microsoft-webui-wasm 0.0.27

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

//! Handler-only WASM exports.

use crate::error::WasmError;
use js_sys::{Function, Object, Reflect, Uint8Array};
use serde_json::Value;
use std::sync::Arc;
use wasm_bindgen::prelude::*;
use webui_handler::plugin::fast_v2::FastV2HydrationPlugin;
use webui_handler::plugin::fast_v3::FastV3HydrationPlugin;
use webui_handler::plugin::webui::WebUIHydrationPlugin;
use webui_handler::{
    BoundaryDescriptor, BoundaryInstanceId, BoundaryKey, BoundaryMode, HandlerError,
    Protocol as HandlerProtocol, RenderOptions, ResponseWriter, SessionOptions,
    StreamStep as HandlerStreamStep, StreamingSession as HandlerStreamingSession, WebUIHandler,
};
#[cfg(test)]
use webui_protocol::WebUIProtocol;

const STREAM_CHUNK_SIZE: usize = 16 * 1024;

webui_handler::define_string_response_writer!(StringWriter, content);

/// A writer that batches rendered fragments before crossing into JavaScript.
struct CallbackWriter<'a> {
    on_chunk: &'a Function,
    buffer: String,
}

impl<'a> CallbackWriter<'a> {
    fn new(on_chunk: &'a Function) -> Self {
        Self {
            on_chunk,
            buffer: String::with_capacity(STREAM_CHUNK_SIZE),
        }
    }

    fn flush(&mut self) -> webui_handler::Result<()> {
        if self.buffer.is_empty() {
            return Ok(());
        }

        let chunk = std::mem::replace(&mut self.buffer, String::with_capacity(STREAM_CHUNK_SIZE));
        self.on_chunk
            .call1(&JsValue::UNDEFINED, &JsValue::from_str(&chunk))
            .map(|_| ())
            .map_err(|error| HandlerError::Writer(format!("{error:?}")))
    }
}

impl ResponseWriter for CallbackWriter<'_> {
    fn write(&mut self, content: &str) -> webui_handler::Result<()> {
        self.buffer.push_str(content);
        if self.buffer.len() >= STREAM_CHUNK_SIZE {
            self.flush()?;
        }
        Ok(())
    }

    fn write_attribute(&mut self, name: &str, value: &str) -> webui_handler::Result<()> {
        webui_handler::append_attribute_to_string(&mut self.buffer, name, value);
        if self.buffer.len() >= STREAM_CHUNK_SIZE {
            self.flush()?;
        }
        Ok(())
    }

    fn write_boolean_attribute(&mut self, name: &str) -> webui_handler::Result<()> {
        webui_handler::append_boolean_attribute_to_string(&mut self.buffer, name);
        if self.buffer.len() >= STREAM_CHUNK_SIZE {
            self.flush()?;
        }
        Ok(())
    }

    fn end(&mut self) -> webui_handler::Result<()> {
        self.flush()
    }
}

#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub(crate) enum HandlerPluginKind {
    FastV2,
    FastV3,
    WebUI,
}

impl HandlerPluginKind {
    fn parse(name: &str) -> Result<Self, WasmError> {
        match name {
            "fast" | "fast-v2" => Ok(Self::FastV2),
            "fast-v3" => Ok(Self::FastV3),
            "webui" => Ok(Self::WebUI),
            other => Err(WasmError::UnknownPlugin(other.to_string())),
        }
    }
}

struct WasmRenderOptions {
    entry: String,
    request_path: String,
}

impl Default for WasmRenderOptions {
    fn default() -> Self {
        Self {
            entry: "index.html".to_string(),
            request_path: "/".to_string(),
        }
    }
}

/// A decoded protocol with reusable indices for repeated WASM renders.
#[wasm_bindgen]
pub struct Protocol {
    inner: Arc<HandlerProtocol>,
    handler: Arc<WebUIHandler>,
}

