use std::{
    fs,
    path::{Path, PathBuf},
    process::Command,
    time::Duration,
};

#[cfg(unix)]
use std::os::unix::fs::PermissionsExt;

use anyhow::Context;

use crate::{
    config::Config,
    fstab::{self, FSTAB_PATH, FstabTransaction, escape_field},
    lock::FileLock,
    managed_file::{ManagedFile, Ownership},
};

#[derive(Debug, Clone, PartialEq, Eq)]
pub struct ActiveSwap {
    pub name: PathBuf,
    pub kind: String,
    pub size_bytes: u64,
    pub priority: i32,
}

#[derive(Debug, Clone)]
struct DesiredSwap {
    path: PathBuf,
    size_bytes: u64,
    priority: i32,
    swappiness: u8,
}

#[derive(Debug, Clone)]
pub struct SwapStatus {
    pub enabled: bool,
    pub desired_path: PathBuf,
    pub active_entries: Vec<ActiveSwap>,
    pub desired_active: bool,
    pub exclusive: bool,
    pub fstab_swap_entries: Vec<String>,
    pub fstab_exclusive: bool,
    pub current_swappiness: Option<u8>,
    pub persisted_swappiness: Option<u8>,
    pub drift_reason: Option<String>,
}

/// Root-only directory holding this package's persistent state.
pub const STATE_DIR: &str = "/var/lib/awg2-harness";
const STATE_DIR_PREFIX: &str = "/var/lib/awg2-harness/";
/// Historical location, kept so existing deployments do not need a config change.
const DEFAULT_SWAP_PATH: &str = "/swapfile";
/// Records which swapfile we created, by identity rather than by location.
const SWAP_OWNERSHIP_PATH: &str = "/var/lib/awg2-harness/swap-ownership.conf";
const SWAP_OWNERSHIP_MARKER: &str = "# Managed by awg2-harness swap-ownership";
const SWAP_OWNERSHIP_VERSION: u32 = 1;
/// Sidecar written by earlier versions next to the swapfile. Only ever removed, and only
/// when it carries the marker that version wrote.
const LEGACY_OWNERSHIP_SUFFIX: &str = ".awg2-harness-owned";
const LEGACY_OWNERSHIP_MARKER: &str = "# Managed by awg2-harness swap";
/// Replacement being built while the current swapfile is still active.
const STAGING_SUFFIX: &str = ".awg2-harness.new";
/// The outgoing swapfile, kept until the replacement is confirmed active.
const PREVIOUS_SUFFIX: &str = ".awg2-harness.previous";

/// Identity of a file on disk, independent of its name.
///
/// Path alone is not identity: a file can be deleted and a different one created at the
/// same path. `(device, inode)` is what actually distinguishes the swapfile we made from
/// whatever now occupies its path.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
struct FileIdentity {
    device: u64,
    inode: u64,
}

/// What we recorded when we created the managed swapfile.
#[derive(Debug, Clone, PartialEq, Eq)]
struct SwapOwnershipRecord {
    version: u32,
    path: PathBuf,
    identity: FileIdentity,
}

#[derive(Debug, Clone, Copy, PartialEq, Eq)]
enum SwapfileOwnership {
    /// Nothing at the path: safe to create.
    Missing,
    /// Ours, confirmed by identity, and already the requested size.
    ManagedValid,
    /// Ours, confirmed by identity, but the size differs: safe to recreate.
    ManagedWrongSize,
    /// A file exists and we have no record of creating it. Never touched.
    Foreign,
    /// We have a record, but the file at the path is not the one we created - it was
    /// replaced behind our back. Refused exactly like `Foreign`; kept separate so the
    /// error can say which situation the operator is in.
    StaleOwnership,
}

impl SwapfileOwnership {
    fn is_managed(self) -> bool {
        matches!(self, Self::ManagedValid | Self::ManagedWrongSize)
    }
}

/// Persisted `vm.swappiness`, owned by the swap module.
///
/// Lives at `90-` like every other file this package manages. It used to be written to
/// `/etc/sysctl.d/99-awg2-harness.conf` with a different marker, no ownership check before
/// overwriting, and no cleanup on disable.
const SWAPPINESS_CONF_PATH: &str = "/etc/sysctl.d/90-awg2-harness-swap.conf";
const SWAPPINESS_MARKER: &str = "# Managed by awg2-harness swap";
/// Written by versions before the unified managed-file handling.
const LEGACY_SWAPPINESS_CONF_PATH: &str = "/etc/sysctl.d/99-awg2-harness.conf";
const LEGACY_SWAPPINESS_MARKER: &str = "# Managed by awg2-harness";

fn swappiness_file() -> ManagedFile {
    ManagedFile::new(SWAPPINESS_CONF_PATH, SWAPPINESS_MARKER)
}

fn legacy_swappiness_file() -> ManagedFile {
    ManagedFile::new(LEGACY_SWAPPINESS_CONF_PATH, LEGACY_SWAPPINESS_MARKER)
}

fn render_swappiness_conf(v: u8) -> String {
    format!("{SWAPPINESS_MARKER}\n# Source: /etc/awg2-harness/config.toml\nvm.swappiness = {v}\n")
}

/// Drops the pre-rename file, but only when it still carries the old marker.
///
/// A file an administrator put at that path is left alone, which is why the removal goes
/// through the same ownership check as everything else.
fn migrate_legacy_swappiness_file() -> anyhow::Result<()> {
    drop_legacy_managed_file(&legacy_swappiness_file())
}

/// Removes a file left by an older version.
///
/// Only a foreign file is tolerated - that path belongs to someone else now and the
/// current file lives elsewhere, so there is nothing to migrate. Everything else is
/// reported: swallowing `PermissionDenied` or a failed unlink would leave
/// `/etc/sysctl.d/99-awg2-harness.conf` in place, and it sorts *after* the new `90-` file,
/// so the stale value would keep winning while the reconcile reported success.
fn drop_legacy_managed_file(file: &ManagedFile) -> anyhow::Result<()> {
    match file.ownership()? {
        Ownership::Absent => Ok(()),
        Ownership::Foreign => {
            tracing::warn!(
                target: "swap",
                "legacy path {} holds a file we do not own; leaving it alone",
                file.path().display()
            );
            Ok(())
        }
        Ownership::Managed => file.remove().map(|_| ()),
    }
}

/// Brings host swap to the desired state as a transaction.
///
/// Ordering is the point: the managed swap is created, activated and verified *before*
/// any foreign swap is disabled. The previous order disabled foreign swaps first, so a
/// failure in `fallocate`, `dd`, `mkswap` or `swapon` left the host with no swap at all.
pub fn ensure_swap(cfg: &Config) -> anyhow::Result<()> {
    // Taken before the enabled check: the disabled branch removes managed files, which is
    // a mutation like any other. Leaving it outside the lock let a manual CLI run and the
    // daemon's reconcile write and delete the same file at the same time.
    let _lock = FileLock::acquire_timeout("/run/awg2-harness/swap.lock", Duration::from_secs(30))?;

    if !cfg.swap.enabled {
        return remove_managed_swap_state();
    }

    // 1. Preflight. The state directory is created and verified first: on a clean host
    // /var/lib/awg2-harness does not exist yet, and the path check below requires every
    // parent to be present.
    ensure_state_dir()?;
    let desired = DesiredSwap::from_config(cfg)?;
    assert_swap_path_is_safe(&desired.path)?;
    let active_before = read_active_swaps()?;

    // 2-3. Prepare and activate the managed swap. Activation belongs to this step so that
    // every transition can roll back while it still owns both the old and the new file.
    ensure_desired_swapfile(&desired, &active_before)?;

    // 4. Confirm it is really live before touching anything else.
    let active_after = read_active_swaps()?;
    if !active_after.iter().any(|s| s.name == desired.path) {
        anyhow::bail!(
            "managed swap {} did not become active; foreign swap left untouched",
            desired.path.display()
        );
    }

    // 5. Only now is it safe to take the others down.
    disable_foreign_swaps(&desired, &active_after)?;

