servo/components/script/dom/htmliframeelement.rs

/* This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at https://mozilla.org/MPL/2.0/. */

use crate::document_loader::{LoadBlocker, LoadType};
use crate::dom::attr::Attr;
use crate::dom::bindings::cell::DomRefCell;
use crate::dom::bindings::codegen::Bindings::HTMLIFrameElementBinding;
use crate::dom::bindings::codegen::Bindings::HTMLIFrameElementBinding::HTMLIFrameElementMethods;
use crate::dom::bindings::codegen::Bindings::WindowBinding::WindowBinding::WindowMethods;
use crate::dom::bindings::inheritance::Castable;
use crate::dom::bindings::refcounted::Trusted;
use crate::dom::bindings::reflector::DomObject;
use crate::dom::bindings::root::{DomRoot, LayoutDom, MutNullableDom};
use crate::dom::bindings::str::{DOMString, USVString};
use crate::dom::document::Document;
use crate::dom::domtokenlist::DOMTokenList;
use crate::dom::element::{AttributeMutation, Element, RawLayoutElementHelpers};
use crate::dom::eventtarget::EventTarget;
use crate::dom::globalscope::GlobalScope;
use crate::dom::htmlelement::HTMLElement;
use crate::dom::node::{
    document_from_node, window_from_node, BindContext, Node, NodeDamage, UnbindContext,
};
use crate::dom::virtualmethods::VirtualMethods;
use crate::dom::window::ReflowReason;
use crate::dom::windowproxy::WindowProxy;
use crate::script_thread::ScriptThread;
use crate::task_source::TaskSource;
use dom_struct::dom_struct;
use euclid::Size2D;
use html5ever::{LocalName, Prefix};
use ipc_channel::ipc;
use msg::constellation_msg::{BrowsingContextId, PipelineId, TopLevelBrowsingContextId};
use net_traits::request::Referrer;
use profile_traits::ipc as ProfiledIpc;
use script_layout_interface::message::ReflowGoal;
use script_traits::IFrameSandboxState::{IFrameSandboxed, IFrameUnsandboxed};
use script_traits::{
    HistoryEntryReplacement, IFrameLoadInfo, IFrameLoadInfoWithData, JsEvalResult, LoadData,
    LoadOrigin, UpdatePipelineIdReason, WindowSizeData,
};
use script_traits::{NewLayoutInfo, ScriptMsg};
use servo_url::ServoUrl;
use std::cell::Cell;
use style::attr::{AttrValue, LengthOrPercentageOrAuto};

bitflags! {
    #[derive(JSTraceable, MallocSizeOf)]
    struct SandboxAllowance: u8 {
        const ALLOW_NOTHING = 0x00;
        const ALLOW_SAME_ORIGIN = 0x01;
        const ALLOW_TOP_NAVIGATION = 0x02;
        const ALLOW_FORMS = 0x04;
        const ALLOW_SCRIPTS = 0x08;
        const ALLOW_POINTER_LOCK = 0x10;
        const ALLOW_POPUPS = 0x20;
    }
}

#[derive(PartialEq)]
pub enum NavigationType {
    InitialAboutBlank,
    Regular,
}

#[derive(PartialEq)]
enum ProcessingMode {
    FirstTime,
    NotFirstTime,
}

#[dom_struct]
pub struct HTMLIFrameElement {
    htmlelement: HTMLElement,
    top_level_browsing_context_id: Cell<Option<TopLevelBrowsingContextId>>,
    browsing_context_id: Cell<Option<BrowsingContextId>>,
    pipeline_id: Cell<Option<PipelineId>>,
    pending_pipeline_id: Cell<Option<PipelineId>>,
    about_blank_pipeline_id: Cell<Option<PipelineId>>,
    sandbox: MutNullableDom<DOMTokenList>,
    sandbox_allowance: Cell<Option<SandboxAllowance>>,
    load_blocker: DomRefCell<Option<LoadBlocker>>,
    visibility: Cell<bool>,
    name: DomRefCell<DOMString>,
}

impl HTMLIFrameElement {
    pub fn is_sandboxed(&self) -> bool {
        self.sandbox_allowance.get().is_some()
    }

