cratefield-module-notifications 0.2.0

//! The fan-out API other modules call, and the drain that delivers it
//! (issue #182).
//!
//! # The shape of a send
//!
//! [`Notifier::notify`] does **not** write. It returns [`Enqueued`], whose
//! statements the caller appends to its own `db.batch_atomic(..)`, so a booking's
//! confirmation row and the notification that announces it commit together
//! or not at all — the core [`Outbox`] contract (#128). Then
//! [`Notifier::deliver_now`] uses `Defer` to *attempt* immediate delivery,
//! and the venture's scheduled entry point drains whatever that missed.
//!
//! The order matters and is not interchangeable: deferring the drain from
//! inside `notify` would race the caller's own commit, and a drain that
//! ran first would find no row.
//!
//! # Where the preference is checked
//!
//! In the **drain**, immediately before `Push::send`. That is what makes a
//! late opt-out win: a preference checked only when the row was written
//! would still deliver everything already queued. `notify` also skips
//! enqueueing for a category that is already off, but that is an
//! optimisation over rows the drain would drop anyway — never the
//! authority.

use std::sync::{Arc, OnceLock};
use std::time::Duration;

use cratefield_core::{
    Database, DbError, Kid, MailError, Message as Mail, ModuleConfig, ModuleContext, Notification,
    Outbox, OutboxRecord, Payload, Push, PushError, PushOutcome, Scope, SendOutcome, Statement,
};
use cratefield_i18n::{Catalog, LanguageIdentifier, direction};
use futures_util::StreamExt as _;
use serde::{Deserialize, Serialize};
use serde_json::{Value, json};

use crate::clock::{now_iso, plus_secs};
use crate::locale::{self, Localised};
use crate::message::{Localizable, Message};
use crate::store::{self, Channels, DeadLetterReason, Subscription, Transport};
use crate::{Category, Settings};

/// The outbox topic a push row carries.
pub const TOPIC_SEND: &str = "notifications.send";
/// The outbox topic an email row carries (#189).
///
/// One row per notification per **account**, not per device: a coach's
/// notes are one email however many phones the account has.
pub(crate) const TOPIC_EMAIL: &str = "notifications.email";

/// The event a venture can subscribe to instead of taking a crate
/// dependency on this module.
pub const EVENT_REQUESTED: &str = "notifications.requested";

/// Emitted when the provider told us a recipient is gone and the
/// subscription was deleted. Carries no recipient: a serialised
/// [`cratefield_core::Recipient`] is credential material (ADR 0015).
pub const EVENT_SUBSCRIPTION_PRUNED: &str = "notifications.subscription_pruned";

/// Emitted when a device that was one account's became another's.
///
/// A device token is not an authenticator (see
/// [`crate::Notifications::rehome_max_per_hour`]), so the venture is told
/// every time a subscription changes hands and can act on it — mail the
/// previous owner, raise it to whoever watches for abuse. Carries no
/// recipient: like the prune event, a serialised
/// [`cratefield_core::Recipient`] is credential material (ADR 0015).
pub const EVENT_SUBSCRIPTION_REHOMED: &str = "notifications.subscription_rehomed";

/// The first retry delay; each further attempt doubles it.
const BACKOFF_BASE_SECS: u64 = 30;
/// The ceiling on the doubling.
const BACKOFF_MAX_SECS: u64 = 3_600;
/// How long a claimed row stays leased to one drainer.
pub(crate) const LEASE_SECS: i64 = 300;

/// One unit of work in `notifications_outbox`.
///
/// The recipient is **not** in it. It is credential material, and an
/// outbox payload is ordinary data that ends up in exports and in
/// diagnostics; the drain reads the subscription row instead. That also
/// gives the right behaviour for free: a device that signed out between
/// the commit and the drain has no subscription, so its queued
/// notification is dropped rather than sent to a stranger's phone.
#[derive(Debug, Clone, Serialize, Deserialize, PartialEq)]
pub(crate) struct SendJob {
    pub notification_id: String,
    pub account_id: String,
    pub category: String,
    pub subscription_id: String,
    /// The notification as it will be sent, unless `localizable` says
    /// otherwise: for a rendered caller this is exactly what it wrote, and
    /// for a localizable one it is the venture default locale's render,
    /// which is what a device whose language resolves to nothing gets.
    pub notification: Notification,
    /// The message id and arguments, for a fan-out the drain localises per
    /// device (#190). Absent for a caller that passed rendered text, which
    /// is what makes a single-language venture's payload byte-identical to
    /// the one it had before this existed — and what lets a row queued
    /// before this deployment drain unchanged afterwards.
    #[serde(default, skip_serializing_if = "Option::is_none")]
    pub localizable: Option<Localizable>,
}

/// One queued email, the account-level sibling of [`SendJob`].
#[derive(Debug, Clone, Serialize, Deserialize)]
pub(crate) struct EmailJob {
    pub notification_id: String,
    pub account_id: String,
    pub category: String,
    pub notification: Notification,
    #[serde(default, skip_serializing_if = "Option::is_none")]
    pub localizable: Option<Localizable>,
}

/// The room one inbox row is announced in, and the bytes announced.
type Room = (String, Vec<u8>);

/// One in-app inbox row, as [`Notifier::notify`] has it at that point.
#[derive(Clone, Copy)]
struct InboxWrite<'a> {
    account_id: &'a str,
    notification_id: &'a str,
    category: &'a Category,
    notification: &'a Notification,
    localizable: Option<&'a Localizable>,
    /// The canonical tag the title and body above were rendered in, or
    /// `None` for a venture that wrote them itself.
    locale: Option<&'a str>,
    now: &'a str,
}

/// Why [`Notifier::notify`] produced no statements.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum Skipped {
    /// The account has this category switched off.
    PreferenceOff,
    /// The account has no registered device.
    NoSubscriptions,
}

/// The outbox inserts one [`Notifier::notify`] produced.
///
/// Append [`Enqueued::statements`] to the caller's own `db.batch_atomic(..)`.
/// Nothing is written until that batch runs.
#[derive(Debug, Clone)]
pub struct Enqueued {
    notification_id: String,
    statements: Vec<Statement>,
    /// How many of `statements` are push rows. Counted rather than derived
    /// from the length: since #187 the batch may also carry the in-app
    /// inbox insert, and `len()` means devices.
    devices: usize,
    inbox: bool,
    /// The room and payload for the live announcement, present exactly
    /// when an inbox row is in `statements`. Built here so
    /// [`Notifier::announce`] is a broadcast and nothing else.
    announcement: Option<(String, Vec<u8>)>,
    skipped: Option<Skipped>,
    /// The category this fan-out is for, so [`Notifier::announce`] can
    /// name it when it reports a missing translation.
    category: String,
    /// What the catalog could not render (#190). Reported by
    /// [`Notifier::announce`], which is the first point at which there is
    /// a [`Scope`] to emit an event in.
    missing: Vec<crate::locale::MissingTranslation>,
}

impl Enqueued {
    /// The `INSERT`s to append to the caller's batch, in subscription
    /// order.
    #[must_use]
    pub fn statements(&self) -> &[Statement] {
        &self.statements
    }

    /// The statements, consumed.
    #[must_use]
    pub fn into_statements(self) -> Vec<Statement> {
        self.statements
    }

    /// The id shared by every row of this fan-out, and carried to the
    /// device in `data.notification_id` so a client that is reached on
    /// two devices can dedupe.
    #[must_use]
    pub fn notification_id(&self) -> &str {
        &self.notification_id
    }

    /// How many devices this notification will be attempted on.
    ///
    /// Devices, not statements: the batch may also carry the in-app inbox
    /// insert, which is not a send.
    #[must_use]
    pub fn len(&self) -> usize {
        self.devices
    }

    /// Whether nothing will be *sent*. An inbox row may still be written —
    /// see [`Enqueued::wrote_inbox`].
    #[must_use]
    pub fn is_empty(&self) -> bool {
        self.devices == 0
    }

    /// Whether the batch carries an in-app inbox row (#187).
    ///
    /// Independent of [`Enqueued::is_empty`]: the inbox is written for an
    /// account with no device and for one whose *push* preference is off,
    /// because those switch off push, not the record.
    #[must_use]
    pub fn wrote_inbox(&self) -> bool {
        self.inbox
    }

    /// Why nothing will be **pushed**, when nothing will be.
    #[must_use]
    pub fn skipped(&self) -> Option<Skipped> {
        self.skipped
    }