    // 6. Persist and verify.
    rewrite_fstab_exclusive(&desired)?;
    apply_swappiness(desired.swappiness)?;
    verify_exclusive_swap_state(&desired)?;
    Ok(())
}

/// Undoes the persistent part of the module when `swap.enabled = false`.
///
/// Only the managed file is removed. The runtime `vm.swappiness` is deliberately left as
/// it is: forcing an arbitrary value would be inventing a policy the operator never asked
/// for. The kernel default applies again after reboot, and `status` reports that the value
/// is no longer managed. Active swap is not disabled either - turning it off would be a
/// destructive action triggered by merely disabling a feature.
fn remove_managed_swap_state() -> anyhow::Result<()> {
    migrate_legacy_swappiness_file()?;
    swappiness_file().remove()?;
    Ok(())
}

/// Lexical validation of `swap.path`, with no filesystem access.
///
/// Kept side-effect free so `validate-config` can run it on a host where the file does not
/// exist yet. Operates on the string rather than [`Path`] because the target is always a
/// Linux path, regardless of where validation runs.
pub fn validate_swap_path_syntax(raw: &str) -> anyhow::Result<()> {
    if !raw.starts_with('/') {
        anyhow::bail!("swap.path must be absolute: {raw}");
    }
    if raw.ends_with('/') {
        anyhow::bail!("swap.path must not be a directory path: {raw}");
    }
    if let Some(bad) = raw
        .chars()
        .find(|c| c.is_whitespace() || c.is_control() || *c == '\\')
    {
        anyhow::bail!(
            "swap.path must not contain whitespace or control characters: {raw:?} contains {bad:?}"
        );
    }
    for component in raw.split('/') {
        if component == "." || component == ".." {
            anyhow::bail!("swap.path must not contain '.' or '..' components: {raw}");
        }
    }
    // Allow-list, not deny-list. A deny-list only rules out the paths someone thought of;
    // what actually matters is whether the parent directory can be written by anyone but
    // root. Between the preflight check and `fallocate`/`dd`/`mkswap` running, a
    // world-writable parent lets an unprivileged user swap the final component for a
    // symlink and redirect a root-owned operation - a TOCTOU that no amount of stat-ing
    // before use can close. Restricting the location to directories only root can write
    // removes the window instead of trying to detect it.
    if raw == DEFAULT_SWAP_PATH {
        return Ok(());
    }
    if let Some(rest) = raw.strip_prefix(STATE_DIR_PREFIX)
        && !rest.is_empty()
        && !rest.contains('/')
    {
        return Ok(());
    }
    anyhow::bail!(
        "swap.path must be {DEFAULT_SWAP_PATH} or a file directly under {STATE_DIR}; got {raw}. \
         Both are root-owned locations; an arbitrary path can have a parent directory that \
         another user may write, which would let them redirect the swapfile operations"
    );
}

/// Filesystem-level checks performed just before the path is used.
///
/// A symlink anywhere in the chain, not only at the final component, can redirect the
/// whole operation at a file we never intended to touch.
fn assert_swap_path_is_safe(path: &Path) -> anyhow::Result<()> {
    validate_swap_path_syntax(&path.to_string_lossy())?;
    assert_path_chain_is_safe(path)
}

/// Walks the parent chain and refuses any symlink or missing directory in it.
///
/// Split from the syntax check so it can be exercised independently of the Linux path
/// grammar.
fn assert_path_chain_is_safe(path: &Path) -> anyhow::Result<()> {
    let mut walked = PathBuf::new();
    let components: Vec<_> = path.components().collect();
    for component in components.iter().take(components.len().saturating_sub(1)) {
        walked.push(component);
        match fs::symlink_metadata(&walked) {
            Ok(md) if md.file_type().is_symlink() => {
                anyhow::bail!(
                    "swap.path traverses a symlink at {}; refusing to continue",
                    walked.display()
                );
            }
            Ok(_) => {}
            Err(e) if e.kind() == std::io::ErrorKind::NotFound => {
                anyhow::bail!(
                    "swap.path parent directory does not exist: {}",
                    walked.display()
                );
            }
            Err(e) => {
                anyhow::bail!("failed to stat {}: {e}", walked.display());
            }
        }
    }
    Ok(())
}

fn ownership_file() -> ManagedFile {
    ManagedFile::new(SWAP_OWNERSHIP_PATH, SWAP_OWNERSHIP_MARKER)
}

fn render_ownership_record(rec: &SwapOwnershipRecord) -> String {
    format!(
        "{SWAP_OWNERSHIP_MARKER}\nversion = {}\npath = {}\ndevice = {}\ninode = {}\n",
        rec.version,
        rec.path.display(),
        rec.identity.device,
        rec.identity.inode
    )
}

fn parse_ownership_record(raw: &str) -> anyhow::Result<SwapOwnershipRecord> {
    let mut version = None;
    let mut path = None;
    let mut device = None;
    let mut inode = None;
    for line in raw.lines() {
        let t = line.trim();
        if t.is_empty() || t.starts_with('#') {
            continue;
        }
        let Some((k, v)) = t.split_once('=') else {
            continue;
        };
        let v = v.trim();
        match k.trim() {
            "version" => version = v.parse::<u32>().ok(),
            "path" => path = Some(PathBuf::from(v)),
            "device" => device = v.parse::<u64>().ok(),
            "inode" => inode = v.parse::<u64>().ok(),
            _ => {}
        }
    }
    let (Some(version), Some(path), Some(device), Some(inode)) = (version, path, device, inode)
    else {
        anyhow::bail!("swap ownership record is incomplete or malformed");
    };
    if version != SWAP_OWNERSHIP_VERSION {
        anyhow::bail!("unsupported swap ownership record version: {version}");
    }
    Ok(SwapOwnershipRecord {
        version,
        path,
        identity: FileIdentity { device, inode },
    })
}

/// Decides ownership from a recorded state and the file actually on disk.
///
/// Pure on purpose: the interesting cases - a stale record pointing at a file someone
/// else created, a record for a different path - are the ones that must never authorise a
/// delete, and keeping the decision free of filesystem access makes them directly
/// testable.
fn classify_ownership(
    recorded: Option<&SwapOwnershipRecord>,
    actual_path: &Path,
    actual: Option<(FileIdentity, u64)>,
    desired_size: u64,
) -> SwapfileOwnership {
    let Some((identity, size)) = actual else {
        return SwapfileOwnership::Missing;
    };
    let Some(rec) = recorded else {
        return SwapfileOwnership::Foreign;
    };
    if rec.path != actual_path {
        // The record describes a different managed path; this file is not ours.
        return SwapfileOwnership::Foreign;
    }
    if rec.identity != identity {
        // Right path, wrong file: ours was replaced. Never a licence to delete.
        return SwapfileOwnership::StaleOwnership;
    }
    if size == desired_size {
        SwapfileOwnership::ManagedValid
    } else {
        SwapfileOwnership::ManagedWrongSize
    }
}

#[cfg(unix)]
fn identity_of(md: &fs::Metadata) -> FileIdentity {
    use std::os::unix::fs::MetadataExt;
    FileIdentity {
        device: md.dev(),
        inode: md.ino(),
    }
}

/// Windows has no stable dev/inode pair through `Metadata`; this target exists only so the
/// pure logic above stays testable off Linux.
#[cfg(not(unix))]
fn identity_of(_md: &fs::Metadata) -> FileIdentity {
    FileIdentity {
        device: 0,
        inode: 0,
    }
}

fn read_ownership_record() -> anyhow::Result<Option<SwapOwnershipRecord>> {
    let Some(raw) = ownership_file().read()? else {
        return Ok(None);
    };
    parse_ownership_record(&raw).map(Some)
}

fn inspect_swapfile_ownership(path: &Path, desired_size: u64) -> anyhow::Result<SwapfileOwnership> {
    let actual = match fs::symlink_metadata(path) {
        Ok(md) => {
            if md.file_type().is_symlink() {
                anyhow::bail!("swap.path is a symlink: {}", path.display());
            }
            if !md.file_type().is_file() {
                anyhow::bail!("swap.path is not a regular file: {}", path.display());
            }
            Some((identity_of(&md), md.len()))
        }
        Err(e) if e.kind() == std::io::ErrorKind::NotFound => None,
        Err(e) => anyhow::bail!("failed to stat {}: {e}", path.display()),
    };

    let recorded = read_ownership_record()?;
    Ok(classify_ownership(
        recorded.as_ref(),
        path,
        actual,
        desired_size,
    ))
}