    /// <https://html.spec.whatwg.org/multipage/#otherwise-steps-for-iframe-or-frame-elements>,
    /// step 1.
    fn get_url(&self) -> ServoUrl {
        let element = self.upcast::<Element>();
        element
            .get_attribute(&ns!(), &local_name!("src"))
            .and_then(|src| {
                let url = src.value();
                if url.is_empty() {
                    None
                } else {
                    document_from_node(self).base_url().join(&url).ok()
                }
            })
            .unwrap_or_else(|| ServoUrl::parse("about:blank").unwrap())
    }

    pub fn navigate_or_reload_child_browsing_context(
        &self,
        mut load_data: LoadData,
        nav_type: NavigationType,
        replace: HistoryEntryReplacement,
    ) {
        let sandboxed = if self.is_sandboxed() {
            IFrameSandboxed
        } else {
            IFrameUnsandboxed
        };

        let browsing_context_id = match self.browsing_context_id() {
            None => return warn!("Navigating unattached iframe."),
            Some(id) => id,
        };

        let top_level_browsing_context_id = match self.top_level_browsing_context_id() {
            None => return warn!("Navigating unattached iframe."),
            Some(id) => id,
        };

        let document = document_from_node(self);

        let mut load_blocker = self.load_blocker.borrow_mut();
        // Any oustanding load is finished from the point of view of the blocked
        // document; the new navigation will continue blocking it.
        LoadBlocker::terminate(&mut load_blocker);

        if load_data.url.scheme() == "javascript" {
            let window_proxy = self.GetContentWindow();
            if let Some(window_proxy) = window_proxy {
                // Important re security. See https://github.com/servo/servo/issues/23373
                // TODO: check according to https://w3c.github.io/webappsec-csp/#should-block-navigation-request
                if ScriptThread::check_load_origin(&load_data.load_origin, &document.url().origin())
                {
                    ScriptThread::eval_js_url(&window_proxy.global(), &mut load_data);
                }
            }
        }

        match load_data.js_eval_result {
            Some(JsEvalResult::NoContent) => (),
            _ => {
                *load_blocker = Some(LoadBlocker::new(
                    &*document,
                    LoadType::Subframe(load_data.url.clone()),
                ));
            },
        };

        let window = window_from_node(self);
        let old_pipeline_id = self.pipeline_id();
        let new_pipeline_id = PipelineId::new();
        self.pending_pipeline_id.set(Some(new_pipeline_id));

        let global_scope = window.upcast::<GlobalScope>();
        let load_info = IFrameLoadInfo {
            parent_pipeline_id: global_scope.pipeline_id(),
            browsing_context_id: browsing_context_id,
            top_level_browsing_context_id: top_level_browsing_context_id,
            new_pipeline_id: new_pipeline_id,
            is_private: false, // FIXME
            replace: replace,
        };

        match nav_type {
            NavigationType::InitialAboutBlank => {
                let (pipeline_sender, pipeline_receiver) = ipc::channel().unwrap();

                self.about_blank_pipeline_id.set(Some(new_pipeline_id));

                let load_info = IFrameLoadInfoWithData {
                    info: load_info,
                    load_data: load_data.clone(),
                    old_pipeline_id: old_pipeline_id,
                    sandbox: sandboxed,
                };
                global_scope
                    .script_to_constellation_chan()
                    .send(ScriptMsg::ScriptNewIFrame(load_info, pipeline_sender))
                    .unwrap();