    /// Whether the catalog could not render part of this message (#190).
    ///
    /// The notification is still queued — with the message id as its text,
    /// so it is visible from both ends — and
    /// [`Notifier::announce`] says so on the bus.
    #[must_use]
    pub fn has_missing_translations(&self) -> bool {
        !self.missing.is_empty()
    }
}

/// Why a fan-out could not be prepared.
#[derive(Debug)]
pub enum NotifyError {
    /// The category is not one the venture declared. A typo in a caller
    /// is a bug, not a silent no-op.
    UnknownCategory(String),
    /// `Notification::data` is neither absent nor a JSON object, so the
    /// module cannot add `category` and `notification_id` to it.
    InvalidData,
    /// The module's router has not been built yet, so it has no ports.
    NotMounted,
    /// A port the module requires is absent from this deployment.
    MissingPort(&'static str),
    /// A [`Message::Localizable`] was sent to a venture that configured no
    /// catalog (#190). An error rather than an English fallback: the
    /// venture named a message it has no strings for, and delivering the
    /// id to a person is worse than telling the developer now.
    NoCatalog(String),
    Database(DbError),
}

impl std::fmt::Display for NotifyError {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        match self {
            NotifyError::UnknownCategory(category) => write!(
                f,
                "notifications: category {category:?} is not declared by this venture"
            ),
            NotifyError::InvalidData => f.write_str(
                "notifications: Notification::data must be a JSON object or absent, because the \
                 module adds `category` and `notification_id` to it",
            ),
            NotifyError::NotMounted => f.write_str(
                "notifications: the module's router has not been built, so it has no ports yet",
            ),
            NotifyError::MissingPort(port) => {
                write!(f, "notifications: this deployment provides no {port} port")
            }
            NotifyError::NoCatalog(key) => write!(
                f,
                "notifications: message {key:?} is localizable, but this venture configured no \
                 catalog — call `.catalog(..)` on the module, or send a rendered Notification"
            ),
            NotifyError::Database(err) => write!(f, "notifications: {err}"),
        }
    }
}

impl std::error::Error for NotifyError {}

impl From<DbError> for NotifyError {
    fn from(err: DbError) -> Self {
        NotifyError::Database(err)
    }
}

/// What one drain pass did.
#[derive(Debug, Clone, Copy, Default, PartialEq, Eq)]
pub struct DrainReport {
    /// Rows this drainer leased.
    pub claimed: usize,
    /// Accepted by the provider.
    pub delivered: usize,
    /// Rescheduled after a transient failure.
    pub retried: usize,
    /// Subscriptions deleted because the provider said they were gone.
    pub pruned: usize,
    /// Rows moved to `notifications_dead_letters`.
    pub dead_lettered: usize,
    /// Rows dropped without sending: the category was switched off after
    /// the row was written, the device signed out, or the device now
    /// belongs to another account.
    pub dropped: usize,
    /// Rows whose own database call failed. The pass carries on: those
    /// rows keep their lease and become due again when it expires, and
    /// the rows behind them are not held hostage to one transient error.
    pub failed: usize,
}

/// What happened to one leased row.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
enum Outcome {
    Delivered,
    Retried,
    Pruned,
    DeadLettered,
    Dropped,
}

impl DrainReport {
    fn record(&mut self, outcome: Outcome) {
        let counter = match outcome {
            Outcome::Delivered => &mut self.delivered,
            Outcome::Retried => &mut self.retried,
            Outcome::Pruned => &mut self.pruned,
            Outcome::DeadLettered => &mut self.dead_lettered,
            Outcome::Dropped => &mut self.dropped,
        };
        *counter += 1;
    }
}

/// The handle other modules call.
///
/// A venture gets one from [`crate::Notifications::notifier`] and hands it
/// to its own modules; it is a cheap clone and shares the mounted module's
/// ports, so it is usable only after `Harness::router` has built the
/// module's router.
#[derive(Clone)]
pub struct Notifier {
    pub(crate) cell: Arc<OnceLock<Arc<ModuleContext>>>,
    /// The module's settings as they were **finished**, parked when its
    /// router is built. Shared with the module, so a handle taken before
    /// the last `.category(..)` cannot serve a smaller set than the one
    /// the module validated, mounted and lists over `GET /preferences`.
    pub(crate) settings_cell: Arc<OnceLock<Settings>>,
    /// What the builder had when this handle was taken: all a handle has
    /// to go on before the module is mounted.
    pub(crate) settings_at_handout: Settings,
}

impl std::fmt::Debug for Notifier {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        f.debug_struct("Notifier")
            .field("mounted", &self.cell.get().is_some())
            .field("categories", &self.settings().categories.len())
            .finish_non_exhaustive()
    }
}

impl Notifier {
    fn ctx(&self) -> Result<&Arc<ModuleContext>, NotifyError> {
        self.cell.get().ok_or(NotifyError::NotMounted)
    }

    /// The module's settings: the finished set once it is mounted, and the
    /// set this handle was taken from before that.
    ///
    /// The indirection is the fix for a whole class of silent
    /// divergence. `Settings::categories` is an `Arc<Vec<..>>` that
    /// `.category(..)` mutates through `Arc::make_mut`, so taking the
    /// handle before the last one used to fork the vector: the module
    /// kept both categories, the handle kept one, and `notify` failed
    /// `UnknownCategory` at runtime for a category `validate_config`
    /// accepted and `GET /preferences` listed. Reading through the shared
    /// cell makes the two agree by construction.
    pub(crate) fn settings(&self) -> &Settings {
        self.settings_cell
            .get()
            .unwrap_or(&self.settings_at_handout)
    }

    /// The declared category, or an error naming the unknown one.
    fn category(&self, name: &str) -> Result<&Category, NotifyError> {
        self.settings()
            .categories
            .iter()
            .find(|category| category.name == name)
            .ok_or_else(|| NotifyError::UnknownCategory(name.to_owned()))
    }

    fn now(&self) -> String {
        now_iso(self.cell.get().and_then(|ctx| ctx.ports.clock.as_ref()))
    }

    fn new_id(&self) -> String {
        crate::clock::new_id(self.cell.get().and_then(|ctx| ctx.ports.id_gen.as_ref()))
    }

    /// The mounted context, once `Harness::router` has built the module's
    /// router. `None` before that.
    #[must_use]
    pub fn context(&self) -> Option<&Arc<ModuleContext>> {
        self.cell.get()
    }

    /// How many attempts a transient failure gets. The builder's value
    /// unless the deployment overrides it.
    fn max_attempts(&self, ctx: &ModuleContext) -> i64 {
        let configured = ModuleConfig::new(crate::MODULE_NAME, &*ctx.config)
            .get_u32("MAX_ATTEMPTS", self.settings().max_attempts);
        i64::from(configured.max(1))
    }

    /// How many rows one drain pass leases.
    fn drain_batch(&self, ctx: &ModuleContext) -> u64 {
        let settings = self.settings();
        let configured = ModuleConfig::new(crate::MODULE_NAME, &*ctx.config).get_u32(
            "DRAIN_BATCH",
            u32::try_from(settings.drain_batch).unwrap_or(u32::MAX),
        );
        u64::from(configured).max(1)
    }

    /// How many rows one drain pass has in flight at a time.
    ///
    /// The drain is one `wait_until` against a 300 s lease, and every row
    /// is a provider round-trip: fifty of them end to end is fifty
    /// latencies added up, for work that shares nothing between rows.
    /// Bounded rather than unbounded because each in-flight row is a
    /// subrequest, and a runtime counts those.
    fn drain_concurrency(&self, ctx: &ModuleContext) -> usize {
        let configured = ModuleConfig::new(crate::MODULE_NAME, &*ctx.config)
            .get_u32("DRAIN_CONCURRENCY", self.settings().drain_concurrency);
        usize::try_from(configured.max(1)).unwrap_or(1)
    }

    /// Whether `account_id` wants an inbox row for `category`.
    ///
    /// Two switches, both of which must be on: the venture's, which says
    /// whether the category is worth keeping at all, and the account's.
    async fn in_app_allowed(
        &self,
        db: &dyn Database,
        account_id: &str,
        category: &Category,
    ) -> Result<bool, DbError> {
        if !category.defaults.in_app {
            return Ok(false);
        }
        // Falls back to `true`, not to `default_enabled`: that is the
        // *push* default, and a category whose push is off by default —
        // `coach_notes` — is exactly the one whose record the account
        // still wants. A push opt-out switches off the interruption, not
        // the history (#187).
        Ok(store::preference(db, account_id, &category.name)
            .await?
            .is_none_or(|channels: Channels| channels.in_app))
    }