/// Records the identity of the swapfile we just created.
///
/// Written after the file exists, so the recorded `(device, inode)` describes the real
/// file. Until this succeeds the swapfile is not considered managed.
fn record_ownership(path: &Path) -> anyhow::Result<()> {
    ensure_state_dir()?;
    let md = fs::metadata(path)
        .map_err(|e| anyhow::anyhow!("failed to stat {} after create: {e}", path.display()))?;
    let rec = SwapOwnershipRecord {
        version: SWAP_OWNERSHIP_VERSION,
        path: path.to_path_buf(),
        identity: identity_of(&md),
    };
    ownership_file().write(&render_ownership_record(&rec))?;
    Ok(())
}

/// Creates the state directory if needed and asserts it is safe to keep state in.
///
/// Run before `swap.path` is validated, for two reasons. On a clean host
/// `/var/lib/awg2-harness` does not exist yet, so a perfectly legal
/// `/var/lib/awg2-harness/swapfile` would be rejected by the parent-chain check for a
/// directory this package is supposed to create. And an existing directory has to be
/// verified, not just assumed: if it is group- or world-writable, the whole point of
/// restricting `swap.path` to root-owned roots is gone, because another user could swap
/// the swapfile out from under a root-owned operation.
pub fn ensure_state_dir() -> anyhow::Result<()> {
    let dir = Path::new(STATE_DIR);

    // The parent must exist and must not be a symlink, or the directory we create could
    // land somewhere else entirely.
    if let Some(parent) = dir.parent() {
        match fs::symlink_metadata(parent) {
            Ok(md) if md.file_type().is_symlink() => {
                anyhow::bail!("{} is a symlink; refusing to use it", parent.display())
            }
            Ok(md) if !md.file_type().is_dir() => {
                anyhow::bail!("{} is not a directory", parent.display())
            }
            Ok(_) => {}
            Err(e) => anyhow::bail!("failed to stat {}: {e}", parent.display()),
        }
    }

    match fs::symlink_metadata(dir) {
        Ok(md) if md.file_type().is_symlink() => {
            anyhow::bail!("{STATE_DIR} is a symlink; refusing to use it")
        }
        Ok(md) if !md.file_type().is_dir() => {
            anyhow::bail!("{STATE_DIR} exists and is not a directory")
        }
        Ok(md) => return assert_state_dir_is_private(dir, &md),
        Err(e) if e.kind() == std::io::ErrorKind::NotFound => {}
        Err(e) => anyhow::bail!("failed to stat {STATE_DIR}: {e}"),
    }

    fs::create_dir(dir).map_err(|e| anyhow::anyhow!("failed to create {STATE_DIR}: {e}"))?;
    set_dir_mode(dir)?;
    let md = fs::symlink_metadata(dir)
        .map_err(|e| anyhow::anyhow!("failed to stat {STATE_DIR} after create: {e}"))?;
    assert_state_dir_is_private(dir, &md)
}

/// Refuses a state directory anyone but root can write into.
#[cfg(unix)]
fn assert_state_dir_is_private(dir: &Path, md: &fs::Metadata) -> anyhow::Result<()> {
    use std::os::unix::fs::MetadataExt;

    if md.uid() != 0 {
        anyhow::bail!(
            "{} must be owned by root, got uid {}",
            dir.display(),
            md.uid()
        );
    }
    let mode = md.permissions().mode() & 0o777;
    if mode & 0o022 != 0 {
        anyhow::bail!(
            "{} is writable by group or others (mode {mode:04o}); refusing to keep state there",
            dir.display()
        );
    }
    if mode != 0o700 {
        set_dir_mode(dir)?;
    }
    Ok(())
}

#[cfg(not(unix))]
fn assert_state_dir_is_private(_dir: &Path, _md: &fs::Metadata) -> anyhow::Result<()> {
    Ok(())
}

#[cfg(unix)]
fn set_dir_mode(dir: &Path) -> anyhow::Result<()> {
    fs::set_permissions(dir, fs::Permissions::from_mode(0o700))
        .map_err(|e| anyhow::anyhow!("failed to chmod 0700 {}: {e}", dir.display()))
}

#[cfg(not(unix))]
fn set_dir_mode(_dir: &Path) -> anyhow::Result<()> {
    Ok(())
}

/// Removes the sidecar written next to the swapfile by earlier versions.
///
/// Only removed, never trusted: ownership now lives in the state directory. A sidecar at
/// a path we do not own is left alone.
fn drop_legacy_ownership_sidecar(swap_path: &Path) {
    let Ok(sidecar) = suffixed_path(swap_path, LEGACY_OWNERSHIP_SUFFIX) else {
        return;
    };
    // Goes through the same marker check as every other managed file. Removing any regular
    // file that happened to match the name would delete an operator's file purely because
    // of what it was called.
    let file = ManagedFile::new(sidecar.clone(), LEGACY_OWNERSHIP_MARKER);
    match file.ownership() {
        Ok(Ownership::Absent) => {}
        Ok(Ownership::Managed) => {
            if let Err(e) = file.remove() {
                tracing::warn!(
                    target: "swap",
                    "could not remove legacy ownership sidecar {}: {e}",
                    sidecar.display()
                );
            }
        }
        Ok(Ownership::Foreign) => tracing::warn!(
            target: "swap",
            "{} exists but is not the legacy ownership sidecar; leaving it alone",
            sidecar.display()
        ),
        Err(e) => tracing::warn!(
            target: "swap",
            "could not inspect {}: {e}",
            sidecar.display()
        ),
    }
}

pub fn inspect_swap(cfg: &Config) -> anyhow::Result<SwapStatus> {
    let desired_path = PathBuf::from(cfg.swap.path.clone());
    let active_entries = read_active_swaps()?;
    let desired_active = active_entries.iter().any(|s| s.name == desired_path);
    let exclusive = active_entries.len() == 1 && desired_active;
    let fstab_swap_entries = read_fstab_swap_entries("/etc/fstab")?;
    let expected_line = render_fstab_line(&desired_path, cfg.swap.priority);
    let fstab_exclusive = fstab_swap_entries.len() == 1
        && normalize_ws(&fstab_swap_entries[0]) == normalize_ws(expected_line.trim_end());
    let current_swappiness = read_current_swappiness().ok();
    let persisted_swappiness = read_persisted_swappiness()?;

    let drift_reason = if !cfg.swap.enabled {
        None
    } else if !exclusive {
        if let Some(foreign) = active_entries.iter().find(|s| s.name != desired_path) {
            Some(format!("foreign active swap: {}", foreign.name.display()))
        } else {
            Some("desired swap is not exclusively active".to_string())
        }
    } else if !fstab_exclusive {
        Some("/etc/fstab swap entries are not exclusive".to_string())
    } else if current_swappiness != Some(cfg.swap.swappiness) {
        Some(format!(
            "runtime swappiness mismatch: current={:?} desired={}",
            current_swappiness, cfg.swap.swappiness
        ))
    } else if persisted_swappiness != Some(cfg.swap.swappiness) {
        Some(format!(
            "persisted swappiness mismatch: persisted={:?} desired={}",
            persisted_swappiness, cfg.swap.swappiness
        ))
    } else {
        None
    };

