer_app as gst_app;

static SPOOL_SEQUENCE: AtomicU64 = AtomicU64::new(1);
static SPOOL_TEMP_SEQUENCE: AtomicU64 = AtomicU64::new(1);
const MAX_MJPEG_FRAME_BYTES: usize = 8 * 1024 * 1024;
const DEFAULT_UVC_MJPEG_BUDGET_BYTES_PER_SEC: u32 = 10_000_000;
const DEFAULT_UVC_ISOCHRONOUS_LIMIT_PCT: u32 = 85;
const HIGH_SPEED_ISOCHRONOUS_MICROFRAMES_PER_SEC: u32 = 8_000;
const CONFIGFS_UVC_BASE: &str = "/sys/kernel/config/usb_gadget/lesavka/functions/uvc.usb0";

#[derive(Clone, Copy)]
pub(super) struct MjpegSpoolTiming {
    pub profile: &'static str,
    pub source_pts_us: Option<u64>,
    pub decoded_pts_us: Option<u64>,
}

impl MjpegSpoolTiming {
    /// Build metadata for direct MJPEG ingress.
    ///
    /// Inputs: the upstream packet PTS in microseconds. Output: timing metadata
    /// labeled as passthrough MJPEG. Why: direct MJPEG and decoded HEVC share
    /// the same spool file, so future diagnostics need to distinguish them.
    pub(super) fn mjpeg_passthrough(source_pts_us: u64) -> Self {
        Self {
            profile: "mjpeg-passthrough",
            source_pts_us: Some(source_pts_us),
            decoded_pts_us: None,
        }
    }

    /// Build metadata for direct MJPEG after local decode/re-encode.
    ///
    /// Inputs: the upstream packet PTS in microseconds. Output: timing metadata
    /// labeled as normalized MJPEG. Why: browser-visible UVC corruption can
    /// happen after a syntactically valid camera JPEG, so probes need to know
    /// when the server has intentionally emitted a clean re-encoded frame.
    pub(super) fn mjpeg_normalized(source_pts_us: u64) -> Self {
        Self {
            profile: "mjpeg-normalized",
            source_pts_us: Some(source_pts_us),
            decoded_pts_us: None,
        }
    }

    /// Build metadata for decoded HEVC entering the MJPEG UVC helper.
    ///
    /// Inputs: upstream packet PTS plus the decoded appsink buffer PTS.
    /// Output: timing metadata labeled as HEVC-decoded MJPEG. Why: the
    /// remaining HEVC sync jitter appears after transport, so we need a
    /// low-overhead marker at the decode-to-UVC handoff boundary.
    pub(super) fn hevc_decoded_mjpeg(source_pts_us: u64, decoded_pts_us: Option<u64>) -> Self {
        Self {
            profile: "hevc-decoded-mjpeg",
            source_pts_us: Some(source_pts_us),
            decoded_pts_us,
        }
    }
}

/// Decide whether the UVC helper file-spool path should own MJPEG emission.
///
/// Inputs: `LESAVKA_UVC_MJPEG_SPOOL`. Output: true unless explicitly disabled.
/// Why: the helper path prevents two processes from fighting over the UVC
/// gadget node, while preserving a direct `v4l2sink` fallback for diagnostics.
pub(super) fn mjpeg_spool_enabled() -> bool {
    std::env::var("LESAVKA_UVC_MJPEG_SPOOL")
        .ok()
        .map(|value| {
            let trimmed = value.trim();
            !(trimmed.eq_ignore_ascii_case("0")
                || trimmed.eq_ignore_ascii_case("false")
                || trimmed.eq_ignore_ascii_case("no")
                || trimmed.eq_ignore_ascii_case("off"))
        })
        .unwrap_or(true)
}

