jerrycan_db/

lib.rs

//! Database extension: one URL-driven `Db` over SQLite and Postgres
//! (sea-orm's `DatabaseConnection`), module-owned dual-dialect migrations, and a
//! deterministic `?`→`$n` translator (placeholders are library-owned; ours is
//! quote-blind and safe because generated SQL never embeds string literals).
#![forbid(unsafe_code)]

use jerrycan_core::{App, Error, Extension, Result};
use sea_orm::{ConnectionTrait, Database, DatabaseConnection, Statement, TransactionTrait};

/// The reserved 64-bit Postgres advisory-lock key that serializes a migration
/// run. Concurrent migrators (e.g. several app nodes booting at once) all take
/// `pg_advisory_xact_lock(MIGRATION_ADVISORY_KEY)` at the top of the migration
/// transaction; the first holder applies the DDL and the others block, then
/// proceed and find every migration already recorded (applying nothing). The
/// lock auto-releases at COMMIT. Distinct from `jerrycan_jobs`'
/// `JOBS_CRON_ADVISORY_KEY` so a migration and a cron tick never contend.
/// Value is an arbitrary jerrycan-migrate magic constant ("jCmig" + 0001).
pub const MIGRATION_ADVISORY_KEY: i64 = 0x6A_43_6D_69_67_00_00_01;

// Connections are driven by sea-orm; generated repos build ALL SQL through
// sea-query (dialect rendering is library-owned: placeholders, RETURNING,
// quoting). Re-exported so generated crates depend on `jerrycan` alone.
pub use sea_orm;
pub use sea_query;
pub use sea_query_binder;

/// Which engine the connection speaks. Generated code branches on this for the
/// few statements that genuinely differ (insert-id strategies, DDL).
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum Backend {
    Sqlite,
    Postgres,
}

/// The database dependency: a cloneable connection handle. Register app-wide
/// with `App::new().extend(db)` (or `.provide(db)` — `extend` is the §6 seam).
#[derive(Clone)]
pub struct Db {
    conn: DatabaseConnection,
    backend: Backend,
    url: String,
}

impl Db {
    /// Connect by URL: `sqlite::memory:`, `sqlite://path.db`, `postgres://…`.
    pub async fn connect(url: &str) -> Result<Self> {
        let backend = if url.starts_with("postgres") {
            Backend::Postgres
        } else if url.starts_with("sqlite") {
            Backend::Sqlite
        } else {
            return Err(Error::internal(format!(
                "unsupported database url scheme: `{url}` (sqlite:// or postgres:// in v0)"
            )));
        };
        // Decision #4: one connection for sqlite (memory correctness + writer lock),
        // small default pool for postgres.
        let max = match backend {
            Backend::Sqlite => 1,
            Backend::Postgres => 5,
        };
        let mut opts = sea_orm::ConnectOptions::new(url.to_string());
        opts.max_connections(max);
        if backend == Backend::Sqlite {
            // FK enforcement is a framework guarantee, so pin it rather than
            // inherit it: sqlx-sqlite happens to default `foreign_keys=ON`
            // today, but a guarantee must not rest on an upstream default.
            // Set through connect options so EVERY pooled connection carries
            // it by construction (a post-connect `PRAGMA` would be lost on
            // pool reconnect). Postgres enforces FKs natively — nothing to pin.
            opts.map_sqlx_sqlite_opts(|o| o.foreign_keys(true));
        }
        let conn = Database::connect(opts).await.map_err(db_error)?;
        Ok(Self {
            conn,
            backend,
            url: url.to_string(),
        })
    }

    /// `JERRYCAN_DATABASE_URL`, defaulting to `sqlite::memory:` for dev.
    pub async fn from_env() -> Result<Self> {
        let url = std::env::var("JERRYCAN_DATABASE_URL")
            .unwrap_or_else(|_| "sqlite::memory:".to_string());
        Self::connect(&url).await
    }

    /// The underlying sea-orm connection. Generated repos and migrations execute
    /// through this handle (`execute_unprepared`, `query_one`, …).
    pub fn conn(&self) -> &DatabaseConnection {
        &self.conn
    }

    pub fn backend(&self) -> Backend {
        self.backend
    }

    /// The URL this handle connected with. Extension crates (jerrycan-realtime)
    /// use it to open sessions the pool cannot serve: LISTEN connections, the
    /// replication socket, and long-held advisory-lock sessions.
    pub fn url(&self) -> &str {
        &self.url
    }