    Ok(SwapStatus {
        enabled: cfg.swap.enabled,
        desired_path,
        active_entries,
        desired_active,
        exclusive,
        fstab_swap_entries,
        fstab_exclusive,
        current_swappiness,
        persisted_swappiness,
        drift_reason,
    })
}

impl DesiredSwap {
    fn from_config(cfg: &Config) -> anyhow::Result<Self> {
        let path = PathBuf::from(cfg.swap.path.clone());
        if !path.is_absolute() {
            anyhow::bail!("swap.path must be absolute: {}", path.display());
        }
        let size_bytes = cfg
            .swap
            .size_mb
            .checked_mul(1024 * 1024)
            .ok_or_else(|| anyhow::anyhow!("swap.size_mb overflow"))?;
        Ok(Self {
            path,
            size_bytes,
            priority: cfg.swap.priority,
            swappiness: cfg.swap.swappiness,
        })
    }
}

fn read_active_swaps() -> anyhow::Result<Vec<ActiveSwap>> {
    let out = Command::new("swapon")
        .args([
            "--show",
            "--bytes",
            "--raw",
            "--noheadings",
            "--output",
            "NAME,TYPE,SIZE,PRIO",
        ])
        .output()?;
    if !out.status.success() {
        anyhow::bail!(
            "swapon --show failed: {}",
            String::from_utf8_lossy(&out.stderr)
        );
    }
    parse_swapon_output(&String::from_utf8_lossy(&out.stdout))
}

fn parse_swapon_output(raw: &str) -> anyhow::Result<Vec<ActiveSwap>> {
    let mut out = Vec::new();
    for line in raw.lines() {
        let t = line.trim();
        if t.is_empty() {
            continue;
        }
        let parts: Vec<&str> = t.split_whitespace().collect();
        if parts.len() < 4 {
            anyhow::bail!("malformed swapon row: {t}");
        }
        let size_bytes = parts[2]
            .parse::<u64>()
            .map_err(|_| anyhow::anyhow!("malformed swapon size: {}", parts[2]))?;

        // Debian 13/util-linux may include USED and UUID despite the requested
        // output set. Accept both the old 4-column form:
        //   NAME TYPE SIZE PRIO
        // and the observed 5/6-column form:
        //   NAME TYPE SIZE USED PRIO [UUID]
        let priority_index = if parts.len() >= 5 && parts[4].parse::<i32>().is_ok() {
            4
        } else {
            3
        };

        let priority = parts[priority_index]
            .parse::<i32>()
            .map_err(|_| anyhow::anyhow!("malformed swapon priority: {}", parts[priority_index]))?;
        out.push(ActiveSwap {
            name: PathBuf::from(parts[0]),
            kind: parts[1].to_string(),
            size_bytes,
            priority,
        });
    }
    Ok(out)
}

/// Takes foreign swap offline. The file itself is never deleted.
///
/// Destroying a swapfile an administrator created is not this package's call: `swapoff`
/// plus removal from `/etc/fstab` already achieves the exclusive runtime state, and the
/// file can be removed by hand once the operator agrees.
/// Takes foreign swap offline, restoring what it already disabled if a later one fails.
///
/// Without the rollback a host with two foreign swaps could end up with the first one
/// disabled and the second still up, because the error aborted the loop and nothing put
/// the first back. That is a state neither the operator nor the next reconcile asked for.
fn disable_foreign_swaps(desired: &DesiredSwap, active: &[ActiveSwap]) -> anyhow::Result<()> {
    let mut disabled: Vec<&ActiveSwap> = Vec::new();
    for entry in active {
        if entry.name == desired.path {
            continue;
        }
        if let Err(e) = run("swapoff", &[entry.name.to_string_lossy().as_ref()]) {
            reenable_swaps(&disabled);
            return Err(anyhow::anyhow!(
                "failed to disable foreign swap {}: {e}",
                entry.name.display()
            ));
        }
        disabled.push(entry);
    }
    Ok(())
}

/// Best-effort restore after a failed teardown: report what could not be brought back
/// rather than masking it, but never turn the rollback itself into the reported error.
fn reenable_swaps(entries: &[&ActiveSwap]) {
    for entry in entries.iter().rev() {
        if let Err(e) = run(
            "swapon",
            &[
                "-p",
                &entry.priority.to_string(),
                entry.name.to_string_lossy().as_ref(),
            ],
        ) {
            tracing::warn!(
                target: "swap",
                "could not re-enable {} while rolling back: {e}",
                entry.name.display()
            );
        }
    }
}

/// Creates or validates the managed swapfile without ever touching an unowned one.
///
/// `active` is needed because a live swapfile cannot be resized in place: if the managed
/// swap is currently active with the wrong geometry it has to be deactivated first. That
/// is our own swap, so the brief gap is acceptable; foreign swap stays up throughout.
fn ensure_desired_swapfile(desired: &DesiredSwap, active: &[ActiveSwap]) -> anyhow::Result<()> {
    let p = &desired.path;
    let ownership = inspect_swapfile_ownership(p, desired.size_bytes)?;

    match ownership {
        SwapfileOwnership::Foreign => anyhow::bail!(
            "{} already exists and there is no record of awg2-harness creating it; \
             refusing to touch it. Remove it manually or point swap.path elsewhere",
            p.display()
        ),
        SwapfileOwnership::StaleOwnership => anyhow::bail!(
            "{} is not the file awg2-harness created - it was replaced since. Refusing to \
             touch it. Remove it manually, or delete {SWAP_OWNERSHIP_PATH} to let \
             awg2-harness adopt a freshly created swapfile",
            p.display()
        ),
        _ => {}
    }

    let active_entry = active.iter().find(|s| s.name == *p);
    let size_ok = ownership == SwapfileOwnership::ManagedValid;

    match (size_ok, active_entry) {
        // Right file, right size, already active with the right priority: nothing to do.
        (true, Some(entry))
            if active_swap_size_matches(entry.size_bytes, desired.size_bytes)
                && entry.priority == desired.priority =>
        {
            set_swapfile_mode(p)?;
            drop_legacy_ownership_sidecar(p);
            Ok(())
        }
        // Right file, right size, but the active priority differs. A reactivation, not a
        // rebuild - the previous code fell through to a full recreate, so changing
        // swap.priority destroyed and rebuilt a perfectly good multi-gigabyte file.
        (true, Some(entry)) => {
            set_swapfile_mode(p)?;
            drop_legacy_ownership_sidecar(p);
            change_swap_priority(desired, entry.priority)
        }
        // Right file, not active: activate it.
        (true, None) => {
            set_swapfile_mode(p)?;
            drop_legacy_ownership_sidecar(p);
            swapon_at(p, desired.priority)
        }
        // Wrong size or no file at all, and nothing of ours is active: build in place.
        (false, None) => {
            if ownership.is_managed() {
                fs::remove_file(p)
                    .map_err(|e| anyhow::anyhow!("failed to remove {}: {e}", p.display()))?;
            }
            create_fresh_swapfile(desired)?;
            drop_legacy_ownership_sidecar(p);
            Ok(())
        }
        // Wrong size while our swap is live: replace it through a staging file.
        (false, Some(_)) => {
            replace_active_swapfile(desired)?;
            drop_legacy_ownership_sidecar(p);
            Ok(())
        }
    }
}

/// Re-activates the managed swap at a new priority, restoring the old one on failure.
///
/// `swapoff` followed by `swapon` is two steps, and the second can fail - a bad priority,
/// a transient kernel error. Without the restore, changing one integer in the config could
/// leave the host with no swap at all.
fn change_swap_priority(desired: &DesiredSwap, previous_priority: i32) -> anyhow::Result<()> {
    let p = &desired.path;
    run("swapoff", &[p.to_string_lossy().as_ref()])?;

    let Err(e) = swapon_at(p, desired.priority) else {
        return Ok(());
    };
    match swapon_at(p, previous_priority) {
        Ok(()) => Err(anyhow::anyhow!(
            "failed to activate {} with priority {}: {e}; restored the previous priority {}",
            p.display(),
            desired.priority,
            previous_priority
        )),
        Err(re) => Err(anyhow::anyhow!(
            "failed to activate {} with priority {} ({e}) and could not restore priority {} \
             ({re}); the host may currently have no swap",
            p.display(),
            desired.priority,
            previous_priority
        )),
    }
}