                let new_layout_info = NewLayoutInfo {
                    parent_info: Some(global_scope.pipeline_id()),
                    new_pipeline_id: new_pipeline_id,
                    browsing_context_id: browsing_context_id,
                    top_level_browsing_context_id: top_level_browsing_context_id,
                    opener: None,
                    load_data: load_data,
                    pipeline_port: pipeline_receiver,
                    window_size: WindowSizeData {
                        initial_viewport: {
                            let rect = self.upcast::<Node>().bounding_content_box_or_zero();
                            Size2D::new(rect.size.width.to_f32_px(), rect.size.height.to_f32_px())
                        },
                        device_pixel_ratio: window.device_pixel_ratio(),
                    },
                };

                self.pipeline_id.set(Some(new_pipeline_id));
                ScriptThread::process_attach_layout(new_layout_info, document.origin().clone());
            },
            NavigationType::Regular => {
                let load_info = IFrameLoadInfoWithData {
                    info: load_info,
                    load_data: load_data,
                    old_pipeline_id: old_pipeline_id,
                    sandbox: sandboxed,
                };
                global_scope
                    .script_to_constellation_chan()
                    .send(ScriptMsg::ScriptLoadedURLInIFrame(load_info))
                    .unwrap();
            },
        }
    }

    /// <https://html.spec.whatwg.org/multipage/#process-the-iframe-attributes>
    fn process_the_iframe_attributes(&self, mode: ProcessingMode) {
        // TODO: srcdoc

        let window = window_from_node(self);

        // https://html.spec.whatwg.org/multipage/#attr-iframe-name
        // Note: the spec says to set the name 'when the nested browsing context is created'.
        // The current implementation sets the name on the window,
        // when the iframe attributes are first processed.
        if mode == ProcessingMode::FirstTime {
            if let Some(window) = self.GetContentWindow() {
                window.set_name(self.name.borrow().clone())
            }
        }

        // https://github.com/whatwg/html/issues/490
        if mode == ProcessingMode::FirstTime &&
            !self.upcast::<Element>().has_attribute(&local_name!("src"))
        {
            let this = Trusted::new(self);
            let pipeline_id = self.pipeline_id().unwrap();
            // FIXME(nox): Why are errors silenced here?
            let _ = window.task_manager().dom_manipulation_task_source().queue(
                task!(iframe_load_event_steps: move || {
                    this.root().iframe_load_event_steps(pipeline_id);
                }),
                window.upcast(),
            );
            return;
        }

        let url = self.get_url();

        // TODO: check ancestor browsing contexts for same URL

        let creator_pipeline_id = if url.as_str() == "about:blank" {
            Some(window.upcast::<GlobalScope>().pipeline_id())
        } else {
            None
        };

        let document = document_from_node(self);
        let load_data = LoadData::new(
            LoadOrigin::Script(document.origin().immutable().clone()),
            url,
            creator_pipeline_id,
            Some(Referrer::ReferrerUrl(document.url())),
            document.get_referrer_policy(),
        );

        let pipeline_id = self.pipeline_id();
        // If the initial `about:blank` page is the current page, load with replacement enabled,
        // see https://html.spec.whatwg.org/multipage/#the-iframe-element:about:blank-3
        let is_about_blank =
            pipeline_id.is_some() && pipeline_id == self.about_blank_pipeline_id.get();
        let replace = if is_about_blank {
            HistoryEntryReplacement::Enabled
        } else {
            HistoryEntryReplacement::Disabled
        };
        self.navigate_or_reload_child_browsing_context(load_data, NavigationType::Regular, replace);
    }

