jmap-server 0.1.2

//! Shared backend infrastructure for all JMAP server crates.
//!
//! Re-exports the marker traits from `jmap-types` and adds the result types,
//! `BackendChangesError`, and [`JmapBackend`] supertrait. Domain crates add
//! their write-side methods and domain-specific error variants on top.

pub use jmap_types::{GetObject, JmapObject, QueryObject, SetObject};

// ---------------------------------------------------------------------------
// SetError — RFC 8620 §5.3 per-object set-method error
// ---------------------------------------------------------------------------

/// A per-item error in a `/set` response (`notCreated`, `notUpdated`,
/// `notDestroyed` maps) (RFC 8620 §5.3).
///
/// Construct with [`SetError::new`] and chain the builder methods as needed.
#[non_exhaustive]
#[derive(Debug, Clone, PartialEq, serde::Serialize, serde::Deserialize)]
#[serde(rename_all = "camelCase")]
pub struct SetError {
    /// The machine-readable error type.
    #[serde(rename = "type")]
    pub error_type: SetErrorType,
    /// Optional human-readable description of the error.
    #[serde(skip_serializing_if = "Option::is_none")]
    pub description: Option<String>,
    /// Property names that caused the error (for `invalidProperties`).
    #[serde(skip_serializing_if = "Option::is_none")]
    pub properties: Option<Vec<String>>,
    /// The existing object id (for `alreadyExists` — RFC 8621 §5.7).
    #[serde(rename = "existingId", skip_serializing_if = "Option::is_none")]
    pub existing_id: Option<jmap_types::Id>,
    /// Maximum recipients allowed (for `tooManyRecipients` — RFC 8621 §7.5).
    #[serde(rename = "maxRecipients", skip_serializing_if = "Option::is_none")]
    pub max_recipients: Option<u64>,
    /// Invalid recipient addresses (for `invalidRecipients` — RFC 8621 §7.5).
    #[serde(rename = "invalidRecipients", skip_serializing_if = "Option::is_none")]
    pub invalid_recipients: Option<Vec<String>>,
    /// Missing blob IDs (for `blobNotFound` — RFC 8621 §5.5).
    #[serde(rename = "notFound", skip_serializing_if = "Option::is_none")]
    pub not_found: Option<Vec<jmap_types::Id>>,
    /// Maximum message size in octets (for `tooLarge` on EmailSubmission — RFC 8621 §7.5).
    #[serde(rename = "maxSize", skip_serializing_if = "Option::is_none")]
    pub max_size: Option<u64>,
    /// Catch-all for extension-defined SetError fields not covered by
    /// the typed members above.
    ///
    /// JMAP extensions sometimes ship error variants whose wire shape
    /// includes additional structured fields beyond the RFC 8620 §5.3
    /// base set — e.g. JMAP Chat's `rateLimited` SetError carries a
    /// `serverRetryAfter` UTCDate telling the client when it may
    /// retry, and `mdnAlreadySent` (RFC 8621 §7.7) is a typed
    /// extension error variant. This map preserves any such field
    /// across serialize / deserialize round-trip, mirroring the
    /// extras-preservation policy on the client-side
    /// [`jmap_types::SetError`] type.
    ///
    /// Use [`SetError::with_extra`] to populate from handler code:
    ///
    /// ```ignore
    /// SetError::new(SetErrorType::custom("rateLimited"))
    ///     .with_description("Slow mode is active for this chat")
    ///     .with_extra("serverRetryAfter", json!(retry_after_str))
    /// ```
    ///
    /// Per workspace AGENTS.md "Extras-preservation policy" — wire
    /// format is byte-identical to a pre-extras SetError when the
    /// map is empty (the `skip_serializing_if` collapses it).
    #[serde(flatten, default, skip_serializing_if = "serde_json::Map::is_empty")]
    pub extra: serde_json::Map<String, serde_json::Value>,
}

impl SetError {
    /// Construct a [`SetError`] with the given type and all optional fields `None`.
    pub fn new(error_type: SetErrorType) -> Self {
        Self {
            error_type,
            description: None,
            properties: None,
            existing_id: None,
            max_recipients: None,
            invalid_recipients: None,
            not_found: None,
            max_size: None,
            extra: serde_json::Map::new(),
        }
    }

    /// Set the human-readable description.
    pub fn with_description(mut self, desc: impl Into<String>) -> Self {
        self.description = Some(desc.into());
        self
    }

    /// Set the list of property names that caused the error.
    pub fn with_properties<I, S>(mut self, props: I) -> Self
    where
        I: IntoIterator<Item = S>,
        S: Into<String>,
    {
        self.properties = Some(props.into_iter().map(|s| s.into()).collect());
        self
    }

    /// Set the existing object id (used with `alreadyExists`).
    pub fn with_existing_id(mut self, id: jmap_types::Id) -> Self {
        self.existing_id = Some(id);
        self
    }

    /// Set the maximum recipients (used with `tooManyRecipients` — RFC 8621 §7.5).
    pub fn with_max_recipients(mut self, n: u64) -> Self {
        self.max_recipients = Some(n);
        self
    }