/// Resolve the frame path consumed by the UVC helper.
///
/// Inputs: `LESAVKA_UVC_FRAME_PATH`. Output: filesystem path for the newest
/// MJPEG frame. Why: the helper polls a single atomic frame file, so both direct
/// MJPEG and decoded HEVC output need to agree on the handoff location.
pub(super) fn mjpeg_spool_path() -> PathBuf {
    std::env::var("LESAVKA_UVC_FRAME_PATH")
        .map(PathBuf::from)
        .unwrap_or_else(|_| PathBuf::from("/run/lesavka-uvc-frame.mjpg"))
}

fn env_u32_opt(name: &str) -> Option<u32> {
    std::env::var(name)
        .ok()
        .and_then(|value| value.trim().parse::<u32>().ok())
}

fn env_flag_enabled(name: &str, default: bool) -> bool {
    std::env::var(name)
        .ok()
        .map(|value| {
            let trimmed = value.trim();
            if trimmed.eq_ignore_ascii_case("0")
                || trimmed.eq_ignore_ascii_case("false")
                || trimmed.eq_ignore_ascii_case("no")
                || trimmed.eq_ignore_ascii_case("off")
            {
                false
            } else if trimmed.eq_ignore_ascii_case("1")
                || trimmed.eq_ignore_ascii_case("true")
                || trimmed.eq_ignore_ascii_case("yes")
                || trimmed.eq_ignore_ascii_case("on")
            {
                true
            } else {
                default
            }
        })
        .unwrap_or(default)
}

fn read_u32_file(path: impl AsRef<Path>) -> Option<u32> {
    fs::read_to_string(path)
        .ok()
        .and_then(|value| value.trim().parse::<u32>().ok())
}

fn uvc_bulk_transfer_enabled() -> bool {
    if !env_flag_enabled("LESAVKA_UVC_BULK", true) {
        return false;
    }
    let base = Path::new(CONFIGFS_UVC_BASE);
    !(base.exists() && !base.join("streaming_bulk").exists())
}

fn uvc_streaming_maxpacket(bulk: bool) -> u32 {
    let mut maxpacket = env_u32_opt("LESAVKA_UVC_MAXPACKET").unwrap_or(1024);
    if let Some(live) = read_u32_file(Path::new(CONFIGFS_UVC_BASE).join("streaming_maxpacket")) {
        maxpacket = maxpacket.min(live);
    }
    if bulk {
        maxpacket.min(512)
    } else {
        maxpacket.min(1024)
    }
}

fn uvc_isochronous_budget_bytes_per_sec(maxpacket: u32) -> u32 {
    let pct = env_u32_opt("LESAVKA_UVC_ISOCHRONOUS_LIMIT_PCT")
        .unwrap_or(DEFAULT_UVC_ISOCHRONOUS_LIMIT_PCT)
        .clamp(1, 100);
    let bytes = u64::from(maxpacket)
        .saturating_mul(u64::from(HIGH_SPEED_ISOCHRONOUS_MICROFRAMES_PER_SEC))
        .saturating_mul(u64::from(pct))
        / 100;
    bytes.min(u64::from(u32::MAX)) as u32
}

fn effective_mjpeg_budget_bytes_per_sec() -> u32 {
    let configured = env_u32_opt("LESAVKA_UVC_MJPEG_BUDGET_BYTES_PER_SEC")
        .unwrap_or(DEFAULT_UVC_MJPEG_BUDGET_BYTES_PER_SEC)
        .max(1);
    if uvc_bulk_transfer_enabled() {
        configured
    } else {
        configured
            .min(uvc_isochronous_budget_bytes_per_sec(uvc_streaming_maxpacket(false)))
            .max(1)
    }
}

/// Resolve the MJPEG byte budget used before publishing to the helper.
///
/// Inputs: active FPS plus `LESAVKA_UVC_FRAME_MAX_BYTES`,
/// `LESAVKA_UVC_MJPEG_BUDGET_BYTES_PER_SEC`, and
/// `LESAVKA_UVC_FRAME_SIZE_GUARD`. Output: maximum accepted frame bytes.
/// Why: oversized MJPEG frames are a common source of host-visible UVC tearing;
/// a short freeze is better than letting the USB gadget emit partial pictures.
pub(super) fn mjpeg_spool_frame_max_bytes(fps: u32) -> usize {
    if !env_flag_enabled("LESAVKA_UVC_FRAME_SIZE_GUARD", true) {
        return MAX_MJPEG_FRAME_BYTES;
    }
    if let Some(limit) = env_u32_opt("LESAVKA_UVC_FRAME_MAX_BYTES")
        && limit > 0
    {
        return (limit as usize).min(MAX_MJPEG_FRAME_BYTES);
    }

    let fps = fps.max(1);
    let budget_per_sec = effective_mjpeg_budget_bytes_per_sec();
    let per_frame = (budget_per_sec / fps).max(64 * 1024);
    per_frame.min(MAX_MJPEG_FRAME_BYTES as u32) as usize
}