#[wasm_bindgen]
impl Protocol {
    /// Decode protobuf bytes once for repeated rendering.
    #[wasm_bindgen(constructor)]
    pub fn new(protocol_bytes: &[u8], plugin: Option<String>) -> Result<Protocol, JsValue> {
        let plugin = parse_optional_plugin(plugin.as_deref())
            .map_err(|error| JsValue::from_str(&error.to_string()))?;
        let inner = HandlerProtocol::from_protobuf(protocol_bytes)
            .map_err(|error| JsValue::from_str(&format!("Protocol error: {error}")))?;
        Ok(Self {
            inner: Arc::new(inner),
            handler: Arc::new(create_handler(plugin)),
        })
    }

    /// Render from an existing JSON string.
    #[wasm_bindgen(js_name = render)]
    pub fn render(&self, state_json: &str, options: Option<Object>) -> Result<String, JsValue> {
        let options =
            parse_render_options(options).map_err(|error| JsValue::from_str(&error.to_string()))?;
        let state =
            parse_state_json(state_json).map_err(|error| JsValue::from_str(&error.to_string()))?;
        render_protocol_to_string_value(&self.handler, &self.inner, &state, &options)
            .map_err(|error| JsValue::from_str(&error.to_string()))
    }

    /// Stream from an existing JSON string in bounded chunks.
    #[wasm_bindgen(js_name = renderStream)]
    pub fn render_stream(
        &self,
        state_json: &str,
        on_chunk: &Function,
        options: Option<Object>,
    ) -> Result<(), JsValue> {
        let options =
            parse_render_options(options).map_err(|error| JsValue::from_str(&error.to_string()))?;
        let state =
            parse_state_json(state_json).map_err(|error| JsValue::from_str(&error.to_string()))?;
        render_protocol_to_callback_value(&self.handler, &self.inner, &state, &options, on_chunk)
            .map_err(|error| JsValue::from_str(&error.to_string()))
    }

    /// Produce a complete partial-navigation response.
    #[wasm_bindgen(js_name = renderPartial)]
    pub fn render_partial(
        &self,
        state_json: &str,
        entry_id: &str,
        request_path: &str,
        inventory_hex: &str,
    ) -> Result<String, JsValue> {
        self.inner
            .render_partial(state_json, entry_id, request_path, inventory_hex)
            .map_err(|error| JsValue::from_str(&format!("render_partial failed: {error}")))
    }

    /// Return component template payloads for requested component tags.
    #[wasm_bindgen(js_name = renderComponentTemplates)]
    pub fn render_component_templates(
        &self,
        component_tags: JsValue,
        inventory_hex: &str,
    ) -> Result<String, JsValue> {
        let tags: Vec<String> = serde_wasm_bindgen::from_value(component_tags)
            .map_err(|error| JsValue::from_str(&format!("invalid component tags: {error}")))?;
        let tag_refs: Vec<&str> = tags.iter().map(String::as_str).collect();
        let result = self
            .inner
            .render_component_templates(&tag_refs, inventory_hex)
            .map_err(|error| {
                JsValue::from_str(&format!("render_component_templates failed: {error}"))
            })?;
        serde_json::to_string(&result)
            .map_err(|error| JsValue::from_str(&format!("JSON serialize error: {error}")))
    }

    /// Return CSS token names in build order.
    #[wasm_bindgen(js_name = tokens)]
    pub fn tokens(&self) -> Result<JsValue, JsValue> {
        serde_wasm_bindgen::to_value(self.inner.tokens())
            .map_err(|error| JsValue::from_str(&format!("Serialization error: {error}")))
    }

    /// Open a host-driven progressive response for a streaming entry.
    ///
    /// Unlike `renderStream`, which pushes every chunk through one callback
    /// during a single synchronous call, the returned session hands each chunk
    /// back so the host owns the socket, the write order, and backpressure.
    #[wasm_bindgen(js_name = streamResponse)]
    pub fn stream_response(
        &self,
        entry: String,
        request_path: String,
        options: Option<Object>,
    ) -> Result<StreamingSession, JsValue> {
        let mut session_options = SessionOptions::new(entry, request_path);
        if let Some(options) = options {
            session_options.nonce = optional_string_property(&options, "nonce")?;
            session_options.head_inject = optional_string_property(&options, "headInject")?;
            session_options.body_inject = optional_string_property(&options, "bodyInject")?;
        }