    /// Set the invalid recipient addresses (used with `invalidRecipients` — RFC 8621 §7.5).
    pub fn with_invalid_recipients<I, S>(mut self, addrs: I) -> Self
    where
        I: IntoIterator<Item = S>,
        S: Into<String>,
    {
        self.invalid_recipients = Some(addrs.into_iter().map(|s| s.into()).collect());
        self
    }

    /// Set the missing blob IDs (used with `blobNotFound` — RFC 8621 §5.5).
    pub fn with_not_found(mut self, ids: Vec<jmap_types::Id>) -> Self {
        self.not_found = Some(ids);
        self
    }

    /// Set the maximum message size in octets (used with `tooLarge` on EmailSubmission — RFC 8621 §7.5).
    pub fn with_max_size(mut self, n: u64) -> Self {
        self.max_size = Some(n);
        self
    }

    /// Insert an extension-defined field into [`Self::extra`].
    ///
    /// Used by handlers to attach typed wire fields that no `with_*`
    /// builder covers — for example JMAP Chat's `rateLimited` SetError
    /// must carry a `serverRetryAfter` UTCDate:
    ///
    /// ```ignore
    /// SetError::new(SetErrorType::custom("rateLimited"))
    ///     .with_description("Slow mode is active for this chat")
    ///     .with_extra("serverRetryAfter", serde_json::json!(retry_after_str))
    /// ```
    ///
    /// The serialized wire shape merges `key`/`value` at the same
    /// level as the typed fields (via `#[serde(flatten)]` on
    /// [`Self::extra`]). Calling `with_extra("type", ...)`,
    /// `with_extra("properties", ...)`, or any other reserved
    /// wire-name will produce a malformed SetError on the wire —
    /// callers are responsible for choosing extension-namespace keys
    /// that do not collide with the typed-field wire names.
    pub fn with_extra(mut self, key: &str, value: serde_json::Value) -> Self {
        self.extra.insert(key.to_owned(), value);
        self
    }
}

impl std::fmt::Display for SetError {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        write!(f, "{}", self.error_type)?;
        if let Some(ref desc) = self.description {
            write!(f, ": {desc}")?;
        }
        Ok(())
    }
}

/// The machine-readable type for a [`SetError`] (RFC 8620 §5.3 and RFC 8621).
///
/// Extension crates define their own error strings via [`SetErrorType::custom`]
/// rather than adding variants here. This keeps the base crate stable as new
/// JMAP extension crates (calendar, contacts, etc.) are added.
#[non_exhaustive]
#[derive(Debug, Clone, PartialEq)]
pub enum SetErrorType {
    /// The action would violate an ACL or other access control policy.
    Forbidden,
    /// Creating or modifying the object would exceed a server quota.
    OverQuota,
    /// The object is too large to be stored by the server.
    TooLarge,
    /// The server is rate-limiting this client.
    RateLimit,
    /// The object to be updated or destroyed does not exist.
    NotFound,
    /// The patch object is not a valid JSON Merge Patch or cannot be applied.
    InvalidPatch,
    /// The client requested destruction of an object that will be destroyed
    /// implicitly when another object is destroyed.
    WillDestroy,
    /// One or more properties have invalid values.
    InvalidProperties,
    /// The object type is a singleton and cannot be created or destroyed.
    Singleton,
    /// An object with the same unique key already exists.
    AlreadyExists,
    /// RFC 8621 §2.5 — Mailbox has child mailboxes and cannot be destroyed.
    MailboxHasChild,
    /// RFC 8621 §2.5 — Mailbox contains emails and `onDestroyRemoveEmails` is false.
    MailboxHasEmail,
    /// RFC 8621 §5.5 — Too many keywords on the Email.
    TooManyKeywords,
    /// RFC 8621 §5.5 — Email is in too many mailboxes.
    TooManyMailboxes,
    /// RFC 8621 §5.5 — A referenced blob was not found.
    BlobNotFound,
    /// RFC 8621 §6.3 — The `from` address is not permitted for this Identity.
    ForbiddenFrom,
    /// RFC 8621 §7.5 — The Email is invalid for submission.
    InvalidEmail,
    /// RFC 8621 §7.5 — Too many recipients.
    TooManyRecipients,
    /// RFC 8621 §7.5 — No recipients specified.
    NoRecipients,
    /// RFC 8621 §7.5 — One or more recipient addresses are invalid.
    InvalidRecipients,
    /// RFC 8621 §7.5 — The MAIL FROM address is not permitted.
    ForbiddenMailFrom,
    /// RFC 8621 §7.5 — The user does not have send permission.
    ForbiddenToSend,
    /// RFC 8621 §7.5 — The submission cannot be undone.
    CannotUnsend,
    /// An extension-defined error type not covered by the variants above.
    /// Serializes as the inner string directly (e.g. `"mdnAlreadySent"`).
    Custom(String),
}

impl SetErrorType {
    /// Construct a [`SetErrorType::Custom`] from any string.
    ///
    /// Use this in extension crates to emit domain-specific error types
    /// without adding variants to this enum.
    pub fn custom(s: impl Into<String>) -> Self {
        Self::Custom(s.into())
    }
}