    fn create_nested_browsing_context(&self) {
        // Synchronously create a new context and navigate it to about:blank.
        let url = ServoUrl::parse("about:blank").unwrap();
        let document = document_from_node(self);
        let window = window_from_node(self);
        let pipeline_id = Some(window.upcast::<GlobalScope>().pipeline_id());
        let load_data = LoadData::new(
            LoadOrigin::Script(document.origin().immutable().clone()),
            url,
            pipeline_id,
            Some(Referrer::ReferrerUrl(document.url().clone())),
            document.get_referrer_policy(),
        );
        let browsing_context_id = BrowsingContextId::new();
        let top_level_browsing_context_id = window.window_proxy().top_level_browsing_context_id();
        self.pipeline_id.set(None);
        self.pending_pipeline_id.set(None);
        self.top_level_browsing_context_id
            .set(Some(top_level_browsing_context_id));
        self.browsing_context_id.set(Some(browsing_context_id));
        self.navigate_or_reload_child_browsing_context(
            load_data,
            NavigationType::InitialAboutBlank,
            HistoryEntryReplacement::Disabled,
        );
    }

    fn destroy_nested_browsing_context(&self) {
        self.pipeline_id.set(None);
        self.pending_pipeline_id.set(None);
        self.about_blank_pipeline_id.set(None);
        self.top_level_browsing_context_id.set(None);
        self.browsing_context_id.set(None);
    }

    pub fn update_pipeline_id(&self, new_pipeline_id: PipelineId, reason: UpdatePipelineIdReason) {
        if self.pending_pipeline_id.get() != Some(new_pipeline_id) &&
            reason == UpdatePipelineIdReason::Navigation
        {
            return;
        }

        self.pipeline_id.set(Some(new_pipeline_id));

        // Only terminate the load blocker if the pipeline id was updated due to a traversal.
        // The load blocker will be terminated for a navigation in iframe_load_event_steps.
        if reason == UpdatePipelineIdReason::Traversal {
            let mut blocker = self.load_blocker.borrow_mut();
            LoadBlocker::terminate(&mut blocker);
        }

        self.upcast::<Node>().dirty(NodeDamage::OtherNodeDamage);
        let window = window_from_node(self);
        window.reflow(ReflowGoal::Full, ReflowReason::FramedContentChanged);
    }

    fn new_inherited(
        local_name: LocalName,
        prefix: Option<Prefix>,
        document: &Document,
    ) -> HTMLIFrameElement {
        HTMLIFrameElement {
            htmlelement: HTMLElement::new_inherited(local_name, prefix, document),
            browsing_context_id: Cell::new(None),
            top_level_browsing_context_id: Cell::new(None),
            pipeline_id: Cell::new(None),
            pending_pipeline_id: Cell::new(None),
            about_blank_pipeline_id: Cell::new(None),
            sandbox: Default::default(),
            sandbox_allowance: Cell::new(None),
            load_blocker: DomRefCell::new(None),
            visibility: Cell::new(true),
            name: DomRefCell::new(DOMString::new()),
        }
    }

    #[allow(unrooted_must_root)]
    pub fn new(
        local_name: LocalName,
        prefix: Option<Prefix>,
        document: &Document,
    ) -> DomRoot<HTMLIFrameElement> {
        Node::reflect_node(
            Box::new(HTMLIFrameElement::new_inherited(
                local_name, prefix, document,
            )),
            document,
            HTMLIFrameElementBinding::Wrap,
        )
    }

    #[inline]
    pub fn pipeline_id(&self) -> Option<PipelineId> {
        self.pipeline_id.get()
    }

    #[inline]
    pub fn browsing_context_id(&self) -> Option<BrowsingContextId> {
        self.browsing_context_id.get()
    }

    #[inline]
    pub fn top_level_browsing_context_id(&self) -> Option<TopLevelBrowsingContextId> {
        self.top_level_browsing_context_id.get()
    }

    pub fn change_visibility_status(&self, visibility: bool) {
        if self.visibility.get() != visibility {
            self.visibility.set(visibility);
        }
    }

    /// https://html.spec.whatwg.org/multipage/#iframe-load-event-steps steps 1-4
    pub fn iframe_load_event_steps(&self, loaded_pipeline: PipelineId) {
        // TODO(#9592): assert that the load blocker is present at all times when we
        //              can guarantee that it's created for the case of iframe.reload().
        if Some(loaded_pipeline) != self.pending_pipeline_id.get() {
            return;
        }