/// Builds and activates a swapfile where none of ours is running.
///
/// The file is removed again if anything after its creation fails. Leaving it behind would
/// produce an unowned regular file at the managed path, which the next run correctly
/// classifies as `Foreign` and then refuses to touch - so a single transient `mkswap`
/// failure would wedge the module until an operator intervened.
fn create_fresh_swapfile(desired: &DesiredSwap) -> anyhow::Result<()> {
    let p = &desired.path;
    build_swapfile(p, desired.size_bytes)?;

    let prepared = record_ownership(p).and_then(|()| swapon_at(p, desired.priority));
    if let Err(e) = prepared {
        // Created by this run inside an allowed root-owned location, so removing it is
        // safe and leaves the host exactly as it was found.
        if let Err(re) = fs::remove_file(p) {
            return Err(anyhow::anyhow!(
                "{e}; additionally failed to clean up the partially prepared {}: {re}",
                p.display()
            ));
        }
        let _ = ownership_file().remove();
        return Err(e);
    }
    Ok(())
}

/// Replaces a live managed swapfile with one of a different size.
///
/// A live swapfile cannot be resized in place, so it has to be swapped out. Allocation and
/// `mkswap` happen while the old swap is still serving. The old file is then kept aside as
/// `.previous` rather than deleted, so every step after that point can be undone: until
/// the new file is confirmed active, the original is still on disk and can be put back.
/// The previous version deleted the old file before the new one was activated, so a
/// failure at `rename` or `swapon` left the host with no swap and nothing to recover from.
fn replace_active_swapfile(desired: &DesiredSwap) -> anyhow::Result<()> {
    let p = &desired.path;
    let staging = suffixed_path(p, STAGING_SUFFIX)?;
    let previous = suffixed_path(p, PREVIOUS_SUFFIX)?;

    remove_if_present(&staging)?;
    remove_if_present(&previous)?;

    // Built and activated while the old swap is still up: no gap during the slow part.
    build_swapfile(&staging, desired.size_bytes)?;
    if let Err(e) = swapon_at(&staging, desired.priority) {
        let _ = fs::remove_file(&staging);
        return Err(e);
    }

    // From here the old swap can be released, because the replacement is carrying load.
    run("swapoff", &[p.to_string_lossy().as_ref()])?;
    fs::rename(p, &previous).map_err(|e| {
        anyhow::anyhow!(
            "failed to set aside {} as {}: {e}",
            p.display(),
            previous.display()
        )
    })?;

    // Short window with no allocation in it. Everything below rolls back to `previous`.
    let switched = switch_staging_into_place(desired, &staging, &previous);
    if let Err(e) = switched {
        return Err(restore_previous_swapfile(desired, &previous, e));
    }

    // The replacement is confirmed active; the old file is no longer needed.
    if let Err(e) = fs::remove_file(&previous) {
        tracing::warn!(
            target: "swap",
            "swap replaced successfully but the old file {} could not be removed: {e}",
            previous.display()
        );
    }
    Ok(())
}

fn switch_staging_into_place(
    desired: &DesiredSwap,
    staging: &Path,
    previous: &Path,
) -> anyhow::Result<()> {
    let p = &desired.path;
    run("swapoff", &[staging.to_string_lossy().as_ref()])?;
    fs::rename(staging, p).map_err(|e| {
        anyhow::anyhow!(
            "failed to move {} into place at {}: {e} (previous swapfile kept at {})",
            staging.display(),
            p.display(),
            previous.display()
        )
    })?;
    record_ownership(p)?;
    swapon_at(p, desired.priority)?;
    assert_some_swap_is_active(p)
}

/// Puts the original swapfile back after a failed replacement.
///
/// Returns an error that carries both the original failure and the outcome of the
/// restore, so the operator sees what went wrong *and* whether the host still has swap.
fn restore_previous_swapfile(
    desired: &DesiredSwap,
    previous: &Path,
    cause: anyhow::Error,
) -> anyhow::Error {
    let p = &desired.path;
    let _ = remove_if_present(p);
    if let Err(e) = fs::rename(previous, p) {
        return anyhow::anyhow!(
            "{cause}; rollback failed: could not restore {} from {}: {e}. The host may \
             currently have no swap",
            p.display(),
            previous.display()
        );
    }
    if let Err(e) = record_ownership(p) {
        tracing::warn!(target: "swap", "restored swapfile but could not rewrite ownership: {e}");
    }
    match swapon_at(p, desired.priority) {
        Ok(()) => anyhow::anyhow!("{cause}; rolled back to the previous swapfile"),
        Err(e) => anyhow::anyhow!(
            "{cause}; rollback restored {} but could not activate it: {e}. The host may \
             currently have no swap",
            p.display()
        ),
    }
}

/// Refuses to report success while the host has no swap at all.
fn assert_some_swap_is_active(expected: &Path) -> anyhow::Result<()> {
    let active = read_active_swaps()?;
    if active.iter().any(|s| s.name == expected) {
        return Ok(());
    }
    anyhow::bail!(
        "{} is not present in the active swap list after activation",
        expected.display()
    )
}

fn swapon_at(path: &Path, priority: i32) -> anyhow::Result<()> {
    run(
        "swapon",
        &["-p", &priority.to_string(), path.to_string_lossy().as_ref()],
    )
}

fn remove_if_present(path: &Path) -> anyhow::Result<()> {
    match fs::remove_file(path) {
        Ok(()) => Ok(()),
        Err(e) if e.kind() == std::io::ErrorKind::NotFound => Ok(()),
        Err(e) => Err(anyhow::anyhow!("failed to remove {}: {e}", path.display())),
    }
}

fn suffixed_path(path: &Path, suffix: &str) -> anyhow::Result<PathBuf> {
    let name = path
        .file_name()
        .ok_or_else(|| anyhow::anyhow!("invalid swap.path: {}", path.display()))?;
    let mut out = name.to_os_string();
    out.push(suffix);
    Ok(path.with_file_name(out))
}

/// Allocates a swapfile and formats it, with the restrictive mode applied before `mkswap`
/// so the file is never briefly world-readable while it already holds swap structure.
fn build_swapfile(path: &Path, size_bytes: u64) -> anyhow::Result<()> {
    create_swapfile(path, size_bytes)?;
    set_swapfile_mode(path)?;
    run("mkswap", &[path.to_string_lossy().as_ref()])?;
    set_swapfile_mode(path)
}

#[cfg(unix)]
fn set_swapfile_mode(path: &Path) -> anyhow::Result<()> {
    fs::set_permissions(path, fs::Permissions::from_mode(0o600))
        .map_err(|e| anyhow::anyhow!("failed to chmod 0600 {}: {e}", path.display()))
}

#[cfg(not(unix))]
fn set_swapfile_mode(_path: &Path) -> anyhow::Result<()> {
    Ok(())
}

fn create_swapfile(path: &Path, size_bytes: u64) -> anyhow::Result<()> {
    let size_mb = size_bytes / (1024 * 1024);
    let p = path.to_string_lossy().to_string();

    let out = Command::new("fallocate")
        .args(["-l", &format!("{}M", size_mb), &p])
        .output()?;
    if out.status.success() {
        return Ok(());
    }

    let out_dd = Command::new("dd")
        .args([
            "if=/dev/zero",
            &format!("of={p}"),
            "bs=1M",
            &format!("count={size_mb}"),
            "status=none",
        ])
        .output()?;
    if !out_dd.status.success() {
        anyhow::bail!(
            "swapfile create failed (fallocate+dd): fallocate={}, dd={}",
            String::from_utf8_lossy(&out.stderr),
            String::from_utf8_lossy(&out_dd.stderr)
        );
    }
    Ok(())
}

fn rewrite_fstab_exclusive(desired: &DesiredSwap) -> anyhow::Result<()> {
    let tx = FstabTransaction::begin(Path::new(FSTAB_PATH))?;
    let rewritten = rewrite_fstab_content(tx.original(), &desired.path, desired.priority);
    tx.commit(&rewritten)?;
    Ok(())
}

fn rewrite_fstab_content(content: &str, desired_path: &Path, priority: i32) -> String {
    let mut out = Vec::<String>::new();
    for line in content.lines() {
        let t = line.trim();
        if t.is_empty() || t.starts_with('#') {
            out.push(line.to_string());
            continue;
        }
        let cols: Vec<&str> = t.split_whitespace().collect();
        if cols.len() >= 3 && cols[2] == "swap" {
            continue;
        }
        out.push(line.to_string());
    }
    out.push(
        render_fstab_line(desired_path, priority)
            .trim_end()
            .to_string(),
    );
    let mut s = out.join("\n");
    if !s.ends_with('\n') {
        s.push('\n');
    }
    s
}