        HandlerStreamingSession::new(
            Arc::clone(&self.handler),
            Arc::clone(&self.inner),
            session_options,
        )
        .map(|inner| StreamingSession { inner })
        .map_err(streaming_error)
    }
}

/// A progressive HTML response driven one semantic step at a time from JavaScript.
///
/// `start()`, `resume()`, and `advance()` return
/// `{ bytes, done, boundary? }`, where `bytes` is a `Uint8Array` and a boundary is
/// `{ instanceId, declarationId, owner, name, key }`. Boundary keys retain
/// their authored JSON type: strings are JavaScript strings and finite numbers
/// are JavaScript numbers.
///
/// ```js
/// const session = protocol.streamResponse('index.html', '/');
/// let step = session.start(JSON.stringify(shellState));
/// controller.enqueue(step.bytes);
/// while (!step.done) {
///   const { instanceId, name, key } = step.boundary;
///   const state = await loadBoundary(name, key);
///   step = session.resume(instanceId, JSON.stringify(state), 'updatable');
///   controller.enqueue(step.bytes);
///   step = session.advance();
///   controller.enqueue(step.bytes);
/// }
/// ```
#[wasm_bindgen]
pub struct StreamingSession {
    inner: HandlerStreamingSession,
}

#[wasm_bindgen]
impl StreamingSession {
    /// Render until the first runtime boundary occurrence or terminal.
    #[wasm_bindgen(js_name = start)]
    pub fn start(&mut self, state_json: &str) -> Result<Object, JsValue> {
        let state = session_state(state_json)?;
        let step = self.inner.start(&state).map_err(streaming_error)?;
        stream_step_object(step)
    }

    /// Commit the pending occurrence through its checkpoint, then stop.
    ///
    /// `mode` is `"final"` (default) or `"updatable"`. Only updatable
    /// boundaries accept later `update()` calls.
    #[wasm_bindgen(js_name = resume)]
    pub fn resume(
        &mut self,
        instance_id: u32,
        state_json: &str,
        mode: Option<String>,
    ) -> Result<Object, JsValue> {
        let state = session_state(state_json)?;
        let mode = parse_boundary_mode(mode.as_deref())?;
        let step = self
            .inner
            .resume(BoundaryInstanceId::from_raw(instance_id), &state, mode)
            .map_err(streaming_error)?;
        stream_step_object(step)
    }

    /// Write the parent bytes after the committed occurrence.
    ///
    /// Valid only after `resume()`. Returns the next boundary occurrence or
    /// completes the document tail.
    #[wasm_bindgen(js_name = advance)]
    pub fn advance(&mut self) -> Result<Object, JsValue> {
        let step = self.inner.advance().map_err(streaming_error)?;
        stream_step_object(step)
    }

    /// Push a projected state patch to a committed updatable boundary.
    #[wasm_bindgen(js_name = update)]
    pub fn update(&mut self, instance_id: u32, patch_json: &str) -> Result<Vec<u8>, JsValue> {
        let patch = session_state(patch_json)?;

        self.inner
            .update(BoundaryInstanceId::from_raw(instance_id), &patch)
            .map_err(streaming_error)
    }
}

fn stream_step_object(step: HandlerStreamStep) -> Result<Object, JsValue> {
    let result = Object::new();
    let bytes = Uint8Array::from(step.bytes.as_slice());
    set_object_property(&result, "bytes", bytes.as_ref())?;
    set_object_property(&result, "done", &JsValue::from_bool(step.done))?;
    if let Some(boundary) = step.boundary {
        let boundary = boundary_object(boundary)?;
        set_object_property(&result, "boundary", boundary.as_ref())?;
    }
    Ok(result)
}

fn boundary_object(boundary: BoundaryDescriptor) -> Result<Object, JsValue> {
    let result = Object::new();
    set_object_property(
        &result,
        "instanceId",
        &JsValue::from_f64(f64::from(boundary.instance_id.raw())),
    )?;
    set_object_property(
        &result,
        "declarationId",
        &JsValue::from_f64(f64::from(boundary.declaration_id)),
    )?;
    set_object_property(&result, "owner", &JsValue::from_str(&boundary.owner))?;
    set_object_property(&result, "name", &JsValue::from_str(&boundary.name))?;
    if let Some(key) = boundary.key {
        set_object_property(&result, "key", &boundary_key_value(key)?)?;
    }
    Ok(result)
}

