webui_handler/streaming/

session.rs

// Copyright (c) Microsoft Corporation.
// Licensed under the MIT license.

//! Public borrowed session API and shared owned continuation state.

use std::borrow::Cow;
use std::collections::HashSet;
use std::sync::Arc;

use serde_json::{Number, Value};

use super::error::{boundary_order_error, state_update_type_error};
use super::state::{
    increment_state_revision, increment_streaming_record_sequence, overlay_full_state,
    overlay_full_state_owned, overlay_selected_state, overlay_selected_state_owned,
    selected_state_snapshot, selected_state_snapshot_owned, StreamingProgress,
    StreamingRenderState,
};
use super::vm::{ContinuationVm, StepGoal};
use super::StreamingSink;
use crate::plugin::HandlerPlugin;
use crate::route_handler::Protocol;
use crate::{
    FlushWriter, HandlerError, RenderOptions, ResponseWriter, Result, WebUIHandler,
    WebUIProcessContext,
};

/// Maximum continuation frames retained by one response.
pub const MAX_CONTINUATION_DEPTH: usize = 256;
/// Maximum nested unfinished component spans.
pub const MAX_SPAN_NESTING: usize = 32;
/// Maximum runtime boundary occurrences in one response.
pub const MAX_BOUNDARY_OCCURRENCES: usize = 512;
/// Maximum runtime boundary occurrences one response may commit as
/// [`BoundaryMode::Updatable`].
///
/// Mirrors the browser coordinator's retained-boundary cap: the client refuses
/// to retain a 129th updatable occurrence, so the server refuses to emit one
/// rather than stream a checkpoint the page would fail on.
pub(crate) const MAX_UPDATABLE_OCCURRENCES: usize = 128;
/// Maximum keyed runtime occurrences tracked for uniqueness.
pub const MAX_KEYED_INSTANCES: usize = 512;
/// Maximum top-level state keys retained by a continuation snapshot.
pub(crate) const MAX_FROZEN_STATE_KEYS: usize = 1_024;

/// Response-local runtime boundary occurrence identity.
#[derive(Clone, Copy, Debug, Eq, Hash, PartialEq)]
pub struct BoundaryInstanceId(u32);

impl BoundaryInstanceId {
    /// Rebuild an ID round-tripped through a host binding.
    #[must_use]
    pub const fn from_raw(raw: u32) -> Self {
        Self(raw)
    }

    /// Return the wire integer.
    #[must_use]
    pub const fn raw(self) -> u32 {
        self.0
    }

    pub(crate) fn index(self) -> Result<usize> {
        usize::try_from(self.0).map_err(|_| {
            HandlerError::Invariant("boundary instance ID does not fit usize".to_string())
        })
    }
}

/// Response-local generated component span identity.
#[derive(Clone, Copy, Debug, Eq, Hash, PartialEq)]
pub struct SpanInstanceId(u32);

impl SpanInstanceId {
    /// Rebuild an ID round-tripped through a host binding.
    #[must_use]
    pub const fn from_raw(raw: u32) -> Self {
        Self(raw)
    }

    /// Return the wire integer.
    #[must_use]
    pub const fn raw(self) -> u32 {
        self.0
    }

    pub(crate) const fn new(raw: u32) -> Self {
        Self(raw)
    }
}

/// Valid evaluated key for a repeated boundary declaration.
#[derive(Clone, Debug, Eq, Hash, PartialEq)]
pub enum BoundaryKey {
    /// Authored key resolved to a JSON string.
    String(String),
    /// Authored key resolved to a finite JSON number.
    Number(Number),
}

impl BoundaryKey {
    pub(crate) fn diagnostic(&self) -> String {
        match self {
            Self::String(value) => {
                let mut out = String::with_capacity(value.len() + 2);
                out.push('"');
                out.push_str(value);
                out.push('"');
                out
            }
            Self::Number(value) => value.to_string(),
        }
    }
}

/// Borrowed or owned JSON state supplied to a streaming step.
///
/// `Value`, `&Value`, and references to smart pointers such as `&Arc<Value>`
/// convert implicitly, so callers use one method name without losing ownership
/// control.
pub enum StreamingState<'a> {
    /// State retained by the caller; the session clones only its projection.
    Borrowed(&'a Value),
    /// State transferred to the session so changed subtrees can move.
    Owned(Value),
}

impl<'a, T> From<&'a T> for StreamingState<'a>
where
    T: std::borrow::Borrow<Value> + ?Sized,
{
    fn from(state: &'a T) -> Self {
        Self::Borrowed(std::borrow::Borrow::borrow(state))
    }
}

impl<'a, T> From<&'a mut T> for StreamingState<'a>
where
    T: std::borrow::Borrow<Value> + ?Sized,
{
    fn from(state: &'a mut T) -> Self {
        Self::Borrowed(std::borrow::Borrow::borrow(&*state))
    }
}

impl From<Value> for StreamingState<'_> {
    fn from(state: Value) -> Self {
        Self::Owned(state)
    }
}

/// Runtime occurrence returned when traversal suspends.
#[derive(Clone, Debug, Eq, PartialEq)]
pub struct BoundaryDescriptor {
    /// Gapless response-local occurrence ID.
    pub instance_id: BoundaryInstanceId,
    /// Stable compiler declaration ID.
    pub declaration_id: u32,
    /// Entry or component template that owns the declaration.
    ///
    /// Interned per protocol, so producing a descriptor shares the compiled
    /// string instead of allocating a copy per occurrence.
    pub owner: Arc<str>,
    /// Free-form authored declaration name.
    ///
    /// Interned per protocol alongside [`Self::owner`].
    pub name: Arc<str>,
    /// Evaluated repeat key, when authored.
    pub key: Option<BoundaryKey>,
}

/// Whether a committed occurrence may receive later state updates.
#[derive(Clone, Copy, Debug, Eq, PartialEq)]
pub enum BoundaryMode {
    /// Hydrate once and release boundary-local roots.
    Final,
    /// Retain live roots until terminal for later [`StreamingResponse::update`].
    Updatable,
}