        // TODO A cross-origin child document would not be easily accessible
        //      from this script thread. It's unclear how to implement
        //      steps 2, 3, and 5 efficiently in this case.
        // TODO Step 2 - check child document `mute iframe load` flag
        // TODO Step 3 - set child document  `mut iframe load` flag

        // Step 4
        self.upcast::<EventTarget>().fire_event(atom!("load"));

        let mut blocker = self.load_blocker.borrow_mut();
        LoadBlocker::terminate(&mut blocker);

        // TODO Step 5 - unset child document `mut iframe load` flag

        let window = window_from_node(self);
        window.reflow(ReflowGoal::Full, ReflowReason::IFrameLoadEvent);
    }
}

pub trait HTMLIFrameElementLayoutMethods {
    fn pipeline_id(&self) -> Option<PipelineId>;
    fn browsing_context_id(&self) -> Option<BrowsingContextId>;
    fn get_width(&self) -> LengthOrPercentageOrAuto;
    fn get_height(&self) -> LengthOrPercentageOrAuto;
}

impl HTMLIFrameElementLayoutMethods for LayoutDom<HTMLIFrameElement> {
    #[inline]
    #[allow(unsafe_code)]
    fn pipeline_id(&self) -> Option<PipelineId> {
        unsafe { (*self.unsafe_get()).pipeline_id.get() }
    }

    #[inline]
    #[allow(unsafe_code)]
    fn browsing_context_id(&self) -> Option<BrowsingContextId> {
        unsafe { (*self.unsafe_get()).browsing_context_id.get() }
    }

    #[allow(unsafe_code)]
    fn get_width(&self) -> LengthOrPercentageOrAuto {
        unsafe {
            (*self.upcast::<Element>().unsafe_get())
                .get_attr_for_layout(&ns!(), &local_name!("width"))
                .map(AttrValue::as_dimension)
                .cloned()
                .unwrap_or(LengthOrPercentageOrAuto::Auto)
        }
    }

    #[allow(unsafe_code)]
    fn get_height(&self) -> LengthOrPercentageOrAuto {
        unsafe {
            (*self.upcast::<Element>().unsafe_get())
                .get_attr_for_layout(&ns!(), &local_name!("height"))
                .map(AttrValue::as_dimension)
                .cloned()
                .unwrap_or(LengthOrPercentageOrAuto::Auto)
        }
    }
}

impl HTMLIFrameElementMethods for HTMLIFrameElement {
    // https://html.spec.whatwg.org/multipage/#dom-iframe-src
    make_url_getter!(Src, "src");

    // https://html.spec.whatwg.org/multipage/#dom-iframe-src
    make_url_setter!(SetSrc, "src");

    // https://html.spec.whatwg.org/multipage/#dom-iframe-sandbox
    fn Sandbox(&self) -> DomRoot<DOMTokenList> {
        self.sandbox
            .or_init(|| DOMTokenList::new(self.upcast::<Element>(), &local_name!("sandbox")))
    }

    // https://html.spec.whatwg.org/multipage/#dom-iframe-contentwindow
    fn GetContentWindow(&self) -> Option<DomRoot<WindowProxy>> {
        self.browsing_context_id
            .get()
            .and_then(|browsing_context_id| ScriptThread::find_window_proxy(browsing_context_id))
    }

    // https://html.spec.whatwg.org/multipage/#dom-iframe-contentdocument
    // https://html.spec.whatwg.org/multipage/#concept-bcc-content-document
    fn GetContentDocument(&self) -> Option<DomRoot<Document>> {
        // Step 1.
        let pipeline_id = self.pipeline_id.get()?;

        // Step 2-3.
        // Note that this lookup will fail if the document is dissimilar-origin,
        // so we should return None in that case.
        let document = ScriptThread::find_document(pipeline_id)?;