fn boundary_key_value(key: BoundaryKey) -> Result<JsValue, JsValue> {
    match key {
        BoundaryKey::String(value) => Ok(JsValue::from_str(&value)),
        BoundaryKey::Number(value) => value.as_f64().map(JsValue::from_f64).ok_or_else(|| {
            JsValue::from_str("boundary key cannot be represented as a JavaScript number")
        }),
    }
}

fn set_object_property(object: &Object, key: &str, value: &JsValue) -> Result<(), JsValue> {
    let written = Reflect::set(object.as_ref(), &JsValue::from_str(key), value)
        .map_err(|_| JsValue::from_str(&format!("failed to set StreamStep '{key}' property")))?;
    if written {
        Ok(())
    } else {
        Err(JsValue::from_str(&format!(
            "failed to set StreamStep '{key}' property"
        )))
    }
}

fn session_state(state_json: &str) -> Result<Value, JsValue> {
    parse_state_json(state_json).map_err(|error| JsValue::from_str(&error.to_string()))
}

fn parse_boundary_mode(mode: Option<&str>) -> Result<BoundaryMode, JsValue> {
    match mode {
        None | Some("final") => Ok(BoundaryMode::Final),
        Some("updatable") => Ok(BoundaryMode::Updatable),
        Some(other) => Err(JsValue::from_str(&format!(
            "unknown boundary mode '{other}'; expected 'final' or 'updatable'"
        ))),
    }
}

fn streaming_error(error: HandlerError) -> JsValue {
    JsValue::from_str(&error.to_string())
}

fn optional_string_property(options: &Object, key: &str) -> Result<Option<String>, JsValue> {
    let value = Reflect::get(options, &JsValue::from_str(key))
        .map_err(|_| JsValue::from_str(&format!("failed to read '{key}' option")))?;
    if value.is_undefined() || value.is_null() {
        return Ok(None);
    }
    value
        .as_string()
        .map(Some)
        .ok_or_else(|| JsValue::from_str(&format!("'{key}' must be a string")))
}

#[cfg(test)]
pub(crate) fn render_protocol_to_string(
    protocol: &WebUIProtocol,
    state_json: &str,
    entry: &str,
    request_path: &str,
    plugin: Option<HandlerPluginKind>,
) -> Result<String, WasmError> {
    let state = parse_state_json(state_json)?;
    let options = WasmRenderOptions {
        entry: entry.to_string(),
        request_path: request_path.to_string(),
    };
    let protocol = HandlerProtocol::new(protocol.clone());
    let handler = create_handler(plugin);
    render_protocol_to_string_value(&handler, &protocol, &state, &options)
}

fn parse_state_json(state_json: &str) -> Result<Value, WasmError> {
    serde_json::from_str(state_json).map_err(WasmError::State)
}

fn render_protocol_to_string_value(
    handler: &WebUIHandler,
    protocol: &HandlerProtocol,
    state: &Value,
    options: &WasmRenderOptions,
) -> Result<String, WasmError> {
    let mut writer = StringWriter::with_capacity(4096);
    handler.render(
        protocol,
        state,
        &RenderOptions::new(&options.entry, &options.request_path),
        &mut writer,
    )?;
    Ok(writer.content)
}

fn render_protocol_to_callback_value(
    handler: &WebUIHandler,
    protocol: &HandlerProtocol,
    state: &Value,
    options: &WasmRenderOptions,
    on_chunk: &Function,
) -> Result<(), WasmError> {
    let mut writer = CallbackWriter::new(on_chunk);
    handler.render(
        protocol,
        state,
        &RenderOptions::new(&options.entry, &options.request_path),
        &mut writer,
    )?;
    writer.flush()?;
    Ok(())
}

pub(crate) fn parse_optional_plugin(
    plugin: Option<&str>,
) -> Result<Option<HandlerPluginKind>, WasmError> {
    plugin.map(HandlerPluginKind::parse).transpose()
}