/// Borrowed-writer result of one semantic streaming step.
///
/// The pair of fields names exactly one state:
///
/// | `boundary` | `done` | meaning |
/// |------------|--------|---------|
/// | `Some`     | `false`| the occurrence is waiting for [`StreamingResponse::resume`] |
/// | `None`     | `false`| the committed occurrence flushed; call [`StreamingResponse::advance`] |
/// | `None`     | `true` | the terminal record and writer end completed |
#[derive(Clone, Debug, Eq, PartialEq)]
pub struct StreamStatus {
    /// The next occurrence waiting for [`StreamingResponse::resume`].
    pub boundary: Option<BoundaryDescriptor>,
    /// True after the terminal record and writer end completed.
    pub done: bool,
}

/// A progressive response that writes directly through a [`FlushWriter`].
///
/// Steps alternate: [`Self::start`] writes the shell prefix and stops before
/// the first occurrence, [`Self::resume`] writes exactly one occurrence through
/// its checkpoint, and [`Self::advance`] writes the ordinary parent bytes that
/// follow it until the next occurrence or the terminal. Every step is one
/// independently writable, independently flushed segment.
pub struct StreamingResponse<'a, W: FlushWriter + ?Sized> {
    handler: &'a WebUIHandler,
    protocol: &'a Protocol,
    options: RenderOptions<'a>,
    sink: StreamingSink<'a, W>,
    pub(crate) core: SessionCore,
}

impl WebUIHandler {
    /// Create a runtime-discovered progressive response.
    pub fn stream_response<'a, W: FlushWriter + ?Sized>(
        &'a self,
        protocol: &'a Protocol,
        options: &RenderOptions<'a>,
        writer: &'a mut W,
    ) -> Result<StreamingResponse<'a, W>> {
        protocol.ensure_style_metadata()?;
        let core = SessionCore::new(self, protocol, options.entry_id)?;
        Ok(StreamingResponse {
            handler: self,
            protocol,
            options: RenderOptions {
                entry_id: options.entry_id,
                request_path: options.request_path,
                nonce: options.nonce,
                head_inject: options.head_inject,
                body_inject: options.body_inject,
            },
            sink: StreamingSink {
                transport: writer,
                component_opening: None,
                written: 0,
                flushed: 0,
            },
            core,
        })
    }
}

impl<W: FlushWriter + ?Sized> StreamingResponse<'_, W> {
    /// Render until the first runtime boundary occurrence or terminal.
    ///
    /// Pass an owned [`Value`] to move it into the continuation, or a reference
    /// to retain caller ownership and clone only the required projection.
    pub fn start<'state>(
        &mut self,
        state: impl Into<StreamingState<'state>>,
    ) -> Result<StreamStatus> {
        let (core, call) = self.parts();
        match state.into() {
            StreamingState::Borrowed(state) => core.start(call, state),
            StreamingState::Owned(state) => core.start_owned(call, state),
        }
    }

    /// Commit the pending occurrence through its checkpoint, then stop.
    ///
    /// The bytes written by this call are exactly the occurrence's own record —
    /// no parent or tail bytes follow it — so the host can release the
    /// occurrence the moment it resolves. Call [`Self::advance`] next.
    ///
    /// [`BoundaryMode::Updatable`] is refused once the response has committed
    /// as many updatable occurrences as the browser retains. The refusal is
    /// raised before any byte or state moves, so the same occurrence stays
    /// pending and can be committed with [`BoundaryMode::Final`] instead.
    /// Pass an owned [`Value`] to move changed subtrees, or a reference to retain
    /// caller ownership. Use [`Self::resume_current`] when no state changed.
    pub fn resume<'state>(
        &mut self,
        instance_id: BoundaryInstanceId,
        state: impl Into<StreamingState<'state>>,
        mode: BoundaryMode,
    ) -> Result<StreamStatus> {
        let (core, call) = self.parts();
        match state.into() {
            StreamingState::Borrowed(state) => core.resume(call, instance_id, state, mode),
            StreamingState::Owned(state) => core.resume_owned(call, instance_id, state, mode),
        }
    }

    /// Commit the pending occurrence against the retained state snapshot.
    ///
    /// This preserves the checkpoint flush and async pause point while avoiding
    /// an overlay when no top-level state changed since the preceding step.
    pub fn resume_current(
        &mut self,
        instance_id: BoundaryInstanceId,
        mode: BoundaryMode,
    ) -> Result<StreamStatus> {
        let (core, call) = self.parts();
        core.resume_current(call, instance_id, mode)
    }

    /// Write the ordinary parent bytes that follow a committed occurrence.
    ///
    /// Valid only after [`Self::resume`]. Renders the shell until the next
    /// occurrence suspends or the terminal record completes.
    pub fn advance(&mut self) -> Result<StreamStatus> {
        let (core, call) = self.parts();
        core.advance(call)
    }

    /// Emit one projected state update for a committed updatable occurrence.
    ///
    /// Valid between [`Self::resume`] and [`Self::advance`] as well as after
    /// the response has moved on, so a host can revise the occurrence it just
    /// committed while the response is still open.
    pub fn update(&mut self, instance_id: BoundaryInstanceId, patch: &Value) -> Result<()> {
        let (core, call) = self.parts();
        core.update(call, instance_id, patch)
    }

    pub(crate) fn render_all(&mut self, state: &Value) -> Result<()> {
        let (core, call) = self.parts();
        core.render_all(call, state)
    }

    /// Whether the response has emitted its terminal and ended its writer.
    #[must_use]
    pub fn is_done(&self) -> bool {
        self.core.done
    }