/// Decide whether frame spool metadata should be published.
///
/// Inputs: `LESAVKA_UVC_FRAME_META`. Output: false unless explicitly enabled.
/// Why: the metadata is useful for HEVC boundary diagnostics, but it adds one
/// extra atomic sidecar write per frame and should stay opt-in during calls.
pub(super) fn mjpeg_spool_metadata_enabled() -> bool {
    std::env::var("LESAVKA_UVC_FRAME_META")
        .ok()
        .map(|value| {
            let trimmed = value.trim();
            trimmed.eq_ignore_ascii_case("1")
                || trimmed.eq_ignore_ascii_case("true")
                || trimmed.eq_ignore_ascii_case("yes")
                || trimmed.eq_ignore_ascii_case("on")
        })
        .unwrap_or(false)
}

/// Resolve the metadata sidecar path for the UVC helper spool.
///
/// Inputs: frame path plus `LESAVKA_UVC_FRAME_META_PATH`. Output: sidecar path.
/// Why: keeping this path explicit lets capture scripts fetch timing evidence
/// without guessing where the virtual webcam helper found the frame.
pub(super) fn mjpeg_spool_metadata_path(frame_path: &Path) -> PathBuf {
    std::env::var("LESAVKA_UVC_FRAME_META_PATH")
        .map(PathBuf::from)
        .unwrap_or_else(|_| frame_path.with_extension("mjpg.meta.json"))
}

/// Resolve the optional JSONL metadata log for full-probe diagnostics.
///
/// Inputs: `LESAVKA_UVC_FRAME_META_LOG_PATH`. Output: an append-only log path
/// when configured. Why: a latest-frame sidecar is enough for spot checks, but
/// client-to-RCT HEVC probes need the whole decode/spool timing sequence.
pub(super) fn mjpeg_spool_metadata_log_path() -> Option<PathBuf> {
    std::env::var("LESAVKA_UVC_FRAME_META_LOG_PATH")
        .ok()
        .map(|value| value.trim().to_string())
        .filter(|value| !value.is_empty())
        .map(PathBuf::from)
}

/// Bound how long one HEVC handoff may wait for decoded MJPEG output.
///
/// Inputs: `LESAVKA_UVC_HEVC_SPOOL_PULL_TIMEOUT_MS`, clamped to 0..=50ms.
/// Output: the timeout used by appsink polling.
/// Why: decoded frames should be published when they are due, but the video
/// handoff worker must not build a WAN-sized backlog while waiting on decode.
pub(super) fn decoded_mjpeg_pull_timeout() -> gst::ClockTime {
    let timeout_ms = std::env::var("LESAVKA_UVC_HEVC_SPOOL_PULL_TIMEOUT_MS")
        .ok()
        .and_then(|value| value.trim().parse::<u64>().ok())
        .unwrap_or(20)
        .min(50);
    gst::ClockTime::from_mseconds(timeout_ms)
}

/// Drain the decoded-MJPEG appsink down to its freshest sample.
///
/// Inputs: the appsink owned by the HEVC-to-MJPEG branch. Output: the newest
/// available sample, if any. Why: the UVC helper should see the latest decoded
/// frame rather than letting stale decode output accumulate during CPU spikes.
#[cfg(not(coverage))]
pub(super) fn freshest_mjpeg_sample(sink: &gst_app::AppSink) -> Option<gst::Sample> {
    let mut newest = sink.try_pull_sample(decoded_mjpeg_pull_timeout());
    while let Some(sample) = sink.try_pull_sample(gst::ClockTime::ZERO) {
        newest = Some(sample);
    }
    newest
}

fn unix_now_ns() -> u128 {
    SystemTime::now()
        .duration_since(UNIX_EPOCH)
        .map(|duration| duration.as_nanos())
        .unwrap_or(0)
}

fn json_number_or_null(value: Option<u64>) -> String {
    value
        .map(|value| value.to_string())
        .unwrap_or_else(|| "null".to_string())
}