        // Step 4.
        let current = GlobalScope::current()
            .expect("No current global object")
            .as_window()
            .Document();
        if !current.origin().same_origin_domain(document.origin()) {
            return None;
        }
        // Step 5.
        Some(document)
    }

    // https://html.spec.whatwg.org/multipage/#attr-iframe-allowfullscreen
    make_bool_getter!(AllowFullscreen, "allowfullscreen");
    // https://html.spec.whatwg.org/multipage/#attr-iframe-allowfullscreen
    make_bool_setter!(SetAllowFullscreen, "allowfullscreen");

    // https://html.spec.whatwg.org/multipage/#dom-dim-width
    make_getter!(Width, "width");
    // https://html.spec.whatwg.org/multipage/#dom-dim-width
    make_dimension_setter!(SetWidth, "width");

    // https://html.spec.whatwg.org/multipage/#dom-dim-height
    make_getter!(Height, "height");
    // https://html.spec.whatwg.org/multipage/#dom-dim-height
    make_dimension_setter!(SetHeight, "height");

    // https://html.spec.whatwg.org/multipage/#other-elements,-attributes-and-apis:attr-iframe-frameborder
    make_getter!(FrameBorder, "frameborder");
    // https://html.spec.whatwg.org/multipage/#other-elements,-attributes-and-apis:attr-iframe-frameborder
    make_setter!(SetFrameBorder, "frameborder");

    // https://html.spec.whatwg.org/multipage/#dom-iframe-name
    fn SetName(&self, name: DOMString) {
        *self.name.borrow_mut() = name.clone();
        if let Some(window) = self.GetContentWindow() {
            window.set_name(name)
        }
    }

    // https://html.spec.whatwg.org/multipage/#dom-iframe-name
    fn Name(&self) -> DOMString {
        if let Some(window) = self.GetContentWindow() {
            window.get_name()
        } else {
            self.name.borrow().clone()
        }
    }
}

impl VirtualMethods for HTMLIFrameElement {
    fn super_type(&self) -> Option<&dyn VirtualMethods> {
        Some(self.upcast::<HTMLElement>() as &dyn VirtualMethods)
    }

    fn attribute_mutated(&self, attr: &Attr, mutation: AttributeMutation) {
        self.super_type().unwrap().attribute_mutated(attr, mutation);
        match attr.local_name() {
            &local_name!("sandbox") => {
                self.sandbox_allowance
                    .set(mutation.new_value(attr).map(|value| {
                        let mut modes = SandboxAllowance::ALLOW_NOTHING;
                        for token in value.as_tokens() {
                            modes |= match &*token.to_ascii_lowercase() {
                                "allow-same-origin" => SandboxAllowance::ALLOW_SAME_ORIGIN,
                                "allow-forms" => SandboxAllowance::ALLOW_FORMS,
                                "allow-pointer-lock" => SandboxAllowance::ALLOW_POINTER_LOCK,
                                "allow-popups" => SandboxAllowance::ALLOW_POPUPS,
                                "allow-scripts" => SandboxAllowance::ALLOW_SCRIPTS,
                                "allow-top-navigation" => SandboxAllowance::ALLOW_TOP_NAVIGATION,
                                _ => SandboxAllowance::ALLOW_NOTHING,
                            };
                        }
                        modes
                    }));
            },
            &local_name!("src") => {
                // https://html.spec.whatwg.org/multipage/#the-iframe-element
                // "Similarly, whenever an iframe element with a non-null nested browsing context
                // but with no srcdoc attribute specified has its src attribute set, changed, or removed,
                // the user agent must process the iframe attributes,"
                // but we can't check that directly, since the child browsing context
                // may be in a different script thread. Instread, we check to see if the parent
                // is in a document tree and has a browsing context, which is what causes
                // the child browsing context to be created.
                if self.upcast::<Node>().is_connected_with_browsing_context() {
                    debug!("iframe src set while in browsing context.");
                    self.process_the_iframe_attributes(ProcessingMode::NotFirstTime);
                }
            },
            &local_name!("name") => {
                let new_value = mutation.new_value(attr);
                let value = new_value.as_ref().map_or("", |v| &v);
                self.SetName(DOMString::from(value.to_owned()));
            },
            _ => {},
        }
    }