    /// The outbox insert for one device.
    #[allow(
        clippy::too_many_arguments,
        reason = "one row's worth of columns, named rather than bundled"
    )]
    fn push_row(
        &self,
        account_id: &str,
        notification_id: &str,
        category: &Category,
        notification: &Notification,
        localizable: Option<&Localizable>,
        subscription_id: &str,
        now: &str,
    ) -> Result<Statement, NotifyError> {
        let job = SendJob {
            notification_id: notification_id.to_owned(),
            account_id: account_id.to_owned(),
            category: category.name.clone(),
            subscription_id: subscription_id.to_owned(),
            notification: notification.clone(),
            localizable: localizable.cloned(),
        };
        let payload = serde_json::to_string(&job).map_err(|err| {
            NotifyError::Database(DbError::Execute(format!(
                "notification does not serialise: {err}"
            )))
        })?;
        Ok(Outbox::new(store::OUTBOX).enqueue_statement(
            &self.new_id(),
            TOPIC_SEND,
            &payload,
            Some(account_id),
            now,
        ))
    }

    /// The outbox insert for one account's email.
    ///
    /// One row per notification per account, not per device: a coach's
    /// notes are one email however many phones the account has.
    fn email_row(
        &self,
        account_id: &str,
        notification_id: &str,
        category: &Category,
        notification: &Notification,
        localizable: Option<&Localizable>,
        now: &str,
    ) -> Result<Statement, NotifyError> {
        let job = EmailJob {
            notification_id: notification_id.to_owned(),
            account_id: account_id.to_owned(),
            category: category.name.clone(),
            notification: notification.clone(),
            localizable: localizable.cloned(),
        };
        let payload = serde_json::to_string(&job).map_err(|err| {
            NotifyError::Database(DbError::Execute(format!(
                "notification does not serialise: {err}"
            )))
        })?;
        Ok(Outbox::new(store::OUTBOX).enqueue_statement(
            &self.new_id(),
            TOPIC_EMAIL,
            &payload,
            Some(account_id),
            now,
        ))
    }

    /// Whether `account_id` should be emailed for `category`.
    ///
    /// Three things, all of which must hold: the venture opted the
    /// category into email, the account did not switch it off, and the
    /// account has a verified address it has not unsubscribed. All three
    /// are re-checked in the drain, because an answer that changes after
    /// the row is written must still win.
    async fn email_allowed(
        &self,
        db: &dyn Database,
        account_id: &str,
        category: &Category,
    ) -> Result<bool, DbError> {
        if !category.defaults.email {
            return Ok(false);
        }
        let wanted = store::preference(db, account_id, &category.name)
            .await?
            .is_none_or(|channels: Channels| channels.email);
        if !wanted {
            return Ok(false);
        }
        Ok(store::email_target(db, account_id)
            .await?
            .is_some_and(|target| target.mailable()))
    }

    /// The in-app inbox insert and the live announcement that goes with
    /// it: one row, written once, in the account's own language.
    fn inbox_row(&self, write: &InboxWrite<'_>) -> Result<(Statement, Room), NotifyError> {
        let InboxWrite {
            account_id,
            notification_id,
            category,
            notification,
            localizable,
            locale,
            now,
        } = *write;
        let serialisation = |err: serde_json::Error| {
            NotifyError::Database(DbError::Execute(format!(
                "notification does not serialise: {err}"
            )))
        };
        let inbox_id = self.new_id();
        let announcement = (
            format!("{INBOX_ROOM_PREFIX}{account_id}"),
            serde_json::to_vec(&json!({
                "type": "notification",
                "id": inbox_id,
                "notification_id": notification_id,
                "category": category.name,
                "title": notification.title,
                "body": notification.body,
                "url": notification.url,
                // So a live-updating client can set `lang` and `dir` on the
                // item it inserts, the same way it does for a row it read
                // from the list.
                "locale": locale,
                "dir": locale
                    .and_then(cratefield_i18n::parse_locale)
                    .map(|locale| direction(&locale).as_str()),
                "created_at": now,
            }))
            .map_err(serialisation)?,
        );
        // The message beside the rendered text, so a client that wants
        // another language can re-render. The module never re-renders a
        // stored row itself.
        let loc_args = localizable
            .filter(|message| !message.args.is_empty())
            .map(|message| serde_json::Value::Object(message.args.clone()).to_string());
        let statement = store::insert_inbox_statement(&store::NewInboxRow {
            id: &inbox_id,
            account_id,
            notification_id,
            category: &category.name,
            title: &notification.title,
            body: &notification.body,
            url: notification.url.as_deref(),
            icon: notification.icon.as_deref(),
            data_json: &serde_json::to_string(&notification.data).map_err(serialisation)?,
            locale,
            loc_key: localizable.map(|message| message.key.as_str()),
            loc_args_json: loc_args.as_deref(),
            now,
        });
        Ok((statement, announcement))
    }

    /// Whether `account_id` wants push for `category` **right now**.
    async fn push_allowed(
        &self,
        db: &dyn Database,
        account_id: &str,
        category: &Category,
    ) -> Result<bool, DbError> {
        Ok(store::preference(db, account_id, &category.name)
            .await?
            .map_or(category.defaults.push, |channels: Channels| channels.push))
    }

    /// The venture's default locale: what a recipient who has never
    /// expressed a language is written to.
    ///
    /// Read through the mounted context when there is one, so
    /// `NOTIFICATIONS_DEFAULT_LOCALE` can move it per deployment; the
    /// compiled-in answer otherwise.
    fn venture_locale(&self) -> LanguageIdentifier {
        self.cell.get().map_or_else(
            || self.settings().venture_locale(),
            |ctx| self.settings().configured_locale(ctx),
        )
    }

    /// The language this **account** reads: its own answer, else the
    /// venture's default. What the inbox and the mailbox use — there is
    /// one of each, whatever the account's devices say.
    async fn account_locale(
        &self,
        db: &dyn Database,
        account_id: &str,
    ) -> Result<LanguageIdentifier, DbError> {
        Ok(locale::resolve(
            None,
            store::account_locale(db, account_id).await?,
            &self.venture_locale(),
        ))
    }

    /// The venture's catalog, or the error that names the message it could
    /// not render.
    fn catalog(&self, key: &str) -> Result<Arc<dyn Catalog>, NotifyError> {
        self.settings()
            .catalog
            .clone()
            .ok_or_else(|| NotifyError::NoCatalog(key.to_owned()))
    }

    /// Prepares one notification for every device `account_id` has.
    ///
    /// Writes nothing: append [`Enqueued::statements`] to your own
    /// `db.batch_atomic(..)` so the notification is durable exactly when your
    /// state change is, then call [`Notifier::deliver_now`].
    ///
    /// `message` is either a [`Notification`] the caller wrote — today's
    /// path, unchanged, and the one a single-language venture uses — or a
    /// [`Localizable`] naming a message in the venture's catalog, which
    /// the drain renders **per recipient** (#190).
    ///
    /// # Errors
    ///
    /// [`NotifyError::UnknownCategory`] for a category the venture did
    /// not declare, [`NotifyError::InvalidData`] for a non-object
    /// `data`, [`NotifyError::NoCatalog`] for a localizable message in a
    /// venture with no catalog, and [`NotifyError::Database`] when a read
    /// fails.
    pub async fn notify(
        &self,
        db: &dyn Database,
        account_id: &str,
        category: &str,
        message: impl Into<Message>,
    ) -> Result<Enqueued, NotifyError> {
        let category = self.category(category)?.clone();
        let notification_id = self.new_id();

        // The account's own language, for the two channels there is one of
        // per account: the inbox it scrolls and the mailbox it reads. A
        // device's own language is resolved later, in the drain, because
        // one account can have an English browser and a Bahasa phone.
        let (notification, localizable, rendered, missing) = match message.into() {
            Message::Rendered(notification) => (*notification, None, None, Vec::new()),
            Message::Localizable(message) => {
                let catalog = self.catalog(&message.key)?;
                let asked = self.account_locale(db, account_id).await?;
                let Localised {
                    notification,
                    locale,
                    missing,
                    ..
                } = locale::render(&*catalog, &message, &asked, false);
                (notification, Some(*message), Some(locale), missing)
            }
        };

        // Prepared before either channel decides anything, so the row the
        // inbox keeps and the payload a device receives are the same
        // notification, and an invalid `data` is one error either way.
        let notification = prepare(notification, &category, &notification_id)?;
        let locale_tag = rendered.as_ref().map(ToString::to_string);
        let now = self.now();
        let mut statements = Vec::new();