    /// Backend-correct placeholders for a `?`-style query string.
    pub fn sql(&self, query: &str) -> String {
        translate_placeholders(query, self.backend)
    }

    /// The sea-query builder matching this connection's dialect. Generated repos
    /// pass it to `build_any` so one builder call renders correct SQL
    /// (placeholders, RETURNING, quoting) for whichever engine is connected.
    pub fn query_builder(&self) -> &'static dyn sea_query::QueryBuilder {
        match self.backend {
            Backend::Sqlite => &sea_query::SqliteQueryBuilder,
            Backend::Postgres => &sea_query::PostgresQueryBuilder,
        }
    }

    /// The sea-orm backend tag for this connection — selects the dialect when
    /// constructing a [`Statement`] from raw SQL and bound values.
    fn backend_db(&self) -> sea_orm::DatabaseBackend {
        match self.backend {
            Backend::Sqlite => sea_orm::DatabaseBackend::Sqlite,
            Backend::Postgres => sea_orm::DatabaseBackend::Postgres,
        }
    }
}

/// One migration, both dialects. Generated apps embed these via the tool-owned
/// `app/src/migrations.rs`; modules own the .sql files (spec §5 anatomy).
#[derive(Debug, Clone, Copy)]
pub struct Migration {
    pub name: &'static str,
    pub sqlite: &'static str,
    pub postgres: &'static str,
}

/// Runtime-loaded migration (CLI `jerrycan db migrate` reads module files from
/// disk). The owned twin of [`Migration`]; both delegate to the same runner.
#[derive(Debug, Clone)]
pub struct OwnedMigration {
    pub name: String,
    pub sqlite: String,
    pub postgres: String,
}

impl Db {
    /// Apply pending migrations in slice order; returns the names applied.
    /// Tracking table `_jerrycan_migrations` remembers what ran. The whole run
    /// is **atomic and concurrency-safe**: it runs in one transaction guarded by
    /// a Postgres advisory lock, so several app instances booting at once can't
    /// race the not-yet-applied check and double-apply the DDL — a failure rolls
    /// the entire run back (all-or-nothing; no half-migrated state).
    pub async fn migrate(&self, migrations: &[Migration]) -> Result<Vec<String>> {
        self.migrate_iter(migrations.iter().map(|m| (m.name, m.sqlite, m.postgres)))
            .await
    }

    /// Owned-migration twin of [`migrate`](Self::migrate) — same runner.
    pub async fn migrate_owned(&self, migrations: &[OwnedMigration]) -> Result<Vec<String>> {
        self.migrate_iter(
            migrations
                .iter()
                .map(|m| (m.name.as_str(), m.sqlite.as_str(), m.postgres.as_str())),
        )
        .await
    }

    /// The shared core: apply each `(name, sqlite, postgres)` in order, skipping
    /// already-recorded names. The whole run is one transaction; on Postgres a
    /// transaction-scoped advisory lock serializes concurrent migrators so the
    /// not-yet-applied check and the (non-`IF NOT EXISTS`) DDL can't race. A
    /// failure rolls the transaction back — all-or-nothing.
    async fn migrate_iter<'a>(
        &self,
        items: impl Iterator<Item = (&'a str, &'a str, &'a str)>,
    ) -> Result<Vec<String>> {
        // One transaction for the whole run: atomic, and the pinned connection
        // lets the Postgres advisory lock span every statement. On SQLite the
        // single writer (pool max = 1) already serializes; the transaction just
        // makes the run atomic.
        let txn = self.conn.begin().await.map_err(db_error)?;

        if self.backend == Backend::Postgres {
            // Serialize concurrent migrators: the first node holds the lock and
            // migrates; the rest block here, then proceed and find every name
            // already recorded (applying nothing). Auto-released at COMMIT.
            txn.execute(Statement::from_string(
                sea_orm::DatabaseBackend::Postgres,
                format!("SELECT pg_advisory_xact_lock({MIGRATION_ADVISORY_KEY})"),
            ))
            .await
            .map_err(db_error)?;
        }

        txn.execute_unprepared(
            "CREATE TABLE IF NOT EXISTS _jerrycan_migrations (name TEXT PRIMARY KEY, applied_at TEXT NOT NULL)",
        )
        .await
        .map_err(db_error)?;