    /// Split the response into its continuation and one call's borrowed
    /// runtime, so every entry point shares the same wiring.
    fn parts(&mut self) -> (&mut SessionCore, SessionCall<'_, '_>) {
        let Self {
            handler,
            protocol,
            options,
            sink,
            core,
        } = self;
        (
            core,
            SessionCall {
                handler,
                protocol,
                options,
                writer: sink,
            },
        )
    }
}

pub(crate) struct SessionCore {
    pub(crate) vm: ContinuationVm,
    frozen_keys: Arc<[Box<str>]>,
    requires_full_state: bool,
    frozen_state: Value,
    started: bool,
    pub(crate) done: bool,
    failed: bool,
    /// True between a committed occurrence and the `advance` that writes the
    /// parent bytes following it.
    awaiting_advance: bool,
    local_vars: crate::ScopeMap,
    component_attrs: crate::ScopeMap,
    route_base: Option<String>,
    rendered_components: HashSet<String>,
    document_style_resources: HashSet<String>,
    shadow_style_roots: Vec<crate::ShadowStyleRoot>,
    plugin: Option<Box<dyn HandlerPlugin>>,
    route_children: crate::RouteChildren<'static>,
    head_end_emitted: bool,
    body_start_emitted: bool,
    component_asset_styles_emitted: bool,
    body_end_emitted: bool,
    route_chain_index: usize,
    route_chain: Option<Vec<crate::route_handler::RouteChainEntry>>,
    route_document_style_targets: Vec<bool>,
    reachable_components: Option<Vec<String>>,
    streaming: Option<StreamingProgress>,
    json_scratch: Vec<u8>,
    scope_pool: Vec<crate::ScopeMap>,
}

pub(crate) struct SessionCall<'call, 'data> {
    pub(crate) handler: &'call WebUIHandler,
    pub(crate) protocol: &'data Protocol,
    pub(crate) options: &'call RenderOptions<'data>,
    pub(crate) writer: &'call mut dyn ResponseWriter,
}

impl SessionCore {
    pub(crate) fn new(handler: &WebUIHandler, protocol: &Protocol, entry_id: &str) -> Result<Self> {
        if !protocol.protocol().fragments.contains_key(entry_id) {
            return Err(HandlerError::MissingFragment(entry_id.to_string()));
        }
        let state_plan = protocol.continuation_state_plan(entry_id)?.resolve()?;
        Ok(Self {
            vm: ContinuationVm::new(entry_id, protocol)?,
            frozen_keys: Arc::clone(&state_plan.keys),
            requires_full_state: state_plan.requires_full_state,
            frozen_state: Value::Object(serde_json::Map::new()),
            started: false,
            done: false,
            failed: false,
            awaiting_advance: false,
            local_vars: crate::ScopeMap::default(),
            component_attrs: crate::ScopeMap::default(),
            route_base: None,
            rendered_components: HashSet::new(),
            document_style_resources: HashSet::new(),
            shadow_style_roots: Vec::new(),
            plugin: handler.plugin_factory.map(|factory| factory()),
            route_children: Cow::Owned(Vec::new()),
            head_end_emitted: false,
            body_start_emitted: false,
            component_asset_styles_emitted: false,
            body_end_emitted: false,
            route_chain_index: 0,
            route_chain: None,
            route_document_style_targets: Vec::new(),
            reachable_components: None,
            streaming: Some(StreamingProgress::new(
                protocol.component_index().len(),
                protocol.style_resource_index().len(),
            )),
            json_scratch: Vec::new(),
            scope_pool: Vec::new(),
        })
    }

    pub(crate) fn start(
        &mut self,
        call: SessionCall<'_, '_>,
        state: &Value,
    ) -> Result<StreamStatus> {
        self.require_usable("start")?;
        if self.started {
            return Err(boundary_order_error(
                "start",
                "the streaming response has already started",
            ));
        }
        // The value is moved into each render context and back, so protocols
        // requiring full projection pay for exactly one response-local clone.
        self.frozen_state = if self.requires_full_state {
            state.clone()
        } else {
            selected_state_snapshot(state, &self.frozen_keys)
        };
        self.started = true;
        self.run_step(call, StepGoal::NextBoundary, None)
    }

    pub(crate) fn start_owned(
        &mut self,
        call: SessionCall<'_, '_>,
        state: Value,
    ) -> Result<StreamStatus> {
        self.require_usable("start")?;
        if self.started {
            return Err(boundary_order_error(
                "start",
                "the streaming response has already started",
            ));
        }
        self.frozen_state = if self.requires_full_state {
            state
        } else {
            selected_state_snapshot_owned(state, &self.frozen_keys)
        };
        self.started = true;
        self.run_step(call, StepGoal::NextBoundary, None)
    }

    pub(crate) fn resume(
        &mut self,
        call: SessionCall<'_, '_>,
        instance_id: BoundaryInstanceId,
        state: &Value,
        mode: BoundaryMode,
    ) -> Result<StreamStatus> {
        self.require_resumable()?;
        self.vm.validate_resume(instance_id)?;
        self.vm.validate_resume_mode(mode)?;
        let changed = if self.requires_full_state {
            overlay_full_state(&mut self.frozen_state, state)
        } else {
            overlay_selected_state(&mut self.frozen_state, state, &self.frozen_keys)
        };
        self.record_state_change(changed)?;
        self.run_step(call, StepGoal::CommitBoundary, Some((instance_id, mode)))
    }

    pub(crate) fn resume_owned(
        &mut self,
        call: SessionCall<'_, '_>,
        instance_id: BoundaryInstanceId,
        state: Value,
        mode: BoundaryMode,
    ) -> Result<StreamStatus> {
        self.require_resumable()?;
        self.vm.validate_resume(instance_id)?;
        self.vm.validate_resume_mode(mode)?;
        let changed = if self.requires_full_state {
            overlay_full_state_owned(&mut self.frozen_state, state)
        } else {
            overlay_selected_state_owned(&mut self.frozen_state, state, &self.frozen_keys)
        };
        self.record_state_change(changed)?;
        self.run_step(call, StepGoal::CommitBoundary, Some((instance_id, mode)))
    }

    pub(crate) fn resume_current(
        &mut self,
        call: SessionCall<'_, '_>,
        instance_id: BoundaryInstanceId,
        mode: BoundaryMode,
    ) -> Result<StreamStatus> {
        self.require_resumable()?;
        self.vm.validate_resume(instance_id)?;
        self.vm.validate_resume_mode(mode)?;
        self.run_step(call, StepGoal::CommitBoundary, Some((instance_id, mode)))
    }

    pub(crate) fn advance(&mut self, call: SessionCall<'_, '_>) -> Result<StreamStatus> {
        self.require_advanceable()?;
        self.run_step(call, StepGoal::NextBoundary, None)
    }