        // In-app first, and unconditionally with respect to push: an
        // account with no device, or with push switched off, still gets
        // the record. Those switch off push, not the inbox.
        let inbox = self.in_app_allowed(db, account_id, &category).await?;
        let mut announcement = None;
        if inbox {
            let (statement, room) = self.inbox_row(&InboxWrite {
                account_id,
                notification_id: &notification_id,
                category: &category,
                notification: &notification,
                localizable: localizable.as_ref(),
                locale: locale_tag.as_deref(),
                now: &now,
            })?;
            statements.push(statement);
            announcement = Some(room);
        }

        // Email, also ahead of the push early returns and for the same
        // reason: an account with no device can still have a mailbox.
        if self.email_allowed(db, account_id, &category).await? {
            statements.push(self.email_row(
                account_id,
                &notification_id,
                &category,
                &notification,
                localizable.as_ref(),
                &now,
            )?);
        }

        // Cheap skip, not the check that counts: the drain re-reads the
        // preference immediately before sending, so an opt-out that
        // arrives after this line still wins.
        if !self.push_allowed(db, account_id, &category).await? {
            return Ok(Enqueued {
                notification_id,
                statements,
                devices: 0,
                inbox,
                announcement,
                skipped: Some(Skipped::PreferenceOff),
                category: category.name,
                missing,
            });
        }

        let subscriptions = store::subscriptions_for_account(db, account_id).await?;
        if subscriptions.is_empty() {
            return Ok(Enqueued {
                notification_id,
                statements,
                devices: 0,
                inbox,
                announcement,
                skipped: Some(Skipped::NoSubscriptions),
                category: category.name,
                missing,
            });
        }

        statements.reserve(subscriptions.len());
        for subscription in &subscriptions {
            statements.push(self.push_row(
                account_id,
                &notification_id,
                &category,
                &notification,
                localizable.as_ref(),
                &subscription.id,
                &now,
            )?);
        }
        Ok(Enqueued {
            devices: subscriptions.len(),
            notification_id,
            statements,
            inbox,
            announcement,
            skipped: None,
            category: category.name,
            missing,
        })
    }

    /// [`Notifier::notify`] for a caller with no state change of its own:
    /// commits the outbox rows in their own batch and attempts immediate
    /// delivery. The event-bus entry point is this.
    ///
    /// # Errors
    ///
    /// As [`Notifier::notify`], plus [`NotifyError::Database`] when the
    /// batch fails.
    pub async fn notify_now(
        &self,
        db: &dyn Database,
        scope: &Scope,
        account_id: &str,
        category: &str,
        message: impl Into<Message>,
    ) -> Result<Enqueued, NotifyError> {
        let enqueued = self.notify(db, account_id, category, message).await?;
        // `is_empty()` is about devices, and an account with none can
        // still have an inbox row waiting in this batch. Commit on the
        // statements, not on the device count.
        if !enqueued.statements().is_empty() {
            db.batch_atomic(enqueued.statements()).await?;
        }
        // Outside that `if`, because it also reports a missing
        // translation (#190) — and the fan-out that queued nothing at all
        // is exactly the one whose broken message would otherwise be
        // silent everywhere.
        self.announce(scope, &enqueued).await;
        if !enqueued.is_empty() {
            self.deliver_now(scope);
        }
        Ok(enqueued)
    }

    /// Announces a just-committed inbox row to the account's room, when
    /// the venture wired `Realtime`.
    ///
    /// Call it **after** your batch commits, for the same reason
    /// [`Notifier::deliver_now`] says so: a client that received the event
    /// will read the inbox, and the row has to be there.
    /// [`Notifier::notify_now`] does it for you.
    ///
    /// Deliberately not part of the drain. The drain runs only when there
    /// are push rows, and the account this matters most to — no device, or
    /// push refused — has none. A failure is logged and swallowed: a live
    /// update is an optimisation over the client's own polling, and losing
    /// one must never fail the caller's write.
    /// Announces a just-committed inbox row, **and** says on the bus that
    /// the catalog could not render part of the message (#190).
    ///
    /// The two are here together because this is the first point in the
    /// batch API at which there is a [`Scope`] to emit an event in:
    /// [`Notifier::notify`] runs inside somebody else's transaction and
    /// has none. A caller that batches its own statements and never calls
    /// this still learns about a missing translation from the drain, which
    /// reports one per delivery — but an account with no device and no
    /// mailbox has no delivery, so call it.
    pub async fn announce(&self, scope: &Scope, enqueued: &Enqueued) {
        if !enqueued.missing.is_empty()
            && let Some(ctx) = self.cell.get()
        {
            locale::report_missing(ctx, scope, &enqueued.category, &enqueued.missing);
        }
        let Some((room, payload)) = enqueued.announcement.as_ref() else {
            return;
        };
        let Some(realtime) = self.cell.get().and_then(|ctx| ctx.ports.realtime.clone()) else {
            return;
        };
        if let Err(err) = realtime.broadcast(room, payload).await {
            tracing::warn!(
                error = %err,
                request = %scope.request_id,
                "the inbox row is written; announcing it live failed"
            );
        }
    }

    /// Attempts immediate delivery on the request's `Defer`.
    ///
    /// Call it **after** your batch commits. Nothing is lost if the
    /// isolate dies first: the venture's scheduled drain picks the rows
    /// up on the next tick.
    pub fn deliver_now(&self, scope: &Scope) {
        let notifier = self.clone();
        let scope_for_drain = scope.clone();
        scope.defer.wait_until(Box::pin(async move {
            match notifier.drain(&scope_for_drain).await {
                Ok(report) if report.claimed > 0 => {
                    tracing::debug!(?report, "notifications drained");
                }
                Ok(_) => {}
                Err(err) => tracing::error!(error = %err, "notifications drain failed"),
            }
        }));
    }

    /// Delivers every due outbox row this drainer can lease, through the
    /// **mounted** module's ports.
    ///
    /// The whole delivery policy lives here: `Delivered` completes the
    /// row, `Unregistered` deletes the row **and** the subscription,
    /// `Rejected` and `NotConfigured` dead-letter it, and `Transient`
    /// retries with backoff — never before the `retry_after` the provider
    /// asked for — to a bound, then dead-letters.
    ///
    /// # Errors
    ///
    /// [`NotifyError`] when the module is not mounted, a port is missing,
    /// or the claim itself fails. A per-row failure never aborts the
    /// pass: it is counted in [`DrainReport::failed`] and the row keeps
    /// its lease until it expires.
    pub async fn drain(&self, scope: &Scope) -> Result<DrainReport, NotifyError> {
        self.drain_with(self.ctx()?, scope).await
    }

    /// [`Notifier::drain`] through the ports of the context handed in.
    ///
    /// This is what scheduled work must use. A cron invocation builds no
    /// router, so on a cold isolate there is no mounted context to drain
    /// through at all, and on a warm one draining through the parked
    /// context means using some earlier request's database handle.
    ///
    /// # Errors
    ///
    /// As [`Notifier::drain`], minus [`NotifyError::NotMounted`]: this
    /// entry point needs no mounted module.
    pub async fn drain_with(
        &self,
        ctx: &ModuleContext,
        scope: &Scope,
    ) -> Result<DrainReport, NotifyError> {
        let db = ctx
            .ports
            .db
            .clone()
            .ok_or(NotifyError::MissingPort("Database"))?;
        let push = ctx
            .ports
            .push
            .clone()
            .ok_or(NotifyError::MissingPort("Push"))?;

        let now = now_iso(ctx.ports.clock.as_ref());
        let lease_until = plus_secs(&now, LEASE_SECS);
        let records = Outbox::new(store::OUTBOX)
            .claim_due(&*db, &now, &lease_until, self.drain_batch(ctx))
            .await?;

        let mut report = DrainReport {
            claimed: records.len(),
            ..DrainReport::default()
        };
        let (db, push, now) = (&*db, &*push, now.as_str());
        let pending: Vec<_> = records
            .iter()
            .map(|record| async move {
                (
                    record.id.as_str(),
                    self.deliver_one(ctx, db, push, scope, record, now).await,
                )
            })
            .collect();
        let mut deliveries =
            futures_util::stream::iter(pending).buffer_unordered(self.drain_concurrency(ctx));
        while let Some((row, result)) = deliveries.next().await {
            match result {
                Ok(outcome) => report.record(outcome),
                // The row keeps its lease and becomes due again when that
                // expires. Abandoning the rest of the pass over one
                // transient error would leave every row behind it leased
                // and untouched for the whole lease.
                Err(error) => {
                    report.failed += 1;
                    tracing::error!(row = %row, %error, "delivering a notification failed");
                }
            }
        }
        // Coalesced summaries (#232), on the same tick: this is the only
        // path the scheduled entry point is guaranteed to run, so it is
        // the only place a summary is ever sent from.
        if let Err(error) = self.summarise_suppressed(ctx, db, now).await {
            tracing::error!(%error, "sending the suppressed-email summaries failed");
        }
        Ok(report)
    }