        let mut applied = Vec::new();
        for (name, sqlite, postgres) in items {
            let seen = txn
                .query_one(Statement::from_sql_and_values(
                    self.backend_db(),
                    self.sql("SELECT name FROM _jerrycan_migrations WHERE name = ?"),
                    [name.into()],
                ))
                .await
                .map_err(db_error)?;
            if seen.is_some() {
                continue;
            }
            let statement = match self.backend {
                Backend::Sqlite => sqlite,
                Backend::Postgres => postgres,
            };
            txn.execute_unprepared(statement).await.map_err(|e| {
                eprintln!("jerrycan-db: migration `{name}` failed");
                db_error(e)
            })?;
            txn.execute(Statement::from_sql_and_values(
                self.backend_db(),
                self.sql("INSERT INTO _jerrycan_migrations (name, applied_at) VALUES (?, ?)"),
                [name.into(), chrono_free_timestamp().into()],
            ))
            .await
            .map_err(db_error)?;
            applied.push(name.to_string());
        }
        txn.commit().await.map_err(db_error)?;
        Ok(applied)
    }
}

/// RFC3339-ish UTC timestamp without a chrono dependency (seconds precision).
fn chrono_free_timestamp() -> String {
    let secs = std::time::SystemTime::now()
        .duration_since(std::time::UNIX_EPOCH)
        .map(|d| d.as_secs())
        .unwrap_or(0);
    format!("unix:{secs}")
}

/// `?` → `$1, $2, …` for Postgres; identity for SQLite. Quote-blind by design:
/// generated SQL never embeds string literals (binds carry all values).
pub fn translate_placeholders(query: &str, backend: Backend) -> String {
    match backend {
        Backend::Sqlite => query.to_string(),
        Backend::Postgres => {
            let mut out = String::with_capacity(query.len() + 8);
            let mut n = 0;
            for ch in query.chars() {
                if ch == '?' {
                    n += 1;
                    out.push('$');
                    out.push_str(&n.to_string());
                } else {
                    out.push(ch);
                }
            }
            out
        }
    }
}

/// Map any sea-orm error to a stable JC code without leaking internals; the
/// underlying detail goes to stderr for the operator. Unique-key violations
/// are the client's fault (a re-POSTed id), not a server fault — they map to
/// 409 JC0409 so duplicate writes can't pollute 5xx alerting.
pub fn db_error(e: sea_orm::DbErr) -> Error {
    eprintln!("jerrycan-db: {e}");
    if matches!(
        e.sql_err(),
        Some(sea_orm::SqlErr::UniqueConstraintViolation(_))
    ) {
        return Error::conflict("conflict: a row with this key already exists");
    }
    Error::new(
        jerrycan_core::http::StatusCode::INTERNAL_SERVER_ERROR,
        "JC0510",
        "database error",
    )
}

impl Extension for Db {
    fn register(self, app: App) -> App {
        app.provide(self)
    }
}

/// Re-exported for generated code that still reaches for sqlx types directly;
/// route crates never declare sqlx themselves.
pub use sqlx;

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

    #[tokio::test]
    async fn db_exposes_its_connection_url() {
        let db = Db::connect("sqlite::memory:").await.unwrap();
        assert_eq!(db.url(), "sqlite::memory:");
    }

    #[tokio::test]
    async fn connects_and_executes_via_sea_orm() {
        // Decision #4: sqlite connections are single-connection — otherwise every
        // pooled connection of sqlite::memory: is its OWN empty database.
        let db = Db::connect("sqlite::memory:").await.unwrap();
        assert_eq!(db.backend(), Backend::Sqlite);
        db.conn()
            .execute_unprepared("CREATE TABLE t (id INTEGER PRIMARY KEY)")
            .await
            .unwrap();
    }

    #[test]
    fn placeholder_translation_is_backend_aware() {
        assert_eq!(
            translate_placeholders("INSERT INTO t (a, b) VALUES (?, ?)", Backend::Postgres),
            "INSERT INTO t (a, b) VALUES ($1, $2)"
        );
        assert_eq!(
            translate_placeholders("INSERT INTO t (a, b) VALUES (?, ?)", Backend::Sqlite),
            "INSERT INTO t (a, b) VALUES (?, ?)"
        );
    }

    #[tokio::test]
    async fn from_env_defaults_to_sqlite_memory() {
        // JERRYCAN_DATABASE_URL unset in the test env → default.
        let db = Db::from_env().await.unwrap();
        assert_eq!(db.backend(), Backend::Sqlite);
    }

