pg_proto/

pipeline.rs

//! Bounded, payload-free orchestration for proxy request pipelines.
//!
//! The ledger in this module records protocol obligations, not wire messages.
//! Applications retain ownership of decoded messages until [`FrontendAction`] or
//! [`BackendAction`] tells them to forward, emit, retry, or discard the value.
//! [`Pipeline::accept_frontend`] and [`Pipeline::accept_backend`] are the canonical
//! ledger interface. Their typed counterparts additionally dispatch accepted
//! messages through phase-specific middleware before committing them.
//! Callers receiving [`crate::demux::SessionItem`] values should first consume
//! any pooling or attribution evidence they need, then convert the item with
//! [`crate::demux::SessionItem::into_backend_message`] before backend acceptance.

use std::{collections::VecDeque, convert::Infallible, sync::Arc};

use tokio::sync::Notify;

use crate::{
    codec::{BackendMessage, FrontendMessage},
    demux::Demux,
    grammar::backend,
    middleware::{
        AsynchronousBackendMessage, ChainError, MessageMiddleware, Middleware,
        ReconstructableMessage as _, Then,
    },
};

/// Stable identity of an accepted frontend operation.
#[derive(Clone, Copy, Debug, Eq, Hash, Ord, PartialEq, PartialOrd)]
pub(crate) struct OperationId(u64);

/// Pipeline policy which preserves the historical lock-step behaviour.
#[derive(Clone, Copy, Debug, Default, Eq, PartialEq)]
pub struct NoPipeline;

/// Configuration for a bounded frontend operation pipeline.
#[derive(Clone, Copy, Debug, Eq, PartialEq)]
pub struct BoundedPipeline {
    max_operations: usize,
}

impl BoundedPipeline {
    /// Creates a pipeline with a non-zero operation-count limit.
    ///
    /// # Errors
    ///
    /// Returns an error when `max_operations` is zero.
    pub fn new(max_operations: usize) -> Result<Self, PipelineConfigError> {
        if max_operations == 0 {
            return Err(PipelineConfigError);
        }
        Ok(Self { max_operations })
    }

    /// Returns the maximum number of incomplete operations.
    #[must_use]
    pub const fn max_operations(self) -> usize {
        self.max_operations
    }
}

/// A zero operation-count limit is not a usable pipeline configuration.
#[derive(Clone, Copy, Debug, Eq, PartialEq)]
pub struct PipelineConfigError;

impl std::fmt::Display for PipelineConfigError {
    fn fmt(&self, formatter: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        formatter.write_str("pipeline operation limit must be non-zero")
    }
}

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

mod private {
    pub trait Sealed {}
}

/// Sealed configuration accepted by the intermediary builder.
pub trait PipelinePolicy: private::Sealed + Copy {
    /// Maximum number of incomplete operation records.
    fn operation_limit(self) -> usize;
}

impl private::Sealed for NoPipeline {}
impl PipelinePolicy for NoPipeline {
    fn operation_limit(self) -> usize {
        1
    }
}

impl private::Sealed for BoundedPipeline {}
impl PipelinePolicy for BoundedPipeline {
    fn operation_limit(self) -> usize {
        self.max_operations
    }
}

/// Whether an accepted frontend operation is locally handled or forwarded.
#[derive(Clone, Copy, Debug, Eq, PartialEq)]
pub(crate) enum FrontendHandling {
    /// Send the returned message to the upstream connection.
    Forward,
    /// Do not send the request upstream; application code will synthesize its response.
    Local,
}

/// Application action for one successfully projected frontend message.
#[derive(Debug, Eq, PartialEq)]
pub(crate) enum FrontendAction {
    /// Forward the owned message upstream.
    Forward {
        /// Accepted operation identity.
        id: OperationId,
        /// Original, unretained frontend message.
        message: FrontendMessage,
    },
    /// The operation is locally handled and the message can be discarded.
    Discard {
        /// Accepted operation identity.
        id: OperationId,
    },
}

/// Position of a successfully accepted operation.
#[derive(Debug, Eq, PartialEq)]
pub(crate) enum FrontendAdmission {
    /// Nothing earlier prevents this operation's response from being emitted.
    Immediate(FrontendAction),
    /// The operation was accepted but an earlier response must be emitted first.
    Waiting(FrontendAction),
}

impl FrontendAdmission {
    /// Returns the application action, discarding only the positional annotation.
    #[must_use]
    pub(crate) fn into_action(self) -> FrontendAction {
        match self {
            Self::Immediate(action) | Self::Waiting(action) => action,
        }
    }
}

/// Why a frontend message could not be accepted.
#[derive(Debug, Eq, PartialEq)]
pub enum FrontendProjectionError {
    /// The bounded ledger is full; the unchanged message may be retried.
    Capacity(Box<FrontendMessage>),
    /// The message is not legal in the projected frontend protocol state.
    Illegal {
        /// Projected state at rejection.
        state: PipelineState,
        /// Unchanged illegal message.
        message: Box<FrontendMessage>,
    },
}

/// Application action for a backend message.
#[derive(Debug, Eq, PartialEq)]
pub(crate) enum BackendAction {
    /// Emit this owned message to the downstream client now.
    Emit(BackendMessage),
    /// An earlier operation must complete; retry this unchanged message later.
    Deferred(BackendMessage),
}