    /// One summary mail per `(account, category)` burst the cooldown
    /// suppressed, for every burst whose window has rolled.
    ///
    /// A burst is due once the trailing window holds fewer than `cap`
    /// sends — the same comparison, the other way round, that suppressed
    /// it. Everything else is the same re-check a normal send gets
    /// (#232): category still declared, preference still on, address
    /// still verified and not unsubscribed. An answer that turned
    /// `false` during the window clears the burst silently — a summary
    /// announcing mail the account has just opted out of is the flood in
    /// miniature.
    ///
    /// The rows are cleared in the same batch as the send record, which
    /// is what bounds the summary to one send: a second tick finds no
    /// burst. The mail also carries an idempotency key derived from the
    /// **window** — the burst's first suppression — so a provider retry
    /// cannot turn into a second copy either.
    async fn summarise_suppressed(
        &self,
        ctx: &ModuleContext,
        db: &dyn Database,
        now: &str,
    ) -> Result<(), NotifyError> {
        let cap = self.email_max_per_window(ctx);
        // The channel is off, not full: nothing is waiting for a window
        // that will never roll.
        if cap == 0 {
            return Ok(());
        }
        let since = plus_secs(now, -self.email_window(ctx));
        let limit = self.drain_batch(ctx);
        for burst in store::suppressed_email_bursts(db, limit).await? {
            if store::emails_since(db, &burst.account_id, &burst.category, &since).await?
                >= i64::from(cap)
            {
                continue;
            }
            // The clear is over the ids the count covered, by identity.
            // A suppression that lands mid-send is not in that set —
            // `suppressed_at` has one-second resolution, so a timestamp
            // bound could not separate it from the counted rows — and the
            // next pass sees it: its read of the trailing window holds
            // the summary this pass just recorded, so the burst waits for
            // the window to roll again rather than mailing twice.
            let burst_ids = burst.ids.clone();
            let clear = || store::clear_email_suppression_statement(&burst_ids);
            let (Some(category), Some(target)) = (
                self.category(&burst.category).cloned().ok(),
                store::email_target(db, &burst.account_id)
                    .await?
                    .filter(store::EmailTarget::mailable),
            ) else {
                db.execute(&clear()).await?;
                continue;
            };
            if !self.email_allowed(db, &burst.account_id, &category).await? {
                db.execute(&clear()).await?;
                continue;
            }
            let Some(mailer) = ctx.ports.mailer.clone() else {
                // Leave the burst for the next tick rather than losing it:
                // unlike a normal send there is no dead letter here, and a
                // mailer wired late should still earn its summary.
                tracing::warn!(
                    account = %burst.account_id,
                    category = %burst.category,
                    "a suppressed-email summary is due but no Mailer port is wired"
                );
                continue;
            };
            let mail = self
                .compose_summary(ctx, db, &burst, &category, &target.email)
                .await?;
            match mailer.send(mail).await {
                Ok(SendOutcome::Sent { .. }) => {
                    db.batch_atomic(&[
                        store::record_email_statement(
                            &self.new_id(),
                            &burst.account_id,
                            &burst.category,
                            now,
                        ),
                        clear(),
                    ])
                    .await?;
                }
                // NotConfigured dead-letters a normal send because the
                // notification will not fix itself; a burst will, so it
                // waits instead. Retries are bounded by retention.
                outcome => {
                    tracing::warn!(
                        account = %burst.account_id,
                        category = %burst.category,
                        ?outcome,
                        "the suppressed-email summary could not be sent; it stays queued"
                    );
                }
            }
        }
        Ok(())
    }

    /// The mail one burst is coalesced into, through the same [`Self::compose`]
    /// a normal send uses — the unsubscribe links, the RFC 8058 headers and
    /// the `Content-Language` fall out of that, and cannot drift from it.
    ///
    /// The identity inside the job is the **window** (the burst's first
    /// suppression), not any one notification: it is what the mailer's
    /// idempotency key is built from, so a retried tick sends the same
    /// mail, and a later, different burst sends a different one.
    async fn compose_summary(
        &self,
        ctx: &ModuleContext,
        db: &dyn Database,
        burst: &store::SuppressedBurst,
        category: &Category,
        to: &str,
    ) -> Result<Mail, NotifyError> {
        let locale = self.account_locale(db, &burst.account_id).await?;
        // No category in the subject: it only has its slug, and
        // "3 new coach_notes notifications" is not a subject line a
        // person should read. The count carries the news; the in-app
        // inbox, which labels by category properly, carries the rest.
        let title = format!("You have {} new notifications", burst.count);
        let notification = Notification::new(
            title,
            format!(
                "{} notifications arrived while your email limit was full. They are in your \
                 in-app notifications.",
                burst.count
            ),
        );
        let job = EmailJob {
            notification_id: format!(
                "summary-{}-{}-{}",
                burst.account_id, burst.category, burst.first_suppressed_at
            ),
            account_id: burst.account_id.clone(),
            category: burst.category.clone(),
            notification: notification.clone(),
            localizable: None,
        };
        Ok(Self::compose(
            ctx,
            &job,
            category,
            to,
            &notification,
            &locale,
            None,
        ))
    }

    async fn deliver_one(
        &self,
        ctx: &ModuleContext,
        db: &dyn Database,
        push: &dyn Push,
        scope: &Scope,
        record: &OutboxRecord,
        now: &str,
    ) -> Result<Outcome, NotifyError> {
        if record.topic == TOPIC_EMAIL {
            return self.deliver_email(ctx, db, scope, record, now).await;
        }

        let Ok(job) = serde_json::from_str::<SendJob>(&record.payload) else {
            tracing::error!(row = %record.id, "outbox row is not a notifications job");
            self.dead_letter(
                db,
                record,
                DeadLetterReason::Malformed,
                "outbox payload is not a notifications job",
                now,
            )
            .await?;
            return Ok(Outcome::DeadLettered);
        };

        let Some(subscription) = store::subscription_by_id(db, &job.subscription_id).await? else {
            // Signed out between the commit and the drain. Not a failure:
            // there is nothing left to deliver to.
            db.execute(&store::delete_outbox_statement(&record.id))
                .await?;
            return Ok(Outcome::Dropped);
        };

        // The device changed hands between the commit and the drain: a
        // sign-out deletes the row, but signing *another* account in on
        // the same device only re-homes it. Sending anyway would deliver
        // this account's notification to whoever holds the device now,
        // judged against this account's preferences — the row's account
        // and the job's account have to be the same account or there is
        // nothing here to deliver.
        if subscription.account_id != job.account_id {
            tracing::info!(
                row = %record.id,
                subscription = %subscription.id,
                "dropped a notification whose device now belongs to another account"
            );
            db.execute(&store::delete_outbox_statement(&record.id))
                .await?;
            return Ok(Outcome::Dropped);
        }

        // The check that counts. A preference read now, not when the row
        // was written, is what makes a late opt-out win.
        let Ok(category) = self.category(&job.category).cloned() else {
            // The venture stopped declaring the category between the commit
            // and the drain. Silence is the safe answer.
            db.execute(&store::delete_outbox_statement(&record.id))
                .await?;
            return Ok(Outcome::Dropped);
        };
        if !self.push_allowed(db, &job.account_id, &category).await? {
            db.execute(&store::delete_outbox_statement(&record.id))
                .await?;
            return Ok(Outcome::Dropped);
        }