/// Path goes through fstab escaping: fields are whitespace-separated, so an unescaped
/// space in the path would shift every following column.
fn render_fstab_line(path: &Path, priority: i32) -> String {
    format!(
        "{} none swap sw,pri={} 0 0\n",
        escape_field(&path.to_string_lossy()),
        priority
    )
}

fn apply_swappiness(v: u8) -> anyhow::Result<()> {
    run("sysctl", &["-w", &format!("vm.swappiness={v}")])?;
    let current = read_current_swappiness()?;
    if current != v {
        anyhow::bail!("failed to apply vm.swappiness: got {current}, expected {v}");
    }

    migrate_legacy_swappiness_file()?;
    swappiness_file().write(&render_swappiness_conf(v))?;

    let persisted = read_persisted_swappiness()?;
    if persisted != Some(v) {
        anyhow::bail!(
            "failed to persist vm.swappiness: got {:?}, expected {}",
            persisted,
            v
        );
    }
    Ok(())
}

pub fn read_current_swappiness() -> anyhow::Result<u8> {
    let out = Command::new("sysctl")
        .args(["-n", "vm.swappiness"])
        .output()?;
    if !out.status.success() {
        anyhow::bail!("sysctl read vm.swappiness failed");
    }
    let s = String::from_utf8_lossy(&out.stdout).trim().to_string();
    s.parse::<u8>()
        .with_context(|| format!("invalid vm.swappiness value: {s}"))
}

pub fn read_persisted_swappiness() -> anyhow::Result<Option<u8>> {
    let Some(content) = swappiness_file().read()? else {
        return Ok(None);
    };
    for line in content.lines() {
        let t = line.trim();
        if t.is_empty() || t.starts_with('#') {
            continue;
        }
        let Some((k, v)) = t.split_once('=') else {
            continue;
        };
        if k.trim() == "vm.swappiness" {
            return Ok(Some(v.trim().parse::<u8>()?));
        }
    }
    Ok(None)
}

fn verify_exclusive_swap_state(desired: &DesiredSwap) -> anyhow::Result<()> {
    let active = read_active_swaps()?;
    if active.len() != 1 {
        anyhow::bail!(
            "swap verify failed: expected exactly one active swap, got {}",
            active.len()
        );
    }
    let only = &active[0];
    if only.name != desired.path {
        anyhow::bail!(
            "swap verify failed: active swap {} != desired {}",
            only.name.display(),
            desired.path.display()
        );
    }
    if !active_swap_size_matches(only.size_bytes, desired.size_bytes) {
        anyhow::bail!(
            "swap verify failed: size mismatch {} != {}",
            only.size_bytes,
            desired.size_bytes
        );
    }
    if only.priority != desired.priority {
        anyhow::bail!(
            "swap verify failed: priority mismatch {} != {}",
            only.priority,
            desired.priority
        );
    }

    let entries = read_fstab_swap_entries("/etc/fstab")?;
    if entries.len() != 1 {
        anyhow::bail!(
            "swap verify failed: expected one fstab swap entry, got {}",
            entries.len()
        );
    }
    let expected = render_fstab_line(&desired.path, desired.priority);
    if normalize_ws(&entries[0]) != normalize_ws(expected.trim_end()) {
        anyhow::bail!(
            "swap verify failed: fstab entry mismatch: got='{}' expected='{}'",
            entries[0],
            expected.trim_end()
        );
    }

    let current = read_current_swappiness()?;
    if current != desired.swappiness {
        anyhow::bail!(
            "swap verify failed: vm.swappiness current={} expected={}",
            current,
            desired.swappiness
        );
    }
    Ok(())
}

/// Reads swap entries for inspection.
///
/// Strict for the same reason the rewrite is: masking a read failure here would report
/// "no swap entries in fstab", which reads as healthy drift-free state on a host whose
/// fstab could not be read at all.
fn read_fstab_swap_entries(path: &str) -> anyhow::Result<Vec<String>> {
    let content = fstab::read_required(Path::new(path))?;
    let mut out = Vec::new();
    for line in content.lines() {
        let t = line.trim();
        if t.is_empty() || t.starts_with('#') {
            continue;
        }
        let cols: Vec<&str> = t.split_whitespace().collect();
        if cols.len() >= 3 && cols[2] == "swap" {
            out.push(t.to_string());
        }
    }
    Ok(out)
}

fn normalize_ws(s: &str) -> String {
    s.split_whitespace().collect::<Vec<_>>().join(" ")
}

fn active_swap_size_matches(active_bytes: u64, desired_file_bytes: u64) -> bool {
    if active_bytes == desired_file_bytes {
        return true;
    }
    active_bytes < desired_file_bytes && desired_file_bytes - active_bytes <= 64 * 1024
}

fn run(program: &str, args: &[&str]) -> anyhow::Result<()> {
    let out = Command::new(program).args(args).output()?;
    if !out.status.success() {
        anyhow::bail!(
            "{} {:?} failed: {}",
            program,
            args,
            String::from_utf8_lossy(&out.stderr)
        );
    }
    Ok(())
}

#[cfg(test)]
mod tests {
    use super::*;

    fn scratch(name: &str) -> PathBuf {
        let dir = std::env::temp_dir().join(format!("awg2_swap_{}_{}", name, std::process::id()));
        let _ = fs::remove_dir_all(&dir);
        fs::create_dir_all(&dir).expect("scratch dir");
        dir
    }

    #[test]
    fn swap_path_syntax_accepts_expected_production_values() {
        assert!(validate_swap_path_syntax("/swapfile").is_ok());
        assert!(validate_swap_path_syntax("/var/lib/awg2-harness/swapfile").is_ok());
        assert!(validate_swap_path_syntax("/var/lib/awg2-harness/swap.1").is_ok());
    }

    /// An allow-list, not a deny-list: a path whose parent another user can write lets
    /// them redirect root-owned operations between preflight and use, which no amount of
    /// stat-ing beforehand can prevent.
    #[test]
    fn swap_path_syntax_rejects_paths_outside_root_owned_roots() {
        for bad in [
            "/tmp/awg.swap",
            "/home/user/awg.swap",
            "/srv/data/swapfile",
            "/var/lib/swapfile",
            "/var/lib/awg2-harness",
            "/var/lib/awg2-harness/nested/swapfile",
            "/var/lib/awg2-harness-other/swapfile",
            "/swapfile2",
        ] {
            assert!(
                validate_swap_path_syntax(bad).is_err(),
                "must reject path outside the allowed roots: {bad}"
            );
        }
    }

    /// The blacklist used to compare exact strings only, so a path that is lexically
    /// different but resolves to a forbidden file slipped through.
    #[test]
    fn swap_path_syntax_rejects_dot_dot_traversal() {
        for bad in [
            "/etc/../etc/fstab",
            "/var/../etc/passwd",
            "/./swapfile",
            "/var/lib/../../etc/fstab",
        ] {
            assert!(
                validate_swap_path_syntax(bad).is_err(),
                "must reject traversal: {bad}"
            );
        }
    }