    /// Render a one-shot streaming response in one prepared process context.
    ///
    /// The synchronous helper has no async host handoff, so retaining the
    /// caller's state borrow for the complete traversal avoids both a state clone
    /// and per-boundary context teardown while preserving every transport flush.
    pub(crate) fn render_all(&mut self, call: SessionCall<'_, '_>, state: &Value) -> Result<()> {
        self.require_usable("render_streaming")?;
        if self.started {
            return Err(boundary_order_error(
                "render_streaming",
                "the streaming response has already started",
            ));
        }
        self.started = true;
        let handler = call.handler;
        let protocol = call.protocol;
        let result = self.with_context(call, state, |vm, context| {
            let mut status = vm.advance(StepGoal::NextBoundary, handler, protocol, context)?;
            while !status.done {
                context.writer.stream_flush()?;
                let Some(boundary) = status.boundary.as_ref() else {
                    return Err(HandlerError::Invariant(
                        "unfinished streaming step has no pending boundary".to_string(),
                    ));
                };
                vm.validate_resume(boundary.instance_id)?;
                vm.validate_resume_mode(BoundaryMode::Final)?;
                vm.begin_resume(boundary.instance_id, BoundaryMode::Final, context)?;
                status = vm.advance(StepGoal::NextBoundary, handler, protocol, context)?;
            }
            Ok(())
        });
        match result {
            Ok(()) if !self.shadow_style_roots.is_empty() => {
                self.failed = true;
                Err(HandlerError::Invariant(
                    "a Shadow CSS tree escaped its component instance".to_string(),
                ))
            }
            Ok(()) => {
                self.done = true;
                Ok(())
            }
            Err(error) => {
                self.failed = true;
                Err(error)
            }
        }
    }

    pub(crate) fn update(
        &mut self,
        call: SessionCall<'_, '_>,
        instance_id: BoundaryInstanceId,
        patch: &Value,
    ) -> Result<()> {
        self.require_usable("update")?;
        self.require_started("update")?;
        if self.done {
            return Err(boundary_order_error(
                "update",
                "the streaming response has already completed",
            ));
        }
        if !patch.is_object() {
            return Err(state_update_type_error());
        }
        let target = self.vm.validate_update(instance_id)?;
        let handler = call.handler;
        let result = self.with_context(call, patch, |_, context| {
            let sequence = super::streaming_state(context)?.next_record_sequence;
            handler.emit_streaming_state_update(sequence, target, context)?;
            increment_streaming_record_sequence("update", super::streaming_state(context)?)
        });
        if result.is_err() {
            self.failed = true;
        }
        result
    }

    fn run_step(
        &mut self,
        call: SessionCall<'_, '_>,
        goal: StepGoal,
        resume: Option<(BoundaryInstanceId, BoundaryMode)>,
    ) -> Result<StreamStatus> {
        let state = std::mem::replace(
            &mut self.frozen_state,
            Value::Object(serde_json::Map::new()),
        );
        let handler = call.handler;
        let protocol = call.protocol;
        let result = self.with_context(call, &state, |vm, context| {
            if let Some((instance_id, mode)) = resume {
                vm.begin_resume(instance_id, mode, context)?;
            }
            let status = vm.advance(goal, handler, protocol, context)?;
            if !status.done {
                context.writer.stream_flush()?;
            }
            Ok(status)
        });
        self.frozen_state = state;
        match result {
            Ok(status) => {
                if status.done && !self.shadow_style_roots.is_empty() {
                    self.failed = true;
                    return Err(HandlerError::Invariant(
                        "a Shadow CSS tree escaped its component instance".to_string(),
                    ));
                }
                self.done = status.done;
                self.awaiting_advance = goal == StepGoal::CommitBoundary && !status.done;
                Ok(status)
            }
            Err(error) => {
                self.failed = true;
                Err(error)
            }
        }
    }

    fn record_state_change(&mut self, changed: bool) -> Result<()> {
        if changed {
            increment_state_revision(self.streaming.as_mut().ok_or_else(missing_progress_error)?)?;
        }
        Ok(())
    }