/// A backend message was not legal for any outstanding operation.
#[derive(Debug, Eq, PartialEq)]
pub struct BackendProjectionError {
    /// Current response-side state.
    pub state: PipelineState,
    /// Unchanged illegal message.
    pub message: BackendMessage,
}

/// Public summary of the pipeline's projected frontend state.
#[derive(Clone, Copy, Debug, Eq, PartialEq)]
pub enum PipelineState {
    /// A simple or extended cycle may begin.
    Ready,
    /// Extended-query messages are being accepted.
    Extended,
    /// An extended error discards messages through `Sync`.
    ExtendedError,
    /// COPY IN accepts frontend data.
    CopyIn,
    /// COPY OUT accepts only backend data.
    CopyOut,
    /// COPY BOTH accepts data in both directions.
    CopyBoth,
    /// The connection has terminated.
    Terminated,
}

/// Error returned while dispatching a pipeline message through typed middleware.
#[derive(Debug)]
pub(crate) enum PipelineMiddlewareError<MiddlewareError, ProjectionError> {
    /// Middleware rejected the phase-typed message.
    Middleware(MiddlewareError),
    /// The pipeline rejected the original or rewritten message.
    Projection(ProjectionError),
}

macro_rules! frontend_pipeline_phases {
    ($consumer:ident) => {
        $consumer! {
            Ready => frontend_ready => backend::ReadyExternalMessage,
            Building => frontend_building => backend::BuildingExternalMessage,
            ExtendedError => frontend_extended_error => backend::ExtendedErrorExternalMessage,
            SimpleCopyIn => frontend_simple_copy_in => backend::SimpleCopyInExternalMessage,
            ExtendedCopyIn => frontend_extended_copy_in => backend::ExtendedCopyInExternalMessage,
            SimpleCopyBoth => frontend_simple_copy_both => backend::SimpleCopyBothExternalMessage,
            ExtendedCopyBoth => frontend_extended_copy_both => backend::ExtendedCopyBothExternalMessage,
        }
    };
}

macro_rules! backend_pipeline_phases {
    ($consumer:ident) => {
        $consumer! {
            Asynchronous => backend_asynchronous => AsynchronousBackendMessage,
            Simple => backend_simple => backend::SimpleInternalMessage,
            SimpleError => backend_simple_error => backend::SimpleErrorInternalMessage,
            ParseResponse => backend_parse_response => backend::ParseResponseInternalMessage,
            BindResponse => backend_bind_response => backend::BindResponseInternalMessage,
            DescribeResponse => backend_describe_response => backend::DescribeResponseInternalMessage,
            ExecuteResponse => backend_execute_response => backend::ExecuteResponseInternalMessage,
            CloseResponse => backend_close_response => backend::CloseResponseInternalMessage,
            SyncResponse => backend_sync_response => backend::SyncResponseInternalMessage,
            FunctionResponse => backend_function_response => backend::FunctionResponseInternalMessage,
            FunctionReady => backend_function_ready => backend::FunctionReadyInternalMessage,
            SimpleCopyInDone => backend_simple_copy_in_done => backend::SimpleCopyInDoneInternalMessage,
            SimpleCopyInFailed => backend_simple_copy_in_failed => backend::SimpleCopyInFailedInternalMessage,
            SimpleCopyOut => backend_simple_copy_out => backend::SimpleCopyOutInternalMessage,
            SimpleCopyOutDone => backend_simple_copy_out_done => backend::SimpleCopyOutDoneInternalMessage,
            SimpleCopyReady => backend_simple_copy_ready => backend::SimpleCopyReadyInternalMessage,
            ExtendedCopyInDone => backend_extended_copy_in_done => backend::ExtendedCopyInDoneInternalMessage,
            ExtendedCopyInFailed => backend_extended_copy_in_failed => backend::ExtendedCopyInFailedInternalMessage,
            ExtendedCopyOut => backend_extended_copy_out => backend::ExtendedCopyOutInternalMessage,
            ExtendedCopyOutDone => backend_extended_copy_out_done => backend::ExtendedCopyOutDoneInternalMessage,
            SimpleCopyBoth => backend_simple_copy_both => backend::SimpleCopyBothInternalMessage,
            SimpleCopyBothClientDone => backend_simple_copy_both_client_done => backend::SimpleCopyBothClientDoneInternalMessage,
            SimpleCopyBothDone => backend_simple_copy_both_done => backend::SimpleCopyBothDoneInternalMessage,
            SimpleCopyBothFailed => backend_simple_copy_both_failed => backend::SimpleCopyBothFailedInternalMessage,
            ExtendedCopyBoth => backend_extended_copy_both => backend::ExtendedCopyBothInternalMessage,
            ExtendedCopyBothClientDone => backend_extended_copy_both_client_done => backend::ExtendedCopyBothClientDoneInternalMessage,
            ExtendedCopyBothDone => backend_extended_copy_both_done => backend::ExtendedCopyBothDoneInternalMessage,
            ExtendedCopyBothFailed => backend_extended_copy_both_failed => backend::ExtendedCopyBothFailedInternalMessage,
        }
    };
}

macro_rules! declare_pipeline_hooks {
    ($($phase:ident => $method:ident => $message:path),+ $(,)?) => {
        $(
            #[doc = concat!("Intercepts backend messages in generated `", stringify!($message), "` phase.")]
            async fn $method(
                &mut self,
                _state: &mut State,
                message: $message,
            ) -> Result<$message, Self::Error> {
                Ok(message)
            }
        )+
    };
}