impl std::fmt::Display for SetErrorType {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        let s: &str = match self {
            Self::Forbidden => "forbidden",
            Self::OverQuota => "overQuota",
            Self::TooLarge => "tooLarge",
            Self::RateLimit => "rateLimit",
            Self::NotFound => "notFound",
            Self::InvalidPatch => "invalidPatch",
            Self::WillDestroy => "willDestroy",
            Self::InvalidProperties => "invalidProperties",
            Self::Singleton => "singleton",
            Self::AlreadyExists => "alreadyExists",
            Self::MailboxHasChild => "mailboxHasChild",
            Self::MailboxHasEmail => "mailboxHasEmail",
            Self::TooManyKeywords => "tooManyKeywords",
            Self::TooManyMailboxes => "tooManyMailboxes",
            Self::BlobNotFound => "blobNotFound",
            Self::ForbiddenFrom => "forbiddenFrom",
            Self::InvalidEmail => "invalidEmail",
            Self::TooManyRecipients => "tooManyRecipients",
            Self::NoRecipients => "noRecipients",
            Self::InvalidRecipients => "invalidRecipients",
            Self::ForbiddenMailFrom => "forbiddenMailFrom",
            Self::ForbiddenToSend => "forbiddenToSend",
            Self::CannotUnsend => "cannotUnsend",
            Self::Custom(s) => s.as_str(),
        };
        f.write_str(s)
    }
}

impl serde::Serialize for SetErrorType {
    fn serialize<S: serde::Serializer>(&self, s: S) -> Result<S::Ok, S::Error> {
        s.serialize_str(&self.to_string())
    }
}

impl<'de> serde::Deserialize<'de> for SetErrorType {
    fn deserialize<D: serde::Deserializer<'de>>(d: D) -> Result<Self, D::Error> {
        struct Visitor;
        impl serde::de::Visitor<'_> for Visitor {
            type Value = SetErrorType;
            fn expecting(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
                f.write_str("a JMAP SetError type string")
            }
            fn visit_str<E: serde::de::Error>(self, v: &str) -> Result<Self::Value, E> {
                Ok(match v {
                    "forbidden" => SetErrorType::Forbidden,
                    "overQuota" => SetErrorType::OverQuota,
                    "tooLarge" => SetErrorType::TooLarge,
                    "rateLimit" => SetErrorType::RateLimit,
                    "notFound" => SetErrorType::NotFound,
                    "invalidPatch" => SetErrorType::InvalidPatch,
                    "willDestroy" => SetErrorType::WillDestroy,
                    "invalidProperties" => SetErrorType::InvalidProperties,
                    "singleton" => SetErrorType::Singleton,
                    "alreadyExists" => SetErrorType::AlreadyExists,
                    "mailboxHasChild" => SetErrorType::MailboxHasChild,
                    "mailboxHasEmail" => SetErrorType::MailboxHasEmail,
                    "tooManyKeywords" => SetErrorType::TooManyKeywords,
                    "tooManyMailboxes" => SetErrorType::TooManyMailboxes,
                    "blobNotFound" => SetErrorType::BlobNotFound,
                    "forbiddenFrom" => SetErrorType::ForbiddenFrom,
                    "invalidEmail" => SetErrorType::InvalidEmail,
                    "tooManyRecipients" => SetErrorType::TooManyRecipients,
                    "noRecipients" => SetErrorType::NoRecipients,
                    "invalidRecipients" => SetErrorType::InvalidRecipients,
                    "forbiddenMailFrom" => SetErrorType::ForbiddenMailFrom,
                    "forbiddenToSend" => SetErrorType::ForbiddenToSend,
                    "cannotUnsend" => SetErrorType::CannotUnsend,
                    other => SetErrorType::Custom(other.to_owned()),
                })
            }
        }
        d.deserialize_str(Visitor)
    }
}

/// Error type returned by create/update/destroy backend methods.
#[non_exhaustive]
#[derive(Debug)]
pub enum BackendSetError<E> {
    /// A well-typed JMAP [`SetError`] to place verbatim in the
    /// `notCreated`/`notUpdated`/`notDestroyed` map.
    SetError(SetError),
    /// An unexpected storage-layer error.
    Other(E),
}

impl<E: std::fmt::Display> std::fmt::Display for BackendSetError<E> {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        match self {
            Self::SetError(se) => write!(f, "set error: {se}"),
            Self::Other(e) => write!(f, "{e}"),
        }
    }
}

impl<E: std::error::Error + 'static> std::error::Error for BackendSetError<E> {
    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
        match self {
            Self::Other(e) => Some(e),
            _ => None,
        }
    }
}

impl<E> From<SetError> for BackendSetError<E> {
    fn from(e: SetError) -> Self {
        Self::SetError(e)
    }
}

// ---------------------------------------------------------------------------
// Backend error envelopes
// ---------------------------------------------------------------------------