    #[test]
    fn db_errors_are_jc0510_and_leak_nothing() {
        let e = db_error(sea_orm::DbErr::Custom("boom".into()));
        assert_eq!(e.code(), "JC0510");
        assert_eq!(e.message(), "database error");
    }

    /// The whole generated-repo chain in one place: sea-query renders the SQL
    /// and binds the values; the connection is only the executor. If this
    /// breaks, every generated repo breaks with it.
    #[tokio::test]
    async fn sea_query_builds_and_executes_via_the_connection() {
        use sea_query::{Alias, Expr, Query};

        let db = Db::connect("sqlite::memory:").await.unwrap();
        db.conn()
            .execute_unprepared("CREATE TABLE sq (id INTEGER PRIMARY KEY, title TEXT NOT NULL)")
            .await
            .unwrap();

        let (sql, values) = Query::insert()
            .into_table(Alias::new("sq"))
            .columns([Alias::new("id"), Alias::new("title")])
            .values_panic([7.into(), "hello".into()])
            .returning(Query::returning().columns([Alias::new("id")]))
            .build_any(db.query_builder());
        let row = db
            .conn()
            .query_one(Statement::from_sql_and_values(db.backend_db(), sql, values))
            .await
            .unwrap()
            .expect("RETURNING id row");
        assert_eq!(
            row.try_get::<i64>("", "id").unwrap(),
            7,
            "RETURNING id round-trips"
        );

        let (sql, values) = Query::select()
            .columns([Alias::new("id"), Alias::new("title")])
            .from(Alias::new("sq"))
            .and_where(Expr::col(Alias::new("id")).eq(7))
            .build_any(db.query_builder());
        let row = db
            .conn()
            .query_one(Statement::from_sql_and_values(db.backend_db(), sql, values))
            .await
            .unwrap()
            .expect("select row");
        assert_eq!(row.try_get::<String>("", "title").unwrap(), "hello");
    }

    /// A duplicate key is the CLIENT's fault: it must surface as 409 JC0409,
    /// not a 500 — a re-POSTed id must never trip server-fault alerting.
    #[tokio::test]
    async fn unique_violations_map_to_409_conflict() {
        let db = Db::connect("sqlite::memory:").await.unwrap();
        db.conn()
            .execute_unprepared("CREATE TABLE u (id INTEGER PRIMARY KEY, t TEXT)")
            .await
            .unwrap();
        db.conn()
            .execute_unprepared("INSERT INTO u VALUES (1, 'a')")
            .await
            .unwrap();
        let dup = db
            .conn()
            .execute_unprepared("INSERT INTO u VALUES (1, 'b')")
            .await
            .expect_err("duplicate pk must fail");
        let e = db_error(dup);
        assert_eq!(e.code(), "JC0409");
        assert_eq!(e.status().as_u16(), 409);
        // Still no internals in the message.
        assert!(!e.message().contains("sqlite"), "{}", e.message());
    }

    /// SQLite FK enforcement is a FRAMEWORK guarantee, pinned in `Db::connect`
    /// via `map_sqlx_sqlite_opts(|o| o.foreign_keys(true))` — not an inherited
    /// sqlx default. This test is the proof: if the pin is ever lost (or an
    /// upstream default flip would otherwise silently disable enforcement),
    /// it turns red. Asserts all three faces of the guarantee through the
    /// pooled connection: the pragma reads ON, an orphan insert is rejected
    /// as an FK violation, and `ON DELETE CASCADE` fires.
    #[tokio::test]
    async fn sqlite_foreign_keys_are_enforced_through_the_pool() {
        let db = Db::connect("sqlite::memory:").await.unwrap();

        // (a) The pragma is ON on the pooled connection itself.
        let row = db
            .conn()
            .query_one(Statement::from_string(
                sea_orm::DatabaseBackend::Sqlite,
                "PRAGMA foreign_keys",
            ))
            .await
            .unwrap()
            .expect("PRAGMA foreign_keys returns a row");
        let on: i64 = row
            .try_get::<i64>("", "foreign_keys")
            .or_else(|_| row.try_get::<i32>("", "foreign_keys").map(i64::from))
            .unwrap();
        assert_eq!(on, 1, "foreign_keys must be ON through the pool");

        db.conn()
            .execute_unprepared("CREATE TABLE parents (id INTEGER PRIMARY KEY)")
            .await
            .unwrap();
        db.conn()
            .execute_unprepared(
                "CREATE TABLE children (id INTEGER PRIMARY KEY, \
                 parent_id INTEGER NOT NULL REFERENCES parents(id) ON DELETE CASCADE)",
            )
            .await
            .unwrap();
        db.conn()
            .execute_unprepared("INSERT INTO parents (id) VALUES (1)")
            .await
            .unwrap();