fn parse_render_options(options: Option<Object>) -> Result<WasmRenderOptions, WasmError> {
    let mut parsed = WasmRenderOptions::default();
    let Some(options) = options else {
        return Ok(parsed);
    };

    if let Some(entry) = optional_string_field(options.as_ref(), "entry")? {
        parsed.entry = entry;
    }
    if let Some(request_path) = optional_string_field(options.as_ref(), "requestPath")? {
        parsed.request_path = request_path;
    }
    Ok(parsed)
}

fn optional_string_field(options: &JsValue, field: &str) -> Result<Option<String>, WasmError> {
    let value = Reflect::get(options, &JsValue::from_str(field)).map_err(|_| {
        WasmError::InvalidOptions(format!("failed to read `{field}` from options object"))
    })?;
    if value.is_null() || value.is_undefined() {
        return Ok(None);
    }
    value.as_string().map(Some).ok_or_else(|| {
        WasmError::InvalidOptions(format!("`{field}` must be a string when provided"))
    })
}

fn create_handler(plugin: Option<HandlerPluginKind>) -> WebUIHandler {
    match plugin {
        Some(HandlerPluginKind::FastV2) => {
            WebUIHandler::with_plugin(|| Box::new(FastV2HydrationPlugin::new()))
        }
        Some(HandlerPluginKind::FastV3) => {
            WebUIHandler::with_plugin(|| Box::new(FastV3HydrationPlugin::new()))
        }
        Some(HandlerPluginKind::WebUI) => {
            WebUIHandler::with_plugin(|| Box::new(WebUIHydrationPlugin::new()))
        }
        None => WebUIHandler::new(),
    }
}

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

    fn structural_fragment(value: &str) -> webui_protocol::WebUIFragment {
        let mut token = String::with_capacity("}}}webui:".len() + value.len());
        token.push_str("}}}webui:");
        token.push_str(value);
        webui_protocol::WebUIFragment::signal(token, true)
    }

    #[test]
    fn parse_plugin_keeps_fast_aliases_parser_free() {
        assert_eq!(
            parse_optional_plugin(Some("fast")).unwrap(),
            Some(HandlerPluginKind::FastV2)
        );
        assert_eq!(
            parse_optional_plugin(Some("fast-v2")).unwrap(),
            Some(HandlerPluginKind::FastV2)
        );
        assert_eq!(
            parse_optional_plugin(Some("fast-v3")).unwrap(),
            Some(HandlerPluginKind::FastV3)
        );
        assert_eq!(
            parse_optional_plugin(Some("webui")).unwrap(),
            Some(HandlerPluginKind::WebUI)
        );
    }

    #[test]
    fn parse_plugin_rejects_unknown_names() {
        let err = parse_optional_plugin(Some("unknown")).unwrap_err();
        assert_eq!(
            err.to_string(),
            "Unknown plugin: unknown. Use \"webui\", \"fast-v3\", \"fast-v2\", or \"fast\"."
        );
    }

    #[test]
    fn protocol_reuses_decoded_protocol() {
        use std::collections::HashMap;
        use webui_protocol::{FragmentList, WebUIFragment};

        let mut fragments = HashMap::new();
        fragments.insert(
            "index.html".to_string(),
            FragmentList {
                fragments: vec![WebUIFragment::signal("name".to_string(), true)],
                contains_boundary: false,
            },
        );
        let bytes = WebUIProtocol::new(fragments)
            .to_protobuf()
            .expect("protocol should serialize");
        let protocol = Protocol::new(&bytes, None).expect("protocol should load");

        let first = protocol
            .render(r#"{"name":"first"}"#, None)
            .expect("first render should succeed");
        let second = protocol
            .render(r#"{"name":"second"}"#, None)
            .expect("second render should succeed");

        assert_eq!(first, "first");
        assert_eq!(second, "second");
    }

    #[test]
    fn render_projects_state_to_component_hydration_keys() {
        use std::collections::HashMap;
        use webui_protocol::{
            ComponentData, FragmentList, InitialStateStrategy, StateProjectionMode, WebUIFragment,
        };