/// Error type returned by [`JmapBackend::get_changes`] and
/// [`JmapBackend::query_changes`].
#[non_exhaustive]
#[derive(Debug)]
pub enum BackendChangesError<E> {
    /// The server cannot supply incremental changes for the given `sinceState`.
    ///
    /// Two sub-cases share this variant:
    ///
    /// - **`limit > 0`** — maps to `tooManyChanges` in the JMAP response, with
    ///   `limit` as the suggested maximum. The client may retry with a smaller
    ///   window.
    ///
    /// - **`limit: 0`** — maps to `cannotCalculateChanges`. This signals a
    ///   **full state reset**: the client MUST discard ALL locally cached
    ///   objects for the affected type, reset its local state token to the
    ///   empty string, and perform a full resync (`/get` with `ids: null`).
    ///   Partial recovery is not permitted — the server has no usable
    ///   change log for this state window. (Source: RFC 8620 §5.6; authoritative
    ///   behavior documented in jmapio/jmap-js `mail-model.js`.)
    TooManyChanges {
        /// Maximum window size the server can supply in a single
        /// `/changes` response. A value of `0` signals a full state
        /// reset is required per RFC 8620 §5.6; any non-zero value is
        /// the suggested maximum the client may retry with.
        limit: u64,
    },
    /// An unexpected storage-layer error.
    Other(E),
}

impl<E: std::fmt::Display> std::fmt::Display for BackendChangesError<E> {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        match self {
            Self::TooManyChanges { limit: 0 } => write!(f, "cannot calculate changes"),
            Self::TooManyChanges { limit } => write!(f, "too many changes (limit: {limit})"),
            Self::Other(e) => write!(f, "{e}"),
        }
    }
}

impl<E: std::error::Error + 'static> std::error::Error for BackendChangesError<E> {
    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
        match self {
            Self::Other(e) => Some(e),
            _ => None,
        }
    }
}

impl<E> From<E> for BackendChangesError<E> {
    fn from(e: E) -> Self {
        Self::Other(e)
    }
}

impl<E: std::error::Error> From<BackendChangesError<E>> for jmap_types::JmapError {
    fn from(e: BackendChangesError<E>) -> Self {
        match e {
            BackendChangesError::TooManyChanges { limit: 0 } => {
                jmap_types::JmapError::cannot_calculate_changes()
            }
            BackendChangesError::TooManyChanges { limit } => {
                jmap_types::JmapError::too_many_changes_with_limit(limit)
            }
            BackendChangesError::Other(inner) => {
                jmap_types::JmapError::server_fail(inner.to_string())
            }
        }
    }
}

// ---------------------------------------------------------------------------
// Result types
// ---------------------------------------------------------------------------

/// Result of a `/changes` call (RFC 8620 §5.2).
#[derive(Debug)]
#[non_exhaustive]
pub struct ChangesResult {
    /// Ids of objects that were created since `sinceState`.
    pub created: Vec<jmap_types::Id>,
    /// Ids of objects that were updated since `sinceState`.
    pub updated: Vec<jmap_types::Id>,
    /// Ids of objects that were destroyed since `sinceState`.
    pub destroyed: Vec<jmap_types::Id>,
    /// `true` if there are more changes beyond this batch.
    pub has_more_changes: bool,
    /// The current state token after applying all reported changes.
    pub new_state: jmap_types::State,
}

impl ChangesResult {
    /// Construct a [`ChangesResult`].
    pub fn new(
        created: Vec<jmap_types::Id>,
        updated: Vec<jmap_types::Id>,
        destroyed: Vec<jmap_types::Id>,
        has_more_changes: bool,
        new_state: jmap_types::State,
    ) -> Self {
        Self {
            created,
            updated,
            destroyed,
            has_more_changes,
            new_state,
        }
    }
}

/// Result of a `/query` call (RFC 8620 §5.5).
#[derive(Debug)]
#[non_exhaustive]
pub struct QueryResult {
    /// The ordered list of matching object ids.
    pub ids: Vec<jmap_types::Id>,
    /// The 0-based index of the first returned id in the complete result list.
    pub position: i64,
    /// Total number of results, if the backend can calculate it.
    pub total: Option<u64>,
    /// Opaque query state token for subsequent `/queryChanges` calls.
    pub query_state: jmap_types::State,
    /// Whether the backend supports `/queryChanges` for this query.
    pub can_calculate_changes: bool,
}

impl QueryResult {
    /// Construct a [`QueryResult`].
    pub fn new(
        ids: Vec<jmap_types::Id>,
        position: i64,
        total: Option<u64>,
        query_state: jmap_types::State,
        can_calculate_changes: bool,
    ) -> Self {
        Self {
            ids,
            position,
            total,
            query_state,
            can_calculate_changes,
        }
    }
}

/// One entry in the `added` list of a `/queryChanges` response (RFC 8620 §5.6).
#[derive(Debug)]
#[non_exhaustive]
pub struct AddedItem {
    /// The id of the newly-added object.
    pub id: jmap_types::Id,
    /// Its 0-based position in the result list after applying all changes.
    pub index: u64,
}

impl AddedItem {
    /// Construct an [`AddedItem`].
    pub fn new(id: jmap_types::Id, index: u64) -> Self {
        Self { id, index }
    }
}