    fn parse_plain_attribute(&self, name: &LocalName, value: DOMString) -> AttrValue {
        match name {
            &local_name!("sandbox") => AttrValue::from_serialized_tokenlist(value.into()),
            &local_name!("width") => AttrValue::from_dimension(value.into()),
            &local_name!("height") => AttrValue::from_dimension(value.into()),
            _ => self
                .super_type()
                .unwrap()
                .parse_plain_attribute(name, value),
        }
    }

    fn bind_to_tree(&self, context: &BindContext) {
        if let Some(ref s) = self.super_type() {
            s.bind_to_tree(context);
        }

        let tree_connected = context.tree_connected;
        let iframe = Trusted::new(self);
        document_from_node(self).add_delayed_task(task!(IFrameDelayedInitialize: move || {
            let this = iframe.root();
            // https://html.spec.whatwg.org/multipage/#the-iframe-element
            // "When an iframe element is inserted into a document that has
            // a browsing context, the user agent must create a new
            // browsing context, set the element's nested browsing context
            // to the newly-created browsing context, and then process the
            // iframe attributes for the "first time"."
            if this.upcast::<Node>().is_connected_with_browsing_context() {
                debug!("iframe bound to browsing context.");
                debug_assert!(tree_connected, "is_connected_with_bc, but not tree_connected");
                this.create_nested_browsing_context();
                this.process_the_iframe_attributes(ProcessingMode::FirstTime);
            }
        }));
    }

    fn unbind_from_tree(&self, context: &UnbindContext) {
        self.super_type().unwrap().unbind_from_tree(context);

        let mut blocker = self.load_blocker.borrow_mut();
        LoadBlocker::terminate(&mut blocker);

        // https://html.spec.whatwg.org/multipage/#a-browsing-context-is-discarded
        let window = window_from_node(self);
        let (sender, receiver) =
            ProfiledIpc::channel(self.global().time_profiler_chan().clone()).unwrap();

        // Ask the constellation to remove the iframe, and tell us the
        // pipeline ids of the closed pipelines.
        let browsing_context_id = match self.browsing_context_id() {
            None => return warn!("Unbinding already unbound iframe."),
            Some(id) => id,
        };
        debug!("Unbinding frame {}.", browsing_context_id);

        let msg = ScriptMsg::RemoveIFrame(browsing_context_id, sender);
        window
            .upcast::<GlobalScope>()
            .script_to_constellation_chan()
            .send(msg)
            .unwrap();
        let exited_pipeline_ids = receiver.recv().unwrap();

        // The spec for discarding is synchronous,
        // so we need to discard the browsing contexts now, rather than
        // when the `PipelineExit` message arrives.
        for exited_pipeline_id in exited_pipeline_ids {
            // https://html.spec.whatwg.org/multipage/#a-browsing-context-is-discarded
            if let Some(exited_document) = ScriptThread::find_document(exited_pipeline_id) {
                debug!(
                    "Discarding browsing context for pipeline {}",
                    exited_pipeline_id
                );
                let exited_window = exited_document.window();
                exited_window.discard_browsing_context();
                for exited_iframe in exited_document.iter_iframes() {
                    debug!("Discarding nested browsing context");
                    exited_iframe.destroy_nested_browsing_context();
                }
            }
        }

        // Resetting the pipeline_id to None is required here so that
        // if this iframe is subsequently re-added to the document
        // the load doesn't think that it's a navigation, but instead
        // a new iframe. Without this, the constellation gets very
        // confused.
        self.destroy_nested_browsing_context();
    }
}