/// Atomically write a text sidecar beside the current frame.
///
/// Inputs: a destination path and complete text payload. Output: success or
/// filesystem error. Why: the latest-frame metadata sidecar should never be
/// observed half-written while RCT probe scripts are collecting artifacts.
fn write_atomic_text(path: &Path, data: &str) -> anyhow::Result<()> {
    if let Some(parent) = path.parent() {
        fs::create_dir_all(parent)?;
    }
    let tmp = path.with_extension(format!("json.{}.tmp", std::process::id()));
    fs::write(&tmp, data)?;
    fs::rename(&tmp, path)?;
    Ok(())
}

/// Append one timing record to the optional full-probe metadata log.
///
/// Inputs: a JSONL path and already formatted metadata record. Output: success
/// or filesystem error. Why: HEVC/RCT debugging needs every decoded-MJPEG
/// handoff timestamp, while the latest sidecar only preserves the newest frame.
fn append_metadata_log(path: &Path, record: &str) -> anyhow::Result<()> {
    if let Some(parent) = path.parent() {
        fs::create_dir_all(parent)?;
    }
    OpenOptions::new()
        .create(true)
        .append(true)
        .open(path)?
        .write_all(record.as_bytes())?;
    Ok(())
}

/// Render one metadata record for a spooled MJPEG frame.
///
/// Inputs: a sequence number, frame size, and timing labels. Output: compact
/// JSON suitable for sidecar artifacts. Why: keeping the format deterministic
/// makes later client-to-RCT scripts able to compare server decode/spool timing
/// against final RCT observations without parsing log prose.
pub(super) fn format_mjpeg_spool_metadata(
    sequence: u64,
    bytes: usize,
    timing: MjpegSpoolTiming,
) -> String {
    format!(
        "{{\"schema\":\"lesavka.uvc-mjpeg-spool-meta.v1\",\"sequence\":{},\"profile\":\"{}\",\"bytes\":{},\"source_pts_us\":{},\"decoded_pts_us\":{},\"spool_unix_ns\":{}}}\n",
        sequence,
        timing.profile,
        bytes,
        json_number_or_null(timing.source_pts_us),
        json_number_or_null(timing.decoded_pts_us),
        unix_now_ns()
    )
}

/// Atomically publish one MJPEG frame plus optional timing metadata.
///
/// Inputs: destination path, JPEG bytes, and optional timing metadata. Output:
/// success or filesystem error. Why: HEVC transport debugging needs to know
/// whether residual jitter happens before or after the decoded-MJPEG handoff,
/// while the default runtime path should remain identical when metadata is off.
pub(super) fn spool_mjpeg_frame_with_timing(
    path: &Path,
    data: &[u8],
    timing: Option<MjpegSpoolTiming>,
) -> anyhow::Result<()> {
    if let Some(parent) = path.parent() {
        fs::create_dir_all(parent)?;
    }
    let tmp = path.with_extension(format!(
        "mjpg.{}.{}.tmp",
        std::process::id(),
        SPOOL_TEMP_SEQUENCE.fetch_add(1, Ordering::Relaxed)
    ));
    fs::write(&tmp, data)?;
    fs::rename(&tmp, path)?;

    if mjpeg_spool_metadata_enabled()
        && let Some(timing) = timing
    {
        let sequence = SPOOL_SEQUENCE.fetch_add(1, Ordering::Relaxed);
        let record = format_mjpeg_spool_metadata(sequence, data.len(), timing);
        write_atomic_text(&mjpeg_spool_metadata_path(path), &record)?;
        if let Some(log_path) = mjpeg_spool_metadata_log_path() {
            append_metadata_log(&log_path, &record)?;
        }
    }