/// Result of a `/queryChanges` call (RFC 8620 §5.6).
#[derive(Debug)]
#[non_exhaustive]
pub struct QueryChangesResult {
    /// The query state token supplied by the client in `sinceQueryState`.
    pub old_query_state: jmap_types::State,
    /// The current query state token.
    pub new_query_state: jmap_types::State,
    /// Total number of results in the new query, if the backend can calculate it.
    pub total: Option<u64>,
    /// Ids removed from the result set since `oldQueryState`.
    pub removed: Vec<jmap_types::Id>,
    /// Ids added to the result set since `oldQueryState`, with their positions.
    pub added: Vec<AddedItem>,
}

impl QueryChangesResult {
    /// Construct a [`QueryChangesResult`].
    pub fn new(
        old_query_state: jmap_types::State,
        new_query_state: jmap_types::State,
        total: Option<u64>,
        removed: Vec<jmap_types::Id>,
        added: Vec<AddedItem>,
    ) -> Self {
        Self {
            old_query_state,
            new_query_state,
            total,
            removed,
            added,
        }
    }
}

// ---------------------------------------------------------------------------
// JmapBackend — the read-side supertrait
// ---------------------------------------------------------------------------

/// Read-side backend supertrait shared by all JMAP server crates.
///
/// Domain-specific backend traits (`MailBackend`, `ChatBackend`, etc.) require
/// this trait as a supertrait and add write-side methods on top.
///
/// Only the read operations that have an identical signature across all JMAP
/// object types belong here. Write operations (`create_object`, `update_object`,
/// `destroy_object`) and domain-specific operations remain in the domain crate.
///
/// The `collapse_threads` parameter on `query_changes` is included for
/// `Email/queryChanges` (RFC 8621 §4.5). Non-mail backends should pass `false`
/// and may ignore the parameter.
///
/// This trait is not object-safe by design (generic methods). Use
/// `Arc<impl JmapBackend>` when sharing across tasks.
///
/// # CallerCtx
///
/// Every backend method takes a `caller: &Self::CallerCtx` parameter as the
/// first argument after `&self`. This is the per-request authentication /
/// authorisation context produced by the caller's auth layer and forwarded
/// unchanged through [`crate::Dispatcher::dispatch`] → [`crate::JmapHandler`]
/// → the registered closure → the backend.
///
/// Implementations that do not need an auth identity can use the unit type:
///
/// ```rust,ignore
/// impl JmapBackend for MyBackend {
///     type Error = MyError;
///     type CallerCtx = ();
///     // ...
/// }
/// ```
///
/// Implementations that do need to differentiate behaviour per caller (e.g.
/// applying per-user visibility rules, or rejecting reads with
/// `forbidden` when the caller is not the owner of the account) read the
/// `caller` parameter to decide.
///
/// The trait bound `Clone + Send + 'static` is what [`crate::Dispatcher`]
/// requires; the bound is repeated here so the supertrait can stand on its
/// own without depending on the dispatcher.
pub trait JmapBackend: Send + Sync + 'static {
    /// The error type returned by storage operations.
    ///
    /// # Security
    ///
    /// The `Display` impl of this type is surfaced through
    /// [`BackendSetError::Other`]'s and [`BackendChangesError::Other`]'s
    /// own `Display` impls, which in turn flow into
    /// [`crate::request_error`]'s `RequestError::Display` output. When a
    /// downstream consumer wires tracing-style logging on top, the
    /// formatted error text lands in operator logs verbatim.
    ///
    /// Implementations MUST NOT include any of the following in this
    /// type's `Display` output:
    ///
    /// - **Credential material** — auth tokens, passwords, push
    ///   verification codes, invite codes, session cookies, or anything
    ///   derived byte-for-byte from an `Authorization`-header value.
    /// - **Blob content** — email bodies, sieve scripts, file contents,
    ///   or any user-supplied opaque payload. An error like
    ///   `"sieve parse error at line 42: <script excerpt>"` violates
    ///   this — emit the line number and a short type-only summary
    ///   ("sieve parse error at line 42: unexpected token") and let the
    ///   server log the full script body separately under a redacted
    ///   path.
    /// - **PII shaped like an email address** in any code path that an
    ///   unauthenticated caller can trigger. Wrapping a downstream
    ///   service error that interpolates the caller's email is the
    ///   common foot-gun.
    ///
    /// Errors that wrap a downstream-service failure should sanitize
    /// the downstream error text — or strip it entirely and replace it
    /// with a static summary — before constructing the `Display`
    /// string. The same rule applies to every extension `*Backend`
    /// trait that inherits this associated type by transitivity:
    /// `MailBackend::Error`, `ChatBackend::Error`,
    /// `CalendarsBackend::Error`, `TasksBackend::Error`,
    /// `ContactsBackend::Error`, `FileNodeBackend::Error`, and
    /// `SharingBackend::Error` are all the same `JmapBackend::Error`
    /// associated type — the contract here governs all of them.
    ///
    /// Precedent: bd:JMAP-sc1b.79 redacted `BearerAuth` and `BasicAuth`
    /// at the type-derive level; bd:JMAP-sc1b.100 documents the
    /// equivalent contract at the trait-associated-type level.
    type Error: std::error::Error + Send + Sync + 'static;