/// Async middleware for frontend messages selected from the runtime ledger phase.
#[allow(async_fn_in_trait)]
pub(crate) trait FrontendPipelineMiddleware<State> {
    /// An error which prevents the message from continuing through the pipeline.
    type Error;
    frontend_pipeline_phases!(declare_pipeline_hooks);
}

/// Async middleware for backend messages selected from the runtime ledger phase.
#[allow(async_fn_in_trait)]
pub(crate) trait BackendPipelineMiddleware<State> {
    /// An error which prevents the message from continuing through the pipeline.
    type Error;
    backend_pipeline_phases!(declare_pipeline_hooks);
}

impl<State> FrontendPipelineMiddleware<State> for crate::middleware::Identity {
    type Error = Infallible;
}

impl<State> BackendPipelineMiddleware<State> for crate::middleware::Identity {
    type Error = Infallible;
}

macro_rules! chained_pipeline_hooks {
    ($($phase:ident => $method:ident => $message:ty),+ $(,)?) => {
        $(
            async fn $method(
                &mut self,
                state: &mut State,
                message: $message,
            ) -> Result<$message, Self::Error> {
                let (first, second) = self.parts_mut();
                let message = first
                    .$method(state, message)
                    .await
                    .map_err(ChainError::First)?;
                second
                    .$method(state, message)
                    .await
                    .map_err(ChainError::Second)
            }
        )+
    };
}

impl<State, First, Second> FrontendPipelineMiddleware<State> for Then<First, Second>
where
    First: FrontendPipelineMiddleware<State>,
    Second: FrontendPipelineMiddleware<State>,
{
    type Error = ChainError<First::Error, Second::Error>;

    frontend_pipeline_phases!(chained_pipeline_hooks);
}

impl<State, First, Second> BackendPipelineMiddleware<State> for Then<First, Second>
where
    First: BackendPipelineMiddleware<State>,
    Second: BackendPipelineMiddleware<State>,
{
    type Error = ChainError<First::Error, Second::Error>;
    backend_pipeline_phases!(chained_pipeline_hooks);
}

/// Adapts direction-wide async middleware to every typed pipeline hook.
pub(crate) struct PipelineWireAdapter<Handler> {
    handler: Handler,
}

impl<Handler> PipelineWireAdapter<Handler> {
    /// Wraps direction-wide middleware for runtime phase dispatch.
    pub(crate) const fn new(handler: Handler) -> Self {
        Self { handler }
    }

    /// Returns the wrapped direction-wide middleware.
    pub(crate) fn into_inner(self) -> Handler {
        self.handler
    }
}

/// Failure from direction-wide middleware adapted to typed pipeline dispatch.
#[derive(Debug)]
pub(crate) enum FrontendPipelineWireAdapterError<Error> {
    /// The wrapped middleware rejected a message.
    Middleware(Error),
    /// The wrapped middleware returned a frontend message illegal in the selected phase.
    IllegalFrontend(FrontendMessage),
}

/// Failure from backend wire middleware adapted to typed pipeline dispatch.
#[derive(Debug)]
pub(crate) enum BackendPipelineWireAdapterError<Error> {
    /// The wrapped middleware rejected a message.
    Middleware(Error),
    /// The wrapped middleware returned a message illegal in the selected phase.
    Illegal(BackendMessage),
}

macro_rules! pipeline_adapter_frontend_hooks {
    ($($phase:ident => $method:ident => $message:ty),+ $(,)?) => {
        $(
            async fn $method(
                &mut self,
                state: &mut State,
                message: $message,
            ) -> Result<$message, Self::Error> {
                let message: FrontendMessage = message.into();
                let message = self
                    .handler
                    .intercept(state, message)
                    .await
                    .map_err(FrontendPipelineWireAdapterError::Middleware)?;
                <$message>::try_from(message)
                    .map_err(FrontendPipelineWireAdapterError::IllegalFrontend)
            }
        )+
    };
}

macro_rules! pipeline_adapter_backend_hooks {
    ($ignored:ident => $async_method:ident => AsynchronousBackendMessage, $($phase:ident => $method:ident => $message:ty),+ $(,)?) => {
        async fn $async_method(
            &mut self,
            state: &mut State,
            message: AsynchronousBackendMessage,
        ) -> Result<AsynchronousBackendMessage, Self::Error> {
            let message = self.handler.intercept(state, message.into_wire()).await
                .map_err(BackendPipelineWireAdapterError::Middleware)?;
            AsynchronousBackendMessage::try_from(message)
                .map_err(BackendPipelineWireAdapterError::Illegal)
        }
        $(
            async fn $method(
                &mut self,
                state: &mut State,
                message: $message,
            ) -> Result<$message, Self::Error> {
                let message: BackendMessage = message.into();
                let message = self
                    .handler
                    .intercept(state, message)
                    .await
                    .map_err(BackendPipelineWireAdapterError::Middleware)?;
                <$message>::try_from(message).map_err(BackendPipelineWireAdapterError::Illegal)
            }
        )+
    };
}

impl<State, Handler> FrontendPipelineMiddleware<State> for PipelineWireAdapter<Handler>
where
    Handler: MessageMiddleware<FrontendMessage, State>,
{
    type Error = FrontendPipelineWireAdapterError<Handler::Error>;
    frontend_pipeline_phases!(pipeline_adapter_frontend_hooks);
}

impl<State, Handler> BackendPipelineMiddleware<State> for PipelineWireAdapter<Handler>
where
    Handler: MessageMiddleware<BackendMessage, State>,
{
    type Error = BackendPipelineWireAdapterError<Handler::Error>;
    backend_pipeline_phases!(pipeline_adapter_backend_hooks);
}

