//! Connection open posture, notify machinery, bootstrap, and the
//! writer/reader connection architecture — ported from honker-core
//! `lib.rs` @ `f4e53c6` (upstream names preserved; the table family
//! renamed `_honker_*` → `__alkstore_*` per ADR-011; the bootstrap's
//! duplicate-column race swallow re-keyed to `pragma_table_info`
//! per ADR-012 §5; the at-attach notifications pruning cap per
//! ADR-010 §6 / register D-16).

use parking_lot::{Condvar, Mutex};
use rusqlite::functions::FunctionFlags;
use rusqlite::{Connection, OpenFlags};
use std::sync::atomic::{AtomicBool, Ordering};

#[derive(thiserror::Error, Debug)]
pub(crate) enum Error {
    #[error("Database error: {0}")]
    Sqlite(#[from] rusqlite::Error),
}

pub(crate) const DEFAULT_PRAGMAS: &str = "PRAGMA busy_timeout = 5000;
         PRAGMA synchronous = NORMAL;
         PRAGMA foreign_keys = ON;
         PRAGMA cache_size = -32000;
         PRAGMA temp_store = MEMORY;
         PRAGMA wal_autocheckpoint = 10000;";

pub(crate) fn apply_default_pragmas(conn: &Connection) -> rusqlite::Result<()> {
    conn.execute_batch("PRAGMA busy_timeout = 5000;")?;
    set_journal_mode_wal(conn)?;
    conn.execute_batch(DEFAULT_PRAGMAS)
}

fn set_journal_mode_wal(conn: &Connection) -> rusqlite::Result<()> {
    let already_wal = |conn: &Connection| -> rusqlite::Result<bool> {
        let mode: String = conn.query_row("PRAGMA journal_mode", [], |row| row.get(0))?;
        Ok(mode.eq_ignore_ascii_case("wal"))
    };

    if already_wal(conn)? {
        return Ok(());
    }

    let deadline = std::time::Instant::now() + std::time::Duration::from_millis(5000);
    let mut backoff = std::time::Duration::from_millis(1);
    loop {
        match conn.execute_batch("PRAGMA journal_mode = WAL;") {
            Ok(()) => return Ok(()),
            Err(e) => {
                if already_wal(conn).unwrap_or(false) {
                    return Ok(());
                }
                if std::time::Instant::now() >= deadline {
                    return Err(e);
                }
                std::thread::sleep(backoff);
                backoff = (backoff * 2).min(std::time::Duration::from_millis(50));
            }
        }
    }
}

/// Upper bound on `__alkstore_notifications` rows before the next
/// attach prunes oldest-first past it (register D-16 — ADR-010 §6's
/// at-attach cap; engine-internal hygiene, not consumer-visible).
///
/// Not upstream. Upstream's `attach_notify` never prunes ("no magic
/// timer" — callers invoke `prune_notifications` explicitly); this
/// substrate's postures pins hygiene at attach so the engine grows
/// nothing unbounded by deferring to a caller who never prunes.
/// Rows are only trimmed while their senders' transactions are
/// committed — pruning runs on a fresh connection before its
/// `notify()` function is registered, so no in-flight wake row can
/// race the DELETE.
pub(crate) const NOTIFY_ATTACH_MAX_ROWS: i64 = 10_000;

/// Install the `__alkstore_notifications` table and the
/// `notify(channel, payload)` SQL scalar function on `conn`.
/// Idempotent.
pub(crate) fn attach_notify(conn: &Connection) -> Result<(), Error> {
    conn.execute_batch(
        "CREATE TABLE IF NOT EXISTS __alkstore_notifications (
           id INTEGER PRIMARY KEY AUTOINCREMENT,
           channel TEXT NOT NULL,
           payload TEXT NOT NULL,
           created_at INTEGER NOT NULL DEFAULT (unixepoch())
         );
         CREATE INDEX IF NOT EXISTS __alkstore_notifications_recent
           ON __alkstore_notifications(channel, id);",
    )?;

    conn.execute(
        "DELETE FROM __alkstore_notifications
         WHERE id IN (
           SELECT id FROM __alkstore_notifications
           ORDER BY id DESC
           LIMIT -1 OFFSET ?1
         )",
        [NOTIFY_ATTACH_MAX_ROWS],
    )?;

    conn.create_scalar_function("notify", 2, FunctionFlags::SQLITE_UTF8, |ctx| {
        let channel: String = ctx.get(0)?;
        let payload: String = ctx.get(1)?;
        let db = unsafe { ctx.get_connection() }?;
        let mut ins = db.prepare_cached(
            "INSERT INTO __alkstore_notifications (channel, payload) VALUES (?1, ?2)",
        )?;
        let id = ins.insert(rusqlite::params![channel, payload])?;
        Ok(id)
    })?;