    /// The per-request caller context type produced by the auth layer and
    /// forwarded by [`crate::Dispatcher::dispatch`] into every method call.
    ///
    /// Use `()` when no auth context is needed.
    ///
    /// The bound is `Clone + Send + Sync + 'static`:
    /// - `Clone` because [`crate::Dispatcher`] clones the value once per
    ///   method call in the batch.
    /// - `Send + 'static` because each method call is spawned on a
    ///   [`tokio::task`].
    /// - `Sync` because handler method bodies take `&Self::CallerCtx`
    ///   and hold that reference across `.await` boundaries inside a
    ///   `Send` future (a `&T` is `Send` iff `T: Sync`).
    type CallerCtx: Clone + Send + Sync + 'static;

    /// Return `true` if the given account exists in this backend.
    ///
    /// Handlers call this at the start of each method to return
    /// `accountNotFound` (RFC 8620 §3.6.2) rather than surfacing
    /// the wrong error when `accountId` is unknown.
    fn account_exists(
        &self,
        caller: &Self::CallerCtx,
        account_id: &jmap_types::Id,
    ) -> impl std::future::Future<Output = Result<bool, Self::Error>> + Send;

    /// Fetch objects by id (or all objects when `ids` is `None`).
    ///
    /// `properties` is the list of property names requested by the client
    /// (RFC 8620 §5.1). `None` means the client did not send a `properties`
    /// field; the backend should return all properties. When `Some`, the backend
    /// MAY filter the response to only the named properties, but is not required
    /// to — implementations that always return all properties are correct.
    ///
    /// Returns `(found, not_found)` — objects that exist and ids that do not.
    fn get_objects<O: GetObject + Send + Sync>(
        &self,
        caller: &Self::CallerCtx,
        account_id: &jmap_types::Id,
        ids: Option<&[jmap_types::Id]>,
        properties: Option<&[String]>,
    ) -> impl std::future::Future<Output = Result<(Vec<O>, Vec<jmap_types::Id>), Self::Error>> + Send;

    /// Return the current state token for an object type in the given account.
    fn get_state<O: JmapObject + Send + Sync>(
        &self,
        caller: &Self::CallerCtx,
        account_id: &jmap_types::Id,
    ) -> impl std::future::Future<Output = Result<jmap_types::State, Self::Error>> + Send;

    /// Return changes since `since_state`, up to `max_changes` entries.
    fn get_changes<O: JmapObject + Send + Sync>(
        &self,
        caller: &Self::CallerCtx,
        account_id: &jmap_types::Id,
        since_state: &jmap_types::State,
        max_changes: Option<u64>,
    ) -> impl std::future::Future<Output = Result<ChangesResult, BackendChangesError<Self::Error>>> + Send;

    /// Execute a `/query` and return a page of matching ids.
    ///
    /// `position` may be negative — negative values are relative to the end of
    /// the result set per RFC 8620 §5.5 (e.g. -1 means the last result).
    ///
    /// # Filter and sort handling
    ///
    /// Implementations MUST honour the supplied `filter` and `sort` arguments
    /// efficiently — typically by pushing both into the indexed storage layer
    /// (database WHERE / ORDER BY, search index, etc.). Returning every
    /// matching id and relying on the caller to paginate after the fact
    /// degenerates to O(n) per page for IMAP-migration accounts.
    ///
    /// Handler implementations in `jmap-*-server` crates SHOULD NOT
    /// post-filter or post-sort the backend's result; doing so re-introduces
    /// the O(n) cost this method exists to avoid. The Mailbox handler in
    /// `jmap-mail-server` is the canonical example of pushing filter/sort
    /// fully into the backend.
    #[allow(clippy::too_many_arguments)]
    fn query_objects<O: QueryObject + Send + Sync>(
        &self,
        caller: &Self::CallerCtx,
        account_id: &jmap_types::Id,
        filter: Option<&O::Filter>,
        sort: Option<&[O::Comparator]>,
        limit: Option<u64>,
        position: i64,
    ) -> impl std::future::Future<Output = Result<QueryResult, Self::Error>> + Send;

    /// Execute a `/queryChanges` and return deltas since `since_query_state`.
    ///
    /// `collapse_threads` is only meaningful for `Email/queryChanges`
    /// (RFC 8621 §4.5). Pass `false` for all other object types.
    #[allow(clippy::too_many_arguments)]
    fn query_changes<O: QueryObject + Send + Sync>(
        &self,
        caller: &Self::CallerCtx,
        account_id: &jmap_types::Id,
        since_query_state: &jmap_types::State,
        filter: Option<&O::Filter>,
        sort: Option<&[O::Comparator]>,
        max_changes: Option<u64>,
        up_to_id: Option<&jmap_types::Id>,
        collapse_threads: bool,
    ) -> impl std::future::Future<
        Output = Result<QueryChangesResult, BackendChangesError<Self::Error>>,
    > + Send;