        let mut fragments = HashMap::new();
        fragments.insert(
            "index.html".to_string(),
            FragmentList {
                fragments: vec![
                    WebUIFragment::raw("<html><head>"),
                    structural_fragment("head_end"),
                    WebUIFragment::raw("</head><body>"),
                    WebUIFragment::component("client-card"),
                    structural_fragment("body_end"),
                    WebUIFragment::raw("</body></html>"),
                ],
                contains_boundary: false,
            },
        );
        fragments.insert(
            "client-card".to_string(),
            FragmentList {
                fragments: vec![WebUIFragment::raw("<p>client</p>")],
                contains_boundary: false,
            },
        );
        let mut protocol = WebUIProtocol::new(fragments);
        protocol.initial_state_strategy = InitialStateStrategy::Components as i32;
        protocol.components.insert(
            "client-card".to_string(),
            ComponentData {
                hydration_mode: StateProjectionMode::Keys as i32,
                hydration_keys: vec!["kept".to_string()],
                ..Default::default()
            },
        );

        let rendered = render_protocol_to_string(
            &protocol,
            r#"{"kept":"KEPT_VALUE_WASM","dropped":"DROPPED_VALUE_WASM"}"#,
            "index.html",
            "/",
            Some(HandlerPluginKind::WebUI),
        )
        .expect("render should succeed");

        // Only the hydratable key reaches the bootstrap state block...
        assert!(
            rendered.contains(r#""kept":"KEPT_VALUE_WASM""#),
            "hydratable key missing from bootstrap state:\n{rendered}"
        );
        // ...the non-hydratable key is projected out entirely.
        assert!(
            !rendered.contains("DROPPED_VALUE_WASM"),
            "server-only value leaked into render:\n{rendered}"
        );
        assert!(
            !rendered.contains("dropped"),
            "server-only key name leaked into render:\n{rendered}"
        );
    }

    #[cfg(feature = "parser")]
    mod streaming_tests {
        use std::collections::HashMap;

        use super::*;

        fn session(html: &str) -> HandlerStreamingSession {
            let files = HashMap::from([("index.html".to_string(), html.to_string())]);
            let protocol = crate::parser::parse_to_protocol(&files, "index.html", &[])
                .expect("protocol should parse");
            HandlerStreamingSession::new(
                Arc::new(WebUIHandler::new()),
                Arc::new(HandlerProtocol::new(protocol)),
                SessionOptions::new("index.html", "/"),
            )
            .expect("session should open")
        }

        fn state(json: &str) -> Value {
            parse_state_json(json).expect("state should parse")
        }

        #[test]
        fn streaming_steps_preserve_key_types_and_checkpoint_segments() {
            let mut session = session(concat!(
                "<html><head></head><body>",
                r#"<boundary name="first" key="{{firstId}}"><p>{{firstLabel}}</p></boundary>"#,
                "<span>between</span>",
                r#"<boundary name="second" key="{{secondId}}"><p>{{secondLabel}}</p></boundary>"#,
                "<footer>tail</footer>",
                "</body></html>",
            ));
            let state =
                state(r#"{"firstId":"alpha","firstLabel":"a","secondId":20,"secondLabel":"b"}"#);

            let first = session
                .start(&state)
                .expect("start should discover first boundary");
            assert!(!first.done);
            assert!(!first.bytes.is_empty());
            let first_boundary = first.boundary.expect("first boundary should be returned");
            assert_eq!(first_boundary.instance_id.raw(), 0);
            assert_eq!(first_boundary.declaration_id, 0);
            assert_eq!(first_boundary.owner.as_ref(), "index.html");
            assert_eq!(first_boundary.name.as_ref(), "first");
            assert_eq!(
                first_boundary.key,
                Some(BoundaryKey::String("alpha".to_string()))
            );

            let resumed = session
                .resume(first_boundary.instance_id, &state, BoundaryMode::Final)
                .expect("resume should commit first boundary");
            assert!(!resumed.done);
            assert!(resumed.boundary.is_none());
            let resumed_text =
                std::str::from_utf8(&resumed.bytes).expect("resume output should be UTF-8");
            assert!(resumed_text.contains(">a<"));
            assert!(!resumed_text.contains("between"));