    Ok(())
}

pub(crate) const BOOTSTRAP_ALKSTORE_SQL: &str = "
    CREATE TABLE IF NOT EXISTS __alkstore_live (
      id INTEGER PRIMARY KEY AUTOINCREMENT,
      queue TEXT NOT NULL,
      payload TEXT NOT NULL,
      state TEXT NOT NULL DEFAULT 'pending',
      priority INTEGER NOT NULL DEFAULT 0,
      run_at INTEGER NOT NULL DEFAULT (unixepoch()),
      worker_id TEXT,
      claim_expires_at INTEGER,
      attempts INTEGER NOT NULL DEFAULT 0,
      max_attempts INTEGER NOT NULL DEFAULT 3,
      created_at INTEGER NOT NULL DEFAULT (unixepoch()),
      expires_at INTEGER,
      claimed_at INTEGER,
      visibility_timeout_s INTEGER NOT NULL DEFAULT 300,
      backoff_base_s INTEGER NOT NULL DEFAULT 5,
      dead_letter_retention_s INTEGER
    );
    CREATE INDEX IF NOT EXISTS __alkstore_live_claim
      ON __alkstore_live(queue, priority DESC, run_at, id)
      WHERE state IN ('pending', 'processing');
    CREATE INDEX IF NOT EXISTS __alkstore_live_pending_deadline
      ON __alkstore_live(queue, run_at)
      WHERE state = 'pending';
    CREATE INDEX IF NOT EXISTS __alkstore_live_processing_deadline
      ON __alkstore_live(queue, claim_expires_at)
      WHERE state = 'processing';
    CREATE TABLE IF NOT EXISTS __alkstore_dead (
      id INTEGER PRIMARY KEY,
      queue TEXT NOT NULL,
      payload TEXT NOT NULL,
      priority INTEGER NOT NULL DEFAULT 0,
      run_at INTEGER NOT NULL DEFAULT 0,
      attempts INTEGER NOT NULL DEFAULT 0,
      max_attempts INTEGER NOT NULL DEFAULT 0,
      last_error TEXT,
      created_at INTEGER NOT NULL DEFAULT (unixepoch()),
      died_at INTEGER NOT NULL DEFAULT (unixepoch()),
      visibility_timeout_s INTEGER NOT NULL DEFAULT 300,
      backoff_base_s INTEGER NOT NULL DEFAULT 5,
      dead_letter_retention_s INTEGER,
      claimed_at INTEGER,
      expires_at INTEGER
    );
    CREATE INDEX IF NOT EXISTS __alkstore_dead_retention
      ON __alkstore_dead(queue, died_at);
    CREATE TABLE IF NOT EXISTS __alkstore_locks (
      name TEXT PRIMARY KEY,
      owner TEXT NOT NULL,
      expires_at INTEGER NOT NULL
    );
    CREATE TABLE IF NOT EXISTS __alkstore_scheduler_tasks (
      name TEXT PRIMARY KEY,
      queue TEXT NOT NULL,
      spec TEXT NOT NULL,
      payload TEXT NOT NULL,
      priority INTEGER NOT NULL DEFAULT 0,
      expires_s INTEGER,
      next_fire_at INTEGER NOT NULL,
      enabled INTEGER NOT NULL DEFAULT 1,
      max_attempts INTEGER NOT NULL DEFAULT 3,
      visibility_timeout_s INTEGER NOT NULL DEFAULT 300,
      backoff_base_s INTEGER NOT NULL DEFAULT 5,
      dead_letter_retention_s INTEGER
    );
    CREATE TABLE IF NOT EXISTS __alkstore_stream (
      offset INTEGER PRIMARY KEY AUTOINCREMENT,
      topic TEXT NOT NULL,
      key TEXT,
      payload TEXT NOT NULL,
      created_at INTEGER NOT NULL DEFAULT (unixepoch())
    );
    CREATE INDEX IF NOT EXISTS __alkstore_stream_topic
      ON __alkstore_stream(topic, offset);
    CREATE TABLE IF NOT EXISTS __alkstore_stream_consumers (
      name TEXT NOT NULL,
      topic TEXT NOT NULL,
      offset INTEGER NOT NULL DEFAULT 0,
      PRIMARY KEY (name, topic)
    );
";