        // (b) A child pointing at a nonexistent parent is REJECTED — and
        // specifically as an FK violation, not some other failure.
        let orphan = db
            .conn()
            .execute_unprepared("INSERT INTO children (id, parent_id) VALUES (10, 999)")
            .await
            .expect_err("orphan insert must violate the FK");
        assert!(
            matches!(
                orphan.sql_err(),
                Some(sea_orm::SqlErr::ForeignKeyConstraintViolation(_))
            ),
            "must be an FK violation, got: {orphan}"
        );

        // (c) Deleting the parent CASCADE-removes its children.
        db.conn()
            .execute_unprepared("INSERT INTO children (id, parent_id) VALUES (11, 1)")
            .await
            .unwrap();
        db.conn()
            .execute_unprepared("DELETE FROM parents WHERE id = 1")
            .await
            .unwrap();
        let row = db
            .conn()
            .query_one(Statement::from_string(
                sea_orm::DatabaseBackend::Sqlite,
                "SELECT COUNT(*) AS n FROM children",
            ))
            .await
            .unwrap()
            .unwrap();
        let n: i64 = row
            .try_get::<i64>("", "n")
            .or_else(|_| row.try_get::<i32>("", "n").map(i64::from))
            .unwrap();
        assert_eq!(n, 0, "ON DELETE CASCADE must remove the child rows");
    }

    fn demo_migrations() -> Vec<Migration> {
        vec![
            Migration {
                name: "0001_create_todos",
                sqlite: "CREATE TABLE todos (id INTEGER PRIMARY KEY AUTOINCREMENT, title TEXT NOT NULL)",
                postgres: "CREATE TABLE todos (id BIGSERIAL PRIMARY KEY, title TEXT NOT NULL)",
            },
            Migration {
                name: "0002_add_done",
                sqlite: "ALTER TABLE todos ADD COLUMN done BOOLEAN NOT NULL DEFAULT 0",
                postgres: "ALTER TABLE todos ADD COLUMN done BOOLEAN NOT NULL DEFAULT FALSE",
            },
        ]
    }

    #[tokio::test]
    async fn migrations_apply_in_order_and_only_once() {
        let db = Db::connect("sqlite::memory:").await.unwrap();
        let applied = db.migrate(&demo_migrations()).await.unwrap();
        assert_eq!(applied, vec!["0001_create_todos", "0002_add_done"]);

        // Re-running applies nothing (tracking table remembers).
        let applied = db.migrate(&demo_migrations()).await.unwrap();
        assert!(applied.is_empty());

        // The schema is genuinely there.
        db.conn()
            .execute_unprepared("INSERT INTO todos (title, done) VALUES ('x', 1)")
            .await
            .unwrap();
    }

    #[tokio::test]
    async fn owned_migrations_apply_in_order_and_only_once() {
        let db = Db::connect("sqlite::memory:").await.unwrap();
        let owned = vec![
            OwnedMigration {
                name: "0001_create_todos".into(),
                sqlite:
                    "CREATE TABLE todos (id INTEGER PRIMARY KEY AUTOINCREMENT, title TEXT NOT NULL)"
                        .into(),
                postgres: "CREATE TABLE todos (id BIGSERIAL PRIMARY KEY, title TEXT NOT NULL)"
                    .into(),
            },
            OwnedMigration {
                name: "0002_add_done".into(),
                sqlite: "ALTER TABLE todos ADD COLUMN done BOOLEAN NOT NULL DEFAULT 0".into(),
                postgres: "ALTER TABLE todos ADD COLUMN done BOOLEAN NOT NULL DEFAULT FALSE".into(),
            },
        ];
        let applied = db.migrate_owned(&owned).await.unwrap();
        assert_eq!(applied, vec!["0001_create_todos", "0002_add_done"]);
        // Re-running applies nothing (shares the tracking table with `migrate`).
        let applied = db.migrate_owned(&owned).await.unwrap();
        assert!(applied.is_empty());
    }