    fn with_context<'data, 'state, T>(
        &mut self,
        call: SessionCall<'_, 'data>,
        state: &'state Value,
        operation: impl for<'output> FnOnce(
            &mut ContinuationVm,
            &mut WebUIProcessContext<'data, 'state, 'output>,
        ) -> Result<T>,
    ) -> Result<T> {
        let SessionCall {
            protocol,
            options,
            writer,
            ..
        } = call;
        let component_asset_style_manifest = protocol.component_asset_style_manifest()?;
        let progress = self.streaming.take().ok_or_else(missing_progress_error)?;
        let mut streaming =
            StreamingRenderState::from_progress(progress, protocol.component_reachability());
        let mut context = WebUIProcessContext {
            protocol: protocol.protocol(),
            render_fragments: protocol.render_fragments().resolve(protocol.protocol()),
            component_asset_style_manifest,
            component_asset_style_links: protocol.component_asset_style_links(),
            state,
            writer,
            local_vars: std::mem::take(&mut self.local_vars),
            local_borrowed_vars: super::super::BorrowedScope::default(),
            loop_vars: Vec::new(),
            visible_loop_scope: super::super::VisibleLoopScope::EMPTY,
            component_attrs: std::mem::take(&mut self.component_attrs),
            component_borrowed_attrs: super::super::BorrowedScope::default(),
            collecting_component_attrs: false,
            request_path: options.request_path,
            route_base: self
                .route_base
                .take()
                .map_or(Cow::Borrowed("/"), Cow::Owned),
            rendered_components: std::mem::take(&mut self.rendered_components),
            plugin: self.plugin.take(),
            route_children: std::mem::take(&mut self.route_children),
            entry_id: options.entry_id,
            nonce: options.nonce.filter(|nonce| !nonce.is_empty()),
            component_index: protocol.component_index(),
            style_resource_index: protocol.style_resource_index(),
            style_resources_requiring_escape: protocol.style_resources_requiring_escape(),
            style_chunk_index: protocol.protocol().style_chunk_index(),
            css_strategy: protocol.css_strategy(),
            head_inject: options.head_inject.filter(|html| !html.is_empty()),
            body_inject: options.body_inject.filter(|html| !html.is_empty()),
            state_inject: crate::StateInject::resolve(state),
            head_end_emitted: self.head_end_emitted,
            body_start_emitted: self.body_start_emitted,
            component_asset_styles_emitted: self.component_asset_styles_emitted,
            body_end_emitted: self.body_end_emitted,
            route_index: protocol.route_index(),
            route_chain_index: self.route_chain_index,
            route_chain: std::mem::take(&mut self.route_chain),
            route_document_style_targets: std::mem::take(&mut self.route_document_style_targets),
            reachable_components: std::mem::take(&mut self.reachable_components),
            streaming: Some(&mut streaming),
            json_scratch: std::mem::take(&mut self.json_scratch),
            scope_pool: std::mem::take(&mut self.scope_pool),
            document_style_resources: std::mem::take(&mut self.document_style_resources),
            shadow_style_roots: std::mem::take(&mut self.shadow_style_roots),
            borrowed_scope_pool: Vec::new(),
        };
        let result = operation(&mut self.vm, &mut context);
        self.local_vars = std::mem::take(&mut context.local_vars);
        self.component_attrs = std::mem::take(&mut context.component_attrs);
        self.route_base = match std::mem::replace(&mut context.route_base, Cow::Borrowed("/")) {
            Cow::Owned(base) => Some(base),
            Cow::Borrowed(_) => None,
        };
        self.rendered_components = std::mem::take(&mut context.rendered_components);
        self.plugin = context.plugin.take();
        self.route_children = Cow::Owned(std::mem::take(&mut context.route_children).into_owned());
        self.head_end_emitted = context.head_end_emitted;
        self.body_start_emitted = context.body_start_emitted;
        self.component_asset_styles_emitted = context.component_asset_styles_emitted;
        self.body_end_emitted = context.body_end_emitted;
        self.route_chain_index = context.route_chain_index;
        self.route_chain = std::mem::take(&mut context.route_chain);
        self.route_document_style_targets =
            std::mem::take(&mut context.route_document_style_targets);
        self.reachable_components = std::mem::take(&mut context.reachable_components);
        self.json_scratch = std::mem::take(&mut context.json_scratch);
        self.scope_pool = std::mem::take(&mut context.scope_pool);
        self.shadow_style_roots = std::mem::take(&mut context.shadow_style_roots);
        self.document_style_resources = std::mem::take(&mut context.document_style_resources);
        self.streaming = Some(streaming.into_progress());
        result
    }

    fn require_usable(&self, operation: &str) -> Result<()> {
        if self.failed {
            return Err(boundary_order_error(
                operation,
                "the session is poisoned by a previous render or transport failure; start a new response",
            ));
        }
        Ok(())
    }

    fn require_started(&self, operation: &str) -> Result<()> {
        if !self.started {
            return Err(boundary_order_error(
                operation,
                "start must be called before this operation",
            ));
        }
        Ok(())
    }

    /// Reject a `resume` that is out of order without writing a byte.
    ///
    /// Ordering is checked before any render work, so a rejected call leaves
    /// the response exactly as it was and the host can retry with the right
    /// instance ID or the missing `advance`.
    fn require_resumable(&self) -> Result<()> {
        self.require_usable("resume")?;
        self.require_started("resume")?;
        if self.done {
            return Err(boundary_order_error(
                "resume",
                "the streaming response has already completed",
            ));
        }
        if self.awaiting_advance {
            return Err(boundary_order_error(
                "resume",
                "the previously committed boundary has not been advanced past; call advance \
                 before resuming the next occurrence",
            ));
        }
        Ok(())
    }

    /// Reject an `advance` that is out of order without writing a byte.
    fn require_advanceable(&self) -> Result<()> {
        self.require_usable("advance")?;
        self.require_started("advance")?;
        if self.done {
            return Err(boundary_order_error(
                "advance",
                "the streaming response has already completed",
            ));
        }
        if !self.awaiting_advance {
            return Err(boundary_order_error(
                "advance",
                "there is no committed boundary to advance past; resume the pending occurrence \
                 first",
            ));
        }
        Ok(())
    }
}

#[cold]
#[inline(never)]
fn missing_progress_error() -> HandlerError {
    HandlerError::Invariant("streaming progress is unavailable".to_string())
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::{FlushWriter, ResponseWriter};
    use webui_parser::{ComponentRegistration, HtmlParser};
    use webui_protocol::{ComponentData, InitialStateStrategy, StateProjectionMode, WebUIProtocol};
    use webui_test_utils::test_json;

    const ISLAND_TAG: &str = "state-island";

    struct TestSink {
        output: String,
    }

    impl ResponseWriter for TestSink {
        fn write(&mut self, content: &str) -> Result<()> {
            self.output.push_str(content);
            Ok(())
        }

        fn end(&mut self) -> Result<()> {
            Ok(())
        }
    }

    impl FlushWriter for TestSink {
        fn flush(&mut self) -> Result<()> {
            Ok(())
        }
    }

    /// A sink whose bytes stay readable while the response holds it, so a test
    /// can prove a refused step wrote nothing.
    #[derive(Clone, Default)]
    struct SharedSink(std::rc::Rc<std::cell::RefCell<String>>);

    impl ResponseWriter for SharedSink {
        fn write(&mut self, content: &str) -> Result<()> {
            self.0.borrow_mut().push_str(content);
            Ok(())
        }

        fn end(&mut self) -> Result<()> {
            Ok(())
        }
    }

    impl FlushWriter for SharedSink {
        fn flush(&mut self) -> Result<()> {
            Ok(())
        }
    }

