meerkat_sqlite/

watch.rs

//! Cross-process change watch for one SQLite database file.
//!
//! Another process's commit to a database is invisible to this process's
//! in-memory signals. This watch turns the file-system notifications that can
//! announce a commit (on the database, its `-wal` sidecar and their
//! directory) into coalesced ticks for a consumer that then reads durable
//! state. A connection's own open and close are no change and tick nothing
//! ([`sqlite_watch_event_relevant`]): otherwise a consumer's own read would
//! wake it again.
//!
//! A notification is a HINT, never a guarantee: SQLite writes WAL frames
//! before the WAL index makes the commit visible, so a reader woken by an
//! event can still read the old state, and no later event is promised. The
//! watch therefore also ticks on a bounded sweep interval; the consumer's
//! sweep read is the eventual-wake guarantee. Neither the watch nor the sweep
//! ever defines permission, custody or expiry: correctness rests with the
//! consumer's durable reads and its own fences.
//!
//! Registering for notifications is a hint too, and nothing waits for it. The
//! operating system can hold a registration without bound (macOS FSEvents
//! registration has taken over 30 seconds) and nothing can interrupt it, so
//! the sweep worker starts first and the registration runs on a thread of its
//! own: starting a watch returns at once, and a registration that fails leaves
//! the watch on its sweep alone. [`SqliteChangeWatch::registration`] reports
//! where it stands.
//!
//! This is the contract the mob event bus has used since its external watch
//! landed, extracted so every store shares one implementation.
//!
//! Teardown is explicit. Dropping a watch, or [`SqliteWatchStopHandle::request_stop`],
//! only asks the worker to return and never blocks: a tick already running
//! finishes on its own thread, unjoined. [`SqliteChangeWatch::stop_and_join`]
//! is the one operation that waits for the worker, and it cannot do so from
//! the worker's own thread. Neither waits for a registration that is still
//! running: a stop cancels it, and its thread discards its result when the
//! platform call returns. That thread is never detached: it stays with its
//! watch and, once the watch is dropped, with the process
//! ([`retained_registrations`]) until it returns.

use std::panic::AssertUnwindSafe;
use std::path::{Path, PathBuf};
use std::sync::atomic::{AtomicBool, AtomicUsize, Ordering};
use std::sync::{Arc, Mutex, MutexGuard, OnceLock, PoisonError, mpsc};
use std::thread;
use std::time::{Duration, Instant};

use notify::{RecursiveMode, Watcher as _};

/// Default sweep interval: the bounded eventual-wake guarantee.
pub const SQLITE_WATCH_SWEEP: Duration = Duration::from_millis(5_000);
/// Settle delay after a notification, so a burst of events (a commit touches
/// the WAL and its index) coalesces into one tick.
const SQLITE_WATCH_COALESCE: Duration = Duration::from_millis(10);
const SQLITE_WATCH_RECOVERY_BACKOFF_MIN: Duration = Duration::from_millis(100);
const SQLITE_WATCH_RECOVERY_BACKOFF_MAX: Duration = Duration::from_millis(30_000);
/// Notification registrations a process runs at once. A stalled platform
/// (macOS FSEvents) holds each one for as long as it stalls, with two threads
/// apiece, so a stall holds at most 128 threads; past this many, a new watch
/// runs on its sweep alone instead of adding two more. A watch refused at the
/// cap never registers later: it stays on its sweep for the rest of its life,
/// which for a store held open is the life of the process. That is still
/// correct, since correctness rests on the sweep and a notification only
/// wakes the consumer sooner. The cap fits the bursts of a normal startup,
/// such as parallel store opens. A queue that registers a refused watch once
/// a place frees was considered and closed: the cliff needs more than 64
/// registrations in flight at once.
const MAX_REGISTRATIONS_IN_FLIGHT: usize = 64;
/// Registration threads running now in this process, for watches alive or
/// dropped.
static REGISTRATIONS_IN_FLIGHT: AtomicUsize = AtomicUsize::new(0);

/// Why the consumer is being asked to look.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum SqliteWatchTick {
    /// A relevant file-system notification arrived (coalesced).
    Changed,
    /// The sweep interval elapsed without a notification.
    Sweep,
}

/// What the consumer's tick achieved.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum SqliteWatchControl {
    /// The tick was handled.
    Handled,
    /// The consumer's durable read failed: ticks are suppressed until a
    /// bounded exponential backoff elapses, so a notification storm cannot
    /// hammer a failing store.
    Failed,
    /// The consumer is gone: stop the watch.
    Stop,
}

/// How [`SqliteChangeWatch::stop_and_join`] ended.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
#[must_use]
pub enum SqliteWatchJoin {
    /// The worker returned and was joined: no tick is running and none will
    /// run.
    Joined,
    /// The worker was joined after a tick panicked: no tick is running and
    /// none will run.
    JoinedAfterPanic,
    /// Called on the worker's own thread, from inside a tick. The stop is
    /// requested, but a thread cannot join itself: the worker returns after
    /// the current tick, unjoined. The join handle stays in this watch, so
    /// whoever holds the watch must keep it and join it from another thread.
    /// Never a settled outcome.
    UnjoinedOnWorkerThread,
}

/// Where a watch's notification registration stands
/// ([`SqliteChangeWatch::registration`]). Notifications only shorten a wake:
/// the sweep runs whatever this says.
#[derive(Debug, Clone, PartialEq, Eq)]
#[non_exhaustive]
pub enum SqliteWatchRegistration {
    /// Running on its own thread, which the operating system can hold without
    /// bound. Until it ends, only the sweep wakes the consumer.
    Pending,
    /// Registered. Its notifications stop when the watch does.
    Registered,
    /// Failed, or its thread could not start: only the sweep wakes the
    /// consumer.
    Failed(String),
    /// The watch stopped before the registration ended; its thread discarded
    /// the result.
    Cancelled,
}

/// Asks a watch's worker to return, without owning the worker. Cheap to clone;
/// never blocks and never joins.
#[derive(Debug, Clone)]
pub struct SqliteWatchStopHandle {
    control: Arc<WatchControl>,
}

impl SqliteWatchStopHandle {
    /// Ask the worker to return. Once it observes the request it starts no
    /// further tick; a tick already running finishes first. Never blocks.
    pub fn request_stop(&self) {
        self.control.request_stop();
    }
}

#[derive(Debug)]
struct WatchControl {
    stop_requested: Arc<AtomicBool>,
    wake: mpsc::Sender<()>,
}

impl WatchControl {
    fn request_stop(&self) {
        self.stop_requested.store(true, Ordering::Release);
        // Wake a worker waiting for its next tick. A worker that already
        // returned has dropped the receiver; nothing is left to wake.
        let _ = self.wake.send(());
    }
}

/// Join custody of a watch's worker thread.
enum WorkerCustody {
    Running(thread::JoinHandle<()>),
    Joined(SqliteWatchJoin),
}

/// A registration's result, shared by its thread and its watch.
enum RegistrationSlot {
    Pending,
    /// Registered; the source until the watch stops.
    Registered(Option<Box<dyn WatchSource>>),
    Failed(String),
    Cancelled,
}

/// One running registration's place among [`MAX_REGISTRATIONS_IN_FLIGHT`],
/// given back when its thread ends, or when the thread never starts.
struct InFlightPlace;

impl InFlightPlace {
    /// Take a place, or say how many registrations hold them all.
    fn take() -> Result<Self, usize> {
        REGISTRATIONS_IN_FLIGHT
            .fetch_update(Ordering::AcqRel, Ordering::Acquire, |running| {
                (running < MAX_REGISTRATIONS_IN_FLIGHT).then_some(running + 1)
            })
            .map(|_| Self)
    }
}

impl Drop for InFlightPlace {
    fn drop(&mut self) {
        REGISTRATIONS_IN_FLIGHT.fetch_sub(1, Ordering::AcqRel);
    }
}

/// A registration thread and what it registers.
struct RegistrationThread {
    handle: thread::JoinHandle<()>,
    directory: PathBuf,
    name: String,
    started: Instant,
}

impl RegistrationThread {
    fn describe(&self) -> SqliteWatchRetainedRegistration {
        SqliteWatchRetainedRegistration {
            directory: self.directory.clone(),
            thread_name: self.name.clone(),
            running_for: self.started.elapsed(),
        }
    }
}

/// Custody of a watch's notification registration: its result, and its
/// thread until that is joined.
struct RegistrationCustody {
    slot: Arc<Mutex<RegistrationSlot>>,
    /// `None` once joined, or when the thread never started.
    thread: Option<RegistrationThread>,
}

impl RegistrationCustody {
    /// Register `targets` on a thread of its own, named after the watch. A
    /// thread that cannot start, or a process already running
    /// [`MAX_REGISTRATIONS_IN_FLIGHT`] registrations, leaves the registration
    /// failed: the sweep alone wakes the consumer, for the rest of the
    /// watch's life.
    fn spawn(
        watch_name: &str,
        backend: Box<dyn WatchBackend>,
        targets: WatchTargets,
        wake: mpsc::Sender<()>,
        stop_requested: Arc<AtomicBool>,
    ) -> Self {
        let slot = Arc::new(Mutex::new(RegistrationSlot::Pending));
        let place = match InFlightPlace::take() {
            Ok(place) => place,
            Err(running) => {
                settle_failed(
                    &slot,
                    format!(
                        "{running} sqlite change watch notification registrations are already \
                         pending (at most {MAX_REGISTRATIONS_IN_FLIGHT})"
                    ),
                );
                return Self { slot, thread: None };
            }
        };
        let thread_slot = Arc::clone(&slot);
        let directory = targets.parent.clone();
        let name = format!("{watch_name}-register");
        let started = Instant::now();
        let spawned = thread::Builder::new().name(name.clone()).spawn(move || {
            // Given back as the thread ends, however it ends.
            let _place = place;
            register(
                backend.as_ref(),
                &targets,
                wake,
                &stop_requested,
                &thread_slot,
            );
        });
        match spawned {
            Ok(handle) => Self {
                slot,
                thread: Some(RegistrationThread {
                    handle,
                    directory,
                    name,
                    started,
                }),
            },
            Err(error) => {
                settle_failed(
                    &slot,
                    format!("cannot start the sqlite change watch registration thread: {error}"),
                );
                Self { slot, thread: None }
            }
        }
    }

    fn state(&self) -> SqliteWatchRegistration {
        match &*lock(&self.slot) {
            RegistrationSlot::Pending => SqliteWatchRegistration::Pending,
            RegistrationSlot::Registered(_) => SqliteWatchRegistration::Registered,
            RegistrationSlot::Failed(reason) => SqliteWatchRegistration::Failed(reason.clone()),
            RegistrationSlot::Cancelled => SqliteWatchRegistration::Cancelled,
        }
    }

    /// The registration's thread while the registration is pending. Its
    /// thread settles the slot before it ends, so a pending registration's
    /// thread is running.
    fn pending(&self) -> Option<SqliteWatchRetainedRegistration> {
        if !matches!(*lock(&self.slot), RegistrationSlot::Pending) {
            return None;
        }
        self.thread.as_ref().map(RegistrationThread::describe)
    }

    /// Stop the notifications, once the stop flag is set: unregister a
    /// registered source ([`unregister`]), and join the thread if it has
    /// returned. A registration still running discards its result when it
    /// ends.
    fn stop(&mut self) {
        let source = match &mut *lock(&self.slot) {
            RegistrationSlot::Registered(source) => source.take(),
            _ => None,
        };
        if let Some(source) = source {
            unregister(source);
        }
        if self
            .thread
            .as_ref()
            .is_some_and(|thread| thread.handle.is_finished())
            && let Some(thread) = self.thread.take()
        {
            let _ = thread.handle.join();
        }
    }

    /// The watch is going: stop, and hand a thread still running to the
    /// process's custody.
    fn release(&mut self) {
        self.stop();
        if let Some(thread) = self.thread.take() {
            retain_registration(thread);
        }
    }
}

/// A running watch: custody of its worker thread and of its notification
/// registration.
///
/// Dropping it requests the stop and never joins its worker: the worker
/// returns at its next wake, after any tick that is running, unjoined. Use
/// [`Self::stop_and_join`] to wait for the worker.
pub struct SqliteChangeWatch {
    control: Arc<WatchControl>,
    registration: RegistrationCustody,
    worker: WorkerCustody,
}

impl std::fmt::Debug for SqliteChangeWatch {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        f.debug_struct("SqliteChangeWatch").finish_non_exhaustive()
    }
}

impl SqliteChangeWatch {
    /// A handle that can request this watch's stop after the watch itself has
    /// moved elsewhere.
    pub fn stop_handle(&self) -> SqliteWatchStopHandle {
        SqliteWatchStopHandle {
            control: Arc::clone(&self.control),
        }
    }

    /// Ask the worker to return; see [`SqliteWatchStopHandle::request_stop`].
    pub fn request_stop(&self) {
        self.control.request_stop();
    }

    /// Whether the calling thread is this watch's worker, i.e. the caller is
    /// running inside one of its ticks.
    pub fn on_worker_thread(&self) -> bool {
        self.worker_thread_id() == Some(thread::current().id())
    }

    /// The worker thread's identity until the worker is joined.
    pub fn worker_thread_id(&self) -> Option<thread::ThreadId> {
        match &self.worker {
            WorkerCustody::Running(worker) => Some(worker.thread().id()),
            WorkerCustody::Joined(_) => None,
        }
    }

    /// Whether the worker has returned (joined or not). A returned worker
    /// runs no tick again.
    pub fn is_finished(&self) -> bool {
        match &self.worker {
            WorkerCustody::Running(worker) => worker.is_finished(),
            WorkerCustody::Joined(_) => true,
        }
    }