fn column_present(conn: &Connection, table: &str, column: &str) -> bool {
    let mut stmt = match conn.prepare(&format!(
        "SELECT 1 FROM pragma_table_info('{table}') WHERE name = ?1"
    )) {
        Ok(stmt) => stmt,
        Err(_) => return false,
    };
    stmt.query_row([column], |_| Ok(true)).unwrap_or(false)
}

fn add_column_if_absent(
    conn: &Connection,
    table: &str,
    column: &str,
    ddl: &str,
) -> Result<(), Error> {
    if column_present(conn, table, column) {
        return Ok(());
    }
    match conn.execute(&format!("ALTER TABLE {table} ADD COLUMN {ddl}"), []) {
        Ok(_) => Ok(()),
        Err(e) => {
            if column_present(conn, table, column) {
                Ok(())
            } else {
                Err(e.into())
            }
        }
    }
}

/// Install the alkstore queue schema on `conn`. Idempotent.
///
/// Fresh bootstrap only (ADR-012 §5): a file carrying honker's legacy
/// `_honker_*` tables gets inert orphans — never touched, never read,
/// never migrated. The migrations below exist for the same-schema
/// case: a database this bootstrap already created in an earlier
/// revision, missing a column appended since.
pub(crate) fn bootstrap_schema(conn: &Connection) -> Result<(), Error> {
    conn.execute_batch(BOOTSTRAP_ALKSTORE_SQL)?;
    add_column_if_absent(
        conn,
        "__alkstore_scheduler_tasks",
        "enabled",
        "enabled INTEGER NOT NULL DEFAULT 1",
    )?;
    add_column_if_absent(
        conn,
        "__alkstore_scheduler_tasks",
        "max_attempts",
        "max_attempts INTEGER NOT NULL DEFAULT 3",
    )?;
    add_column_if_absent(
        conn,
        "__alkstore_scheduler_tasks",
        "visibility_timeout_s",
        "visibility_timeout_s INTEGER NOT NULL DEFAULT 300",
    )?;
    add_column_if_absent(
        conn,
        "__alkstore_scheduler_tasks",
        "backoff_base_s",
        "backoff_base_s INTEGER NOT NULL DEFAULT 5",
    )?;
    add_column_if_absent(
        conn,
        "__alkstore_scheduler_tasks",
        "dead_letter_retention_s",
        "dead_letter_retention_s INTEGER",
    )?;
    add_column_if_absent(conn, "__alkstore_live", "claimed_at", "claimed_at INTEGER")?;
    add_column_if_absent(
        conn,
        "__alkstore_live",
        "visibility_timeout_s",
        "visibility_timeout_s INTEGER NOT NULL DEFAULT 300",
    )?;
    add_column_if_absent(
        conn,
        "__alkstore_live",
        "backoff_base_s",
        "backoff_base_s INTEGER NOT NULL DEFAULT 5",
    )?;
    add_column_if_absent(
        conn,
        "__alkstore_live",
        "dead_letter_retention_s",
        "dead_letter_retention_s INTEGER",
    )?;
    add_column_if_absent(
        conn,
        "__alkstore_dead",
        "visibility_timeout_s",
        "visibility_timeout_s INTEGER NOT NULL DEFAULT 300",
    )?;
    add_column_if_absent(
        conn,
        "__alkstore_dead",
        "backoff_base_s",
        "backoff_base_s INTEGER NOT NULL DEFAULT 5",
    )?;
    add_column_if_absent(
        conn,
        "__alkstore_dead",
        "dead_letter_retention_s",
        "dead_letter_retention_s INTEGER",
    )?;
    add_column_if_absent(conn, "__alkstore_dead", "claimed_at", "claimed_at INTEGER")?;
    add_column_if_absent(conn, "__alkstore_dead", "expires_at", "expires_at INTEGER")?;
    Ok(())
}

pub(crate) fn open_conn(path: &str, install_notify: bool) -> Result<Connection, Error> {
    // No SQLITE_OPEN_URI (ADR-023 §3 delta): the path argument is a
    // plain filesystem path — URI-interpreted `?name=value` suffixes
    // would mutate connection semantics outside the engine
    // constructor's opts.
    let conn = Connection::open_with_flags(
        path,
        OpenFlags::SQLITE_OPEN_READ_WRITE | OpenFlags::SQLITE_OPEN_CREATE,
    )?;
    apply_default_pragmas(&conn)?;
    if install_notify {
        attach_notify(&conn)?;
    }
    Ok(conn)
}