    Ok(())
}

#[cfg(test)]
mod tests {
    /// Verifies HEVC decoded-frame polling defaults to a freshness-first wait.
    ///
    /// Input: unset timeout env var. Output: 20ms appsink poll timeout. Why:
    /// server-side hardware decode/JPEG encode often lands inside one 30fps
    /// frame interval, and a 5ms poll was starving Meet-visible UVC output.
    #[test]
    fn decoded_mjpeg_pull_timeout_defaults_to_short_bounded_wait() {
        temp_env::with_var_unset("LESAVKA_UVC_HEVC_SPOOL_PULL_TIMEOUT_MS", || {
            assert_eq!(
                super::decoded_mjpeg_pull_timeout(),
                gstreamer::ClockTime::from_mseconds(20)
            );
        });
    }

    /// Verifies explicit HEVC spool polling overrides stay bounded.
    ///
    /// Input: zero and oversized timeout values. Output: direct zero polling
    /// and a 50ms safety cap. Why: lab tuning may need aggressive polling, but
    /// no override should recreate the multi-second decoded-frame backlog.
    #[test]
    fn decoded_mjpeg_pull_timeout_allows_fast_poll_and_clamps_slow_waits() {
        temp_env::with_var("LESAVKA_UVC_HEVC_SPOOL_PULL_TIMEOUT_MS", Some("0"), || {
            assert_eq!(
                super::decoded_mjpeg_pull_timeout(),
                gstreamer::ClockTime::from_mseconds(0)
            );
        });

        temp_env::with_var("LESAVKA_UVC_HEVC_SPOOL_PULL_TIMEOUT_MS", Some("250"), || {
            assert_eq!(
                super::decoded_mjpeg_pull_timeout(),
                gstreamer::ClockTime::from_mseconds(50)
            );
        });
    }

    #[test]
    fn mjpeg_spool_frame_budget_uses_live_budget_when_zero_or_unset() {
        temp_env::with_vars(
            [
                ("LESAVKA_UVC_FRAME_SIZE_GUARD", None::<&str>),
                ("LESAVKA_UVC_FRAME_MAX_BYTES", None::<&str>),
                ("LESAVKA_UVC_MJPEG_BUDGET_BYTES_PER_SEC", None::<&str>),
            ],
            || {
                assert_eq!(super::mjpeg_spool_frame_max_bytes(30), 333_333);
            },
        );

        temp_env::with_vars(
            [
                ("LESAVKA_UVC_FRAME_SIZE_GUARD", Some("1")),
                ("LESAVKA_UVC_FRAME_MAX_BYTES", Some("0")),
                ("LESAVKA_UVC_MJPEG_BUDGET_BYTES_PER_SEC", Some("9")),
            ],
            || {
                assert_eq!(super::mjpeg_spool_frame_max_bytes(30), 65_536);
            },
        );

        temp_env::with_vars(
            [
                ("LESAVKA_UVC_FRAME_SIZE_GUARD", Some("1")),
                ("LESAVKA_UVC_FRAME_MAX_BYTES", None::<&str>),
                ("LESAVKA_UVC_MJPEG_BUDGET_BYTES_PER_SEC", Some("10000000")),
                ("LESAVKA_UVC_BULK", Some("0")),
                ("LESAVKA_UVC_MAXPACKET", Some("1024")),
                ("LESAVKA_UVC_ISOCHRONOUS_LIMIT_PCT", None::<&str>),
            ],
            || {
                assert_eq!(super::mjpeg_spool_frame_max_bytes(30), 232_106);
            },
        );
    }