    /// Where the notification registration stands. The sweep runs whatever
    /// it says.
    pub fn registration(&self) -> SqliteWatchRegistration {
        self.registration.state()
    }

    /// The notification registration this watch would leave with the
    /// process if it were dropped now, described as
    /// [`retained_registrations`] would list it: `Some` exactly while
    /// [`Self::registration`] is [`SqliteWatchRegistration::Pending`].
    ///
    /// Read after a stop request, it names the registration the stop
    /// cancelled while the operating system still held it: that thread
    /// discards the result when the platform call returns, and until then
    /// stays with this watch or, once the watch is dropped, with the
    /// process.
    pub fn pending_registration(&self) -> Option<SqliteWatchRetainedRegistration> {
        self.registration.pending()
    }

    /// Request the stop, stop the notifications, and wait for the worker to
    /// return: a tick that is running finishes first. Unbounded by design: it
    /// waits exactly as long as that tick runs.
    ///
    /// It never waits for a notification registration that is still running:
    /// the stop cancels it, and its thread stays in this watch until it
    /// returns ([`Self::registration`] says
    /// [`SqliteWatchRegistration::Pending`] until then). A later call joins
    /// that thread once it has.
    ///
    /// On the worker's own thread it cannot wait; it returns
    /// [`SqliteWatchJoin::UnjoinedOnWorkerThread`] and keeps the join handle
    /// in this watch, so a later call from another thread still joins.
    /// Calling it again after a join returns the recorded outcome.
    pub fn stop_and_join(&mut self) -> SqliteWatchJoin {
        self.control.request_stop();
        self.registration.stop();
        if self.on_worker_thread() {
            return SqliteWatchJoin::UnjoinedOnWorkerThread;
        }
        let outcome = match std::mem::replace(
            &mut self.worker,
            WorkerCustody::Joined(SqliteWatchJoin::Joined),
        ) {
            WorkerCustody::Running(worker) => match worker.join() {
                Ok(()) => SqliteWatchJoin::Joined,
                Err(_) => SqliteWatchJoin::JoinedAfterPanic,
            },
            WorkerCustody::Joined(outcome) => outcome,
        };
        self.worker = WorkerCustody::Joined(outcome);
        outcome
    }
}

impl Drop for SqliteChangeWatch {
    fn drop(&mut self) {
        // Never wait here: the last owner can drop a watch on any thread,
        // including its own worker's. A registration still running goes to
        // the process's custody.
        self.control.request_stop();
        self.registration.release();
    }
}

/// Registration threads still running after their watch was dropped.
static RETAINED_REGISTRATIONS: OnceLock<Mutex<Vec<RegistrationThread>>> = OnceLock::new();

/// The process's unregister thread, `sqlite-watch-unregister`: the queue it
/// drops sources from, in turn, and the thread itself, which lives as long as
/// the process. `None` when it could not start.
static UNREGISTER: OnceLock<Option<Unregister>> = OnceLock::new();

struct Unregister {
    queue: mpsc::Sender<Box<dyn WatchSource>>,
    _thread: thread::JoinHandle<()>,
}

/// Drop a stopped watch's notification source. A source whose drop waits on
/// the platform ([`WatchSource::teardown_waits`]) goes to the process's one
/// unregister thread, which drops such sources in turn, so no thread that
/// stops a watch (a runtime worker shutting down, a watch's own worker) waits
/// on the platform. However many watches stop at once, that is one thread;
/// their sources wait in its queue. Until the thread drops it, a source can
/// still notify, which only wakes its own stopped watch: its worker returns
/// at that wake without a tick, and a watch started meanwhile on the same
/// database has its own source and wake. Any other source, or every source
/// when the thread cannot start, is dropped here.
fn unregister(source: Box<dyn WatchSource>) {
    if !source.teardown_waits() {
        drop(source);
        return;
    }
    let unregister = UNREGISTER.get_or_init(|| {
        let (queue, sources) = mpsc::channel::<Box<dyn WatchSource>>();
        thread::Builder::new()
            .name("sqlite-watch-unregister".to_string())
            .spawn(move || {
                for source in sources {
                    // A drop can panic (notify's macOS watcher expects its
                    // run-loop thread to join). Contain it, as a registration's
                    // panic is, so this thread keeps dropping its queue.
                    if std::panic::catch_unwind(AssertUnwindSafe(move || drop(source))).is_err() {
                        tracing::warn!(
                            "dropping a stopped sqlite change watch's notification source panicked"
                        );
                    }
                }
            })
            .map(|thread| Unregister {
                queue,
                _thread: thread,
            })
            .ok()
    });
    let source = match unregister {
        Some(unregister) => match unregister.queue.send(source) {
            Ok(()) => return,
            Err(mpsc::SendError(source)) => source,
        },
        None => source,
    };
    drop(source);
}

fn retained() -> &'static Mutex<Vec<RegistrationThread>> {
    RETAINED_REGISTRATIONS.get_or_init(Mutex::default)
}

/// A notification registration still running after its watch was dropped
/// ([`retained_registrations`]), or one a live watch would leave with the
/// process ([`SqliteChangeWatch::pending_registration`]).
#[derive(Debug, Clone, PartialEq, Eq)]
#[non_exhaustive]
pub struct SqliteWatchRetainedRegistration {
    /// The directory it registers.
    pub directory: PathBuf,
    /// The name of its thread.
    pub thread_name: String,
    /// How long it has been running.
    pub running_for: Duration,
}

/// The notification registrations this process still holds for watches that
/// are gone. Each one is blocked in the operating system, which nothing can
/// interrupt, and discards its result when that call returns. Joins the ones
/// that have returned; never waits.
pub fn retained_registrations() -> Vec<SqliteWatchRetainedRegistration> {
    let mut retained = lock(retained());
    reap(&mut retained);
    retained.iter().map(RegistrationThread::describe).collect()
}

fn retain_registration(thread: RegistrationThread) {
    let mut retained = lock(retained());
    reap(&mut retained);
    retained.push(thread);
}

/// Join the retained threads that have returned.
fn reap(retained: &mut Vec<RegistrationThread>) {
    let (returned, running): (Vec<_>, Vec<_>) = std::mem::take(retained)
        .into_iter()
        .partition(|thread| thread.handle.is_finished());
    *retained = running;
    for thread in returned {
        let _ = thread.handle.join();
    }
}

fn sidecar_path(path: &Path, suffix: &str) -> PathBuf {
    let mut value = path.as_os_str().to_os_string();
    value.push(suffix);
    PathBuf::from(value)
}

/// The database file and its sidecars.
pub fn sqlite_watch_paths(path: &Path) -> Vec<PathBuf> {
    vec![
        path.to_path_buf(),
        sidecar_path(path, "-wal"),
        sidecar_path(path, "-shm"),
    ]
}

fn sqlite_watch_targets(db_path: &Path) -> Result<(PathBuf, Vec<PathBuf>), String> {
    let file_name = db_path
        .file_name()
        .ok_or_else(|| format!("{} has no database filename", db_path.display()))?;
    let parent = db_path
        .parent()
        .filter(|parent| !parent.as_os_str().is_empty())
        .unwrap_or_else(|| Path::new("."));
    let parent = parent.canonicalize().map_err(|error| {
        format!(
            "cannot resolve {} for sqlite watch: {error}",
            parent.display()
        )
    })?;
    // FSEvents reports canonical paths, including /private/var for /var on
    // macOS. Resolve only the existing directory: database and sidecar files
    // may not exist yet, or may have been removed when an event is delivered.
    let watched_paths = sqlite_watch_paths(&parent.join(file_name));
    Ok((parent, watched_paths))
}

/// Whether a notification can announce a change to the database: a commit,
/// a checkpoint, or the database file or its directory replaced or removed.
///
/// `watched_paths` are the database's paths as [`sqlite_watch_paths`] names
/// them in its canonical directory `parent`. A connection's own open and
/// close are no change: the first connection to open a WAL database creates
/// its `-wal` and `-shm` files and writes the `-shm` (the WAL index), and the
/// last to close removes both. A commit appends to the WAL (without one, it
/// writes the database file), and the WAL index changes only through shared
/// memory, which no notification reports. So nothing on the `-shm` file is
/// relevant, nor the creation, removal or metadata of the `-wal`. Every
/// other event on the database file, the `-wal` or the directory itself is
/// (an unknown kind included), and so is an event that names no path (a
/// backend's rescan). An access never is.
///
/// On macOS, FSEvents can also report the directory itself when a file in
/// it is created or removed. The directory stays relevant, since its own
/// removal or rename matters, so there a connection's own open or close may
/// still tick.
pub fn sqlite_watch_event_relevant(
    event: &notify::Event,
    parent: &Path,
    watched_paths: &[PathBuf],
) -> bool {
    if matches!(event.kind, notify::EventKind::Access(_)) {
        return false;
    }
    if event.paths.is_empty() {
        return true;
    }
    event.paths.iter().any(|path| {
        if path == parent {
            return true;
        }
        let Some(watched) = watched_paths.iter().find(|watched| {
            path == *watched
                || (path.parent() == watched.parent() && path.file_name() == watched.file_name())
        }) else {
            return false;
        };
        match WatchedFile::of(watched, watched_paths) {
            WatchedFile::Database => true,
            WatchedFile::Wal => !matches!(
                event.kind,
                notify::EventKind::Create(_)
                    | notify::EventKind::Remove(_)
                    | notify::EventKind::Modify(notify::event::ModifyKind::Metadata(_))
            ),
            WatchedFile::WalIndex => false,
        }
    })
}

/// Which of a database's watched files a watched path is.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
enum WatchedFile {
    Database,
    /// The `-wal` file, which commits append to.
    Wal,
    /// The `-shm` file, the WAL index, which changes through shared memory.
    WalIndex,
}

impl WatchedFile {
    /// `watched`'s place among `watched_paths`: the `-wal` or `-shm` sidecar
    /// of another watched path, or a database.
    fn of(watched: &Path, watched_paths: &[PathBuf]) -> Self {
        for database in watched_paths {
            if sidecar_path(database, "-wal") == watched {
                return Self::Wal;
            }
            if sidecar_path(database, "-shm") == watched {
                return Self::WalIndex;
            }
        }
        Self::Database
    }
}

/// The directory a watch registers, and the database files whose changes in
/// it wake the worker.
struct WatchTargets {
    parent: PathBuf,
    watched_paths: Vec<PathBuf>,
}

/// Creates a watch's notification source: notify's recommended watcher, or a
/// held fake under the `test-support` feature.
trait WatchBackend: Send {
    /// A source that sends on `wake` for each relevant change once it is
    /// registered.
    fn create(
        &self,
        targets: &WatchTargets,
        wake: mpsc::Sender<()>,
    ) -> Result<Box<dyn WatchSource>, String>;
}

/// A created notification source. Dropping it stops its notifications.
trait WatchSource: Send {
    /// Register `directory`. Returns once the platform has registered it,
    /// which has no bound: macOS FSEvents registration can take tens of
    /// seconds.
    fn watch(&mut self, directory: &Path) -> Result<(), String>;

    /// Whether dropping this source can wait on the platform, so a stopped
    /// watch hands it to the unregister thread ([`unregister`]).
    fn teardown_waits(&self) -> bool {
        false
    }
}

/// notify's recommended watcher: FSEvents on macOS, inotify on Linux.
struct NotifyBackend;

impl WatchBackend for NotifyBackend {
    fn create(
        &self,
        targets: &WatchTargets,
        wake: mpsc::Sender<()>,
    ) -> Result<Box<dyn WatchSource>, String> {
        let parent = targets.parent.clone();
        let watched_paths = targets.watched_paths.clone();
        let watcher = notify::recommended_watcher(move |result: notify::Result<notify::Event>| {
            let relevant = matches!(
                &result,
                Ok(event) if sqlite_watch_event_relevant(event, &parent, &watched_paths)
            );
            #[cfg(any(test, feature = "test-support"))]
            test_support::log_raw_event(&parent, &result, relevant);
            match result {
                Ok(_) if relevant => {
                    let _ = wake.send(());
                }
                Ok(_) => {}
                Err(error) => {
                    // The sweep bounds anything a watcher error lost.
                    tracing::warn!(error = %error, "sqlite change watch reported an error");
                }
            }
        })
        .map_err(|error| format!("cannot create the sqlite change watcher: {error}"))?;
        Ok(Box::new(NotifySource(watcher)))
    }
}

struct NotifySource(notify::RecommendedWatcher);

impl WatchSource for NotifySource {
    fn watch(&mut self, directory: &Path) -> Result<(), String> {
        #[cfg(any(test, feature = "test-support"))]
        if let Some(delay) = test_support::platform_registration_delay() {
            thread::sleep(delay);
        }
        self.0
            .watch(directory, RecursiveMode::NonRecursive)
            .map_err(|error| format!("cannot watch {}: {error}", directory.display()))
    }

    /// macOS FSEvents: dropping the watcher joins its run-loop thread, which
    /// makes two round trips to the file-system event service (the current
    /// event id, then a purge up to it) before it lets go.
    fn teardown_waits(&self) -> bool {
        cfg!(target_os = "macos")
    }
}

/// The backend that registers a watch of `parent`: notify's, unless test
/// support holds the registrations in that directory.
#[cfg_attr(not(any(test, feature = "test-support")), allow(unused_variables))]
fn watch_backend(parent: &Path) -> Box<dyn WatchBackend> {
    #[cfg(any(test, feature = "test-support"))]
    if let Some(held) = test_support::held_backend(parent) {
        return held;
    }
    #[cfg(any(test, feature = "test-support"))]
    if let Some(observed) = test_support::observed_backend(parent) {
        return observed;
    }
    Box::new(NotifyBackend)
}

fn lock<T>(mutex: &Mutex<T>) -> MutexGuard<'_, T> {
    mutex.lock().unwrap_or_else(PoisonError::into_inner)
}