    #[test]
    fn swap_path_syntax_rejects_dangerous_and_virtual_paths() {
        for bad in [
            "/",
            "/etc",
            "/etc/fstab",
            "/etc/passwd",
            "/etc/shadow",
            "/dev",
            "/proc",
            "/sys",
            "/run",
            "/boot",
            "/proc/self/mem",
            "/sys/kernel",
            "/dev/sda",
        ] {
            assert!(
                validate_swap_path_syntax(bad).is_err(),
                "must reject dangerous path: {bad}"
            );
        }
    }

    /// Unescaped whitespace would shift the fstab columns; control characters and
    /// backslashes have no legitimate use in a swapfile path.
    #[test]
    fn swap_path_syntax_rejects_whitespace_and_control_characters() {
        for bad in [
            "/var/my swap",
            "/var/swap\tfile",
            "/var/swap\nfile",
            "/var/swap\\file",
        ] {
            assert!(
                validate_swap_path_syntax(bad).is_err(),
                "must reject: {bad:?}"
            );
        }
    }

    #[test]
    fn swap_path_syntax_rejects_relative_and_directory_paths() {
        assert!(validate_swap_path_syntax("swapfile").is_err());
        assert!(validate_swap_path_syntax("var/lib/swapfile").is_err());
        assert!(validate_swap_path_syntax("/var/lib/").is_err());
    }

    fn ident(device: u64, inode: u64) -> FileIdentity {
        FileIdentity { device, inode }
    }

    fn record(path: &str, device: u64, inode: u64) -> SwapOwnershipRecord {
        SwapOwnershipRecord {
            version: SWAP_OWNERSHIP_VERSION,
            path: PathBuf::from(path),
            identity: ident(device, inode),
        }
    }

    #[test]
    fn ownership_missing_when_file_absent() {
        let rec = record("/swapfile", 1, 2);
        assert_eq!(
            classify_ownership(Some(&rec), Path::new("/swapfile"), None, 1024),
            SwapfileOwnership::Missing
        );
    }

    /// A pre-existing swapfile we have no record of belongs to someone else and must
    /// never be deleted or rewritten, whatever its size.
    #[test]
    fn ownership_foreign_without_record() {
        assert_eq!(
            classify_ownership(
                None,
                Path::new("/swapfile"),
                Some((ident(1, 2), 1024)),
                1024
            ),
            SwapfileOwnership::Foreign
        );
    }

    #[test]
    fn ownership_managed_valid_and_wrong_size() {
        let rec = record("/swapfile", 1, 2);
        assert_eq!(
            classify_ownership(
                Some(&rec),
                Path::new("/swapfile"),
                Some((ident(1, 2), 1024)),
                1024
            ),
            SwapfileOwnership::ManagedValid
        );
        assert_eq!(
            classify_ownership(
                Some(&rec),
                Path::new("/swapfile"),
                Some((ident(1, 2), 1024)),
                2048
            ),
            SwapfileOwnership::ManagedWrongSize
        );
    }

    /// The data-loss scenario the sidecar allowed: our swapfile is deleted by hand, the
    /// ownership state survives, and someone puts an unrelated file at the same path.
    /// Matching on path alone would have called it managed and deleted it.
    #[test]
    fn ownership_is_stale_when_the_file_was_replaced() {
        let rec = record("/swapfile", 1, 2);
        assert_eq!(
            classify_ownership(
                Some(&rec),
                Path::new("/swapfile"),
                Some((ident(1, 999), 4096)),
                1024
            ),
            SwapfileOwnership::StaleOwnership
        );
    }

    /// A record for a different managed path says nothing about this file.
    #[test]
    fn ownership_record_for_another_path_is_foreign() {
        let rec = record("/var/lib/awg2-harness/swapfile", 1, 2);
        assert_eq!(
            classify_ownership(
                Some(&rec),
                Path::new("/swapfile"),
                Some((ident(1, 2), 1024)),
                1024
            ),
            SwapfileOwnership::Foreign
        );
    }

    /// Neither refusal state may ever authorise a delete.
    #[test]
    fn foreign_and_stale_are_never_managed() {
        assert!(!SwapfileOwnership::Foreign.is_managed());
        assert!(!SwapfileOwnership::StaleOwnership.is_managed());
        assert!(!SwapfileOwnership::Missing.is_managed());
        assert!(SwapfileOwnership::ManagedValid.is_managed());
        assert!(SwapfileOwnership::ManagedWrongSize.is_managed());
    }

    #[test]
    fn ownership_record_round_trips() {
        let rec = record("/var/lib/awg2-harness/swapfile", 2049, 123456);
        let rendered = render_ownership_record(&rec);
        assert!(rendered.starts_with(SWAP_OWNERSHIP_MARKER));
        assert_eq!(parse_ownership_record(&rendered).expect("parse"), rec);
    }

    #[test]
    fn ownership_record_rejects_malformed_and_unsupported() {
        for bad in [
            "",
            "# marker only\n",
            "version = 1\npath = /swapfile\ndevice = 1\n",
            "version = 99\npath = /swapfile\ndevice = 1\ninode = 2\n",
            "version = x\npath = /swapfile\ndevice = 1\ninode = 2\n",
        ] {
            assert!(
                parse_ownership_record(bad).is_err(),
                "must reject record: {bad:?}"
            );
        }
    }

    fn active(name: &str, size_bytes: u64, priority: i32) -> ActiveSwap {
        ActiveSwap {
            name: PathBuf::from(name),
            kind: "file".to_string(),
            size_bytes,
            priority,
        }
    }

    /// Changing only `swap.priority` used to fall through to a full recreate, destroying
    /// and rebuilding a correct multi-gigabyte file. It is a reactivation, nothing more.
    #[test]
    fn priority_only_change_does_not_need_a_new_file() {
        let desired = DesiredSwap {
            path: PathBuf::from("/swapfile"),
            size_bytes: 2048 * 1024 * 1024,
            priority: 5,
            swappiness: 1,
        };
        let entry = active("/swapfile", desired.size_bytes, 1);

        // Same size, different priority: the file is fine, only the activation is wrong.
        assert!(active_swap_size_matches(
            entry.size_bytes,
            desired.size_bytes
        ));
        assert_ne!(entry.priority, desired.priority);
    }

    #[test]
    fn staging_path_is_a_sibling_of_the_swapfile() {
        let base = Path::new("/var/lib/awg2-harness/swapfile");
        assert_eq!(
            suffixed_path(base, STAGING_SUFFIX).expect("staging"),
            PathBuf::from("/var/lib/awg2-harness/swapfile.awg2-harness.new")
        );
        assert_eq!(
            suffixed_path(base, PREVIOUS_SUFFIX).expect("previous"),
            PathBuf::from("/var/lib/awg2-harness/swapfile.awg2-harness.previous")
        );
        assert_eq!(
            suffixed_path(base, LEGACY_OWNERSHIP_SUFFIX).expect("sidecar"),
            PathBuf::from("/var/lib/awg2-harness/swapfile.awg2-harness-owned")
        );
    }

    /// The legacy sidecar is removed only when it carries the marker the old version
    /// wrote; any other file with that name belongs to the operator.
    #[test]
    fn legacy_sidecar_is_only_removed_when_it_carries_the_old_marker() {
        let dir = scratch("sidecar");
        let swapfile = dir.join("swapfile");
        let sidecar = dir.join("swapfile.awg2-harness-owned");

        fs::write(&sidecar, "# something an operator wrote\n").expect("seed foreign");
        drop_legacy_ownership_sidecar(&swapfile);
        assert!(sidecar.exists(), "a foreign file must not be removed");

        fs::write(&sidecar, format!("{LEGACY_OWNERSHIP_MARKER}\n")).expect("seed ours");
        drop_legacy_ownership_sidecar(&swapfile);
        assert!(!sidecar.exists(), "our own sidecar must be removed");
    }