    #[test]
    fn mjpeg_spool_frame_budget_allows_explicit_limit_or_diagnostic_disable() {
        temp_env::with_vars(
            [
                ("LESAVKA_UVC_FRAME_SIZE_GUARD", Some("1")),
                ("LESAVKA_UVC_FRAME_MAX_BYTES", Some("123456")),
                ("LESAVKA_UVC_MJPEG_BUDGET_BYTES_PER_SEC", Some("1")),
            ],
            || {
                assert_eq!(super::mjpeg_spool_frame_max_bytes(30), 123_456);
            },
        );

        temp_env::with_vars(
            [
                ("LESAVKA_UVC_FRAME_SIZE_GUARD", Some("0")),
                ("LESAVKA_UVC_FRAME_MAX_BYTES", Some("123456")),
                ("LESAVKA_UVC_MJPEG_BUDGET_BYTES_PER_SEC", Some("1")),
            ],
            || {
                assert_eq!(
                    super::mjpeg_spool_frame_max_bytes(30),
                    super::MAX_MJPEG_FRAME_BYTES
                );
            },
        );
    }

    /// Verifies spool metadata remains opt-in and path-configurable.
    ///
    /// Input: default and explicit metadata env vars. Output: disabled by
    /// default plus deterministic sidecar path selection. Why: diagnostics must
    /// not add per-frame writes unless the operator asks for timing evidence.
    #[test]
    fn mjpeg_spool_metadata_is_opt_in_and_path_configurable() {
        temp_env::with_var_unset("LESAVKA_UVC_FRAME_META", || {
            assert!(!super::mjpeg_spool_metadata_enabled());
        });
        temp_env::with_var("LESAVKA_UVC_FRAME_META", Some("yes"), || {
            assert!(super::mjpeg_spool_metadata_enabled());
        });

        let frame = std::path::Path::new("/tmp/lesavka-frame.mjpg");
        temp_env::with_var_unset("LESAVKA_UVC_FRAME_META_PATH", || {
            assert_eq!(
                super::mjpeg_spool_metadata_path(frame),
                std::path::PathBuf::from("/tmp/lesavka-frame.mjpg.meta.json")
            );
        });
        temp_env::with_var(
            "LESAVKA_UVC_FRAME_META_PATH",
            Some("/tmp/custom-meta.json"),
            || {
                assert_eq!(
                    super::mjpeg_spool_metadata_path(frame),
                    std::path::PathBuf::from("/tmp/custom-meta.json")
                );
            },
        );

        temp_env::with_var_unset("LESAVKA_UVC_FRAME_META_LOG_PATH", || {
            assert_eq!(super::mjpeg_spool_metadata_log_path(), None);
        });
        temp_env::with_var("LESAVKA_UVC_FRAME_META_LOG_PATH", Some("  "), || {
            assert_eq!(super::mjpeg_spool_metadata_log_path(), None);
        });
    }

    /// Verifies metadata records carry enough timing evidence for RCT analysis.
    ///
    /// Input: HEVC-decoded spool timing. Output: JSON fields for source and
    /// decoded PTS. Why: future blind end-to-end probes need to tell whether a
    /// bad RCT result came from transport/decode or from the UVC helper/browser.
    #[test]
    fn mjpeg_spool_metadata_formats_timing_fields() {
        let record = super::format_mjpeg_spool_metadata(
            7,
            1234,
            super::MjpegSpoolTiming::hevc_decoded_mjpeg(42_000, Some(43_000)),
        );

        assert!(record.contains("\"schema\":\"lesavka.uvc-mjpeg-spool-meta.v1\""));
        assert!(record.contains("\"sequence\":7"));
        assert!(record.contains("\"profile\":\"hevc-decoded-mjpeg\""));
        assert!(record.contains("\"bytes\":1234"));
        assert!(record.contains("\"source_pts_us\":42000"));
        assert!(record.contains("\"decoded_pts_us\":43000"));
    }

    /// Verifies direct MJPEG metadata explicitly marks passthrough timing.
    ///
    /// Input: an upstream MJPEG packet PTS. Output: metadata with no decoded
    /// PTS. Why: direct MJPEG ingress must remain distinguishable from HEVC
    /// decode when later RCT timing evidence is compared across profiles.
    #[test]
    fn mjpeg_passthrough_metadata_uses_source_pts_and_null_decode_pts() {
        let record = super::format_mjpeg_spool_metadata(
            8,
            99,
            super::MjpegSpoolTiming::mjpeg_passthrough(55_000),
        );

        assert!(record.contains("\"profile\":\"mjpeg-passthrough\""));
        assert!(record.contains("\"source_pts_us\":55000"));
        assert!(record.contains("\"decoded_pts_us\":null"));
    }