/// Start watching `db_path`. `on_tick` runs on a dedicated thread named
/// `thread_name`, once per coalesced notification and once per `sweep`
/// without one, until the watch is stopped or `on_tick` returns
/// [`SqliteWatchControl::Stop`].
///
/// Returns as soon as that worker runs. The notification registration runs
/// on a thread of its own, `<thread_name>-register`, which ends once the
/// platform has registered (at once with inotify);
/// [`SqliteChangeWatch::registration`] reports it. While the platform holds
/// registrations, each one holds that thread, and on macOS notify's run-loop
/// thread too, until the platform call returns. At most 64 run at once in a
/// process: a watch started while 64 are pending runs on its sweep alone
/// (`Failed`) for the rest of its life and logs a warning. Errors only when
/// the database's directory cannot be resolved or the worker cannot start.
pub fn start_sqlite_change_watch(
    db_path: &Path,
    thread_name: &str,
    sweep: Duration,
    on_tick: impl FnMut(SqliteWatchTick) -> SqliteWatchControl + Send + 'static,
) -> Result<SqliteChangeWatch, String> {
    start_with_backend(db_path, thread_name, sweep, on_tick, watch_backend)
}

fn start_with_backend(
    db_path: &Path,
    thread_name: &str,
    sweep: Duration,
    mut on_tick: impl FnMut(SqliteWatchTick) -> SqliteWatchControl + Send + 'static,
    backend: impl FnOnce(&Path) -> Box<dyn WatchBackend>,
) -> Result<SqliteChangeWatch, String> {
    let (parent, watched_paths) = sqlite_watch_targets(db_path)?;
    let backend = backend(&parent);
    let targets = WatchTargets {
        parent,
        watched_paths,
    };
    let (wake_tx, wake_rx) = mpsc::channel::<()>();
    let stop_requested = Arc::new(AtomicBool::new(false));
    let worker_stop_requested = Arc::clone(&stop_requested);

    // The sweep is the guarantee: its worker starts first, before anything
    // that can block.
    let worker = thread::Builder::new()
        .name(thread_name.to_string())
        .spawn(move || {
            let mut recovery_backoff = SQLITE_WATCH_RECOVERY_BACKOFF_MIN;
            let mut recovery_deadline: Option<Instant> = None;
            loop {
                let wait = recovery_deadline
                    .map(|deadline| deadline.saturating_duration_since(Instant::now()))
                    .unwrap_or(sweep);
                let tick = match wake_rx.recv_timeout(wait) {
                    Ok(()) => SqliteWatchTick::Changed,
                    Err(mpsc::RecvTimeoutError::Timeout) => SqliteWatchTick::Sweep,
                    // The watcher (and every sender) is gone: stop.
                    Err(mpsc::RecvTimeoutError::Disconnected) => break,
                };
                if worker_stop_requested.load(Ordering::Acquire) {
                    break;
                }
                if tick == SqliteWatchTick::Changed {
                    thread::sleep(SQLITE_WATCH_COALESCE);
                }
                while wake_rx.try_recv().is_ok() {}
                // A stop requested while the burst settled still precedes
                // this tick.
                if worker_stop_requested.load(Ordering::Acquire) {
                    break;
                }
                if recovery_deadline.is_some_and(|deadline| Instant::now() < deadline) {
                    // A notification storm must not bypass a failed read's
                    // recovery deadline.
                    continue;
                }
                match on_tick(tick) {
                    SqliteWatchControl::Handled => {
                        recovery_deadline = None;
                        recovery_backoff = SQLITE_WATCH_RECOVERY_BACKOFF_MIN;
                    }
                    SqliteWatchControl::Failed => {
                        recovery_deadline = Some(Instant::now() + recovery_backoff);
                        recovery_backoff = recovery_backoff
                            .saturating_mul(2)
                            .min(SQLITE_WATCH_RECOVERY_BACKOFF_MAX);
                    }
                    SqliteWatchControl::Stop => break,
                }
            }
        })
        .map_err(|error| format!("cannot start the sqlite change watch thread: {error}"))?;

    let registration = RegistrationCustody::spawn(
        thread_name,
        backend,
        targets,
        wake_tx.clone(),
        Arc::clone(&stop_requested),
    );
    Ok(SqliteChangeWatch {
        control: Arc::new(WatchControl {
            stop_requested,
            wake: wake_tx,
        }),
        registration,
        worker: WorkerCustody::Running(worker),
    })
}

/// The registration thread: create the source and register it, then hand it
/// to the watch, or discard it when the watch stopped meanwhile.
fn register(
    backend: &dyn WatchBackend,
    targets: &WatchTargets,
    wake: mpsc::Sender<()>,
    stop_requested: &AtomicBool,
    slot: &Mutex<RegistrationSlot>,
) {
    if stop_requested.load(Ordering::Acquire) {
        *lock(slot) = RegistrationSlot::Cancelled;
        return;
    }
    let mut source = match backend.create(targets, wake) {
        Ok(source) => source,
        Err(reason) => {
            settle_error(slot, stop_requested, reason);
            return;
        }
    };
    // notify 8.2.0's inotify watcher ignores a failed start of its event-loop
    // thread; registering then panics, and so would its drop, aborting the
    // process. Contain the panic and leak that source (a sender and a waker)
    // instead of dropping it. An error, unlike a panic, comes from a live
    // event loop, so its source drops normally.
    match std::panic::catch_unwind(AssertUnwindSafe(|| source.watch(&targets.parent))) {
        Ok(Ok(())) => {}
        Ok(Err(reason)) => {
            drop(source);
            settle_error(slot, stop_requested, reason);
            return;
        }
        Err(_) => {
            std::mem::forget(source);
            settle_failed(
                slot,
                "registering the notification source panicked: its event loop is not running"
                    .to_string(),
            );
            return;
        }
    }
    {
        // The stop side sets its flag before it reads the slot, so a flag
        // read under the slot lock never leaves a source live past a stop.
        let mut settled = lock(slot);
        if !stop_requested.load(Ordering::Acquire) {
            *settled = RegistrationSlot::Registered(Some(source));
            return;
        }
    }
    // The watch stopped while the platform registered: discard the source
    // here, so its notifications never start, then settle.
    drop(source);
    *lock(slot) = RegistrationSlot::Cancelled;
}

/// Settle a registration that returned an error: failed, with a warning, while
/// its watch runs on its sweep alone; cancelled, quietly, once the watch has
/// stopped, since nothing is left to run on the sweep.
fn settle_error(slot: &Mutex<RegistrationSlot>, stop_requested: &AtomicBool, reason: String) {
    if stop_requested.load(Ordering::Acquire) {
        *lock(slot) = RegistrationSlot::Cancelled;
    } else {
        settle_failed(slot, reason);
    }
}

fn settle_failed(slot: &Mutex<RegistrationSlot>, reason: String) {
    tracing::warn!(
        %reason,
        "sqlite change watch notifications are unavailable; the watch runs on its sweep alone"
    );
    *lock(slot) = RegistrationSlot::Failed(reason);
}

/// Held watch registrations, for tests of code that starts SQLite change
/// watches. Every watch started on a database in a held directory blocks in
/// its notification registration, as a stalled platform registration (macOS
/// FSEvents) does, until the hold is released.
#[cfg(any(test, feature = "test-support"))]
pub mod test_support {
    use std::collections::HashMap;
    use std::io::Write as _;
    use std::path::{Path, PathBuf};
    use std::sync::atomic::{AtomicBool, AtomicUsize, Ordering};
    use std::sync::{Arc, Condvar, Mutex, OnceLock, PoisonError, mpsc};
    use std::time::{Duration, Instant};

    use super::{WatchBackend, WatchSource, WatchTargets, lock};

    /// The variable that slows every platform registration in a test process.
    pub const PLATFORM_REGISTRATION_DELAY_MS: &str =
        "MEERKAT_TEST_SQLITE_WATCH_REGISTRATION_DELAY_MS";

    /// How long every platform registration in this process waits before it
    /// registers: [`PLATFORM_REGISTRATION_DELAY_MS`] milliseconds, read once
    /// (unset: none). It makes the notify source register as slowly as a
    /// macOS FSEvents registration can (tens of seconds), on any platform.
    pub(super) fn platform_registration_delay() -> Option<Duration> {
        static DELAY: OnceLock<Option<Duration>> = OnceLock::new();
        *DELAY.get_or_init(|| {
            std::env::var(PLATFORM_REGISTRATION_DELAY_MS)
                .ok()
                .and_then(|millis| millis.parse().ok())
                .map(Duration::from_millis)
        })
    }

    /// The variable that names a file this test process appends a line to
    /// for every notification its platform watchers report, before anything
    /// filters it: when it arrived, the watched directory, the event (its
    /// kind, paths and attributes) and whether it was relevant. Rows add
    /// their own lines ([`log_raw`]). Unset: nothing is logged.
    pub const RAW_EVENT_LOG: &str = "MEERKAT_TEST_SQLITE_WATCH_EVENT_LOG";

    fn raw_event_log() -> Option<&'static Mutex<std::fs::File>> {
        static LOG: OnceLock<Option<Mutex<std::fs::File>>> = OnceLock::new();
        LOG.get_or_init(|| {
            let path = std::env::var_os(RAW_EVENT_LOG)?;
            std::fs::OpenOptions::new()
                .create(true)
                .append(true)
                .open(path)
                .ok()
                .map(Mutex::new)
        })
        .as_ref()
    }

    /// Append `line` to the [`RAW_EVENT_LOG`] file, after the time (seconds
    /// and microseconds since the Unix epoch) and this process's id. Without
    /// that variable it does nothing.
    pub fn log_raw(line: std::fmt::Arguments<'_>) {
        let Some(log) = raw_event_log() else {
            return;
        };
        let now = std::time::SystemTime::now()
            .duration_since(std::time::UNIX_EPOCH)
            .unwrap_or_default();
        let _ = writeln!(
            lock(log),
            "{}.{:06} pid={} {line}",
            now.as_secs(),
            now.subsec_micros(),
            std::process::id()
        );
    }

    /// Log one notification a platform watcher of `parent` reported, and
    /// whether the watch took it as relevant.
    pub(super) fn log_raw_event(
        parent: &Path,
        result: &notify::Result<notify::Event>,
        relevant: bool,
    ) {
        if raw_event_log().is_none() {
            return;
        }
        match result {
            Ok(event) => log_raw(format_args!(
                "event dir={} relevant={relevant} {event:?}",
                parent.display()
            )),
            Err(error) => log_raw(format_args!("error dir={} {error}", parent.display())),
        }
    }

    /// Held directories, by canonical path.
    static HOLDS: OnceLock<Mutex<HashMap<PathBuf, Arc<Hold>>>> = OnceLock::new();

    fn holds() -> &'static Mutex<HashMap<PathBuf, Arc<Hold>>> {
        HOLDS.get_or_init(Mutex::default)
    }

    #[derive(Default)]
    struct Hold {
        gate: Mutex<Gate>,
        changed: Condvar,
        live_sources: AtomicUsize,
        dropped_sources: AtomicUsize,
    }

    #[derive(Default)]
    struct Gate {
        released: bool,
        entered: usize,
    }

    /// Holds the notification registrations of the watches on databases in
    /// one directory. Dropping it releases them and ends the hold.
    pub struct HeldRegistrations {
        directory: PathBuf,
        hold: Arc<Hold>,
    }

    /// Hold the registration of every watch started from now on for a
    /// database in `directory` (an existing directory, matched by its
    /// canonical path), until the returned hold is released or dropped. A
    /// held registration has created its source and blocks in registering
    /// it; once released it registers, without ever notifying.
    pub fn hold_registrations_in(directory: &Path) -> std::io::Result<HeldRegistrations> {
        let directory = directory.canonicalize()?;
        let hold = Arc::new(Hold::default());
        let mut holds = lock(holds());
        if holds.contains_key(&directory) {
            return Err(std::io::Error::new(
                std::io::ErrorKind::AlreadyExists,
                format!("{} is already held", directory.display()),
            ));
        }
        holds.insert(directory.clone(), Arc::clone(&hold));
        Ok(HeldRegistrations { directory, hold })
    }

    impl HeldRegistrations {
        /// Wait, at most `bound`, until `count` registrations have entered
        /// the hold. Whether they had.
        pub fn wait_entered(&self, count: usize, bound: Duration) -> bool {
            let deadline = Instant::now() + bound;
            let mut gate = lock(&self.hold.gate);
            while gate.entered < count {
                let Some(left) = deadline.checked_duration_since(Instant::now()) else {
                    return false;
                };
                gate = self
                    .hold
                    .changed
                    .wait_timeout(gate, left)
                    .unwrap_or_else(PoisonError::into_inner)
                    .0;
            }
            true
        }

        /// Let every held registration register, and every later one in the
        /// directory register at once.
        pub fn release(&self) {
            lock(&self.hold.gate).released = true;
            self.hold.changed.notify_all();
        }

        /// Sources created through this hold and not dropped yet.
        pub fn live_sources(&self) -> usize {
            self.hold.live_sources.load(Ordering::SeqCst)
        }

        /// Sources created through this hold and dropped since.
        pub fn dropped_sources(&self) -> usize {
            self.hold.dropped_sources.load(Ordering::SeqCst)
        }
    }

    impl Drop for HeldRegistrations {
        fn drop(&mut self) {
            lock(holds()).remove(&self.directory);
            self.release();
        }
    }