    /// Build a parser-produced entry with `boundaries` runtime occurrences,
    /// each hosting one island component.
    fn boundary_protocol(boundaries: usize, hydration_mode: StateProjectionMode) -> Protocol {
        let mut html = String::from("<!doctype html><html><head></head><body>");
        for sequence in 0..boundaries {
            html.push_str("<boundary name=\"b");
            html.push_str(&sequence.to_string());
            html.push_str("\"><article><");
            html.push_str(ISLAND_TAG);
            html.push_str("></");
            html.push_str(ISLAND_TAG);
            html.push_str("></article></boundary>");
        }
        html.push_str("</body></html>");

        let mut parser = HtmlParser::new();
        match parser
            .component_registry_mut()
            .register_component(ComponentRegistration::new(
                ISLAND_TAG,
                "<button>{{title}}</button>",
                None,
                true,
            )) {
            Ok(()) => {}
            Err(error) => panic!("registering the island failed: {error}"),
        }
        if let Err(error) = parser.parse("index.html", &html) {
            panic!("parsing the streaming entry failed: {error}");
        }
        let mut document = WebUIProtocol::new(parser.into_fragment_records());
        document.initial_state_strategy = InitialStateStrategy::Components as i32;
        document.components.insert(
            ISLAND_TAG.to_string(),
            ComponentData {
                template_json: r#"{"h":"<button></button>","th":1}"#.to_string(),
                uses_shadow_dom: true,
                hydration_mode: hydration_mode as i32,
                hydration_keys: if matches!(hydration_mode, StateProjectionMode::Keys) {
                    vec!["count".to_string(), "title".to_string()]
                } else {
                    Vec::new()
                },
                ..Default::default()
            },
        );
        document.populate_style_closures(&["index.html"]);
        Protocol::new(document)
    }

    /// A state whose payload is large enough that a per-boundary copy would be
    /// unmistakable in both time and allocation.
    fn large_state(rows: usize) -> Value {
        let mut items = Vec::with_capacity(rows);
        for row in 0..rows {
            items.push(test_json!({
                "id": row,
                "label": format!("row-{row}"),
                "tags": ["alpha", "beta", "gamma"],
            }));
        }
        test_json!({
            "count": 42,
            "title": "large state",
            "rows": items,
        })
    }

    /// Heap address of the retained snapshot's `rows` buffer, or `None` when
    /// the snapshot does not hold it.
    fn rows_address(state: &Value) -> Option<usize> {
        state
            .get("rows")
            .and_then(Value::as_array)
            .map(|rows| rows.as_ptr().addr())
    }

    fn options<'a>() -> RenderOptions<'a> {
        RenderOptions::new("index.html", "/")
    }

    #[test]
    fn session_parks_owned_routes_and_retains_scope_pool() -> Result<()> {
        let protocol = boundary_protocol(2, StateProjectionMode::Keys);
        let handler = WebUIHandler::new();
        let state = test_json!({ "count": 3, "title": "pooled" });
        let render_options = options();
        let mut sink = TestSink {
            output: String::new(),
        };
        let mut response = handler.stream_response(&protocol, &render_options, &mut sink)?;

        let first = response.start(&state)?;
        assert!(matches!(response.core.route_children, Cow::Owned(_)));
        let Some(boundary) = first.boundary else {
            panic!("the first boundary should suspend");
        };

        response.resume(boundary.instance_id, &state, BoundaryMode::Final)?;
        assert!(matches!(response.core.route_children, Cow::Owned(_)));
        let pooled_capacity = response
            .core
            .scope_pool
            .first()
            .map(crate::ScopeMap::capacity)
            .unwrap_or_else(|| panic!("the completed component should recycle its scope map"));

        response.advance()?;
        assert!(matches!(response.core.route_children, Cow::Owned(_)));
        assert_eq!(
            response
                .core
                .scope_pool
                .first()
                .map(crate::ScopeMap::capacity),
            Some(pooled_capacity),
            "a suspension step must preserve the scope-map pool"
        );
        Ok(())
    }

    #[test]
    fn render_streaming_projects_full_state_once_per_response() -> Result<()> {
        // Full-state protocols retain the caller's whole tree. Committing each
        // occurrence against the snapshot the response already holds must not
        // re-copy that tree, so the retained buffer keeps its identity for the
        // entire response no matter how many boundaries commit.
        let protocol = boundary_protocol(8, StateProjectionMode::All);
        let handler = WebUIHandler::new();
        let state = large_state(256);
        let render_options = options();
        let mut sink = TestSink {
            output: String::new(),
        };
        let mut response = handler.stream_response(&protocol, &render_options, &mut sink)?;

        let mut status = response.start(&state)?;
        let snapshot = rows_address(&response.core.frozen_state);
        assert!(
            snapshot.is_some(),
            "a full-state protocol must retain the caller's payload"
        );
        assert_ne!(
            snapshot,
            rows_address(&state),
            "the response owns its snapshot rather than borrowing the caller's tree"
        );

        let mut committed = 0usize;
        while !status.done {
            status = match status.boundary.as_ref() {
                Some(boundary) => {
                    let next =
                        response.resume(boundary.instance_id, &state, BoundaryMode::Final)?;
                    committed += 1;
                    assert!(
                        next.boundary.is_none() && !next.done,
                        "a commit step stops at its checkpoint and waits for advance"
                    );
                    assert_eq!(
                        rows_address(&response.core.frozen_state),
                        snapshot,
                        "committing occurrence {committed} must not re-copy the retained snapshot"
                    );
                    next
                }
                None => response.advance()?,
            };
        }
        assert_eq!(committed, 8, "every authored boundary must commit");

        // The one-shot helper fuses commit and advance into one continuation
        // setup, so its bytes must match the split steps it stands in for.
        let mut helper_sink = TestSink {
            output: String::new(),
        };
        handler.render_streaming(&protocol, &state, &render_options, &mut helper_sink)?;
        assert_eq!(
            helper_sink.output, sink.output,
            "render_streaming must resume against the retained snapshot"
        );
        Ok(())
    }