#[derive(Clone, Copy, Debug, Eq, PartialEq)]
enum RequestState {
    Ready,
    Extended { bound: bool },
    ExtendedError,
    CopyIn,
    CopyOut,
    CopyBoth,
    Terminated,
}

impl RequestState {
    const fn public(self) -> PipelineState {
        match self {
            Self::Ready => PipelineState::Ready,
            Self::Extended { .. } => PipelineState::Extended,
            Self::ExtendedError => PipelineState::ExtendedError,
            Self::CopyIn => PipelineState::CopyIn,
            Self::CopyOut => PipelineState::CopyOut,
            Self::CopyBoth => PipelineState::CopyBoth,
            Self::Terminated => PipelineState::Terminated,
        }
    }
}

#[derive(Clone, Copy, Debug, Eq, PartialEq)]
enum Origin {
    Forwarded,
    Local,
}

#[derive(Clone, Copy, Debug, Eq, PartialEq)]
enum OperationKind {
    Query,
    FunctionCall,
    Parse,
    Bind,
    Describe,
    Execute,
    Close,
    Flush,
    Sync,
    CopyData,
    CopyDone,
    CopyFail,
    Terminate,
}

#[derive(Clone, Copy, Debug, Eq, PartialEq)]
enum PreparedResponse {
    Asynchronous,
    Emit {
        head: Operation,
        response_state: backend::RuntimeState,
    },
    Deferred,
    Illegal,
}

#[derive(Clone, Copy, Debug, Eq, PartialEq)]
enum FrontendPhase {
    Ready,
    Building,
    ExtendedError,
    SimpleCopyIn,
    ExtendedCopyIn,
    SimpleCopyBoth,
    ExtendedCopyBoth,
}

#[derive(Clone, Copy, Debug)]
struct PreparedFrontend {
    phase: FrontendPhase,
    request_state: RequestState,
}

#[derive(Clone, Copy, Debug, Eq, PartialEq)]
struct Operation {
    id: OperationId,
    kind: OperationKind,
    origin: Origin,
    discarded: bool,
    response_state: backend::RuntimeState,
}

/// A bounded ledger coordinating independently owned frontend and backend values.
#[derive(Debug)]
pub(crate) struct Pipeline<P = NoPipeline> {
    policy: P,
    operations: VecDeque<Operation>,
    request_state: RequestState,
    response_state: Option<PipelineState>,
    next_id: u64,
    changed: Arc<Notify>,
}

impl Default for Pipeline<NoPipeline> {
    fn default() -> Self {
        Self::new(NoPipeline)
    }
}

impl<P: PipelinePolicy> Pipeline<P> {
    /// Creates an empty pipeline using `policy`.
    #[must_use]
    pub(crate) fn new(policy: P) -> Self {
        Self {
            policy,
            operations: VecDeque::new(),
            request_state: RequestState::Ready,
            response_state: None,
            next_id: 0,
            changed: Arc::new(Notify::new()),
        }
    }

    /// Returns the projected frontend protocol state.
    #[must_use]
    pub(crate) fn state(&self) -> PipelineState {
        self.response_state
            .unwrap_or_else(|| self.request_state.public())
    }

    /// Returns the number of incomplete lightweight operation records.
    #[must_use]
    pub(crate) fn len(&self) -> usize {
        self.operations.len()
    }

    /// Reports whether no operations remain outstanding.
    #[must_use]
    pub(crate) fn is_empty(&self) -> bool {
        self.operations.is_empty()
    }

    /// Projects and accepts one frontend message without retaining its payload.
    ///
    /// Capacity and legality failures return the original owned message. A
    /// capacity failure does not mutate either projected state or the ledger.
    ///
    /// # Errors
    ///
    /// Returns the unchanged boxed message when capacity is exhausted or the
    /// message is illegal in the projected state.
    pub(crate) fn accept_frontend(
        &mut self,
        message: FrontendMessage,
        handling: FrontendHandling,
    ) -> Result<FrontendAdmission, FrontendProjectionError> {
        let prepared = self.prepare_frontend(&message)?;
        Ok(self.commit_frontend(prepared, message, handling))
    }

    fn prepare_frontend(
        &self,
        message: &FrontendMessage,
    ) -> Result<PreparedFrontend, FrontendProjectionError> {
        if self.operations.len() == self.policy.operation_limit()
            && !matches!(
                self.request_state,
                RequestState::CopyIn | RequestState::CopyBoth
            )
        {
            return Err(FrontendProjectionError::Capacity(Box::new(message.clone())));
        }
        if project_frontend(self.request_state, message).is_none() {
            return Err(FrontendProjectionError::Illegal {
                state: self.state(),
                message: Box::new(message.clone()),
            });
        }
        Ok(PreparedFrontend {
            phase: frontend_phase(
                self.request_state,
                self.operations.front().map(|operation| operation.kind),
            ),
            request_state: self.request_state,
        })
    }