    /// The held backend for a watch of `parent`, while a hold covers it.
    pub(super) fn held_backend(parent: &Path) -> Option<Box<dyn WatchBackend>> {
        let hold = lock(holds()).get(parent).cloned()?;
        Some(Box::new(HeldBackend(hold)))
    }

    struct HeldBackend(Arc<Hold>);

    impl WatchBackend for HeldBackend {
        fn create(
            &self,
            _targets: &WatchTargets,
            wake: mpsc::Sender<()>,
        ) -> Result<Box<dyn WatchSource>, String> {
            self.0.live_sources.fetch_add(1, Ordering::SeqCst);
            Ok(Box::new(HeldSource {
                hold: Arc::clone(&self.0),
                _wake: wake,
            }))
        }
    }

    struct HeldSource {
        hold: Arc<Hold>,
        /// Kept for the source's life, as a real source keeps its wake.
        _wake: mpsc::Sender<()>,
    }

    impl WatchSource for HeldSource {
        fn watch(&mut self, _directory: &Path) -> Result<(), String> {
            let mut gate = lock(&self.hold.gate);
            gate.entered += 1;
            self.hold.changed.notify_all();
            while !gate.released {
                gate = self
                    .hold
                    .changed
                    .wait(gate)
                    .unwrap_or_else(PoisonError::into_inner);
            }
            Ok(())
        }
    }

    impl Drop for HeldSource {
        fn drop(&mut self) {
            self.hold.live_sources.fetch_sub(1, Ordering::SeqCst);
            self.hold.dropped_sources.fetch_add(1, Ordering::SeqCst);
        }
    }

    /// Observed directories, by canonical path.
    static OBSERVATIONS: OnceLock<Mutex<HashMap<PathBuf, Arc<Observation>>>> = OnceLock::new();

    fn observations() -> &'static Mutex<HashMap<PathBuf, Arc<Observation>>> {
        OBSERVATIONS.get_or_init(Mutex::default)
    }

    #[derive(Default)]
    struct Observation {
        registered: Mutex<usize>,
        changed: Condvar,
        /// Report a database's WAL created or removed also as modified.
        wal_lifecycle_as_modified: AtomicBool,
        drops: Mutex<SourceDrops>,
        drops_changed: Condvar,
    }

    /// The observed sources' drops, while a test holds them.
    #[derive(Default)]
    struct SourceDrops {
        held: bool,
        finished: usize,
    }

    /// Observes the notification registrations of the watches on databases
    /// in one directory: they register with the platform as usual, and this
    /// counts the ones that did. Dropping it ends the observation.
    pub struct ObservedRegistrations {
        directory: PathBuf,
        observation: Arc<Observation>,
    }

    /// Observe the registration of every watch started from now on for a
    /// database in `directory` (an existing directory, matched by its
    /// canonical path), so a test can order its own file activity after a
    /// watch's registration ([`ObservedRegistrations::wait_registered`]).
    pub fn observe_registrations_in(directory: &Path) -> std::io::Result<ObservedRegistrations> {
        let directory = directory.canonicalize()?;
        let observation = Arc::new(Observation::default());
        let mut observations = lock(observations());
        if observations.contains_key(&directory) {
            return Err(std::io::Error::new(
                std::io::ErrorKind::AlreadyExists,
                format!("{} is already observed", directory.display()),
            ));
        }
        observations.insert(directory.clone(), Arc::clone(&observation));
        Ok(ObservedRegistrations {
            directory,
            observation,
        })
    }

    impl ObservedRegistrations {
        /// From now on, the observed watches also report a database's WAL
        /// created or removed as modified (`Modify(Any)`), as
        /// ReadDirectoryChangesW can report a new or removed file, so a
        /// connection's own open and close notify a WAL change. Each report
        /// still passes the watch's relevance filter.
        pub fn report_wal_lifecycle_as_modified(&self) {
            self.observation
                .wal_lifecycle_as_modified
                .store(true, Ordering::SeqCst);
        }

        /// From now on, dropping an observed source blocks until
        /// [`Self::release_source_drops`], as a macOS FSEvents watcher's drop
        /// waits on the file-system event service.
        pub fn hold_source_drops(&self) {
            lock(&self.observation.drops).held = true;
        }

        /// Let every held source drop, and every later one at once.
        pub fn release_source_drops(&self) {
            lock(&self.observation.drops).held = false;
            self.observation.drops_changed.notify_all();
        }

        /// Observed sources dropped so far.
        pub fn source_drops_finished(&self) -> usize {
            lock(&self.observation.drops).finished
        }

        /// Wait, at most `bound`, until `count` observed sources have been
        /// dropped. Whether they had.
        pub fn wait_source_drops_finished(&self, count: usize, bound: Duration) -> bool {
            let deadline = Instant::now() + bound;
            let mut drops = lock(&self.observation.drops);
            while drops.finished < count {
                let Some(left) = deadline.checked_duration_since(Instant::now()) else {
                    return false;
                };
                drops = self
                    .observation
                    .drops_changed
                    .wait_timeout(drops, left)
                    .unwrap_or_else(PoisonError::into_inner)
                    .0;
            }
            true
        }

        /// Wait, at most `bound`, until `count` registrations have
        /// registered. Whether they had.
        pub fn wait_registered(&self, count: usize, bound: Duration) -> bool {
            let deadline = Instant::now() + bound;
            let mut registered = lock(&self.observation.registered);
            while *registered < count {
                let Some(left) = deadline.checked_duration_since(Instant::now()) else {
                    return false;
                };
                registered = self
                    .observation
                    .changed
                    .wait_timeout(registered, left)
                    .unwrap_or_else(PoisonError::into_inner)
                    .0;
            }
            true
        }
    }

    impl Drop for ObservedRegistrations {
        fn drop(&mut self) {
            lock(observations()).remove(&self.directory);
            self.release_source_drops();
        }
    }

    /// The platform backend, observed, for a watch of `parent` while an
    /// observation covers it.
    pub(super) fn observed_backend(parent: &Path) -> Option<Box<dyn WatchBackend>> {
        let observation = lock(observations()).get(parent).cloned()?;
        Some(Box::new(ObservedBackend(observation)))
    }

    struct ObservedBackend(Arc<Observation>);

    impl WatchBackend for ObservedBackend {
        fn create(
            &self,
            targets: &WatchTargets,
            wake: mpsc::Sender<()>,
        ) -> Result<Box<dyn WatchSource>, String> {
            let inner = if self.0.wal_lifecycle_as_modified.load(Ordering::SeqCst) {
                wal_lifecycle_as_modified_source(targets, wake)?
            } else {
                super::NotifyBackend.create(targets, wake)?
            };
            Ok(Box::new(ObservedSource {
                inner: Some(inner),
                observation: Arc::clone(&self.0),
            }))
        }
    }

    /// The platform watcher, with a database's WAL created or removed also
    /// reported as modified; every report passes the relevance filter.
    fn wal_lifecycle_as_modified_source(
        targets: &WatchTargets,
        wake: mpsc::Sender<()>,
    ) -> Result<Box<dyn WatchSource>, String> {
        use notify::EventKind;
        let parent = targets.parent.clone();
        let watched_paths = targets.watched_paths.clone();
        let wals: Vec<PathBuf> = watched_paths
            .iter()
            .map(|path| super::sidecar_path(path, "-wal"))
            .filter(|wal| watched_paths.contains(wal))
            .collect();
        let watcher = notify::recommended_watcher(move |result: notify::Result<notify::Event>| {
            let Ok(event) = result else {
                log_raw_event(&parent, &result, false);
                return;
            };
            let mut reports = vec![event.clone()];
            if matches!(event.kind, EventKind::Create(_) | EventKind::Remove(_)) {
                for wal in event.paths.iter().filter(|path| wals.contains(path)) {
                    reports.push(
                        notify::Event::new(EventKind::Modify(notify::event::ModifyKind::Any))
                            .add_path(wal.clone()),
                    );
                }
            }
            let relevant = reports
                .iter()
                .any(|report| super::sqlite_watch_event_relevant(report, &parent, &watched_paths));
            log_raw_event(&parent, &Ok(event), relevant);
            if relevant {
                let _ = wake.send(());
            }
        })
        .map_err(|error| format!("cannot create the sqlite change watcher: {error}"))?;
        Ok(Box::new(super::NotifySource(watcher)))
    }

    struct ObservedSource {
        /// `None` once dropped.
        inner: Option<Box<dyn WatchSource>>,
        observation: Arc<Observation>,
    }

    impl WatchSource for ObservedSource {
        fn watch(&mut self, directory: &Path) -> Result<(), String> {
            if let Some(inner) = self.inner.as_mut() {
                inner.watch(directory)?;
            }
            *lock(&self.observation.registered) += 1;
            self.observation.changed.notify_all();
            Ok(())
        }

        /// The platform's answer, or that a test holds this source's drop.
        fn teardown_waits(&self) -> bool {
            self.inner
                .as_ref()
                .is_some_and(|inner| inner.teardown_waits())
                || lock(&self.observation.drops).held
        }
    }

    impl Drop for ObservedSource {
        fn drop(&mut self) {
            let mut drops = lock(&self.observation.drops);
            while drops.held {
                drops = self
                    .observation
                    .drops_changed
                    .wait(drops)
                    .unwrap_or_else(PoisonError::into_inner);
            }
            drop(drops);
            drop(self.inner.take());
            lock(&self.observation.drops).finished += 1;
            self.observation.drops_changed.notify_all();
        }
    }
}

#[cfg(test)]
#[allow(clippy::expect_used, clippy::unwrap_used)]
mod tests {
    use super::*;
    use std::sync::atomic::AtomicUsize;

    /// Wait, within the registration bound, until the watch's registration
    /// ends.
    fn settled_registration(watch: &SqliteChangeWatch) -> SqliteWatchRegistration {
        let deadline = Instant::now() + REGISTRATION_BOUND;
        loop {
            let registration = watch.registration();
            if registration != SqliteWatchRegistration::Pending || Instant::now() >= deadline {
                return registration;
            }
            thread::sleep(Duration::from_millis(2));
        }
    }

    fn retained_in(directory: &Path) -> Vec<SqliteWatchRetainedRegistration> {
        retained_registrations()
            .into_iter()
            .filter(|retained| retained.directory == directory)
            .collect()
    }

    /// Wait, within the hang guard, for a Changed tick, skipping the sweeps: a
    /// sweep that ticked while the registration was pending (a macOS FSEvents
    /// registration can outlast the sweep) comes first. A source that never
    /// notifies leaves sweeps only, and fails.
    fn next_changed(tick_rx: &mpsc::Receiver<SqliteWatchTick>) -> Result<(), String> {
        let deadline = Instant::now() + HANG_GUARD;
        let mut sweeps = 0_usize;
        loop {
            match tick_rx.recv_timeout(deadline.saturating_duration_since(Instant::now())) {
                Ok(SqliteWatchTick::Changed) => return Ok(()),
                Ok(SqliteWatchTick::Sweep) => sweeps += 1,
                Err(error) => {
                    return Err(format!(
                        "no Changed tick within the hang guard, {sweeps} sweeps: {error}"
                    ));
                }
            }
        }
    }

    /// Whether a Changed tick comes within `window`, skipping sweeps.
    fn changed_within(tick_rx: &mpsc::Receiver<SqliteWatchTick>, window: Duration) -> bool {
        let deadline = Instant::now() + window;
        loop {
            match tick_rx.recv_timeout(deadline.saturating_duration_since(Instant::now())) {
                Ok(SqliteWatchTick::Changed) => return true,
                Ok(SqliteWatchTick::Sweep) => {}
                Err(_) => return false,
            }
        }
    }

    /// Append `bytes` to the existing file at `path` and close it, as a
    /// commit appends frames to a WAL that a connection holds open.
    fn append(path: &Path, bytes: &[u8]) {
        use std::io::Write as _;
        let mut file = std::fs::OpenOptions::new().append(true).open(path).unwrap();
        file.write_all(bytes).unwrap();
        drop(file);
        log_row(
            path,
            format_args!("appended {} bytes and closed", bytes.len()),
        );
    }

    /// Log a row's step for the database file or sidecar `path`, naming its
    /// canonical directory as the notifications for it are logged (see
    /// [`test_support::RAW_EVENT_LOG`]).
    fn log_row(path: &Path, step: std::fmt::Arguments<'_>) {
        let dir = path
            .parent()
            .and_then(|parent| parent.canonicalize().ok())
            .unwrap_or_default();
        test_support::log_raw(format_args!(
            "row dir={} file={:?}: {step}",
            dir.display(),
            path.file_name().unwrap_or_default()
        ));
    }

    /// A WAL database's WAL exists before the watch starts, as it does while
    /// any connection holds the database open, so a write to it is a data
    /// change only. (Its creation is a connection's own open, which ticks
    /// nothing: see [`sqlite_watch_event_relevant`].)
    fn assert_database_write_ticks_changed(db: &Path) {
        log_row(db, format_args!("a write row starts on {}", db.display()));
        std::fs::write(db, b"").unwrap();
        let wal = sidecar_path(db, "-wal");
        std::fs::write(&wal, b"").unwrap();
        let (tick_tx, tick_rx) = mpsc::channel();
        let watch = start_sqlite_change_watch(db, "test-watch", SQLITE_WATCH_SWEEP, move |tick| {
            let _ = tick_tx.send(tick);
            SqliteWatchControl::Handled
        })
        .expect("watch starts");
        // Notifications are registered after the start returns: a write
        // before that is seen by the sweep alone.
        assert_eq!(
            settled_registration(&watch),
            SqliteWatchRegistration::Registered
        );
        append(&wal, b"frame");
        next_changed(&tick_rx).expect("the write is observed within the hang guard");
    }