    #[test]
    fn fused_helper_matches_split_step_bytes_and_flushes() -> Result<()> {
        // `render_streaming` skips the host hand-off between a commit and the
        // parent bytes that follow it. That is a scheduling shortcut only: the
        // emitted bytes and the positions at which they are flushed must match
        // the split step machine exactly.
        #[derive(Default)]
        struct FlushSink {
            output: String,
            flushes: Vec<usize>,
        }

        impl ResponseWriter for FlushSink {
            fn write(&mut self, content: &str) -> Result<()> {
                self.output.push_str(content);
                Ok(())
            }

            fn end(&mut self) -> Result<()> {
                Ok(())
            }
        }

        impl FlushWriter for FlushSink {
            fn flush(&mut self) -> Result<()> {
                self.flushes.push(self.output.len());
                Ok(())
            }
        }

        let protocol = boundary_protocol(4, StateProjectionMode::Keys);
        let handler = WebUIHandler::new();
        let state = test_json!({ "count": 3, "title": "fused" });
        let render_options = options();

        let mut split = FlushSink::default();
        {
            let mut response = handler.stream_response(&protocol, &render_options, &mut split)?;
            let mut status = response.start(&state)?;
            while !status.done {
                status = match status.boundary.as_ref() {
                    Some(boundary) => {
                        response.resume(boundary.instance_id, &state, BoundaryMode::Final)?
                    }
                    None => response.advance()?,
                };
            }
        }

        let mut fused = FlushSink::default();
        handler.render_streaming(&protocol, &state, &render_options, &mut fused)?;

        assert_eq!(fused.output, split.output, "bytes must match");
        assert_eq!(fused.flushes, split.flushes, "flush positions must match");
        Ok(())
    }

    #[test]
    fn resume_overlays_new_state_and_reuses_unchanged_subtrees() -> Result<()> {
        // The public resume keeps its patch semantics: a changed key lands in
        // the snapshot, an omitted key survives, and an unchanged subtree is
        // left in place instead of being copied again.
        let protocol = boundary_protocol(2, StateProjectionMode::All);
        let handler = WebUIHandler::new();
        let state = large_state(64);
        let render_options = options();
        let mut sink = TestSink {
            output: String::new(),
        };
        let mut response = handler.stream_response(&protocol, &render_options, &mut sink)?;

        let status = response.start(&state)?;
        let snapshot = rows_address(&response.core.frozen_state);
        let Some(boundary) = status.boundary.as_ref() else {
            panic!("the first occurrence must suspend");
        };

        let mut next = state.clone();
        if let Some(object) = next.as_object_mut() {
            object.insert("title".to_string(), Value::String("second".to_string()));
            object.remove("count");
        }
        let status = response.resume(boundary.instance_id, &next, BoundaryMode::Final)?;

        assert_eq!(
            response.core.frozen_state.get("title"),
            Some(&Value::String("second".to_string())),
            "a changed key must land in the snapshot"
        );
        assert_eq!(
            response.core.frozen_state.get("count"),
            Some(&test_json!(42)),
            "an omitted key keeps the value the snapshot already holds"
        );
        assert_eq!(
            rows_address(&response.core.frozen_state),
            snapshot,
            "an unchanged subtree must not be copied again"
        );
        assert!(
            status.boundary.is_none() && !status.done,
            "the commit step stops at its checkpoint"
        );
        assert!(
            response.advance()?.boundary.is_some(),
            "the second occurrence follows the parent bytes"
        );
        Ok(())
    }

    #[test]
    fn semantic_steps_reuse_projection_scratch() -> Result<()> {
        // The record projection scratch lives in the retained progress as plain
        // integers, so after the first record every later step reuses the same
        // allocation instead of rebuilding one per step.
        let protocol = boundary_protocol(6, StateProjectionMode::Keys);
        let handler = WebUIHandler::new();
        let state = test_json!({ "count": 1, "title": "scratch", "unused": "x" });
        let render_options = options();
        let mut sink = TestSink {
            output: String::new(),
        };
        let mut response = handler.stream_response(&protocol, &render_options, &mut sink)?;

        let mut status = response.start(&state)?;
        let mut retained: Option<(usize, usize)> = None;
        while !status.done {
            let Some(boundary) = status.boundary.as_ref() else {
                status = response.advance()?;
                continue;
            };
            status = response.resume(boundary.instance_id, &state, BoundaryMode::Updatable)?;
            let Some(progress) = response.core.streaming.as_ref() else {
                panic!("a suspended response must retain its progress");
            };
            let observed = (
                progress.state_key_ids.capacity(),
                progress.state_key_ids.as_ptr().addr(),
            );
            assert!(
                observed.0 > 0,
                "the projection scratch must survive the record that filled it"
            );
            match retained {
                None => retained = Some(observed),
                Some(previous) => assert_eq!(
                    observed, previous,
                    "a later step must reuse the projection buffer, not allocate a new one"
                ),
            }
        }
        Ok(())
    }

    #[test]
    fn updates_reuse_their_committed_projection_buffer() -> Result<()> {
        // An update writes through the plan captured at commit time: no key
        // list is rebuilt, so the plan's buffer keeps its identity across every
        // update it serves.
        let protocol = boundary_protocol(2, StateProjectionMode::Keys);
        let handler = WebUIHandler::new();
        let state = test_json!({ "count": 1, "title": "updates" });
        let render_options = options();
        let mut sink = TestSink {
            output: String::new(),
        };
        let mut response = handler.stream_response(&protocol, &render_options, &mut sink)?;

        let status = response.start(&state)?;
        let Some(boundary) = status.boundary.as_ref() else {
            panic!("the first occurrence must suspend");
        };
        let instance_id = boundary.instance_id;
        response.resume(instance_id, &state, BoundaryMode::Updatable)?;

        let mut retained: Option<(usize, usize)> = None;
        for _ in 0..4 {
            response.update(instance_id, &state)?;
            let Some(progress) = response.core.streaming.as_ref() else {
                panic!("a live response must retain its progress");
            };
            let Some(Some(plan)) = progress.update_plans.first() else {
                panic!("an updatable occurrence must retain its projection plan");
            };
            let observed = (plan.key_ids.capacity(), plan.key_ids.as_ptr().addr());
            assert!(!plan.requires_full_state, "keyed islands project keys");
            assert!(observed.0 > 0, "the plan must retain its key buffer");
            match retained {
                None => retained = Some(observed),
                Some(previous) => assert_eq!(
                    observed, previous,
                    "every update must reuse the committed projection buffer"
                ),
            }
        }
        assert_eq!(
            sink.output.matches(",2,0,{").count(),
            4,
            "each update emits exactly one typed state-update record"
        );
        Ok(())
    }