    fn commit_frontend(
        &mut self,
        prepared: PreparedFrontend,
        message: FrontendMessage,
        handling: FrontendHandling,
    ) -> FrontendAdmission {
        let (kind, next_state) = classify_frontend(prepared.request_state, &message)
            .expect("phase-typed frontend replacement has a ledger classification");
        let waiting = !self.operations.is_empty();
        let id = OperationId(self.next_id);
        self.next_id = self.next_id.saturating_add(1);
        self.request_state = next_state;
        let origin = match handling {
            FrontendHandling::Forward => Origin::Forwarded,
            FrontendHandling::Local => Origin::Local,
        };
        if let Some(head) = self.operations.front_mut()
            && let Some(event) = backend::project_external(head.response_state, &message)
            && let Some(transition) = backend::transition(head.response_state, event)
        {
            head.response_state = transition.target;
        }
        self.operations.push_back(Operation {
            id,
            kind,
            origin,
            discarded: matches!(self.request_state, RequestState::ExtendedError)
                && kind != OperationKind::Sync,
            response_state: initial_response_state(kind),
        });
        let action = match handling {
            FrontendHandling::Forward => FrontendAction::Forward { id, message },
            FrontendHandling::Local => FrontendAction::Discard { id },
        };
        let admission = if waiting {
            FrontendAdmission::Waiting(action)
        } else {
            FrontendAdmission::Immediate(action)
        };
        self.remove_inert_heads();
        admission
    }

    /// Projects, asynchronously intercepts, and accepts one frontend message.
    ///
    /// The ledger selects the phase-specific middleware hook at runtime. The
    /// selected hook can only return a message legal in that same phase.
    /// Middleware is not invoked when capacity is exhausted.
    ///
    /// # Errors
    ///
    /// Returns a middleware error, an illegal original or replacement message,
    /// or the unchanged message when capacity is exhausted.
    pub(crate) async fn accept_frontend_typed<State, Handler>(
        &mut self,
        middleware: &mut Middleware<State, Handler>,
        message: FrontendMessage,
        handling: FrontendHandling,
    ) -> Result<FrontendAdmission, PipelineMiddlewareError<Handler::Error, FrontendProjectionError>>
    where
        Handler: FrontendPipelineMiddleware<State>,
    {
        let prepared = self
            .prepare_frontend(&message)
            .map_err(PipelineMiddlewareError::Projection)?;

        let message = self
            .intercept_frontend(prepared.phase, middleware, message)
            .await
            .map_err(PipelineMiddlewareError::Middleware)?;
        if !message.is_reconstructable() {
            return Err(PipelineMiddlewareError::Projection(
                FrontendProjectionError::Illegal {
                    state: self.state(),
                    message: Box::new(message),
                },
            ));
        }
        Ok(self.commit_frontend(prepared, message, handling))
    }

    /// Projects one upstream backend message and preserves response order.
    ///
    /// # Errors
    ///
    /// Returns an unchanged response which cannot belong to any outstanding operation.
    pub(crate) fn accept_backend(
        &mut self,
        message: BackendMessage,
    ) -> Result<BackendAction, BackendProjectionError> {
        self.accept_response(None, message)
    }

    /// Intercepts an emittable backend response through its operation-typed hook.
    ///
    /// Responses belonging to a later operation are returned unchanged as
    /// [`BackendAction::Deferred`] and are intercepted only when retried at the
    /// response head. Asynchronous messages use their non-advancing hook.
    ///
    /// # Errors
    ///
    /// Returns a middleware error or an unchanged response which cannot belong
    /// to any outstanding operation.
    pub(crate) async fn accept_backend_typed<State, Handler>(
        &mut self,
        middleware: &mut Middleware<State, Handler>,
        message: BackendMessage,
    ) -> Result<BackendAction, PipelineMiddlewareError<Handler::Error, BackendProjectionError>>
    where
        Handler: BackendPipelineMiddleware<State>,
    {
        self.accept_response_typed(None, middleware, message).await
    }

    /// Attempts to register and emit a locally synthesized response.
    ///
    /// The message is returned as [`BackendAction::Deferred`] when `id` has not
    /// reached the head. No backend payload is retained by the ledger.
    ///
    /// # Errors
    ///
    /// Returns the unchanged message when it is illegal for the named operation.
    pub(crate) fn try_emit_local(
        &mut self,
        id: OperationId,
        message: BackendMessage,
    ) -> Result<BackendAction, BackendProjectionError> {
        self.accept_response(Some(id), message)
    }

    /// Typed-middleware counterpart to [`Self::try_emit_local`].
    ///
    /// Deferred local responses are not intercepted until their operation reaches
    /// the response head.
    ///
    /// # Errors
    ///
    /// Returns a middleware error or an illegal response for the named operation.
    pub(crate) async fn try_emit_local_typed<State, Handler>(
        &mut self,
        middleware: &mut Middleware<State, Handler>,
        id: OperationId,
        message: BackendMessage,
    ) -> Result<BackendAction, PipelineMiddlewareError<Handler::Error, BackendProjectionError>>
    where
        Handler: BackendPipelineMiddleware<State>,
    {
        self.accept_response_typed(Some(id), middleware, message)
            .await
    }