    /// The transaction idiom is the framework's atomicity guarantee: a closure
    /// returning `Err` must roll back EVERY statement it issued, leaving no
    /// partial writes. If this fails, the sea-orm feature set is wrong — fix the
    /// Cargo features, never weaken the test.
    #[tokio::test]
    async fn transactions_roll_back_on_error() {
        use sea_orm::TransactionTrait;
        let db = Db::connect("sqlite::memory:").await.unwrap();
        db.conn()
            .execute_unprepared("CREATE TABLE t (id INTEGER PRIMARY KEY)")
            .await
            .unwrap();
        let r = db
            .conn()
            .transaction::<_, (), sea_orm::DbErr>(|txn| {
                Box::pin(async move {
                    txn.execute_unprepared("INSERT INTO t VALUES (1)").await?;
                    Err(sea_orm::DbErr::Custom("boom".into()))
                })
            })
            .await;
        assert!(r.is_err());
        let rows = db
            .conn()
            .query_all(sea_orm::Statement::from_string(
                sea_orm::DatabaseBackend::Sqlite,
                "SELECT id FROM t",
            ))
            .await
            .unwrap();
        assert!(rows.is_empty(), "rollback must leave no rows");
    }

    #[tokio::test]
    async fn a_failing_migration_surfaces_jc0510_and_is_not_recorded() {
        let db = Db::connect("sqlite::memory:").await.unwrap();
        let bad = vec![Migration {
            name: "0001_broken",
            sqlite: "CREATE GARBAGE",
            postgres: "CREATE GARBAGE",
        }];
        let err = db.migrate(&bad).await.unwrap_err();
        assert_eq!(err.code(), "JC0510");

        // Fixing it lets the same name apply afresh — failures are not recorded.
        let good = vec![Migration {
            name: "0001_broken",
            sqlite: "CREATE TABLE ok (x BIGINT)",
            postgres: "CREATE TABLE ok (x BIGINT)",
        }];
        let applied = db.migrate(&good).await.unwrap();
        assert_eq!(applied, vec!["0001_broken"]);
    }

    /// Several app instances booting at once all call `migrate()` against the
    /// same Postgres. Without the advisory-lock serialization they race the
    /// not-yet-applied check and double-apply the (non-`IF NOT EXISTS`) DDL —
    /// one node crashes with a unique violation (JC0409/JC0510). With it, every
    /// migrator succeeds and the migration is applied EXACTLY once. Needs a live
    /// Postgres; run with `JERRYCAN_TEST_PG_URL=… cargo test -p jerrycan-db -- --ignored`.
    #[tokio::test(flavor = "multi_thread", worker_threads = 8)]
    #[ignore = "needs a local postgres (set JERRYCAN_TEST_PG_URL)"]
    async fn concurrent_migrators_do_not_race() {
        let Ok(url) = std::env::var("JERRYCAN_TEST_PG_URL") else {
            eprintln!("SKIP: JERRYCAN_TEST_PG_URL not set");
            return;
        };
        // A run-unique table so repeated runs against a persistent DB don't
        // collide (the tracking row is keyed by the unique migration name).
        let nanos = std::time::SystemTime::now()
            .duration_since(std::time::UNIX_EPOCH)
            .unwrap()
            .as_nanos();
        let table = format!("mig_race_{nanos}");
        let name = format!("{table}_0001");
        let migrations = vec![Migration {
            name: Box::leak(name.clone().into_boxed_str()),
            sqlite: "",
            postgres: Box::leak(
                format!("CREATE TABLE {table} (id BIGSERIAL PRIMARY KEY, v TEXT NOT NULL)")
                    .into_boxed_str(),
            ),
        }];
        let migrations = std::sync::Arc::new(migrations);

        // 8 separate connection pools = 8 genuine concurrent "nodes".
        let mut handles = Vec::new();
        for _ in 0..8 {
            let url = url.clone();
            let migrations = migrations.clone();
            handles.push(tokio::spawn(async move {
                let db = Db::connect(&url).await.expect("connect");
                db.migrate(&migrations).await
            }));
        }

        let mut total_applied = 0usize;
        for h in handles {
            let applied = h.await.expect("task").expect("migrate must not error");
            total_applied += applied.len();
        }
        assert_eq!(
            total_applied, 1,
            "exactly one migrator applies the migration; the rest see it recorded"
        );

        // The table exists and is usable.
        let db = Db::connect(&url).await.unwrap();
        db.conn()
            .execute_unprepared(&format!("INSERT INTO {table} (v) VALUES ('ok')"))
            .await
            .unwrap();
        db.conn()
            .execute_unprepared(&format!("DROP TABLE {table}"))
            .await
            .unwrap();
    }
}