        // The language, resolved here and not when the row was written:
        // the subscription's own, then the account's, then the venture's
        // default. A device that changed its language between the commit
        // and the drain is written to in the one it has now.
        let locale = locale::resolve(
            subscription.locale.clone(),
            store::account_locale(db, &job.account_id).await?,
            &self.venture_locale(),
        );
        let notification =
            self.localise_push(ctx, scope, &category, &job, subscription.transport, &locale)?;

        match push.send(&subscription.recipient, &notification).await {
            Ok(PushOutcome::Delivered { .. }) => {
                db.execute(&store::delete_outbox_statement(&record.id))
                    .await?;
                Ok(Outcome::Delivered)
            }
            Ok(PushOutcome::NotConfigured) => {
                self.dead_letter(
                    db,
                    record,
                    DeadLetterReason::NotConfigured,
                    &format!(
                        "no adapter is configured for {}: the module is mounted without the \
                         transport it needs",
                        subscription.transport
                    ),
                    now,
                )
                .await?;
                Ok(Outcome::DeadLettered)
            }
            // A delete instruction, not a failed send, and the only error
            // that is ever allowed to prune (ADR 0015).
            Err(PushError::Unregistered) => {
                self.prune(ctx, db, scope, record, &subscription).await?;
                Ok(Outcome::Pruned)
            }
            Err(PushError::Rejected(message)) => {
                self.dead_letter(db, record, DeadLetterReason::Rejected, &message, now)
                    .await?;
                Ok(Outcome::DeadLettered)
            }
            Err(PushError::Transient {
                message,
                retry_after,
            }) => {
                self.retry_or_give_up(ctx, db, record, &message, retry_after, now)
                    .await
            }
        }
    }

    /// The notification one device actually receives (#190).
    ///
    /// A caller that passed rendered text gets its own notification back
    /// untouched — no catalog is consulted, nothing is re-prepared, and a
    /// `loc` it set itself travels exactly as it did before this existed.
    ///
    /// For a localizable message the render mode decides two things, and
    /// they are separate: whether the app's own loc keys are attached, and
    /// which language the text beside them is in.
    ///
    /// **Web Push has no loc-key mechanism**, so a browser is always
    /// served the rendered text in its own language whatever the category
    /// says. That is not the adapter being lenient — the keys are never
    /// attached for it here, so nothing downstream has to know the rule.
    fn localise_push(
        &self,
        ctx: &ModuleContext,
        scope: &Scope,
        category: &Category,
        job: &SendJob,
        transport: Transport,
        locale: &LanguageIdentifier,
    ) -> Result<Notification, NotifyError> {
        let Some(message) = job.localizable.as_ref() else {
            return Ok(job.notification.clone());
        };
        let catalog = self.catalog(&message.key)?;
        let native = category.render.sends_loc_keys() && transport != Transport::Webpush;
        // `native` alone means the app owns the language, so the text
        // beside the keys is only a fallback for a build too old to have
        // them: the venture's default is the right thing to send, and
        // rendering per recipient would be work nobody reads.
        let asked = if native && !category.render.renders_for_recipient() {
            self.venture_locale()
        } else {
            locale.clone()
        };
        let rendered = locale::render(&*catalog, message, &asked, false);
        locale::report_missing(ctx, scope, &category.name, &rendered.missing);
        let mut notification = prepare(rendered.notification, category, &job.notification_id)?;
        if native {
            notification.loc.clone_from(&message.loc);
        }
        Ok(notification)
    }

    /// The mail one queued notification becomes, in the account's own
    /// language (#190).
    ///
    /// The language is read **now**, not when the row was written: one
    /// inbox and one mailbox per account, and an account that changed its
    /// answer in between gets the one it has.
    ///
    /// A caller that passed rendered text gets its own words, and the
    /// account's locale is still what `Content-Language` and `dir` are set
    /// from — the venture wrote its mail in some language, and saying
    /// nothing about it is what leaves a client guessing.
    async fn localise_mail(
        &self,
        ctx: &ModuleContext,
        db: &dyn Database,
        scope: &Scope,
        job: &EmailJob,
        category: &Category,
        to: &str,
    ) -> Result<Mail, NotifyError> {
        let asked = self.account_locale(db, &job.account_id).await?;
        let Some(message) = job.localizable.as_ref() else {
            return Ok(Self::compose(
                ctx,
                job,
                category,
                to,
                &job.notification,
                &asked,
                None,
            ));
        };
        let catalog = self.catalog(&message.key)?;
        // `want_subject`: mail is the one channel with a third line to
        // write, and a venture that writes none is not missing anything.
        let rendered = locale::render(&*catalog, message, &asked, true);
        locale::report_missing(ctx, scope, &category.name, &rendered.missing);
        let notification = prepare(rendered.notification, category, &job.notification_id)?;
        Ok(Self::compose(
            ctx,
            job,
            category,
            to,
            &notification,
            &rendered.locale,
            rendered.subject,
        ))
    }

    /// One transient-failure policy, shared by both channels: back off to
    /// a bound and then dead-letter, never retrying before the provider's
    /// own `retry_after`.
    async fn retry_or_give_up(
        &self,
        ctx: &ModuleContext,
        db: &dyn Database,
        record: &OutboxRecord,
        message: &str,
        retry_after: Option<Duration>,
        now: &str,
    ) -> Result<Outcome, NotifyError> {
        let attempts = record.attempts.saturating_add(1);
        if attempts >= self.max_attempts(ctx) {
            self.dead_letter(
                db,
                record,
                DeadLetterReason::AttemptsExhausted,
                &format!("gave up after {attempts} attempts: {message}"),
                now,
            )
            .await?;
            return Ok(Outcome::DeadLettered);
        }
        let delay = backoff(attempts).max(retry_after.unwrap_or(Duration::ZERO));
        let seconds = i64::try_from(delay.as_secs())
            .unwrap_or_else(|_| i64::try_from(BACKOFF_MAX_SECS).unwrap_or(i64::MAX));
        Outbox::new(store::OUTBOX)
            .retry_later(db, &record.id, &plus_secs(now, seconds))
            .await?;
        Ok(Outcome::Retried)
    }

    /// Delivers one queued email.
    ///
    /// The same policy shape as push over `Mailer`'s outcomes: a permanent
    /// refusal dead-letters with a reason ops can act on, a transient one
    /// retries with backoff, and `NotConfigured` dead-letters rather than
    /// passing for success — a venture that opted a category into email
    /// and wired no mailer has a bug, not a quiet no-op.
    async fn deliver_email(
        &self,
        ctx: &ModuleContext,
        db: &dyn Database,
        scope: &Scope,
        record: &OutboxRecord,
        now: &str,
    ) -> Result<Outcome, NotifyError> {
        let Ok(job) = serde_json::from_str::<EmailJob>(&record.payload) else {
            tracing::error!(row = %record.id, "outbox row is not a notifications email job");
            self.dead_letter(
                db,
                record,
                DeadLetterReason::Malformed,
                "outbox payload is not a notifications email job",
                now,
            )
            .await?;
            return Ok(Outcome::DeadLettered);
        };

        let Some((category, target)) = self.email_still_wanted(ctx, db, &job, now).await? else {
            db.execute(&store::delete_outbox_statement(&record.id))
                .await?;
            return Ok(Outcome::Dropped);
        };

        let Some(mailer) = ctx.ports.mailer.clone() else {
            self.dead_letter(
                db,
                record,
                DeadLetterReason::NotConfigured,
                "the venture opted a category into email and wired no Mailer port",
                now,
            )
            .await?;
            return Ok(Outcome::DeadLettered);
        };

        let mail = self
            .localise_mail(ctx, db, scope, &job, &category, &target.email)
            .await?;

        match mailer.send(mail).await {
            Ok(SendOutcome::Sent { .. }) => {
                db.batch_atomic(&[
                    store::record_email_statement(
                        &self.new_id(),
                        &job.account_id,
                        &category.name,
                        now,
                    ),
                    store::delete_outbox_statement(&record.id),
                ])
                .await?;
                Ok(Outcome::Delivered)
            }
            Ok(SendOutcome::NotConfigured) => {
                self.dead_letter(
                    db,
                    record,
                    DeadLetterReason::NotConfigured,
                    "the mailer has no API key or no verified sending domain",
                    now,
                )
                .await?;
                Ok(Outcome::DeadLettered)
            }
            // Nothing about this message will ever be accepted.
            Err(
                err @ (MailError::Unauthorized
                | MailError::DomainNotVerified { .. }
                | MailError::Invalid { .. }),
            ) => {
                self.dead_letter(
                    db,
                    record,
                    DeadLetterReason::Rejected,
                    &err.to_string(),
                    now,
                )
                .await?;
                Ok(Outcome::DeadLettered)
            }
            Err(err) => {
                let retry_after = match &err {
                    MailError::RateLimited { retry_after } => *retry_after,
                    _ => None,
                };
                self.retry_or_give_up(ctx, db, record, &err.to_string(), retry_after, now)
                    .await
            }
        }
    }