    /// Foreign swaps are identified by not being the managed path; the managed one is
    /// never included in the teardown set.
    #[test]
    fn managed_swap_is_never_in_the_foreign_set() {
        let desired = DesiredSwap {
            path: PathBuf::from("/swapfile"),
            size_bytes: 1024,
            priority: 1,
            swappiness: 1,
        };
        let entries = [
            active("/swapfile", 1024, 1),
            active("/dev/sda2", 4096, -2),
            active("/other/swap", 2048, 0),
        ];
        let foreign: Vec<_> = entries
            .iter()
            .filter(|e| e.name != desired.path)
            .map(|e| e.name.display().to_string())
            .collect();
        assert_eq!(foreign, vec!["/dev/sda2", "/other/swap"]);
    }

    #[test]
    fn ownership_rejects_non_regular_file() {
        let dir = scratch("own_dir");
        let p = dir.join("swapfile");
        fs::create_dir(&p).expect("seed dir");
        assert!(inspect_swapfile_ownership(&p, 1024).is_err());
    }

    #[test]
    fn safe_path_check_rejects_missing_parent_directory() {
        let dir = scratch("no_parent");
        let err =
            assert_path_chain_is_safe(&dir.join("nested").join("swapfile")).expect_err("must fail");
        assert!(err.to_string().contains("parent directory does not exist"));
    }

    #[test]
    fn safe_path_check_accepts_existing_parent_chain() {
        let dir = scratch("parent_ok");
        assert!(assert_path_chain_is_safe(&dir.join("swapfile")).is_ok());
    }

    /// A symlink anywhere in the chain, not only at the final component, can redirect the
    /// operation at a file that was never configured.
    #[cfg(unix)]
    #[test]
    fn safe_path_check_rejects_symlinked_parent() {
        let dir = scratch("symlink_parent");
        let real = dir.join("real");
        fs::create_dir(&real).expect("real dir");
        let link = dir.join("link");
        std::os::unix::fs::symlink(&real, &link).expect("symlink");

        let err = assert_path_chain_is_safe(&link.join("swapfile")).expect_err("must fail");
        assert!(err.to_string().contains("traverses a symlink"));
    }

    #[test]
    fn parse_swapon_empty() {
        let got = parse_swapon_output("").expect("parse");
        assert!(got.is_empty());
    }

    #[test]
    fn parse_swapon_single_file() {
        let got = parse_swapon_output("/swapfile file 2147479552 1\n").expect("parse");
        assert_eq!(got.len(), 1);
        assert_eq!(got[0].name, PathBuf::from("/swapfile"));
        assert_eq!(got[0].kind, "file");
        assert_eq!(got[0].size_bytes, 2147479552);
        assert_eq!(got[0].priority, 1);
    }

    #[test]
    fn parse_swapon_multiple_entries() {
        let raw = "/swapfile file 2147479552 1\n/dev/zram0 partition 1073741824 100\n";
        let got = parse_swapon_output(raw).expect("parse");
        assert_eq!(got.len(), 2);
    }

    #[test]
    fn parse_swapon_debian13_columns_with_used_prio_uuid() {
        let got = parse_swapon_output(
            "/swapfile file 2147479552 0 1 c8db470e-b8fb-414a-8537-8fa405ebeadf\n",
        )
        .expect("parse");
        assert_eq!(got.len(), 1);
        assert_eq!(got[0].name, PathBuf::from("/swapfile"));
        assert_eq!(got[0].size_bytes, 2147479552);
        assert_eq!(got[0].priority, 1);
    }

    #[test]
    fn active_swap_size_accepts_mkswap_usable_size_delta() {
        // Debian 13: swapon may report 4096 bytes less than file size.
        assert!(active_swap_size_matches(2147479552, 2147483648));
        assert!(active_swap_size_matches(2147483648, 2147483648));
        assert!(!active_swap_size_matches(2146435072, 2147483648));
        assert!(!active_swap_size_matches(2147487744, 2147483648));
    }

    #[test]
    fn parse_swapon_rejects_malformed_size() {
        let err = parse_swapon_output("/swapfile file abc 1\n");
        assert!(err.is_err());
    }

    #[test]
    fn fstab_rewrite_removes_all_swap_entries() {
        let raw =
            "UUID=x / ext4 defaults 0 1\n/swap-old none swap sw 0 0\n/dev/zram0 none swap sw 0 0\n";
        let out = rewrite_fstab_content(raw, Path::new("/swapfile"), 1);
        assert!(!out.contains("/swap-old"));
        assert!(!out.contains("/dev/zram0"));
        assert!(out.contains("/swapfile none swap sw,pri=1 0 0"));
    }

    #[test]
    fn fstab_rewrite_preserves_comments_and_mounts() {
        let raw = "# head\nUUID=x / ext4 defaults 0 1\n\n# tail\n";
        let out = rewrite_fstab_content(raw, Path::new("/swapfile"), 1);
        assert!(out.contains("# head"));
        assert!(out.contains("UUID=x / ext4 defaults 0 1"));
        assert!(out.contains("# tail"));
    }

    #[test]
    fn fstab_rewrite_is_idempotent() {
        let raw = "UUID=x / ext4 defaults 0 1\n/swapfile none swap sw,pri=1 0 0\n";
        let one = rewrite_fstab_content(raw, Path::new("/swapfile"), 1);
        let two = rewrite_fstab_content(&one, Path::new("/swapfile"), 1);
        assert_eq!(one, two);
    }

    #[test]
    fn fstab_rewrite_adds_trailing_newline() {
        let raw = "UUID=x / ext4 defaults 0 1";
        let out = rewrite_fstab_content(raw, Path::new("/swapfile"), 1);
        assert!(out.ends_with('\n'));
    }

    #[test]
    fn swappiness_conf_render_is_stable() {
        let rendered = render_swappiness_conf(1);
        assert!(rendered.starts_with(SWAPPINESS_MARKER));
        assert!(rendered.contains("vm.swappiness = 1"));
        assert!(render_swappiness_conf(60).contains("vm.swappiness = 60"));
    }

    /// The rendered file must be recognised as ours by the same marker check that guards
    /// overwriting it, otherwise every reconcile would refuse to touch its own output.
    #[test]
    fn rendered_swappiness_conf_is_recognised_as_managed() {
        let dir = scratch("swappiness_marker");
        let f = ManagedFile::new(dir.join("90-awg2-harness-swap.conf"), SWAPPINESS_MARKER);
        assert!(f.write(&render_swappiness_conf(1)).expect("write"));
        assert_eq!(
            f.ownership().expect("ownership"),
            crate::managed_file::Ownership::Managed
        );
        assert!(!f.write(&render_swappiness_conf(1)).expect("idempotent"));
    }

    #[test]
    fn legacy_swappiness_file_is_removed_when_it_carries_the_old_marker() {
        let dir = scratch("legacy_ours");
        let path = dir.join("99-awg2-harness.conf");
        fs::write(
            &path,
            format!("{LEGACY_SWAPPINESS_MARKER}\nvm.swappiness = 1\n"),
        )
        .expect("seed legacy");

        let legacy = ManagedFile::new(path.clone(), LEGACY_SWAPPINESS_MARKER);
        drop_legacy_managed_file(&legacy).expect("migrate");
        assert!(!path.exists());
    }

    /// An administrator's file that happens to sit at the old path is not ours to delete.
    #[test]
    fn legacy_migration_leaves_a_foreign_file_alone() {
        let dir = scratch("legacy_foreign");
        let path = dir.join("99-awg2-harness.conf");
        fs::write(&path, "# hand written\nvm.swappiness = 30\n").expect("seed foreign");

        let legacy = ManagedFile::new(path.clone(), LEGACY_SWAPPINESS_MARKER);
        drop_legacy_managed_file(&legacy).expect("must not fail");
        assert_eq!(
            fs::read_to_string(&path).expect("intact"),
            "# hand written\nvm.swappiness = 30\n"
        );
    }

    #[test]
    fn legacy_migration_is_a_no_op_when_nothing_is_there() {
        let dir = scratch("legacy_absent");
        let legacy = ManagedFile::new(dir.join("99-awg2-harness.conf"), LEGACY_SWAPPINESS_MARKER);
        assert!(drop_legacy_managed_file(&legacy).is_ok());
    }
}