    /// Waits until a local operation reaches the response head.
    ///
    /// Cancellation is safe: polling this future never reserves or removes a
    /// ledger entry. The caller should then invoke [`Self::try_emit_local`].
    pub(crate) async fn wait_until_emittable(&self, id: OperationId) {
        loop {
            let notified = self.changed.notified();
            if self
                .operations
                .front()
                .is_some_and(|operation| operation.id == id)
            {
                return;
            }
            notified.await;
        }
    }

    async fn intercept_frontend<State, Handler>(
        &self,
        phase: FrontendPhase,
        middleware: &mut Middleware<State, Handler>,
        message: FrontendMessage,
    ) -> Result<FrontendMessage, Handler::Error>
    where
        Handler: FrontendPipelineMiddleware<State>,
    {
        let (state, handler) = middleware.parts_mut();
        macro_rules! dispatch {
            ($message:expr, $type:path, $handler:ident, $state:ident, $method:ident) => {{
                let Ok(typed) = <$type>::try_from($message) else {
                    unreachable!("frontend message was prevalidated for pipeline phase")
                };
                $handler.$method($state, typed).await?.into()
            }};
        }

        macro_rules! dispatch_catalogue {
            ($($catalogue_phase:ident => $method:ident => $message_type:path),+ $(,)?) => {
                match phase {
                    $(
                        FrontendPhase::$catalogue_phase =>
                            dispatch!(message, $message_type, handler, state, $method),
                    )+
                }
            };
        }

        Ok(frontend_pipeline_phases!(dispatch_catalogue))
    }

    #[allow(clippy::too_many_lines)]
    async fn accept_response_typed<State, Handler>(
        &mut self,
        local_id: Option<OperationId>,
        middleware: &mut Middleware<State, Handler>,
        message: BackendMessage,
    ) -> Result<BackendAction, PipelineMiddlewareError<Handler::Error, BackendProjectionError>>
    where
        Handler: BackendPipelineMiddleware<State>,
    {
        let prepared = self.prepare_response(local_id, &message);
        if matches!(prepared, PreparedResponse::Deferred) {
            return Ok(BackendAction::Deferred(message));
        }
        if matches!(prepared, PreparedResponse::Illegal) {
            return Err(PipelineMiddlewareError::Projection(
                BackendProjectionError {
                    state: self.state(),
                    message,
                },
            ));
        }

        let (state, handler) = middleware.parts_mut();
        let message = match prepared {
            PreparedResponse::Asynchronous => {
                let Ok(typed) = AsynchronousBackendMessage::try_from(message) else {
                    unreachable!("asynchronous response was prevalidated")
                };
                handler
                    .backend_asynchronous(state, typed)
                    .await
                    .map_err(PipelineMiddlewareError::Middleware)?
                    .into_wire()
            }
            PreparedResponse::Emit { response_state, .. } => {
                macro_rules! dispatch {
                    ($message:ty, $method:ident) => {{
                        let typed = match <$message>::try_from(message) {
                            Ok(typed) => typed,
                            Err(message) => {
                                return Err(PipelineMiddlewareError::Projection(
                                    BackendProjectionError {
                                        state: self.state(),
                                        message,
                                    },
                                ));
                            }
                        };
                        handler
                            .$method(state, typed)
                            .await
                            .map_err(PipelineMiddlewareError::Middleware)?
                            .into_wire()
                    }};
                }

                macro_rules! dispatch_catalogue {
                    ($ignored:ident => $ignored_method:ident => AsynchronousBackendMessage,
                     $($catalogue_phase:ident => $method:ident => $message_type:path),+ $(,)?) => {
                        match response_state {
                            $(
                                backend::RuntimeState::$catalogue_phase =>
                                    dispatch!($message_type, $method),
                            )+
                            _ => unreachable!("response phase has no backend-selected transition"),
                        }
                    };
                }

                backend_pipeline_phases!(dispatch_catalogue)
            }
            PreparedResponse::Deferred | PreparedResponse::Illegal => unreachable!(),
        };

        if !message.is_reconstructable() {
            return Err(PipelineMiddlewareError::Projection(
                BackendProjectionError {
                    state: self.state(),
                    message,
                },
            ));
        }
        self.commit_response(prepared, message)
            .map_err(PipelineMiddlewareError::Projection)
    }

    fn prepare_response(
        &self,
        local_id: Option<OperationId>,
        message: &BackendMessage,
    ) -> PreparedResponse {
        if is_asynchronous(message) {
            return PreparedResponse::Asynchronous;
        }
        let Some(head) = self.operations.front().copied() else {
            return PreparedResponse::Illegal;
        };
        if let Some(id) = local_id {
            if head.id != id {
                return PreparedResponse::Deferred;
            }
            if head.origin != Origin::Local {
                return PreparedResponse::Illegal;
            }
        } else if head.origin == Origin::Local {
            return if self.operations.iter().skip(1).any(|operation| {
                operation.origin == Origin::Forwarded && response_fits(*operation, message)
            }) {
                PreparedResponse::Deferred
            } else {
                PreparedResponse::Illegal
            };
        }
        if head.discarded || !response_fits(head, message) {
            return if self
                .operations
                .iter()
                .skip(1)
                .any(|operation| response_fits(*operation, message))
            {
                PreparedResponse::Deferred
            } else {
                PreparedResponse::Illegal
            };
        }
        PreparedResponse::Emit {
            head,
            response_state: head.response_state,
        }
    }

    fn accept_response(
        &mut self,
        local_id: Option<OperationId>,
        message: BackendMessage,
    ) -> Result<BackendAction, BackendProjectionError> {
        let prepared = self.prepare_response(local_id, &message);
        self.commit_response(prepared, message)
    }