    /// Everything that can turn a queued email back into nothing,
    /// re-checked at send time rather than trusted from when the row was
    /// written: the category still exists, the account still wants it, the
    /// address is still mailable, and the cooldown still has room.
    ///
    /// `None` means drop the row. All four are drops rather than retries —
    /// a retry would deliver the moment the answer changed back, and for
    /// the cooldown that is exactly the flood the cap exists to prevent.
    async fn email_still_wanted(
        &self,
        ctx: &ModuleContext,
        db: &dyn Database,
        job: &EmailJob,
        now: &str,
    ) -> Result<Option<(Category, store::EmailTarget)>, NotifyError> {
        let Ok(category) = self.category(&job.category).cloned() else {
            return Ok(None);
        };
        if !self.email_allowed(db, &job.account_id, &category).await? {
            return Ok(None);
        }
        let Some(target) = store::email_target(db, &job.account_id).await? else {
            return Ok(None);
        };
        let cap = self.email_max_per_window(ctx);
        let since = plus_secs(now, -self.email_window(ctx));
        if cap == 0
            || store::emails_since(db, &job.account_id, &category.name, &since).await?
                >= i64::from(cap)
        {
            tracing::info!(
                account = %job.account_id,
                category = %category.name,
                "email suppressed: the category is at its cooldown cap for this account"
            );
            // The trace the summary pass later coalesces from (#232). A
            // cap of 0 is the venture switching the channel off, not a
            // window that will roll, so that drop leaves no trace.
            if cap > 0 && !store::email_suppression_exists(db, &job.notification_id).await? {
                store::record_email_suppression(
                    db,
                    &self.new_id(),
                    &job.account_id,
                    &category.name,
                    &job.notification_id,
                    now,
                )
                .await?;
            }
            return Ok(None);
        }
        Ok(Some((category, target)))
    }

    /// How long the per-category cooldown window is, in seconds.
    fn email_window(&self, ctx: &ModuleContext) -> i64 {
        i64::from(
            ModuleConfig::new(crate::MODULE_NAME, &*ctx.config)
                .get_u32("EMAIL_WINDOW_SECS", self.settings().email_window_secs),
        )
    }

    /// How many mails one account may get for one category in a window.
    /// `0` stops the channel, which is what a venture that wants email off
    /// without editing its categories sets.
    fn email_max_per_window(&self, ctx: &ModuleContext) -> u32 {
        ModuleConfig::new(crate::MODULE_NAME, &*ctx.config)
            .get_u32("EMAIL_MAX_PER_WINDOW", self.settings().email_max_per_window)
    }

    /// The mail one notification becomes.
    ///
    /// Text and HTML together, an unsubscribe link in both, and the RFC
    /// 8058 headers beside it: Gmail and Yahoo have required one-click of
    /// bulk senders since 2024, and a "click here" line in a footer is not
    /// what they check for.
    ///
    /// **The headers go on only when there is a `Signer`** (issue #234).
    /// RFC 8058 requires the advertised URI to accept the POST, and with
    /// no signer the link degrades to a venture front-end path this module
    /// does not serve — which either 404s or, worse, hits a single-page
    /// app that answers the POST with `200 HTML` and looks like success
    /// while doing nothing. A mail with no one-click header is compliant.
    /// One whose header points somewhere that cannot honour it is not, and
    /// it fails silently at exactly the two providers the headers exist
    /// for. The footer link still tells a person where to go.
    #[allow(
        clippy::too_many_arguments,
        reason = "one mail's worth of inputs, named rather than bundled"
    )]
    fn compose(
        ctx: &ModuleContext,
        job: &EmailJob,
        category: &Category,
        to: &str,
        notification: &Notification,
        locale: &LanguageIdentifier,
        localised_subject: Option<String>,
    ) -> Mail {
        let cfg = ModuleConfig::new(crate::MODULE_NAME, &*ctx.config);
        let from = cfg.get_str(
            "MAIL_FROM",
            &format!("no-reply@send.{}", ctx.venture.domain),
        );
        let base = cfg.get_str("PUBLIC_URL", &format!("https://{}", ctx.venture.domain));
        let one_click = Self::unsubscribe_url(ctx, &base, &job.account_id, &category.name);
        // Two links, because they answer different questions. The header
        // one-click stops *this* category, which is what somebody who
        // clicked "unsubscribe" on one kind of mail means. The footer also
        // offers every category, so a reader who wants out entirely does
        // not have to unsubscribe once per category as they arrive.
        let one_click_all = Self::unsubscribe_url(ctx, &base, &job.account_id, UNSUBSCRIBE_ALL);
        // With no signer both fall back to the account's own settings
        // page: a person can still act on it, which is all the footer
        // claims. The header, which claims more, is left off below.
        let settings = format!("{base}/settings/notifications");
        let unsubscribe = one_click.clone().unwrap_or_else(|| settings.clone());
        let unsubscribe_all = one_click_all.unwrap_or(settings);
        // The catalog's own `.subject` wins, because it is the only one of
        // the three that is in the recipient's language. Then the
        // category's template, then the title.
        let subject = localised_subject
            .or_else(|| category.subject_template.clone())
            .unwrap_or_else(|| notification.title.clone());

        let action = notification
            .url
            .as_deref()
            .map_or_else(String::new, |url| format!("\n{url}\n"));
        let text = format!(
            "{}\n\n{}\n{}\nTo stop receiving these, open:\n{}\n\nTo stop every \
             notification email:\n{}\n",
            notification.title, notification.body, action, unsubscribe, unsubscribe_all
        );
        // `lang` and `dir` on a wrapper, not on a `<html>` element this
        // fragment does not have. Both matter and they are different
        // questions: `lang` is what a screen reader pronounces it as, and
        // `dir` is what stops an Arabic paragraph being laid out left to
        // right with its punctuation in the wrong place. Direction comes
        // from `cratefield-i18n`'s explicit list — `unic-langid` carries
        // no directionality data at all.
        let html = format!(
            "<div lang=\"{}\" dir=\"{}\"><p><strong>{}</strong></p><p>{}</p>{}<hr><p><a \
             href=\"{}\">Stop receiving these</a> &middot; <a href=\"{}\">stop all notification \
             email</a></p></div>",
            escape(&locale.to_string()),
            direction(locale).as_str(),
            escape(&notification.title),
            escape(&notification.body),
            notification.url.as_deref().map_or_else(String::new, |url| {
                format!("<p><a href=\"{}\">Open</a></p>", escape(url))
            }),
            escape(&unsubscribe),
            escape(&unsubscribe_all),
        );

        let mut message = Mail::new(to, from, subject, text, html)
            // RFC 3282: what language this mail is in. A filing rule, a
            // screen reader and a translation prompt all read it, and a
            // mail sent in Indonesian that claims nothing is a mail every
            // one of them guesses about.
            .header("Content-Language", locale.to_string())
            // One notification is one mail however often the row is
            // retried: the fan-out id is stable, the outbox row id is not.
            .idempotency_key(format!("{}-email", job.notification_id))
            .tags(["notifications"]);
        // Both headers, or neither: `List-Unsubscribe-Post` is what turns
        // the other into a one-click promise, so advertising it beside a
        // URI that cannot accept the POST is the lie.
        if let Some(url) = one_click {
            message = message
                .header("List-Unsubscribe", format!("<{url}>"))
                .header("List-Unsubscribe-Post", "List-Unsubscribe=One-Click");
        }
        if let Some(reply_to) = cfg.get_opt("MAIL_REPLY_TO") {
            message = message.reply_to(reply_to);
        }
        message
    }

    /// The signed one-click unsubscribe link for one account and category,
    /// or `None` when this deployment has no `Signer`.
    ///
    /// Signed and purpose-bound, so a link for one account cannot switch
    /// another's preference off — the revocable-link pattern of ADR 0014.
    ///
    /// `None` rather than a front-end fallback: the caller decides what a
    /// missing link means for it. A footer can say "open your settings";
    /// a `List-Unsubscribe` header cannot, because the URI it names has to
    /// accept a POST from a mailbox provider (issue #234).
    fn unsubscribe_url(
        ctx: &ModuleContext,
        base: &str,
        account_id: &str,
        category: &str,
    ) -> Option<String> {
        let signer = ctx.ports.signer.as_ref()?;
        let token = signer.sign(&Payload {
            purpose: PURPOSE_UNSUBSCRIBE.to_owned(),
            subject: format!("{account_id}:{category}"),
            exp: None,
            kid: Kid::Cur,
        });
        Some(format!(
            "{base}/v1/notifications/email/unsubscribe?token={token}"
        ))
    }