    /// The caller's stable identity within this account namespace.
    ///
    /// Returns `None` for deployments that have not wired identity
    /// (test fixtures, single-user dev servers). A `None`-returning
    /// backend CANNOT honor JMAP semantics that depend on caller
    /// identity — chat role-hierarchy, calendar ACLs, sharing/myRights,
    /// per-user $seen on shared mailboxes, metadata isPrivate
    /// visibility scoping, etc. Authentication is still the HTTP
    /// layer's job; this method exposes the result of that
    /// authentication to the JMAP layer for in-method semantics.
    ///
    /// Implementations MUST NOT mint identity — they MUST read it
    /// from the `CallerCtx` populated by the HTTP/auth middleware
    /// before `dispatch()` was called.
    ///
    /// Backends that honor identity-dependent semantics MUST override
    /// this method. Handlers and downstream backend traits MAY rely on
    /// it being correct when it returns `Some`.
    fn principal_id(caller: &Self::CallerCtx) -> Option<&jmap_types::Id> {
        let _ = caller;
        None
    }
}

// ---------------------------------------------------------------------------
// Tests
// ---------------------------------------------------------------------------

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

    /// Oracle: BackendChangesError::TooManyChanges { limit: 0 } must map to
    /// cannotCalculateChanges (RFC 8620 §5.6), not tooManyChanges with limit 0.
    ///
    /// limit=0 is the convention for "cannot calculate".
    #[test]
    fn backend_changes_error_limit_zero_maps_to_cannot_calculate() {
        let err = jmap_types::JmapError::from(
            BackendChangesError::<std::convert::Infallible>::TooManyChanges { limit: 0 },
        );
        assert_eq!(
            err.error_type.as_str(),
            "cannotCalculateChanges",
            "limit=0 must produce cannotCalculateChanges; got: {:?}",
            err.error_type
        );
    }

    /// Oracle: BackendChangesError::TooManyChanges { limit: N } (N > 0) maps to
    /// tooManyChanges with the suggested limit.
    #[test]
    fn backend_changes_error_nonzero_limit_maps_to_too_many_changes() {
        let err = jmap_types::JmapError::from(
            BackendChangesError::<std::convert::Infallible>::TooManyChanges { limit: 50 },
        );
        assert_eq!(
            err.error_type.as_str(),
            "tooManyChanges",
            "limit=50 must produce tooManyChanges; got: {:?}",
            err.error_type
        );
    }

    /// Oracle: SetErrorType::Custom("mdnAlreadySent") must serialize as the bare
    /// string "mdnAlreadySent" and deserialize back to Custom("mdnAlreadySent").
    /// Extension crates depend on this round-trip to emit domain-specific errors.
    #[test]
    fn set_error_type_custom_round_trips_as_bare_string() {
        let original = SetErrorType::custom("mdnAlreadySent");
        let serialized = serde_json::to_string(&original).expect("serialize");
        assert_eq!(
            serialized, r#""mdnAlreadySent""#,
            "Custom must serialize as bare string"
        );
        let deserialized: SetErrorType = serde_json::from_str(&serialized).expect("deserialize");
        assert_eq!(
            deserialized, original,
            "Custom must deserialize back to Custom"
        );
    }

    /// Oracle (bd:JMAP-dha0): SetError gains an `extra` map that captures
    /// extension-defined fields not covered by the typed `with_*` builders.
    /// A handler that emits `rateLimited` with `serverRetryAfter` must
    /// see the value round-trip through serialize / deserialize.
    #[test]
    fn set_error_extra_field_round_trips() {
        let original = SetError::new(SetErrorType::custom("rateLimited"))
            .with_description("Slow mode is active")
            .with_extra(
                "serverRetryAfter",
                serde_json::Value::String("2025-12-31T23:59:59Z".to_owned()),
            );

        let wire = serde_json::to_value(&original).expect("serialize");
        assert_eq!(wire["type"], "rateLimited");
        assert_eq!(wire["description"], "Slow mode is active");
        assert_eq!(
            wire["serverRetryAfter"], "2025-12-31T23:59:59Z",
            "extra field must flatten into the SetError wire shape"
        );

        let round: SetError = serde_json::from_value(wire).expect("deserialize");
        assert_eq!(round.error_type, original.error_type);
        assert_eq!(round.description, original.description);
        assert_eq!(
            round.extra.get("serverRetryAfter").and_then(|v| v.as_str()),
            Some("2025-12-31T23:59:59Z"),
            "extra field must survive deserialize"
        );
    }

    /// Oracle (bd:JMAP-dha0): a SetError with no extras serializes to a
    /// wire shape byte-identical to the pre-extras layout. The
    /// `skip_serializing_if` on `extra` collapses the empty map.
    #[test]
    fn set_error_empty_extra_is_invisible_on_the_wire() {
        let err = SetError::new(SetErrorType::Forbidden);
        let wire = serde_json::to_value(&err).expect("serialize");
        let obj = wire.as_object().expect("object");
        assert!(
            !obj.contains_key("extra"),
            "empty `extra` map must not appear on the wire (got {wire})"
        );
        // The only key on the wire for a bare SetError must be `type`.
        assert_eq!(
            obj.len(),
            1,
            "bare SetError must have exactly one key on the wire"
        );
        assert_eq!(obj["type"], "forbidden");
    }