    #[test]
    fn a_write_to_the_database_ticks_changed() {
        let dir = tempfile::tempdir().unwrap();
        assert_database_write_ticks_changed(&dir.path().join("store.sqlite3"));
    }

    #[cfg(unix)]
    #[test]
    fn symlinked_directory_targets_match_canonical_events_for_missing_files() {
        let dir = tempfile::tempdir().unwrap();
        let actual = dir.path().join("actual");
        let alias = dir.path().join("alias");
        std::fs::create_dir(&actual).unwrap();
        std::os::unix::fs::symlink(&actual, &alias).unwrap();
        let db = alias.join("store.sqlite3");
        let (parent, watched_paths) = sqlite_watch_targets(&db).unwrap();
        let canonical_parent = actual.canonicalize().unwrap();
        assert_eq!(parent, canonical_parent);
        assert_eq!(
            watched_paths,
            sqlite_watch_paths(&canonical_parent.join("store.sqlite3"))
        );
        assert!(!db.exists());
        for path in &watched_paths {
            assert!(!path.exists());
        }
        let [database, wal, _index] = <&[PathBuf; 3]>::try_from(watched_paths.as_slice())
            .expect("the database and its two sidecars");
        for kind in [
            notify::EventKind::Create(notify::event::CreateKind::File),
            notify::EventKind::Remove(notify::event::RemoveKind::File),
        ] {
            let event = notify::Event::new(kind).add_path(database.clone());
            assert!(sqlite_watch_event_relevant(&event, &parent, &watched_paths));
        }
        let append = notify::Event::new(notify::EventKind::Modify(
            notify::event::ModifyKind::Data(notify::event::DataChange::Any),
        ))
        .add_path(wal.clone());
        assert!(sqlite_watch_event_relevant(
            &append,
            &parent,
            &watched_paths
        ));
        for path in [
            canonical_parent.join("other.sqlite3-wal"),
            canonical_parent.join("other").join("store.sqlite3-wal"),
        ] {
            let event = notify::Event::new(notify::EventKind::Any).add_path(path);
            assert!(!sqlite_watch_event_relevant(
                &event,
                &parent,
                &watched_paths
            ));
        }
        let access = notify::Event::new(notify::EventKind::Access(notify::event::AccessKind::Any))
            .add_path(watched_paths[0].clone());
        assert!(!sqlite_watch_event_relevant(
            &access,
            &parent,
            &watched_paths
        ));
    }

    /// The `notify` kinds one platform backend reports for a WAL database's
    /// files: a connection's own open and close, and a commit.
    struct BackendShapes {
        /// The first connection to open the database creates its WAL and the
        /// WAL index.
        created: &'static [notify::EventKind],
        /// It writes the index (its truncation and size) and can touch the
        /// sidecars' metadata.
        written: &'static [notify::EventKind],
        /// The last connection to close removes both.
        removed: &'static [notify::EventKind],
        /// A commit appends to the WAL; without a WAL it writes the database.
        appended: &'static [notify::EventKind],
    }

    /// A connection's own open and close of a WAL database are no change,
    /// whichever kinds the backend reports them as; a commit is, and so is
    /// any event on the database file or the directory itself.
    fn assert_only_a_commit_is_a_change(shapes: &BackendShapes) {
        let parent = PathBuf::from("/data");
        let database = parent.join("store.sqlite3");
        let watched_paths = sqlite_watch_paths(&database);
        let wal = sidecar_path(&database, "-wal");
        let index = sidecar_path(&database, "-shm");
        let relevant = |kind: notify::EventKind, path: &Path| {
            sqlite_watch_event_relevant(
                &notify::Event::new(kind).add_path(path.to_path_buf()),
                &parent,
                &watched_paths,
            )
        };
        for &kind in shapes.created.iter().chain(shapes.removed) {
            assert!(
                !relevant(kind, &wal),
                "the WAL created or removed is no change: {kind:?}"
            );
        }
        for &kind in shapes
            .created
            .iter()
            .chain(shapes.written)
            .chain(shapes.removed)
        {
            assert!(
                !relevant(kind, &index),
                "nothing on the WAL index is a change: {kind:?}"
            );
        }
        for &kind in shapes.appended {
            assert!(relevant(kind, &wal), "a WAL append is a change: {kind:?}");
            assert!(
                relevant(kind, &database),
                "a database write is a change: {kind:?}"
            );
        }
        for &kind in shapes.created.iter().chain(shapes.removed) {
            assert!(
                relevant(kind, &database),
                "the database created or removed is a change: {kind:?}"
            );
            assert!(
                relevant(kind, &parent),
                "the directory itself is a change: {kind:?}"
            );
        }
        let rescan = notify::Event::new(notify::EventKind::Other);
        assert!(
            sqlite_watch_event_relevant(&rescan, &parent, &watched_paths),
            "a rescan names no path and is a change"
        );
    }

    /// inotify (Linux): IN_CREATE, IN_MODIFY, IN_ATTRIB and IN_DELETE.
    #[test]
    fn inotify_kinds_of_a_connections_own_open_and_close_are_no_change() {
        use notify::EventKind::{Create, Modify, Remove};
        use notify::event::{CreateKind, DataChange, MetadataKind, ModifyKind, RemoveKind};
        assert_only_a_commit_is_a_change(&BackendShapes {
            created: &[Create(CreateKind::File)],
            written: &[
                Modify(ModifyKind::Data(DataChange::Any)),
                Modify(ModifyKind::Metadata(MetadataKind::Any)),
            ],
            removed: &[Remove(RemoveKind::File)],
            appended: &[Modify(ModifyKind::Data(DataChange::Any))],
        });
    }

    /// FSEvents (macOS): one event per flag of a record, so a record that
    /// merges the WAL's creation with an append still carries the append's
    /// data change.
    #[test]
    fn fsevents_kinds_of_a_connections_own_open_and_close_are_no_change() {
        use notify::EventKind::{Create, Modify, Remove};
        use notify::event::{CreateKind, DataChange, MetadataKind, ModifyKind, RemoveKind};
        assert_only_a_commit_is_a_change(&BackendShapes {
            created: &[Create(CreateKind::File), Create(CreateKind::Any)],
            written: &[
                Modify(ModifyKind::Data(DataChange::Content)),
                Modify(ModifyKind::Metadata(MetadataKind::Any)),
                Modify(ModifyKind::Metadata(MetadataKind::Ownership)),
                Modify(ModifyKind::Metadata(MetadataKind::Extended)),
            ],
            removed: &[Remove(RemoveKind::File), Remove(RemoveKind::Any)],
            appended: &[Modify(ModifyKind::Data(DataChange::Content))],
        });
        let parent = PathBuf::from("/data");
        let watched_paths = sqlite_watch_paths(&parent.join("store.sqlite3"));
        let merged = [
            Create(CreateKind::File),
            Modify(ModifyKind::Metadata(MetadataKind::Any)),
            Modify(ModifyKind::Data(DataChange::Content)),
        ];
        assert!(
            merged.into_iter().any(|kind| sqlite_watch_event_relevant(
                &notify::Event::new(kind).add_path(watched_paths[1].clone()),
                &parent,
                &watched_paths,
            )),
            "a merged record of the WAL's creation and an append is a change"
        );
    }

    /// ReadDirectoryChangesW (Windows): added, modified and removed, with no
    /// finer kinds, so every modification of the WAL stays a change.
    #[test]
    fn read_directory_changes_kinds_of_a_connections_own_open_and_close_are_no_change() {
        use notify::EventKind::{Any, Create, Modify, Remove};
        use notify::event::{CreateKind, ModifyKind, RemoveKind};
        assert_only_a_commit_is_a_change(&BackendShapes {
            created: &[Create(CreateKind::Any)],
            written: &[Modify(ModifyKind::Any)],
            removed: &[Remove(RemoveKind::Any)],
            appended: &[Modify(ModifyKind::Any), Any],
        });
    }

    /// On a WAL database, a connection's own open (the first: it creates the
    /// WAL and its index) and close (the last: it removes them) tick no
    /// watch; its commit does. inotify reports all of it.
    #[cfg(target_os = "linux")]
    #[test]
    fn a_connections_own_open_and_close_tick_nothing_but_its_commit_does() {
        let dir = tempfile::tempdir().unwrap();
        let db = wal_database(dir.path());
        let wal = sidecar_path(&db, "-wal");
        let index = sidecar_path(&db, "-shm");
        assert!(
            !wal.exists() && !index.exists(),
            "the last close removed both"
        );
        let (tick_tx, tick_rx) = mpsc::channel();
        let watch = start_sqlite_change_watch(
            &db,
            "test-own-open",
            Duration::from_secs(3600),
            move |tick| {
                let _ = tick_tx.send(tick);
                SqliteWatchControl::Handled
            },
        )
        .expect("watch starts");
        assert_eq!(
            settled_registration(&watch),
            SqliteWatchRegistration::Registered
        );
        {
            let conn = crate::open(&db, crate::ConnectionProfile::PRIMARY).unwrap();
            let rows: i64 = conn
                .query_row("SELECT COUNT(*) FROM t", [], |row| row.get(0))
                .unwrap();
            assert_eq!(rows, 1);
            assert!(
                wal.exists() && index.exists(),
                "the first connection created the WAL and its index"
            );
        }
        assert!(
            !wal.exists() && !index.exists(),
            "the last connection removed them"
        );
        // The watch coalesces for 10 ms: a tick of the open or the close
        // would have come long before this.
        assert_eq!(
            tick_rx.recv_timeout(Duration::from_millis(500)),
            Err(mpsc::RecvTimeoutError::Timeout),
            "a connection's own open and close are no change"
        );
        {
            let conn = crate::open(&db, crate::ConnectionProfile::PRIMARY).unwrap();
            conn.execute("INSERT INTO t (v) VALUES (2)", []).unwrap();
        }
        assert_eq!(
            tick_rx
                .recv_timeout(HANG_GUARD)
                .expect("the commit ticks within the hang guard"),
            SqliteWatchTick::Changed
        );
    }

    /// How long a commit row waits for a commit's tick before its connection
    /// closes. What it saw is logged (see [`test_support::RAW_EVENT_LOG`]),
    /// never required: a platform can report a write only when the file
    /// written closes.
    const BEFORE_CLOSE_WINDOW: Duration = Duration::from_secs(2);

    /// A commit while another connection (a kept reader) holds a WAL database
    /// open appends to the existing WAL and ticks the watch. The committing
    /// connection closes after its commit, as the stores' connections do.
    #[test]
    fn a_commit_to_an_existing_wal_ticks_changed() {
        let dir = tempfile::tempdir().unwrap();
        let db = wal_database(dir.path());
        log_row(
            &db,
            format_args!("a_commit_to_an_existing_wal_ticks_changed starts"),
        );
        let wal = sidecar_path(&db, "-wal");
        let kept = crate::open(&db, crate::ConnectionProfile::PRIMARY).unwrap();
        let rows: i64 = kept
            .query_row("SELECT COUNT(*) FROM t", [], |row| row.get(0))
            .unwrap();
        assert_eq!(rows, 1);
        assert!(wal.exists(), "the kept connection holds the WAL open");
        let (tick_tx, tick_rx) = mpsc::channel();
        let watch = start_sqlite_change_watch(
            &db,
            "test-commit-existing-wal",
            Duration::from_secs(3600),
            move |tick| {
                let _ = tick_tx.send(tick);
                SqliteWatchControl::Handled
            },
        )
        .expect("watch starts");
        assert_eq!(
            settled_registration(&watch),
            SqliteWatchRegistration::Registered
        );
        let writer = crate::open(&db, crate::ConnectionProfile::PRIMARY).unwrap();
        writer.execute("INSERT INTO t (v) VALUES (2)", []).unwrap();
        let wal_len = std::fs::metadata(&wal).map(|metadata| metadata.len());
        log_row(&wal, format_args!("committed, WAL length {wal_len:?}"));
        let before_close = changed_within(&tick_rx, BEFORE_CLOSE_WINDOW);
        log_row(
            &wal,
            format_args!("Changed before the writer closed: {before_close}"),
        );
        drop(writer);
        if !before_close {
            next_changed(&tick_rx).expect("the commit ticks once its connection closed");
            log_row(&wal, format_args!("Changed after the writer closed"));
        }
        drop(kept);
    }

    /// A connection that opens a WAL database no other connection holds open
    /// creates the WAL, and its commit at once appends to that new WAL: the
    /// watch ticks. Whether it ticks before the connection closes (its close,
    /// the last, then checkpoints into the database file and removes the
    /// WAL) is logged, with every notification (see
    /// [`test_support::RAW_EVENT_LOG`]).
    #[test]
    fn a_commit_that_creates_the_wal_ticks_changed() {
        let dir = tempfile::tempdir().unwrap();
        let db = wal_database(dir.path());
        log_row(
            &db,
            format_args!("a_commit_that_creates_the_wal_ticks_changed starts"),
        );
        let wal = sidecar_path(&db, "-wal");
        assert!(!wal.exists(), "no connection holds the database open");
        let (tick_tx, tick_rx) = mpsc::channel();
        let watch = start_sqlite_change_watch(
            &db,
            "test-commit-creates-wal",
            Duration::from_secs(3600),
            move |tick| {
                let _ = tick_tx.send(tick);
                SqliteWatchControl::Handled
            },
        )
        .expect("watch starts");
        assert_eq!(
            settled_registration(&watch),
            SqliteWatchRegistration::Registered
        );
        let writer = crate::open(&db, crate::ConnectionProfile::PRIMARY).unwrap();
        writer.execute("INSERT INTO t (v) VALUES (2)", []).unwrap();
        let wal_len = std::fs::metadata(&wal).map(|metadata| metadata.len());
        log_row(&wal, format_args!("committed, WAL length {wal_len:?}"));
        let before_close = changed_within(&tick_rx, BEFORE_CLOSE_WINDOW);
        log_row(
            &wal,
            format_args!("Changed before the writer closed: {before_close}"),
        );
        drop(writer);
        assert!(!wal.exists(), "the last connection removed the WAL");
        if !before_close {
            next_changed(&tick_rx).expect("the commit ticks once its connection closed");
            log_row(&wal, format_args!("Changed after the writer closed"));
        }
    }