    /// The provider says this recipient is gone: delete the subscription
    /// and the row in one batch, then say so on the bus.
    async fn prune(
        &self,
        ctx: &ModuleContext,
        db: &dyn Database,
        scope: &Scope,
        record: &OutboxRecord,
        subscription: &Subscription,
    ) -> Result<(), NotifyError> {
        db.batch_atomic(&[
            store::delete_subscription_statement(&subscription.id),
            store::delete_outbox_statement(&record.id),
        ])
        .await?;
        tracing::info!(
            subscription = %subscription.id,
            transport = %subscription.transport,
            "pruned a subscription the provider reported as gone"
        );
        ctx.events.emit_in(
            scope,
            EVENT_SUBSCRIPTION_PRUNED,
            json!({
                "subscription_id": subscription.id,
                "account_id": subscription.account_id,
                "transport": subscription.transport.as_str(),
            }),
        );
        Ok(())
    }

    /// Moves a row that will never succeed out of the work queue, in one
    /// batch so it can never be both queued and dead.
    async fn dead_letter(
        &self,
        db: &dyn Database,
        record: &OutboxRecord,
        reason: DeadLetterReason,
        last_error: &str,
        now: &str,
    ) -> Result<(), NotifyError> {
        tracing::error!(
            row = %record.id,
            reason = reason.as_str(),
            error = last_error,
            "notification dead-lettered"
        );
        // The attempt that just failed counts: `OutboxRecord::attempts` is
        // what had already failed before it.
        let attempts = record.attempts.saturating_add(1);
        // And the row's own enqueue time, so `created_at` and `failed_at`
        // are two facts rather than one written twice.
        let created_at = store::outbox_created_at(db, &record.id)
            .await?
            .unwrap_or_else(|| now.to_owned());
        db.batch_atomic(&[
            store::dead_letter_statement(
                record,
                &store::DeadLetter {
                    reason,
                    last_error,
                    attempts,
                    created_at: &created_at,
                    failed_at: now,
                },
            ),
            store::delete_outbox_statement(&record.id),
        ])
        .await?;
        Ok(())
    }
}

/// The notification as it will be stored and sent: `data` always carries
/// `category` and `notification_id`, and a badge count survives only for a
/// category that asked for one.
fn prepare(
    mut notification: Notification,
    category: &Category,
    notification_id: &str,
) -> Result<Notification, NotifyError> {
    let mut data = match notification.data {
        Value::Null => serde_json::Map::new(),
        Value::Object(map) => map,
        _ => return Err(NotifyError::InvalidData),
    };
    data.insert("category".to_owned(), json!(category.name));
    data.insert("notification_id".to_owned(), json!(notification_id));
    notification.data = Value::Object(data);

    // No badge counts by default: a number that is wrong is worse than no
    // number, and only the venture knows whether it can count.
    if !category.badge {
        notification.badge = None;
    }
    Ok(notification)
}

/// The room one account's live inbox events go to. A client joins
/// `notifications:<account_id>` when the app opens.
///
/// Nothing is published here that the inbox row does not also hold, so a
/// client that missed the event loses nothing by reading the list.
pub(crate) const INBOX_ROOM_PREFIX: &str = "notifications:";

/// The category name that means "every category" in an unsubscribe
/// token's subject.
pub(crate) const UNSUBSCRIBE_ALL: &str = "all";

/// The purpose a one-click unsubscribe token is bound to.
pub(crate) const PURPOSE_UNSUBSCRIBE: &str = "notifications.unsubscribe";

/// The five characters that must not travel into HTML as themselves.
pub(crate) fn escape(raw: &str) -> String {
    raw.replace('&', "&amp;")
        .replace('<', "&lt;")
        .replace('>', "&gt;")
        .replace('"', "&quot;")
        .replace('\'', "&#39;")
}

/// Exponential backoff on the number of failed attempts, capped.
fn backoff(attempts: i64) -> Duration {
    let exponent = u32::try_from(attempts.saturating_sub(1).max(0)).unwrap_or(u32::MAX);
    let secs = BACKOFF_BASE_SECS
        .checked_shl(exponent.min(16))
        .unwrap_or(BACKOFF_MAX_SECS)
        .min(BACKOFF_MAX_SECS);
    Duration::from_secs(secs)
}

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

    #[test]
    fn backoff_doubles_and_stops_at_an_hour() {
        assert_eq!(backoff(1), Duration::from_secs(30));
        assert_eq!(backoff(2), Duration::from_secs(60));
        assert_eq!(backoff(3), Duration::from_secs(120));
        assert_eq!(backoff(9), Duration::from_secs(3_600));
        assert_eq!(backoff(64), Duration::from_secs(3_600));
    }

    #[test]
    fn data_always_carries_the_category_and_the_notification_id() {
        let category = Category::new("booking");
        let mut notification = Notification::new("Booked", "See you Tuesday");
        notification.data = json!({ "booking_id": "b-1" });
        let prepared = prepare(notification, &category, "01JNOTIF").expect("prepared");
        assert_eq!(prepared.data["booking_id"], "b-1");
        assert_eq!(prepared.data["category"], "booking");
        assert_eq!(prepared.data["notification_id"], "01JNOTIF");
    }

    #[test]
    fn a_badge_survives_only_for_a_category_that_asked_for_one() {
        let mut notification = Notification::new("a", "b");
        notification.badge = Some(7);
        let plain = prepare(notification.clone(), &Category::new("booking"), "id").expect("plain");
        assert_eq!(plain.badge, None, "no badge counts by default");
        let opted =
            prepare(notification, &Category::new("booking").badge(true), "id").expect("opted");
        assert_eq!(opted.badge, Some(7));
    }

    #[test]
    fn a_non_object_data_is_refused_rather_than_silently_replaced() {
        let mut notification = Notification::new("a", "b");
        notification.data = json!(["not", "an", "object"]);
        assert!(matches!(
            prepare(notification, &Category::new("booking"), "id"),
            Err(NotifyError::InvalidData)
        ));
    }

    #[test]
    fn a_job_payload_carries_no_recipient() {
        // The regression this pins: a payload with the token in it would
        // put credential material into every export and diagnostic.
        let job = SendJob {
            notification_id: "n".to_owned(),
            account_id: "a".to_owned(),
            category: "booking".to_owned(),
            subscription_id: "s".to_owned(),
            notification: Notification::new("a", "b"),
            localizable: None,
        };
        let json = serde_json::to_string(&job).expect("serialises");
        assert!(!json.contains("device_token"), "{json}");
        assert!(!json.contains("endpoint"), "{json}");
        assert!(!json.contains("recipient"), "{json}");
    }

    #[test]
    fn a_rendered_callers_payload_is_the_one_it_always_was() {
        // "A single-language venture pays nothing" includes the wire: a
        // row queued before this deployment must drain after it, and a row
        // queued after it must be readable by a deployment that rolls
        // back. Both hold exactly while the new field is absent.
        let job = SendJob {
            notification_id: "n".to_owned(),
            account_id: "a".to_owned(),
            category: "booking".to_owned(),
            subscription_id: "s".to_owned(),
            notification: Notification::new("Booked", "See you Tuesday"),
            localizable: None,
        };
        let json = serde_json::to_string(&job).expect("serialises");
        assert!(!json.contains("localizable"), "{json}");

        // And the other direction: a payload written before #190 still
        // deserialises, because the field defaults.
        let old = r#"{"notification_id":"n","account_id":"a","category":"booking",
            "subscription_id":"s","notification":{"title":"Booked","body":"Tuesday"}}"#;
        let parsed: SendJob = serde_json::from_str(old).expect("an older payload still reads");
        assert!(parsed.localizable.is_none());
        assert_eq!(parsed.notification.title, "Booked");
    }

    #[test]
    fn a_localizable_payload_carries_the_message_and_no_recipient() {
        let job = SendJob {
            notification_id: "n".to_owned(),
            account_id: "a".to_owned(),
            category: "booking".to_owned(),
            subscription_id: "s".to_owned(),
            notification: Notification::new("Booked", "Tuesday"),
            localizable: Some(Localizable::new("booking-confirmed").arg("coach", "Sari")),
        };
        let json = serde_json::to_string(&job).expect("serialises");
        assert!(json.contains("booking-confirmed"), "{json}");
        assert!(!json.contains("device_token"), "{json}");
        assert!(!json.contains("recipient"), "{json}");
        let back: SendJob = serde_json::from_str(&json).expect("round trips");
        assert_eq!(back, job);
    }
}