            let next = session
                .advance()
                .expect("advance should discover second boundary");
            assert!(!next.done);
            let next_text =
                std::str::from_utf8(&next.bytes).expect("advance output should be UTF-8");
            assert!(next_text.contains("between"));
            assert!(!next_text.contains(">b<"));
            let second_boundary = next.boundary.expect("second boundary should be returned");
            assert_eq!(second_boundary.instance_id.raw(), 1);
            assert_eq!(second_boundary.declaration_id, 1);
            assert_eq!(second_boundary.name.as_ref(), "second");
            assert_eq!(second_boundary.key, Some(BoundaryKey::Number(20.into())));

            let resumed = session
                .resume(second_boundary.instance_id, &state, BoundaryMode::Final)
                .expect("resume should commit second boundary");
            assert!(!resumed.done);
            assert!(resumed.boundary.is_none());
            let resumed_text =
                std::str::from_utf8(&resumed.bytes).expect("resume output should be UTF-8");
            assert!(resumed_text.contains(">b<"));
            assert!(!resumed_text.contains("tail"));

            let done = session.advance().expect("final advance should complete");
            assert!(done.done);
            assert!(done.boundary.is_none());
            assert!(std::str::from_utf8(&done.bytes)
                .expect("advance output should be UTF-8")
                .contains("tail"));
        }

        #[test]
        fn streaming_update_returns_bytes_for_updatable_occurrence() {
            let mut session = session(concat!(
                "<html><head></head><body>",
                r#"<boundary name="first"><p>{{count}}</p></boundary>"#,
                r#"<boundary name="second"><p>done</p></boundary>"#,
                "</body></html>",
            ));
            let initial = state(r#"{"count":1}"#);
            let first = session
                .start(&initial)
                .expect("start should discover first boundary")
                .boundary
                .expect("first boundary should be returned");
            let resumed = session
                .resume(first.instance_id, &initial, BoundaryMode::Updatable)
                .expect("resume should commit updatable boundary");
            assert!(!resumed.done);
            assert!(resumed.boundary.is_none());

            let update = session
                .update(first.instance_id, &state(r#"{"count":2}"#))
                .expect("update should render");
            assert!(!update.is_empty());
            assert!(std::str::from_utf8(&update)
                .expect("update should be UTF-8")
                .contains(r#""count":2"#));

            let second = session
                .advance()
                .expect("advance should discover second boundary")
                .boundary
                .expect("second boundary should be returned");
            let resumed = session
                .resume(second.instance_id, &state("{}"), BoundaryMode::Final)
                .expect("second resume should commit boundary");
            assert!(!resumed.done);
            assert!(resumed.boundary.is_none());
            let done = session.advance().expect("final advance should complete");
            assert!(done.done);
        }

        #[test]
        fn streaming_advance_rejects_out_of_order_calls() {
            let mut session = session(concat!(
                "<html><head></head><body>",
                r#"<boundary name="first"><p>first</p></boundary>"#,
                "</body></html>",
            ));

            let before_start = session
                .advance()
                .expect_err("advance before start should fail");
            assert!(before_start
                .to_string()
                .contains("start must be called before this operation"));

            let start = session.start(&state("{}")).expect("start should succeed");
            let before_resume = session
                .advance()
                .expect_err("advance before resume should fail");
            assert!(before_resume
                .to_string()
                .contains("there is no committed boundary to advance past"));

            let boundary = start.boundary.expect("first boundary should be returned");
            session
                .resume(boundary.instance_id, &state("{}"), BoundaryMode::Final)
                .expect("resume should still succeed after rejected advance");
            assert!(session.advance().expect("advance should complete").done);
        }

        #[test]
        fn streaming_start_returns_done_for_boundary_free_document() {
            let mut session = session("<html><head></head><body><p>done</p></body></html>");

            let step = session
                .start(&state("{}"))
                .expect("boundary-free start should complete");
            assert!(step.done);
            assert!(step.boundary.is_none());
            assert!(std::str::from_utf8(&step.bytes)
                .expect("output should be UTF-8")
                .contains("<p>done</p>"));
        }
    }
}