    /// Oracle (bd:JMAP-dha0): unknown wire fields on a deserialized
    /// SetError land in `extra`. This means a future spec adding
    /// `someNewSetErrorField` will round-trip through current
    /// versions of the kit losslessly.
    #[test]
    fn set_error_unknown_field_lands_in_extra() {
        let wire = serde_json::json!({
            "type": "forbidden",
            "futureSpecField": "future-value",
            "anotherOne": 42
        });
        let err: SetError = serde_json::from_value(wire).expect("deserialize");
        assert_eq!(err.error_type, SetErrorType::Forbidden);
        assert_eq!(
            err.extra.get("futureSpecField").and_then(|v| v.as_str()),
            Some("future-value")
        );
        assert_eq!(
            err.extra.get("anotherOne").and_then(|v| v.as_u64()),
            Some(42)
        );
    }

    /// Oracle: known SetErrorType variants (e.g. Singleton) must still
    /// serialize as their camelCase wire strings and deserialize back correctly.
    #[test]
    fn set_error_type_known_variant_round_trips() {
        let original = SetErrorType::Singleton;
        let serialized = serde_json::to_string(&original).expect("serialize");
        assert_eq!(
            serialized, r#""singleton""#,
            "Singleton must serialize as \"singleton\""
        );
        let deserialized: SetErrorType = serde_json::from_str(&serialized).expect("deserialize");
        assert_eq!(
            deserialized, original,
            "Singleton must deserialize back to Singleton"
        );
    }

    /// Oracle (bd:JMAP-ga0q.1): `JmapBackend::principal_id` has a default impl
    /// that returns `None`. A backend whose `CallerCtx = ()` and that does NOT
    /// override `principal_id` inherits that default and signals "identity not
    /// wired" to callers. JMAP semantics that depend on caller identity must
    /// treat `None` as a hard "cannot honor".
    #[test]
    fn principal_id_default_impl_returns_none_for_unit_caller_ctx() {
        // Minimal stub backend exercising only the default impl. All other
        // trait methods are stubbed with `unreachable!()` and never invoked.
        struct StubBackend;

        #[derive(Debug)]
        struct StubError;

        impl std::fmt::Display for StubError {
            fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
                f.write_str("stub")
            }
        }
        impl std::error::Error for StubError {}

        impl JmapBackend for StubBackend {
            type Error = StubError;
            type CallerCtx = ();

            async fn account_exists(
                &self,
                _caller: &(),
                _account_id: &jmap_types::Id,
            ) -> Result<bool, Self::Error> {
                unreachable!("only principal_id is exercised in this test")
            }

            async fn get_objects<O: GetObject + Send + Sync>(
                &self,
                _caller: &(),
                _account_id: &jmap_types::Id,
                _ids: Option<&[jmap_types::Id]>,
                _properties: Option<&[String]>,
            ) -> Result<(Vec<O>, Vec<jmap_types::Id>), Self::Error> {
                unreachable!("only principal_id is exercised in this test")
            }

            async fn get_state<O: JmapObject + Send + Sync>(
                &self,
                _caller: &(),
                _account_id: &jmap_types::Id,
            ) -> Result<jmap_types::State, Self::Error> {
                unreachable!("only principal_id is exercised in this test")
            }

            async fn get_changes<O: JmapObject + Send + Sync>(
                &self,
                _caller: &(),
                _account_id: &jmap_types::Id,
                _since_state: &jmap_types::State,
                _max_changes: Option<u64>,
            ) -> Result<ChangesResult, BackendChangesError<Self::Error>> {
                unreachable!("only principal_id is exercised in this test")
            }

            async fn query_objects<O: QueryObject + Send + Sync>(
                &self,
                _caller: &(),
                _account_id: &jmap_types::Id,
                _filter: Option<&O::Filter>,
                _sort: Option<&[O::Comparator]>,
                _limit: Option<u64>,
                _position: i64,
            ) -> Result<QueryResult, Self::Error> {
                unreachable!("only principal_id is exercised in this test")
            }

            async fn query_changes<O: QueryObject + Send + Sync>(
                &self,
                _caller: &(),
                _account_id: &jmap_types::Id,
                _since_query_state: &jmap_types::State,
                _filter: Option<&O::Filter>,
                _sort: Option<&[O::Comparator]>,
                _max_changes: Option<u64>,
                _up_to_id: Option<&jmap_types::Id>,
                _collapse_threads: bool,
            ) -> Result<QueryChangesResult, BackendChangesError<Self::Error>> {
                unreachable!("only principal_id is exercised in this test")
            }
        }

        let caller: <StubBackend as JmapBackend>::CallerCtx = ();
        let id = <StubBackend as JmapBackend>::principal_id(&caller);
        assert!(
            id.is_none(),
            "default principal_id impl must return None; got Some({:?})",
            id
        );
    }
}