/// Open a connection carrying the engine's full per-connection
/// bootstrap: pragmas, notify machinery, this substrate's function
/// set, and schema bootstrap. The engine's `open` constructor runs
/// this on the writer and each pooled reader (engine-sqlite.md's
/// connection architecture: "each connection runs the substrate's
/// bootstrap at open").
///
/// D-30: the lineage's pools open readers through bare `open_conn`
/// (pragmas + notify only), the function set + schema living only on
/// the writer. This substrate extends the pool open path with
/// `attach_alkstore_functions` + `bootstrap_schema` — the engine's
/// reader-slot operations (claim/ack, stream reads, lock ops,
/// scheduler checks) run this substrate's SQL functions on pooled
/// connections, so every pooled connection must carry them.
pub(crate) fn open_conn_bootstrapped(path: &str) -> Result<Connection, Error> {
    let conn = open_conn(path, true)?;
    super::ops::attach_alkstore_functions(&conn)?;
    bootstrap_schema(&conn)?;
    Ok(conn)
}

pub(crate) struct Writer {
    slot: Mutex<Option<Connection>>,
    available: Condvar,
    closed: AtomicBool,
    path: String,
}

impl Writer {
    pub(crate) fn new(path: String, conn: Connection) -> Self {
        Self {
            slot: Mutex::new(Some(conn)),
            available: Condvar::new(),
            closed: AtomicBool::new(false),
            path,
        }
    }

    /// The abandonment arm (the auto-commit seam's panic-safety armor,
    /// review 004 item 1): a connection that cannot return to the slot
    /// — a panicked op leaves its statement state unknown — is dropped,
    /// not released, and the slot is replenished with a fresh
    /// bootstrapped open from the writer's own path. The release's slot
    /// notify re-opens the waiters. A failed replenish leaves the slot
    /// empty (the established arms' tolerance — the tick's
    /// poisoned-disposition arm and the tx seam's `ConnLease` drop arm
    /// both take `if let Ok(fresh)`), with the failure printed for
    /// diagnosability.
    pub(crate) fn replenish(&self) {
        if self.closed.load(Ordering::Acquire) {
            return;
        }
        match open_conn_bootstrapped(&self.path) {
            Ok(fresh) => self.release(fresh),
            Err(e) => eprintln!(
                "alkstore: error: writer slot replenish after an abandoned \
                 operation failed to open a fresh connection: {e}"
            ),
        }
    }

    pub(crate) fn acquire(&self) -> Option<Connection> {
        let mut guard = self.slot.lock();
        loop {
            if self.closed.load(Ordering::Acquire) {
                return None;
            }
            if let Some(c) = guard.take() {
                return Some(c);
            }
            self.available.wait(&mut guard);
        }
    }

    pub(crate) fn release(&self, conn: Connection) {
        if self.closed.load(Ordering::Acquire) {
            return;
        }
        let mut guard = self.slot.lock();
        *guard = Some(conn);
        self.available.notify_one();
    }

    pub(crate) fn close(&self) {
        self.closed.store(true, Ordering::Release);
        let mut guard = self.slot.lock();
        guard.take();
        self.available.notify_all();
    }
}

pub(crate) struct Readers {
    pool: Mutex<Vec<Connection>>,
    outstanding: Mutex<usize>,
    available: Condvar,
    path: String,
    max: usize,
    closed: AtomicBool,
}

impl Readers {
    pub(crate) fn new(path: String, max: usize) -> Self {
        Self {
            pool: Mutex::new(Vec::new()),
            outstanding: Mutex::new(0),
            available: Condvar::new(),
            path,
            max: max.max(1),
            closed: AtomicBool::new(false),
        }
    }

    pub(crate) fn acquire(&self) -> Result<Connection, Error> {
        loop {
            if self.closed.load(Ordering::Acquire) {
                return Err(closed_err());
            }
            let mut pool = self.pool.lock();
            if let Some(c) = pool.pop() {
                return Ok(c);
            }
            let mut out = self.outstanding.lock();
            if *out < self.max {
                *out += 1;
                drop(out);
                drop(pool);
                match open_conn_bootstrapped(&self.path) {
                    Ok(conn) => {
                        if self.closed.load(Ordering::Acquire) {
                            *self.outstanding.lock() -= 1;
                            drop(conn);
                            return Err(closed_err());
                        }
                        return Ok(conn);
                    }
                    Err(e) => {
                        *self.outstanding.lock() -= 1;
                        return Err(e);
                    }
                }
            }
            drop(out);
            self.available.wait(&mut pool);
        }
    }

    pub(crate) fn release(&self, conn: Connection) {
        if self.closed.load(Ordering::Acquire) {
            return;
        }
        let mut pool = self.pool.lock();
        pool.push(conn);
        self.available.notify_one();
    }