    #[cfg(unix)]
    #[test]
    fn a_write_through_a_symlinked_directory_ticks_changed() {
        let dir = tempfile::tempdir().unwrap();
        let actual = dir.path().join("actual");
        let alias = dir.path().join("alias");
        std::fs::create_dir(&actual).unwrap();
        std::os::unix::fs::symlink(&actual, &alias).unwrap();
        assert_database_write_ticks_changed(&alias.join("store.sqlite3"));
    }

    /// The sweep ticks with no notification. Once the watch registers, a
    /// platform can report the watched directory's own recent creation (macOS
    /// FSEvents does, and an event on the directory itself is relevant), so
    /// Changed ticks before the first sweep are skipped.
    #[test]
    fn the_sweep_ticks_without_a_notification() {
        let dir = tempfile::tempdir().unwrap();
        let db = dir.path().join("store.sqlite3");
        let (tick_tx, tick_rx) = mpsc::channel();
        let _watch =
            start_sqlite_change_watch(&db, "test-sweep", Duration::from_millis(50), move |tick| {
                let _ = tick_tx.send(tick);
                SqliteWatchControl::Handled
            })
            .expect("watch starts");
        next_sweep(&tick_rx).expect("the sweep ticks within the hang guard");
    }

    /// Wait, within the hang guard, for a Sweep tick, skipping Changed ticks.
    fn next_sweep(tick_rx: &mpsc::Receiver<SqliteWatchTick>) -> Result<(), String> {
        let deadline = Instant::now() + HANG_GUARD;
        let mut changed = 0_usize;
        loop {
            match tick_rx.recv_timeout(deadline.saturating_duration_since(Instant::now())) {
                Ok(SqliteWatchTick::Sweep) => return Ok(()),
                Ok(SqliteWatchTick::Changed) => changed += 1,
                Err(error) => {
                    return Err(format!(
                        "no Sweep tick within the hang guard, {changed} Changed: {error}"
                    ));
                }
            }
        }
    }

    #[test]
    fn stop_ends_the_worker() {
        let dir = tempfile::tempdir().unwrap();
        let db = dir.path().join("store.sqlite3");
        let (tick_tx, tick_rx) = mpsc::channel();
        let _watch =
            start_sqlite_change_watch(&db, "test-stop", Duration::from_millis(20), move |tick| {
                let _ = tick_tx.send(tick);
                SqliteWatchControl::Stop
            })
            .expect("watch starts");
        tick_rx
            .recv_timeout(Duration::from_secs(10))
            .expect("one tick");
        assert!(
            tick_rx.recv_timeout(Duration::from_millis(500)).is_err(),
            "no tick after Stop: the worker ended (its sender is gone)"
        );
    }

    /// A WAL database holding one row, as the stores keep theirs.
    fn wal_database(dir: &Path) -> PathBuf {
        let db = dir.join("store.sqlite3");
        let conn = crate::open(&db, crate::ConnectionProfile::PRIMARY).unwrap();
        conn.execute_batch("CREATE TABLE t (v INTEGER); INSERT INTO t (v) VALUES (1);")
            .unwrap();
        db
    }

    /// Counts ticks, and reports once when the worker drops its tick closure,
    /// which it does as it returns.
    struct TickProbe {
        ticks: Arc<AtomicUsize>,
        returned: mpsc::Sender<()>,
    }

    impl TickProbe {
        fn tick(&self) {
            self.ticks.fetch_add(1, Ordering::SeqCst);
        }
    }

    impl Drop for TickProbe {
        fn drop(&mut self) {
            let _ = self.returned.send(());
        }
    }

    #[test]
    fn stop_and_join_waits_for_the_tick_in_flight() {
        let dir = tempfile::tempdir().unwrap();
        let db = wal_database(dir.path());
        let (entered_tx, entered_rx) = mpsc::channel();
        let (release_tx, release_rx) = mpsc::channel::<()>();
        let finished = Arc::new(AtomicBool::new(false));
        let tick_finished = Arc::clone(&finished);
        let tick_db = db.clone();
        let mut watch = start_sqlite_change_watch(
            &db,
            "test-held-tick",
            Duration::from_millis(20),
            move |_tick| {
                if tick_finished.load(Ordering::SeqCst) {
                    return SqliteWatchControl::Handled;
                }
                // The held tick keeps a read connection open, as a store
                // catch-up does.
                let conn = crate::open(&tick_db, crate::ConnectionProfile::ReadOnly).unwrap();
                let _ = entered_tx.send(());
                let _ = release_rx.recv();
                let value: i64 = conn
                    .query_row("SELECT v FROM t", [], |row| row.get(0))
                    .unwrap();
                assert_eq!(value, 1);
                drop(conn);
                tick_finished.store(true, Ordering::SeqCst);
                SqliteWatchControl::Handled
            },
        )
        .expect("watch starts");
        entered_rx
            .recv_timeout(Duration::from_secs(10))
            .expect("a tick is held within the hang guard");

        let (stopping_tx, stopping_rx) = mpsc::channel();
        let stopper_finished = Arc::clone(&finished);
        let stopper = thread::spawn(move || {
            let _ = stopping_tx.send(());
            let outcome = watch.stop_and_join();
            (outcome, stopper_finished.load(Ordering::SeqCst))
        });
        stopping_rx.recv().unwrap();
        release_tx.send(()).unwrap();
        let (outcome, finished_at_return) = stopper.join().unwrap();

        assert!(
            finished_at_return,
            "stop_and_join returned while a tick still held its connection"
        );
        assert_eq!(outcome, SqliteWatchJoin::Joined);
        // The joined worker can never open the database again, so nothing
        // recreates a file while the directory goes: no sleep, no retry.
        std::fs::remove_dir_all(dir.path()).unwrap();
        assert!(!dir.path().exists());
    }

    #[test]
    fn stop_and_join_on_the_worker_thread_is_unjoined_and_keeps_custody() {
        let dir = tempfile::tempdir().unwrap();
        let db = dir.path().join("store.sqlite3");
        let custody: Arc<std::sync::Mutex<Option<SqliteChangeWatch>>> = Arc::default();
        let tick_custody = Arc::clone(&custody);
        let (outcome_tx, outcome_rx) = mpsc::channel();
        let watch = start_sqlite_change_watch(
            &db,
            "test-self-stop",
            Duration::from_millis(20),
            move |_| {
                let outcome = tick_custody
                    .lock()
                    .unwrap()
                    .as_mut()
                    .map(SqliteChangeWatch::stop_and_join);
                if let Some(outcome) = outcome {
                    let _ = outcome_tx.send(outcome);
                }
                SqliteWatchControl::Handled
            },
        )
        .expect("watch starts");
        *custody.lock().unwrap() = Some(watch);

        let on_worker = outcome_rx
            .recv_timeout(Duration::from_secs(10))
            .expect("a tick stops its own watch within the hang guard");
        assert_eq!(on_worker, SqliteWatchJoin::UnjoinedOnWorkerThread);

        // The self-stop kept the join handle, so from here the worker is
        // really joined rather than reported as gone.
        let mut watch = custody
            .lock()
            .unwrap()
            .take()
            .expect("the watch stays in custody");
        assert!(!watch.on_worker_thread());
        assert_eq!(watch.stop_and_join(), SqliteWatchJoin::Joined);
        assert!(watch.is_finished());
        assert_eq!(
            watch.stop_and_join(),
            SqliteWatchJoin::Joined,
            "a repeated stop reports the recorded join"
        );
    }

    #[test]
    fn dropping_the_watch_requests_the_stop_without_blocking() {
        let dir = tempfile::tempdir().unwrap();
        let db = dir.path().join("store.sqlite3");
        let (entered_tx, entered_rx) = mpsc::channel();
        let (release_tx, release_rx) = mpsc::channel::<()>();
        let (returned_tx, returned_rx) = mpsc::channel();
        let ticks = Arc::new(AtomicUsize::new(0));
        let probe = TickProbe {
            ticks: Arc::clone(&ticks),
            returned: returned_tx,
        };
        let watch =
            start_sqlite_change_watch(&db, "test-drop", Duration::from_millis(20), move |_| {
                probe.tick();
                let _ = entered_tx.send(());
                let _ = release_rx.recv();
                SqliteWatchControl::Handled
            })
            .expect("watch starts");
        entered_rx
            .recv_timeout(Duration::from_secs(10))
            .expect("a tick is held within the hang guard");

        // Dropping never waits for the held tick: this thread releases the
        // tick only after the drop returned.
        drop(watch);
        release_tx.send(()).unwrap();
        returned_rx
            .recv_timeout(Duration::from_secs(10))
            .expect("the worker returns after the held tick");
        assert_eq!(
            ticks.load(Ordering::SeqCst),
            1,
            "no tick starts after the stop request"
        );
    }

    /// How long a held registration may keep a start from returning, or the
    /// sweep from ticking, before the test calls it held.
    const HELD_BOUND: Duration = Duration::from_secs(5);
    const HANG_GUARD: Duration = Duration::from_secs(10);
    /// How long a row waits for a watch to register with the platform: a hang
    /// guard, not a latency bound. A macOS FSEvents registration can take tens
    /// of seconds ([`WatchSource::watch`]), so a minute; a registration that
    /// never returns still fails the row.
    const REGISTRATION_BOUND: Duration = Duration::from_secs(60);
    /// How long a row waits for released sources to finish their teardown:
    /// a hang guard, not a latency bound. Dropping a macOS FSEvents source
    /// joins its run-loop thread, which makes two round trips to the file
    /// system event service (the current event id, then a purge up to it)
    /// before it lets go ([`WatchSource::teardown_waits`]), and under load two
    /// of those teardowns in turn outlasted the 10 s guard. So a minute; a
    /// teardown that never ends still fails the row.
    const TEARDOWN_BOUND: Duration = Duration::from_secs(60);

    #[test]
    fn a_held_registration_does_not_hold_the_start() {
        let dir = tempfile::tempdir().unwrap();
        let held = test_support::hold_registrations_in(dir.path()).unwrap();
        let db = dir.path().join("store.sqlite3");
        let (started_tx, started_rx) = mpsc::channel();
        let starter = thread::spawn(move || {
            let watch =
                start_sqlite_change_watch(&db, "test-held-start", SQLITE_WATCH_SWEEP, |_| {
                    SqliteWatchControl::Handled
                });
            let _ = started_tx.send(());
            watch
        });
        assert!(
            held.wait_entered(1, HANG_GUARD),
            "the registration reached its platform call"
        );
        let started = started_rx.recv_timeout(HELD_BOUND);
        held.release();
        let watch = starter.join().unwrap().expect("watch starts");
        assert_eq!(
            started,
            Ok(()),
            "the start waited for the held notification registration"
        );
        drop(watch);
    }

    #[test]
    fn the_sweep_ticks_while_the_registration_is_held() {
        let dir = tempfile::tempdir().unwrap();
        let held = test_support::hold_registrations_in(dir.path()).unwrap();
        let db = dir.path().join("store.sqlite3");
        let (tick_tx, tick_rx) = mpsc::channel();
        let starter = thread::spawn(move || {
            start_sqlite_change_watch(
                &db,
                "test-held-sweep",
                Duration::from_millis(20),
                move |tick| {
                    let _ = tick_tx.send(tick);
                    SqliteWatchControl::Handled
                },
            )
        });
        assert!(
            held.wait_entered(1, HANG_GUARD),
            "the registration reached its platform call"
        );
        let tick = tick_rx.recv_timeout(HELD_BOUND);
        held.release();
        let watch = starter.join().unwrap().expect("watch starts");
        assert_eq!(
            tick,
            Ok(SqliteWatchTick::Sweep),
            "no sweep ran while the notification registration was held"
        );
        drop(watch);
    }

    /// A source whose registration fails, as inotify's does past the user's
    /// watch limit.
    struct FailingBackend;

    struct FailingSource;

    impl WatchBackend for FailingBackend {
        fn create(
            &self,
            _targets: &WatchTargets,
            _wake: mpsc::Sender<()>,
        ) -> Result<Box<dyn WatchSource>, String> {
            Ok(Box::new(FailingSource))
        }
    }

    impl WatchSource for FailingSource {
        fn watch(&mut self, directory: &Path) -> Result<(), String> {
            Err(format!(
                "cannot watch {}: the watch limit is reached",
                directory.display()
            ))
        }
    }

    #[test]
    fn a_failed_registration_keeps_the_sweep() {
        let dir = tempfile::tempdir().unwrap();
        let db = dir.path().join("store.sqlite3");
        let (tick_tx, tick_rx) = mpsc::channel();
        let watch = start_with_backend(
            &db,
            "test-failed-registration",
            Duration::from_millis(20),
            move |tick| {
                let _ = tick_tx.send(tick);
                SqliteWatchControl::Handled
            },
            |_| Box::new(FailingBackend),
        )
        .expect("a failed notification registration leaves the watch on its sweep");
        assert_eq!(tick_rx.recv_timeout(HANG_GUARD), Ok(SqliteWatchTick::Sweep));
        drop(watch);
    }