    fn commit_response(
        &mut self,
        prepared: PreparedResponse,
        message: BackendMessage,
    ) -> Result<BackendAction, BackendProjectionError> {
        let PreparedResponse::Emit {
            head,
            response_state,
        } = prepared
        else {
            return match prepared {
                PreparedResponse::Asynchronous => Ok(BackendAction::Emit(message)),
                PreparedResponse::Deferred => Ok(BackendAction::Deferred(message)),
                PreparedResponse::Illegal => Err(BackendProjectionError {
                    state: self.state(),
                    message,
                }),
                PreparedResponse::Emit { .. } => unreachable!(),
            };
        };
        let event = backend::project_internal(response_state, &message)
            .expect("response was validated against its generated backend phase");
        let next_response_state = backend::transition(response_state, event)
            .expect("projected backend event has a generated transition")
            .target;
        let terminal = response_is_terminal(head.kind, &message);
        let error = matches!(message, BackendMessage::ErrorResponse(_));
        let copy_state = response_copy_state(next_response_state);
        if terminal {
            self.operations.pop_front();
            if error && is_extended_kind(head.kind) {
                self.enter_extended_error();
            }
        } else if let Some(head) = self.operations.front_mut() {
            head.response_state = next_response_state;
        }
        if !terminal {
            self.response_state = copy_state.map(RequestState::public);
        }
        if let Some(state) = copy_state
            && matches!(state, RequestState::CopyIn | RequestState::CopyBoth)
        {
            self.request_state = state;
        }
        if terminal {
            self.response_state = None;
            match head.kind {
                OperationKind::Sync | OperationKind::Query => {
                    self.request_state = RequestState::Ready;
                }
                OperationKind::Execute if !error => {
                    self.request_state = RequestState::Extended { bound: true };
                }
                _ => {}
            }
        }
        self.remove_inert_heads();
        self.changed.notify_waiters();
        Ok(BackendAction::Emit(message))
    }

    fn enter_extended_error(&mut self) {
        self.request_state = RequestState::ExtendedError;
        self.response_state = None;
        for operation in &mut self.operations {
            if operation.kind == OperationKind::Sync {
                break;
            }
            operation.discarded = true;
        }
    }

    fn remove_inert_heads(&mut self) {
        let previous_len = self.operations.len();
        while self.operations.front().is_some_and(|operation| {
            operation.kind == OperationKind::Flush
                || operation.kind == OperationKind::CopyData
                || operation.kind == OperationKind::CopyDone
                || operation.kind == OperationKind::CopyFail
                || operation.kind == OperationKind::Terminate
                || operation.discarded
        }) {
            self.operations.pop_front();
        }
        if self.operations.len() != previous_len {
            self.changed.notify_waiters();
        }
    }
}

fn project_frontend(
    state: RequestState,
    message: &FrontendMessage,
) -> Option<(OperationKind, RequestState)> {
    use RequestState as S;
    let generated_state = match state {
        S::Ready => backend::RuntimeState::Ready,
        S::Extended { .. } => backend::RuntimeState::Building,
        S::ExtendedError => backend::RuntimeState::ExtendedError,
        S::CopyIn => backend::RuntimeState::ExtendedCopyIn,
        S::CopyOut => backend::RuntimeState::ExtendedCopyOut,
        S::CopyBoth => backend::RuntimeState::ExtendedCopyBoth,
        S::Terminated => backend::RuntimeState::Terminated,
    };
    backend::project_external(generated_state, message)?;
    classify_frontend(state, message)
}

fn classify_frontend(
    state: RequestState,
    message: &FrontendMessage,
) -> Option<(OperationKind, RequestState)> {
    use FrontendMessage as F;
    use OperationKind as O;
    use RequestState as S;

    match (state, message) {
        (S::Ready, F::Query(_)) => Some((O::Query, S::Ready)),
        (S::Ready, F::FunctionCall(_)) => Some((O::FunctionCall, S::Ready)),
        (S::Ready, F::Parse(_)) => Some((O::Parse, S::Extended { bound: false })),
        (S::Ready | S::Extended { .. }, F::Bind(_)) => Some((O::Bind, S::Extended { bound: true })),
        (S::Ready, F::Describe(_)) => Some((O::Describe, S::Extended { bound: false })),
        (S::Ready | S::Extended { .. }, F::Execute(_)) => {
            Some((O::Execute, S::Extended { bound: true }))
        }
        (S::Ready, F::Close(_)) => Some((O::Close, S::Extended { bound: false })),
        (S::Ready, F::Terminate) => Some((O::Terminate, S::Terminated)),
        (S::Extended { bound }, F::Parse(_)) => Some((O::Parse, S::Extended { bound })),
        (S::Extended { bound }, F::Describe(_)) => Some((O::Describe, S::Extended { bound })),
        (S::Extended { bound }, F::Close(_)) => Some((O::Close, S::Extended { bound })),
        (S::Extended { bound }, F::Flush) => Some((O::Flush, S::Extended { bound })),
        (S::Extended { .. } | S::ExtendedError, F::Sync) => Some((O::Sync, S::Ready)),
        (S::ExtendedError, _) => Some((classify_discard(message)?, S::ExtendedError)),
        (S::CopyIn, F::CopyData(_)) => Some((O::CopyData, S::CopyIn)),
        (S::CopyIn, F::CopyDone) => Some((O::CopyDone, S::Extended { bound: true })),
        (S::CopyIn, F::CopyFail(_)) => Some((O::CopyFail, S::ExtendedError)),
        (S::CopyBoth, F::CopyData(_)) => Some((O::CopyData, S::CopyBoth)),
        (S::CopyBoth, F::CopyDone) => Some((O::CopyDone, S::CopyBoth)),
        _ => None,
    }
}