    /// Drive `count` occurrences to completion in `mode`, returning the status
    /// the response stopped on.
    fn commit_occurrences<W: FlushWriter + ?Sized>(
        response: &mut StreamingResponse<'_, W>,
        status: StreamStatus,
        state: &Value,
        count: usize,
        mode: BoundaryMode,
    ) -> Result<StreamStatus> {
        let mut status = status;
        for committed in 0..count {
            let Some(boundary) = status.boundary.as_ref() else {
                panic!("occurrence {committed} must suspend before it can commit");
            };
            let instance_id = boundary.instance_id;
            response.resume(instance_id, state, mode)?;
            status = response.advance()?;
        }
        Ok(status)
    }

    #[test]
    fn updatable_commits_stop_at_the_browser_retention_cap() -> Result<()> {
        // The browser retains every updatable occurrence for the life of the
        // response and refuses the one past its cap, so the server refuses to
        // emit that checkpoint at all. The refusal lands before a byte is
        // written and before the caller's state reaches the snapshot, leaving
        // the same occurrence pending so the host can commit it as final.
        let protocol = boundary_protocol(MAX_UPDATABLE_OCCURRENCES + 1, StateProjectionMode::Keys);
        let handler = WebUIHandler::new();
        let state = test_json!({ "count": 1, "title": "retained" });
        let render_options = options();
        let mut sink = SharedSink::default();
        let written = SharedSink::clone(&sink).0;
        let mut response = handler.stream_response(&protocol, &render_options, &mut sink)?;

        let status = response.start(&state)?;
        let status = commit_occurrences(
            &mut response,
            status,
            &state,
            MAX_UPDATABLE_OCCURRENCES,
            BoundaryMode::Updatable,
        )?;

        let Some(boundary) = status.boundary.as_ref() else {
            panic!("the occurrence past the cap must suspend like any other");
        };
        let instance_id = boundary.instance_id;
        let bytes_before = written.borrow().len();
        let refused = test_json!({ "count": 2, "title": "refused" });
        let rejected = response.resume(instance_id, &refused, BoundaryMode::Updatable);
        match rejected {
            Err(HandlerError::StreamingBoundary(error)) => {
                assert_eq!(error.signal, "resume");
                assert!(
                    error.reason.contains("BoundaryMode::Final"),
                    "the refusal must name the recovery: {}",
                    error.reason
                );
            }
            _ => panic!("committing past the cap must fail with a typed boundary error"),
        }
        assert_eq!(
            written.borrow().len(),
            bytes_before,
            "a refused commit must not write a byte"
        );
        assert_eq!(
            response.core.frozen_state.get("title"),
            Some(&Value::String("retained".to_string())),
            "a refused commit must not merge its state into the snapshot"
        );

        // The occurrence is untouched, so the same ID commits as final.
        let status = response.resume(instance_id, &state, BoundaryMode::Final)?;
        assert!(
            status.boundary.is_none() && !status.done,
            "the retried commit stops at its own checkpoint"
        );
        assert!(
            written.borrow().len() > bytes_before,
            "the retry writes its checkpoint"
        );
        assert!(
            response.advance()?.done,
            "the response still reaches its terminal"
        );
        Ok(())
    }

    #[test]
    fn final_commits_do_not_consume_the_updatable_cap() -> Result<()> {
        // Only occurrences the browser retains count against the cap: a final
        // boundary releases its roots at hydration, so a response may commit
        // any number of them and still use its full updatable budget.
        let protocol = boundary_protocol(MAX_UPDATABLE_OCCURRENCES + 2, StateProjectionMode::Keys);
        let handler = WebUIHandler::new();
        let state = test_json!({ "count": 1, "title": "mixed" });
        let render_options = options();
        let mut sink = TestSink {
            output: String::new(),
        };
        let mut response = handler.stream_response(&protocol, &render_options, &mut sink)?;

        let status = response.start(&state)?;
        let status = commit_occurrences(&mut response, status, &state, 2, BoundaryMode::Final)?;
        let status = commit_occurrences(
            &mut response,
            status,
            &state,
            MAX_UPDATABLE_OCCURRENCES,
            BoundaryMode::Updatable,
        )?;
        assert!(
            status.done && response.is_done(),
            "final commits must leave the whole updatable budget available"
        );
        Ok(())
    }

    #[test]
    fn owned_sessions_enforce_the_same_updatable_cap() -> Result<()> {
        // The owned session shares the borrowed session's continuation, so the
        // cap, the refusal, and the final-mode retry behave identically.
        let protocol = Arc::new(boundary_protocol(
            MAX_UPDATABLE_OCCURRENCES + 1,
            StateProjectionMode::Keys,
        ));
        let mut session = crate::streaming::StreamingSession::new(
            Arc::new(WebUIHandler::new()),
            protocol,
            crate::streaming::SessionOptions::new("index.html", "/"),
        )?;
        let state = test_json!({ "count": 1, "title": "owned" });

        let mut step = session.start(&state)?;
        for committed in 0..MAX_UPDATABLE_OCCURRENCES {
            let Some(boundary) = step.boundary.as_ref() else {
                panic!("occurrence {committed} must suspend before it can commit");
            };
            let instance_id = boundary.instance_id;
            session.resume(instance_id, &state, BoundaryMode::Updatable)?;
            step = session.advance()?;
        }

        let Some(boundary) = step.boundary.as_ref() else {
            panic!("the occurrence past the cap must suspend like any other");
        };
        let instance_id = boundary.instance_id;
        assert!(
            session
                .resume(instance_id, &state, BoundaryMode::Updatable)
                .is_err(),
            "an owned session refuses the occurrence past the cap"
        );
        let step = session.resume(instance_id, &state, BoundaryMode::Final)?;
        assert!(
            !step.bytes.is_empty(),
            "the retried commit still delivers its checkpoint bytes"
        );
        assert!(
            session.advance()?.done,
            "the owned response still reaches its terminal"
        );
        Ok(())
    }
}