    /// Dropping a watch never waits for its notification source's teardown,
    /// which on macOS waits on the file-system event service; a watch started
    /// at once on the same database registers and sees a write meanwhile, and
    /// the stopped watch ticks no more.
    #[test]
    fn dropping_a_watch_never_waits_for_its_sources_teardown() {
        // It holds the process's unregister thread: alone in its process.
        alone_in_child_process(
            "dropping_a_watch_never_waits_for_its_sources_teardown",
            || {
                let dir = tempfile::tempdir().unwrap();
                let observed = test_support::observe_registrations_in(dir.path()).unwrap();
                let db = dir.path().join("store.sqlite3");
                log_row(
                    &db,
                    format_args!("dropping_a_watch_never_waits_for_its_sources_teardown starts"),
                );
                std::fs::write(&db, b"").unwrap();
                // The WAL exists before the watches start: the write below is
                // a data change only.
                let wal = sidecar_path(&db, "-wal");
                std::fs::write(&wal, b"").unwrap();
                let first_ticks = Arc::new(AtomicUsize::new(0));
                let first =
                    start_sqlite_change_watch(&db, "test-teardown-first", SQLITE_WATCH_SWEEP, {
                        let first_ticks = Arc::clone(&first_ticks);
                        move |_| {
                            first_ticks.fetch_add(1, Ordering::SeqCst);
                            SqliteWatchControl::Handled
                        }
                    })
                    .expect("watch starts");
                assert!(
                    observed.wait_registered(1, REGISTRATION_BOUND),
                    "the first registered: {:?}",
                    first.registration()
                );
                observed.hold_source_drops();

                let (dropped_tx, dropped_rx) = mpsc::channel();
                let dropper = thread::spawn(move || {
                    drop(first);
                    let _ = dropped_tx.send(());
                });
                let returned = dropped_rx.recv_timeout(HANG_GUARD).is_ok();
                if !returned {
                    observed.release_source_drops();
                }
                dropper.join().unwrap();
                assert!(
                    returned,
                    "dropping the watch waited for its source's teardown"
                );
                let first_ticked = first_ticks.load(Ordering::SeqCst);

                // A watch started at once on the same database works while the first
                // watch's source is still held.
                let (tick_tx, tick_rx) = mpsc::channel();
                let second =
                    start_sqlite_change_watch(&db, "test-teardown-second", SQLITE_WATCH_SWEEP, {
                        move |tick| {
                            let _ = tick_tx.send(tick);
                            SqliteWatchControl::Handled
                        }
                    })
                    .expect("watch starts");
                assert!(
                    observed.wait_registered(2, REGISTRATION_BOUND),
                    "the second registered: {:?}",
                    second.registration()
                );
                assert_eq!(
                    observed.source_drops_finished(),
                    0,
                    "the first source is held"
                );
                append(&wal, b"frame");
                next_changed(&tick_rx).expect("the second watch sees the write");
                // The held source still delivers that write, to the stopped watch
                // only, which ticks no more.
                assert_eq!(first_ticks.load(Ordering::SeqCst), first_ticked);

                drop(second);
                observed.release_source_drops();
                assert!(
                    observed.wait_source_drops_finished(2, TEARDOWN_BOUND),
                    "both sources are dropped once released"
                );
            },
        );
    }

    #[test]
    fn stopping_cancels_a_held_registration_and_keeps_its_thread() {
        let dir = tempfile::tempdir().unwrap();
        let held = test_support::hold_registrations_in(dir.path()).unwrap();
        let directory = dir.path().canonicalize().unwrap();
        let db = dir.path().join("store.sqlite3");
        let mut watch =
            start_sqlite_change_watch(&db, "test-held-stop", Duration::from_millis(20), |_| {
                SqliteWatchControl::Handled
            })
            .expect("watch starts");
        assert!(
            held.wait_entered(1, HANG_GUARD),
            "the registration reached its platform call"
        );

        // The stop joins the worker and never waits for the registration.
        assert_eq!(watch.stop_and_join(), SqliteWatchJoin::Joined);
        assert_eq!(watch.registration(), SqliteWatchRegistration::Pending);
        assert_eq!(held.live_sources(), 1);

        // Once the platform call returns, the registration discards its source
        // on its own thread instead of handing it to the stopped watch.
        held.release();
        assert_eq!(
            settled_registration(&watch),
            SqliteWatchRegistration::Cancelled
        );
        assert_eq!((held.live_sources(), held.dropped_sources()), (0, 1));
        // Its thread stayed with this watch, which joins it now.
        assert!(retained_in(&directory).is_empty());
        assert_eq!(watch.stop_and_join(), SqliteWatchJoin::Joined);
    }

    #[test]
    fn dropping_a_watch_retains_its_held_registration_until_it_returns() {
        let dir = tempfile::tempdir().unwrap();
        let held = test_support::hold_registrations_in(dir.path()).unwrap();
        let directory = dir.path().canonicalize().unwrap();
        let db = dir.path().join("store.sqlite3");
        let watch =
            start_sqlite_change_watch(&db, "test-held-drop", Duration::from_millis(20), |_| {
                SqliteWatchControl::Handled
            })
            .expect("watch starts");
        assert!(
            held.wait_entered(1, HANG_GUARD),
            "the registration reached its platform call"
        );

        // Dropping never waits for the registration and never detaches it:
        // the process keeps its thread, and reports it.
        drop(watch);
        let retained = retained_in(&directory);
        assert_eq!(retained.len(), 1, "{retained:?}");
        assert_eq!(retained[0].thread_name, "test-held-drop-register");
        assert_eq!(held.live_sources(), 1);

        // Once the platform call returns, the registration discards its
        // source, and the process joins its thread.
        held.release();
        let deadline = Instant::now() + HANG_GUARD;
        while !retained_in(&directory).is_empty() && Instant::now() < deadline {
            thread::sleep(Duration::from_millis(2));
        }
        assert!(
            retained_in(&directory).is_empty(),
            "the process joins a retained registration once it returns"
        );
        assert_eq!((held.live_sources(), held.dropped_sources()), (0, 1));
    }

    #[test]
    fn a_pending_registration_is_described_as_the_process_retains_it() {
        let dir = tempfile::tempdir().unwrap();
        let held = test_support::hold_registrations_in(dir.path()).unwrap();
        let directory = dir.path().canonicalize().unwrap();
        let db = dir.path().join("store.sqlite3");
        let watch =
            start_sqlite_change_watch(&db, "test-held-pending", Duration::from_millis(20), |_| {
                SqliteWatchControl::Handled
            })
            .expect("watch starts");
        assert!(
            held.wait_entered(1, HANG_GUARD),
            "the registration reached its platform call"
        );

        // The stop cancels the held registration without ending it: it is
        // still pending, and dropping the watch hands exactly it to the
        // process.
        watch.request_stop();
        let pending = watch
            .pending_registration()
            .expect("a held registration is pending");
        assert_eq!(pending.directory, directory);
        assert_eq!(pending.thread_name, "test-held-pending-register");
        drop(watch);
        let retained = retained_in(&directory);
        assert_eq!(retained.len(), 1, "{retained:?}");
        assert_eq!(
            (&retained[0].directory, &retained[0].thread_name),
            (&pending.directory, &pending.thread_name)
        );

        held.release();
        let deadline = Instant::now() + HANG_GUARD;
        while !retained_in(&directory).is_empty() && Instant::now() < deadline {
            thread::sleep(Duration::from_millis(2));
        }
        assert!(retained_in(&directory).is_empty());
    }

    #[test]
    fn a_settled_registration_is_not_pending() {
        let dir = tempfile::tempdir().unwrap();
        // Released before the watch starts: the registration completes at
        // once, without a platform call.
        let held = test_support::hold_registrations_in(dir.path()).unwrap();
        held.release();
        let db = dir.path().join("store.sqlite3");
        let watch =
            start_sqlite_change_watch(&db, "test-settled-pending", SQLITE_WATCH_SWEEP, |_| {
                SqliteWatchControl::Handled
            })
            .expect("watch starts");
        assert_eq!(
            settled_registration(&watch),
            SqliteWatchRegistration::Registered
        );
        assert_eq!(watch.pending_registration(), None);
        watch.request_stop();
        assert_eq!(watch.pending_registration(), None);
    }

    #[test]
    fn a_failed_registration_reports_its_reason() {
        let dir = tempfile::tempdir().unwrap();
        let db = dir.path().join("store.sqlite3");
        let watch = start_with_backend(
            &db,
            "test-failed-reason",
            SQLITE_WATCH_SWEEP,
            |_| SqliteWatchControl::Handled,
            |_| Box::new(FailingBackend),
        )
        .expect("watch starts");
        let registration = settled_registration(&watch);
        assert!(
            matches!(
                &registration,
                SqliteWatchRegistration::Failed(reason)
                    if reason.contains("the watch limit is reached")
            ),
            "{registration:?}"
        );
    }

    /// Set in the child process of the panicking-registration test.
    const PANIC_CHILD: &str = "MEERKAT_TEST_SQLITE_WATCH_PANIC_CHILD";
    /// What that child prints once its watch has survived.
    const PANIC_CHILD_DONE: &str = "SQLITE_WATCH_PANIC_CONTAINED";

    /// A source whose registration panics and whose drop panics again, as
    /// notify 8.2.0's inotify watcher does when its event-loop thread never
    /// started: dropping it while the first panic unwinds aborts the process.
    struct PanickingBackend;

    struct PanickingSource;

    impl WatchBackend for PanickingBackend {
        fn create(
            &self,
            _targets: &WatchTargets,
            _wake: mpsc::Sender<()>,
        ) -> Result<Box<dyn WatchSource>, String> {
            Ok(Box::new(PanickingSource))
        }
    }

    #[allow(clippy::panic)]
    impl WatchSource for PanickingSource {
        fn watch(&mut self, _directory: &Path) -> Result<(), String> {
            panic!("the notification event loop is gone");
        }
    }

    #[allow(clippy::panic)]
    impl Drop for PanickingSource {
        fn drop(&mut self) {
            panic!("the notification event loop is gone");
        }
    }

    /// A registered source whose teardown waits and whose drop panics.
    struct PanicsOnDropBackend;

    struct PanicsOnDropSource;

    impl WatchBackend for PanicsOnDropBackend {
        fn create(
            &self,
            _targets: &WatchTargets,
            _wake: mpsc::Sender<()>,
        ) -> Result<Box<dyn WatchSource>, String> {
            Ok(Box::new(PanicsOnDropSource))
        }
    }

    impl WatchSource for PanicsOnDropSource {
        fn watch(&mut self, _directory: &Path) -> Result<(), String> {
            Ok(())
        }

        fn teardown_waits(&self) -> bool {
            true
        }
    }

    #[allow(clippy::panic)]
    impl Drop for PanicsOnDropSource {
        fn drop(&mut self) {
            panic!("the run-loop thread did not shut down");
        }
    }

    /// The unregister thread contains a panicking source drop: a source whose
    /// drop panics, queued between two others, leaves both dropped, and the
    /// thread still drops a source queued after them.
    #[test]
    fn a_panicking_source_drop_leaves_the_unregister_thread_running() {
        // It holds the process's unregister thread: alone in its process.
        alone_in_child_process(
            "a_panicking_source_drop_leaves_the_unregister_thread_running",
            || {
                let dir = tempfile::tempdir().unwrap();
                let observed = test_support::observe_registrations_in(dir.path()).unwrap();
                let db = dir.path().join("store.sqlite3");
                std::fs::write(&db, b"").unwrap();
                let observed_watch = |name: &str| {
                    start_sqlite_change_watch(&db, name, SQLITE_WATCH_SWEEP, |_| {
                        SqliteWatchControl::Handled
                    })
                    .expect("watch starts")
                };
                let first = observed_watch("test-unregister-first");
                assert!(
                    observed.wait_registered(1, REGISTRATION_BOUND),
                    "the first registered: {:?}",
                    first.registration()
                );
                let panicking = start_with_backend(
                    &dir.path().join("panics.sqlite3"),
                    "test-unregister-panics",
                    SQLITE_WATCH_SWEEP,
                    |_| SqliteWatchControl::Handled,
                    |_| Box::new(PanicsOnDropBackend),
                )
                .expect("watch starts");
                assert_eq!(
                    settled_registration(&panicking),
                    SqliteWatchRegistration::Registered
                );
                let second = observed_watch("test-unregister-second");
                assert!(
                    observed.wait_registered(2, REGISTRATION_BOUND),
                    "the second registered: {:?}",
                    second.registration()
                );

                // Queued in this order behind the first source's held drop.
                observed.hold_source_drops();
                drop(first);
                drop(panicking);
                drop(second);
                observed.release_source_drops();
                assert!(
                    observed.wait_source_drops_finished(2, TEARDOWN_BOUND),
                    "both observed sources are dropped around the panicking one"
                );

                // The thread keeps going: a source queued after the panic goes to it,
                // so its held drop does not hold the dropping thread, and it is
                // dropped once released.
                let third = observed_watch("test-unregister-third");
                assert!(
                    observed.wait_registered(3, REGISTRATION_BOUND),
                    "the third registered: {:?}",
                    third.registration()
                );
                observed.hold_source_drops();
                let (dropped_tx, dropped_rx) = mpsc::channel();
                let dropper = thread::spawn(move || {
                    drop(third);
                    let _ = dropped_tx.send(());
                });
                let returned = dropped_rx.recv_timeout(HANG_GUARD).is_ok();
                observed.release_source_drops();
                dropper.join().unwrap();
                assert!(
                    returned,
                    "a source queued after the panic went to the unregister thread"
                );
                assert!(
                    observed.wait_source_drops_finished(3, TEARDOWN_BOUND),
                    "the unregister thread drops a source queued after the panic"
                );
                let unregister = UNREGISTER
                    .get()
                    .and_then(Option::as_ref)
                    .expect("the unregister thread started");
                assert!(
                    !unregister._thread.is_finished(),
                    "the unregister thread runs"
                );
            },
        );
    }