    /// Verifies normalized direct-MJPEG handoffs are distinguishable.
    ///
    /// Input: an upstream MJPEG packet PTS after decode/re-encode. Output:
    /// metadata with the normalized profile marker. Why: RCT artifact probes
    /// need to separate raw passthrough from the safer browser-facing path.
    #[test]
    fn mjpeg_normalized_metadata_uses_source_pts_and_profile_marker() {
        let record = super::format_mjpeg_spool_metadata(
            9,
            101,
            super::MjpegSpoolTiming::mjpeg_normalized(66_000),
        );

        assert!(record.contains("\"profile\":\"mjpeg-normalized\""));
        assert!(record.contains("\"source_pts_us\":66000"));
        assert!(record.contains("\"decoded_pts_us\":null"));
    }

    /// Verifies frame spooling preserves default behavior unless metadata is enabled.
    ///
    /// Input: a temporary frame path plus disabled metadata env vars. Output:
    /// the frame file is atomically written and no sidecar appears. Why:
    /// diagnostics must not alter the normal UVC helper handoff during calls.
    #[test]
    fn spool_mjpeg_frame_writes_frame_without_default_sidecar() {
        let dir = tempfile::tempdir().expect("tempdir");
        let frame = dir.path().join("nested").join("frame.mjpg");
        let meta = frame.with_extension("mjpg.meta.json");

        temp_env::with_var_unset("LESAVKA_UVC_FRAME_META", || {
            super::spool_mjpeg_frame_with_timing(
                &frame,
                b"jpeg-bytes",
                Some(super::MjpegSpoolTiming::mjpeg_passthrough(10)),
            )
            .expect("spool frame");
        });

        assert_eq!(std::fs::read(&frame).expect("read frame"), b"jpeg-bytes");
        assert!(!meta.exists());
    }

    /// Verifies enabled frame metadata is atomically written beside the frame.
    ///
    /// Input: explicit metadata enablement, custom sidecar path, and HEVC
    /// timing. Output: both frame and sidecar are published. Why: this gives
    /// client-to-RCT probes a precise server decode/spool boundary without
    /// requiring invasive server logging.
    #[test]
    fn spool_mjpeg_frame_writes_enabled_sidecar_with_timing() {
        let dir = tempfile::tempdir().expect("tempdir");
        let frame = dir.path().join("frame.mjpg");
        let meta = dir.path().join("frame-meta.json");
        let log = dir.path().join("frames.jsonl");

        temp_env::with_vars(
            [
                ("LESAVKA_UVC_FRAME_META", Some("on")),
                (
                    "LESAVKA_UVC_FRAME_META_PATH",
                    Some(meta.to_str().expect("utf8 path")),
                ),
                (
                    "LESAVKA_UVC_FRAME_META_LOG_PATH",
                    Some(log.to_str().expect("utf8 path")),
                ),
            ],
            || {
                super::spool_mjpeg_frame_with_timing(
                    &frame,
                    b"decoded-jpeg",
                    Some(super::MjpegSpoolTiming::hevc_decoded_mjpeg(
                        100_000,
                        Some(101_000),
                    )),
                )
                .expect("spool frame with metadata");
            },
        );

        assert_eq!(std::fs::read(&frame).expect("read frame"), b"decoded-jpeg");
        let record = std::fs::read_to_string(&meta).expect("read metadata");
        assert!(record.contains("\"profile\":\"hevc-decoded-mjpeg\""));
        assert!(record.contains("\"bytes\":12"));
        assert!(record.contains("\"source_pts_us\":100000"));
        assert!(record.contains("\"decoded_pts_us\":101000"));

        let log_record = std::fs::read_to_string(&log).expect("read metadata log");
        assert_eq!(log_record.lines().count(), 1);
        assert!(log_record.contains("\"profile\":\"hevc-decoded-mjpeg\""));
    }
}