    /// The abandonment arm (the auto-commit seam's panic-safety
    /// armor, review 004 item 1): the abandoned connection is dropped,
    /// not released. `outstanding` counts connections in existence —
    /// every connection carries its fresh-open increment for life, and
    /// release/pop never decrement — so the abandoned connection's
    /// decrement restores the pool's open-budget, and a waiter wakes
    /// under the pool lock (the release's notify discipline) to
    /// re-evaluate: pool miss + under `max` ⇒ fresh open.
    pub(crate) fn abandon(&self) {
        {
            let mut out = self.outstanding.lock();
            *out = out.saturating_sub(1);
        }
        let _pool = self.pool.lock();
        self.available.notify_one();
    }

    pub(crate) fn close(&self) {
        self.closed.store(true, Ordering::Release);
        self.pool.lock().clear();
        self.available.notify_all();
    }
}

fn closed_err() -> Error {
    Error::Sqlite(rusqlite::Error::SqliteFailure(
        rusqlite::ffi::Error::new(rusqlite::ffi::SQLITE_MISUSE),
        Some("Database is closed".to_string()),
    ))
}

#[cfg(test)]
mod writer_reader_tests {
    use super::*;
    use std::sync::Arc;

    fn mem() -> Connection {
        Connection::open_in_memory().unwrap()
    }

    fn temp_db(name: &str) -> std::path::PathBuf {
        let p = std::env::temp_dir().join(format!(
            "alkstore-{name}-{}-{:?}.db",
            std::process::id(),
            std::thread::current().id()
        ));
        let _ = std::fs::remove_file(&p);
        let _ = std::fs::remove_file(format!("{}-wal", p.display()));
        let _ = std::fs::remove_file(format!("{}-shm", p.display()));
        p
    }

    #[test]
    fn writer_close_drops_idle_connection() {
        let w = Writer::new(":memory:".to_string(), mem());
        w.close();
        assert!(w.acquire().is_none());
    }

    #[test]
    fn writer_close_drops_returned_connection() {
        let w = Writer::new(":memory:".to_string(), mem());
        let conn = w.acquire().unwrap();
        w.close();
        w.release(conn);
        assert!(w.acquire().is_none());
    }

    #[test]
    fn readers_close_returns_closed_err() {
        let tmp = temp_db("readers-close");
        let _ = std::fs::remove_file(&tmp);
        let c = Connection::open(&tmp).unwrap();
        c.execute_batch("PRAGMA journal_mode=WAL;").unwrap();
        drop(c);

        let r = Readers::new(tmp.to_string_lossy().into_owned(), 4);
        let c = r.acquire().unwrap();
        r.release(c);
        r.close();
        match r.acquire() {
            Err(Error::Sqlite(rusqlite::Error::SqliteFailure(_, Some(msg)))) => {
                assert!(msg.contains("Database is closed"));
            }
            other => panic!("expected closed err, got {other:?}"),
        }
        let _ = std::fs::remove_file(&tmp);
    }

    #[test]
    fn readers_open_failure_does_not_leak_capacity() {
        let r = Arc::new(Readers::new(
            "/this/parent/does/not/exist/alkstore-readers-leak.db".into(),
            2,
        ));
        for i in 0..5 {
            let r = r.clone();
            let handle = std::thread::spawn(move || r.acquire());
            let result = handle.join().unwrap();
            assert!(
                result.is_err(),
                "attempt {i}: expected open failure, got Ok"
            );
        }
    }

    #[test]
    fn open_conn_applies_wal_and_default_pragmas() {
        let tmp = temp_db("open-pragmas");
        let conn = open_conn(tmp.to_str().unwrap(), false).unwrap();
        let mode: String = conn
            .pragma_query_value(None, "journal_mode", |r| r.get(0))
            .unwrap();
        assert!(mode.eq_ignore_ascii_case("wal"));
        let busy: i64 = conn
            .pragma_query_value(None, "busy_timeout", |r| r.get(0))
            .unwrap();
        assert_eq!(busy, 5000);
        drop(conn);
        let _ = std::fs::remove_file(&tmp);
        let _ = std::fs::remove_file(format!("{}-wal", tmp.display()));
        let _ = std::fs::remove_file(format!("{}-shm", tmp.display()));
    }