fn frontend_phase(state: RequestState, response_head: Option<OperationKind>) -> FrontendPhase {
    match (state, response_head) {
        (RequestState::Ready, _) => FrontendPhase::Ready,
        (RequestState::Extended { .. }, _) => FrontendPhase::Building,
        (RequestState::ExtendedError, _) => FrontendPhase::ExtendedError,
        (RequestState::CopyIn, Some(OperationKind::Query)) => FrontendPhase::SimpleCopyIn,
        (RequestState::CopyIn, Some(OperationKind::Execute)) => FrontendPhase::ExtendedCopyIn,
        (RequestState::CopyBoth, Some(OperationKind::Query)) => FrontendPhase::SimpleCopyBoth,
        (RequestState::CopyBoth, Some(OperationKind::Execute)) => FrontendPhase::ExtendedCopyBoth,
        (RequestState::CopyIn | RequestState::CopyBoth, _) => {
            unreachable!("COPY phase must belong to Query or Execute")
        }
        (RequestState::CopyOut | RequestState::Terminated, _) => {
            unreachable!("non-accepting frontend phase cannot be prepared")
        }
    }
}

fn classify_discard(message: &FrontendMessage) -> Option<OperationKind> {
    Some(match message {
        FrontendMessage::Parse(_) => OperationKind::Parse,
        FrontendMessage::Bind(_) => OperationKind::Bind,
        FrontendMessage::Describe(_) => OperationKind::Describe,
        FrontendMessage::Execute(_) => OperationKind::Execute,
        FrontendMessage::Close(_) => OperationKind::Close,
        FrontendMessage::Flush => OperationKind::Flush,
        FrontendMessage::Query(_) => OperationKind::Query,
        FrontendMessage::FunctionCall(_) => OperationKind::FunctionCall,
        FrontendMessage::CopyData(_) => OperationKind::CopyData,
        FrontendMessage::CopyDone => OperationKind::CopyDone,
        FrontendMessage::CopyFail(_) => OperationKind::CopyFail,
        FrontendMessage::Terminate => OperationKind::Terminate,
        FrontendMessage::PasswordResponse(_) => return None,
        FrontendMessage::Sync => unreachable!("Sync is classified before discard"),
    })
}

const fn initial_response_state(kind: OperationKind) -> backend::RuntimeState {
    use OperationKind as O;
    match kind {
        O::Query => backend::RuntimeState::Simple,
        O::FunctionCall => backend::RuntimeState::FunctionResponse,
        O::Parse => backend::RuntimeState::ParseResponse,
        O::Bind => backend::RuntimeState::BindResponse,
        O::Describe => backend::RuntimeState::DescribeResponse,
        O::Execute => backend::RuntimeState::ExecuteResponse,
        O::Close => backend::RuntimeState::CloseResponse,
        O::Sync => backend::RuntimeState::SyncResponse,
        O::Flush | O::CopyData | O::CopyDone | O::CopyFail | O::Terminate => {
            backend::RuntimeState::Terminated
        }
    }
}

fn response_fits(operation: Operation, message: &BackendMessage) -> bool {
    backend::project_internal(operation.response_state, message).is_some()
}

fn response_is_terminal(kind: OperationKind, message: &BackendMessage) -> bool {
    use BackendMessage as B;
    use OperationKind as O;
    match kind {
        O::Query | O::FunctionCall | O::Sync => matches!(message, B::ReadyForQuery(_)),
        O::Parse => matches!(message, B::ParseComplete | B::ErrorResponse(_)),
        O::Bind => matches!(message, B::BindComplete | B::ErrorResponse(_)),
        O::Describe => matches!(
            message,
            B::RowDescription(_) | B::NoData | B::ErrorResponse(_)
        ),
        O::Execute => matches!(
            message,
            B::CommandComplete(_) | B::PortalSuspended | B::ErrorResponse(_)
        ),
        O::Close => matches!(message, B::CloseComplete | B::ErrorResponse(_)),
        O::CopyDone => matches!(
            message,
            B::CopyDone | B::CommandComplete(_) | B::ErrorResponse(_)
        ),
        O::CopyFail => matches!(message, B::ErrorResponse(_)),
        O::Flush | O::CopyData | O::Terminate => true,
    }
}

fn is_extended_kind(kind: OperationKind) -> bool {
    !matches!(
        kind,
        OperationKind::Query | OperationKind::FunctionCall | OperationKind::Terminate
    )
}

fn response_copy_state(state: backend::RuntimeState) -> Option<RequestState> {
    use backend::RuntimeState as S;
    match state {
        S::SimpleCopyIn | S::ExtendedCopyIn => Some(RequestState::CopyIn),
        S::SimpleCopyOut | S::ExtendedCopyOut => Some(RequestState::CopyOut),
        S::SimpleCopyBoth
        | S::SimpleCopyBothClientDone
        | S::SimpleCopyBothServerDone
        | S::ExtendedCopyBoth
        | S::ExtendedCopyBothClientDone
        | S::ExtendedCopyBothServerDone => Some(RequestState::CopyBoth),
        _ => None,
    }
}

fn is_asynchronous(message: &BackendMessage) -> bool {
    Demux::is_asynchronous(message)
}