    /// The registration panics in a child process: a panic during a panic
    /// aborts the whole process, so only a child can show the watch survives.
    #[test]
    fn a_panicking_registration_leaves_the_sweep_running() {
        if std::env::var_os(PANIC_CHILD).is_some() {
            let dir = tempfile::tempdir().unwrap();
            let db = dir.path().join("store.sqlite3");
            let (tick_tx, tick_rx) = mpsc::channel();
            let watch = start_with_backend(
                &db,
                "test-panicking-registration",
                Duration::from_millis(20),
                move |tick| {
                    let _ = tick_tx.send(tick);
                    SqliteWatchControl::Handled
                },
                |_| Box::new(PanickingBackend),
            )
            .expect("watch starts");
            let registration = settled_registration(&watch);
            assert!(
                matches!(registration, SqliteWatchRegistration::Failed(_)),
                "{registration:?}"
            );
            assert_eq!(tick_rx.recv_timeout(HANG_GUARD), Ok(SqliteWatchTick::Sweep));
            drop(watch);
            println!("\n{PANIC_CHILD_DONE}");
            return;
        }
        #[allow(clippy::disallowed_methods)] // the test re-runs its own binary
        let output = std::process::Command::new(std::env::current_exe().unwrap())
            .args([
                "--exact",
                "watch::tests::a_panicking_registration_leaves_the_sweep_running",
                "--nocapture",
                "--test-threads=1",
            ])
            .env(PANIC_CHILD, "1")
            .output()
            .unwrap();
        let stdout = String::from_utf8_lossy(&output.stdout);
        assert!(
            output.status.success() && stdout.contains(PANIC_CHILD_DONE),
            "the watch did not survive its panicking registration ({}):\n{stdout}\n{}",
            output.status,
            String::from_utf8_lossy(&output.stderr)
        );
    }

    /// Set in the child process that runs one test of this module alone.
    const ALONE_CHILD: &str = "MEERKAT_TEST_SQLITE_WATCH_ALONE_CHILD";
    /// What that child prints once the test passed.
    const ALONE_CHILD_DONE: &str = "SQLITE_WATCH_ALONE_TEST_PASSED";

    /// Run `body`, the body of this module's test `test`, alone in a child
    /// process of this test binary, and require it to pass there. The
    /// registrations in flight are counted across the process, so a test
    /// that takes every place shares its process with no other watch test,
    /// whatever runs the tests: nextest's process per test, or plain `cargo
    /// test`'s concurrent threads in one process.
    fn alone_in_child_process(test: &str, body: impl FnOnce()) {
        if std::env::var_os(ALONE_CHILD).is_some() {
            body();
            println!("\n{ALONE_CHILD_DONE}");
            return;
        }
        #[allow(clippy::disallowed_methods)] // the test re-runs its own binary
        let output = std::process::Command::new(std::env::current_exe().unwrap())
            .args([
                "--exact",
                &format!("watch::tests::{test}"),
                "--nocapture",
                "--test-threads=1",
            ])
            .env(ALONE_CHILD, "1")
            .output()
            .unwrap();
        let stdout = String::from_utf8_lossy(&output.stdout);
        assert!(
            output.status.success() && stdout.contains(ALONE_CHILD_DONE),
            "{test} failed alone in its child process ({}):\n{stdout}\n{}",
            output.status,
            String::from_utf8_lossy(&output.stderr)
        );
    }

    #[test]
    fn registrations_past_the_cap_leave_new_watches_on_their_sweep() {
        alone_in_child_process(
            "registrations_past_the_cap_leave_new_watches_on_their_sweep",
            || {
                // Alone in its process: no other registration holds a place.
                assert_eq!(REGISTRATIONS_IN_FLIGHT.load(Ordering::Acquire), 0);
                let dir = tempfile::tempdir().unwrap();
                let held = test_support::hold_registrations_in(dir.path()).unwrap();
                let watches: Vec<_> = (0..MAX_REGISTRATIONS_IN_FLIGHT)
                    .map(|index| {
                        start_sqlite_change_watch(
                            &dir.path().join(format!("store-{index}.sqlite3")),
                            "test-cap-held",
                            SQLITE_WATCH_SWEEP,
                            |_| SqliteWatchControl::Handled,
                        )
                        .expect("watch starts")
                    })
                    .collect();
                assert!(
                    held.wait_entered(MAX_REGISTRATIONS_IN_FLIGHT, HANG_GUARD),
                    "every registration under the cap reached its platform call"
                );

                // One more watch starts with no registration thread, on its sweep
                // alone.
                let (tick_tx, tick_rx) = mpsc::channel();
                let over = start_sqlite_change_watch(
                    &dir.path().join("store-over.sqlite3"),
                    "test-cap-over",
                    Duration::from_millis(20),
                    move |tick| {
                        let _ = tick_tx.send(tick);
                        SqliteWatchControl::Handled
                    },
                )
                .expect("a watch past the cap still starts");
                let registration = over.registration();
                assert!(
                    matches!(
                        &registration,
                        SqliteWatchRegistration::Failed(reason) if reason.contains("already pending")
                    ),
                    "{registration:?}"
                );
                assert!(
                    over.registration.thread.is_none(),
                    "no registration thread started past the cap"
                );
                assert_eq!(held.live_sources(), MAX_REGISTRATIONS_IN_FLIGHT);
                assert_eq!(tick_rx.recv_timeout(HANG_GUARD), Ok(SqliteWatchTick::Sweep));

                // Released, every held registration settles and gives its place back,
                // and a fresh watch registers again.
                held.release();
                for watch in &watches {
                    assert_eq!(
                        settled_registration(watch),
                        SqliteWatchRegistration::Registered
                    );
                }
                let deadline = Instant::now() + HANG_GUARD;
                while REGISTRATIONS_IN_FLIGHT.load(Ordering::Acquire) > 0
                    && Instant::now() < deadline
                {
                    thread::sleep(Duration::from_millis(2));
                }
                let fresh_dir = tempfile::tempdir().unwrap();
                let fresh = start_sqlite_change_watch(
                    &fresh_dir.path().join("store.sqlite3"),
                    "test-cap-fresh",
                    SQLITE_WATCH_SWEEP,
                    |_| SqliteWatchControl::Handled,
                )
                .expect("watch starts");
                assert_eq!(
                    settled_registration(&fresh),
                    SqliteWatchRegistration::Registered,
                    "a fresh watch registers once every place is given back"
                );
                drop(watches);
                drop(over);
            },
        );
    }

    /// A burst of 64 registrations held at once, as a parallel startup holds
    /// them while FSEvents stalls, all reach their platform call: none is
    /// refused at the cap and left on its sweep for life. The burst is
    /// written out, not taken from the cap, so a lower cap fails here.
    #[test]
    fn a_held_burst_of_64_registrations_all_reach_their_platform_call() {
        alone_in_child_process(
            "a_held_burst_of_64_registrations_all_reach_their_platform_call",
            || {
                const BURST: usize = 64;
                // Alone in its process: no other registration holds a place.
                assert_eq!(REGISTRATIONS_IN_FLIGHT.load(Ordering::Acquire), 0);
                let dir = tempfile::tempdir().unwrap();
                let held = test_support::hold_registrations_in(dir.path()).unwrap();
                let watches: Vec<_> = (0..BURST)
                    .map(|index| {
                        start_sqlite_change_watch(
                            &dir.path().join(format!("store-{index}.sqlite3")),
                            "test-burst-held",
                            SQLITE_WATCH_SWEEP,
                            |_| SqliteWatchControl::Handled,
                        )
                        .expect("watch starts")
                    })
                    .collect();
                let refused = watches
                    .iter()
                    .filter(|watch| {
                        matches!(watch.registration(), SqliteWatchRegistration::Failed(_))
                    })
                    .count();
                assert_eq!(refused, 0, "registrations of the burst refused at the cap");
                assert!(
                    held.wait_entered(BURST, HANG_GUARD),
                    "every registration of the burst reached its platform call"
                );
                held.release();
                for watch in &watches {
                    assert_eq!(
                        settled_registration(watch),
                        SqliteWatchRegistration::Registered
                    );
                }
            },
        );
    }

    /// Every WARN event this test process emitted, with the name of the
    /// thread that emitted it.
    static WARNINGS: Mutex<Vec<(Option<String>, String)>> = Mutex::new(Vec::new());

    /// A process-wide subscriber that records WARN events into [`WARNINGS`]: a
    /// registration settles on its own thread, which a thread-local
    /// subscriber would never see.
    struct WarningCapture;

    struct MessageField(String);

    impl tracing::field::Visit for MessageField {
        fn record_debug(&mut self, field: &tracing::field::Field, value: &dyn std::fmt::Debug) {
            if field.name() == "message" {
                self.0 = format!("{value:?}");
            }
        }
    }

    impl tracing::Subscriber for WarningCapture {
        fn enabled(&self, metadata: &tracing::Metadata<'_>) -> bool {
            *metadata.level() == tracing::Level::WARN
        }

        fn new_span(&self, _span: &tracing::span::Attributes<'_>) -> tracing::span::Id {
            tracing::span::Id::from_u64(1)
        }

        fn record(&self, _span: &tracing::span::Id, _values: &tracing::span::Record<'_>) {}

        fn record_follows_from(&self, _span: &tracing::span::Id, _follows: &tracing::span::Id) {}

        fn event(&self, event: &tracing::Event<'_>) {
            let mut message = MessageField(String::new());
            event.record(&mut message);
            lock(&WARNINGS).push((thread::current().name().map(str::to_string), message.0));
        }

        fn enter(&self, _span: &tracing::span::Id) {}

        fn exit(&self, _span: &tracing::span::Id) {}
    }

    /// Install [`WarningCapture`] as this process's global subscriber, once.
    fn capture_warnings() {
        static INSTALLED: OnceLock<bool> = OnceLock::new();
        let installed = *INSTALLED
            .get_or_init(|| tracing::subscriber::set_global_default(WarningCapture).is_ok());
        assert!(
            installed,
            "the warning capture is this process's global subscriber"
        );
    }

    fn warnings_from(thread_name: &str) -> Vec<String> {
        lock(&WARNINGS)
            .iter()
            .filter(|(name, _)| name.as_deref() == Some(thread_name))
            .map(|(_, message)| message.clone())
            .collect()
    }

    /// A source whose registration waits for its release, then fails, as an
    /// inotify watch past the user's limit does.
    struct GatedFailingBackend {
        entered: mpsc::Sender<()>,
        release: Mutex<Option<mpsc::Receiver<()>>>,
    }

    struct GatedFailingSource {
        entered: mpsc::Sender<()>,
        release: mpsc::Receiver<()>,
    }

    impl WatchBackend for GatedFailingBackend {
        fn create(
            &self,
            _targets: &WatchTargets,
            _wake: mpsc::Sender<()>,
        ) -> Result<Box<dyn WatchSource>, String> {
            let release = lock(&self.release)
                .take()
                .ok_or_else(|| "created twice".to_string())?;
            Ok(Box::new(GatedFailingSource {
                entered: self.entered.clone(),
                release,
            }))
        }
    }

    impl WatchSource for GatedFailingSource {
        fn watch(&mut self, directory: &Path) -> Result<(), String> {
            let _ = self.entered.send(());
            let _ = self.release.recv();
            Err(format!(
                "cannot watch {}: the watch limit is reached",
                directory.display()
            ))
        }
    }

    /// Start a watch whose registration fails once released. Returns the
    /// watch, the signal that its registration entered, and its release.
    fn start_failing_after_release(
        dir: &Path,
        name: &str,
    ) -> (SqliteChangeWatch, mpsc::Receiver<()>, mpsc::Sender<()>) {
        let (entered_tx, entered_rx) = mpsc::channel();
        let (release_tx, release_rx) = mpsc::channel();
        let backend = GatedFailingBackend {
            entered: entered_tx,
            release: Mutex::new(Some(release_rx)),
        };
        let watch = start_with_backend(
            &dir.join("store.sqlite3"),
            name,
            SQLITE_WATCH_SWEEP,
            |_| SqliteWatchControl::Handled,
            move |_| Box::new(backend),
        )
        .expect("watch starts");
        (watch, entered_rx, release_tx)
    }

    #[test]
    fn a_registration_failing_after_its_watch_stopped_is_cancelled_quietly() {
        capture_warnings();
        // A live watch whose registration fails reports it and warns once:
        // the capture sees registration threads.
        let live_dir = tempfile::tempdir().unwrap();
        let (live, live_entered, live_release) =
            start_failing_after_release(live_dir.path(), "test-fail-live");
        live_entered
            .recv_timeout(HANG_GUARD)
            .expect("the live registration reached its platform call");
        live_release.send(()).unwrap();
        assert!(matches!(
            settled_registration(&live),
            SqliteWatchRegistration::Failed(_)
        ));
        assert_eq!(warnings_from("test-fail-live-register").len(), 1);

        // A watch that stopped first is gone: its failed registration is
        // cancelled, with no warning about a sweep nobody runs any more.
        let stopped_dir = tempfile::tempdir().unwrap();
        let (mut stopped, stopped_entered, stopped_release) =
            start_failing_after_release(stopped_dir.path(), "test-fail-stopped");
        stopped_entered
            .recv_timeout(HANG_GUARD)
            .expect("the registration reached its platform call");
        assert_eq!(stopped.stop_and_join(), SqliteWatchJoin::Joined);
        stopped_release.send(()).unwrap();
        assert_eq!(
            (
                settled_registration(&stopped),
                warnings_from("test-fail-stopped-register")
            ),
            (SqliteWatchRegistration::Cancelled, Vec::<String>::new())
        );
    }
}