    #[test]
    fn open_conn_treats_uri_shaped_path_as_plain_filename() {
        // ADR-023 §3: no SQLITE_OPEN_URI — a `?name=value` suffix must
        // be a literal filename (the file exists with that name), not
        // URI interpretation (which for a bare-relative "file"-less
        // string with query params would error or mutate semantics).
        let dir = std::env::temp_dir().join(format!(
            "alkstore-uri-literal-{}-{:?}",
            std::process::id(),
            std::thread::current().id()
        ));
        std::fs::create_dir_all(&dir).unwrap();
        let path = dir.join("plain.db?mode=memory");
        let conn = open_conn(path.to_str().unwrap(), false).unwrap();
        conn.execute("CREATE TABLE t (x)", []).unwrap();
        drop(conn);
        let plain = dir.join("plain.db?mode=memory");
        assert!(plain.exists(), "literal-named file must exist on disk");
        let _ = std::fs::remove_dir_all(&dir);
    }

    #[test]
    fn open_conn_bootstrapped_carries_the_full_surface_on_every_connection() {
        let dir = std::env::temp_dir().join(format!(
            "alkstore-bootstrapped-{}-{:?}",
            std::process::id(),
            std::thread::current().id()
        ));
        std::fs::create_dir_all(&dir).unwrap();
        let path = dir.join("bootstrapped.db");

        let first = open_conn_bootstrapped(path.to_str().unwrap()).unwrap();
        drop(first);

        for i in 0..3 {
            let conn = open_conn_bootstrapped(path.to_str().unwrap())
                .unwrap_or_else(|e| panic!("bootstrapped open {i} failed: {e}"));
            let mode: String = conn
                .pragma_query_value(None, "journal_mode", |r| r.get(0))
                .unwrap();
            assert!(mode.eq_ignore_ascii_case("wal"), "open {i}: not WAL");
            let notify_fn: i64 = conn
                .query_row("SELECT notify('__alkstore_probe', 'x')", [], |r| r.get(0))
                .unwrap_or(0);
            assert!(notify_fn >= 1, "open {i}: notify function missing");
            let tables: i64 = conn
                .query_row(
                    "SELECT count(*) FROM sqlite_master
                     WHERE type = 'table' AND name LIKE '__alkstore_%'",
                    [],
                    |r| r.get(0),
                )
                .unwrap();
            assert!(
                tables >= 7,
                "open {i}: bootstrap schema missing (saw {tables} tables)"
            );
            let publish_ready: i64 = conn
                .query_row(
                    "SELECT alkstore_stream_publish('__alkstore_probe', NULL, '{}')",
                    [],
                    |r| r.get(0),
                )
                .unwrap_or(0);
            assert!(publish_ready >= 1, "open {i}: function set missing");
            drop(conn);
        }
        let _ = std::fs::remove_dir_all(&dir);
    }

    #[test]
    fn concurrent_opens_never_return_database_is_locked() {
        const ROUNDS: usize = 8;
        const OPENERS: usize = 16;

        for round in 0..ROUNDS {
            let dir = std::env::temp_dir().join(format!(
                "alkstore-open-race-{}-{round}-{:?}",
                std::process::id(),
                std::thread::current().id()
            ));
            std::fs::create_dir_all(&dir).unwrap();
            let path = dir.join("pressure.db");

            let barrier = Arc::new(std::sync::Barrier::new(OPENERS));
            let handles: Vec<_> = (0..OPENERS)
                .map(|_| {
                    let path = path.clone();
                    let barrier = Arc::clone(&barrier);
                    std::thread::spawn(move || {
                        let conn = Connection::open(&path)?;
                        barrier.wait();
                        apply_default_pragmas(&conn)?;
                        let mode: String =
                            conn.query_row("PRAGMA journal_mode", [], |row| row.get(0))?;
                        assert!(
                            mode.eq_ignore_ascii_case("wal"),
                            "expected WAL after open, got {mode}"
                        );
                        Ok::<_, rusqlite::Error>(())
                    })
                })
                .collect();

            let failures: Vec<String> = handles
                .into_iter()
                .filter_map(|h| h.join().unwrap().err().map(|e| e.to_string()))
                .collect();

            std::fs::remove_dir_all(&dir).ok();
            assert!(
                failures.is_empty(),
                "round {round}: concurrent opens failed: {failures:?}"
            );
        }
    }
}