// Copyright 2019 The Fuchsia Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

use {
    crate::{
        capability::{CapabilityProvider, CapabilitySource},
        model::{
            component::{BindReason, ComponentInstance, Runtime, WeakComponentInstance},
            error::ModelError,
            events::error::EventsError,
        },
    },
    anyhow::format_err,
    async_trait::async_trait,
    cm_rust::{CapabilityName, ComponentDecl},
    fidl_fuchsia_diagnostics_types as fdiagnostics,
    fidl_fuchsia_io::{self as fio, DirectoryProxy, NodeProxy},
    fidl_fuchsia_sys2 as fsys, fuchsia_trace as trace, fuchsia_zircon as zx,
    futures::{channel::oneshot, future::BoxFuture, lock::Mutex},
    io_util,
    moniker::{AbsoluteMoniker, ExtendedMoniker},
    rand::random,
    std::{
        collections::HashMap,
        convert::TryFrom,
        fmt,
        sync::{Arc, Weak},
    },
};

/// Defines the `EventType` enum as well as its implementation.
/// |description| is the description of the event that will be a doc comment on that event type.
/// |name| is the name of the event on CamelCase format, capitalized.
/// |string_name| is the name of the event on snake_case format, not capitalized.
macro_rules! events {
    ([$($(#[$description:meta])* ($name:ident, $string_name:ident),)*]) => {
        pub trait HasEventType {
            fn event_type(&self) -> EventType;
        }

        #[derive(Clone, Debug, Eq, PartialEq, Hash)]
        pub enum EventType {
            $(
                $(#[$description])*
                $name,
            )*
        }

        /// Transfers any move-only state out of self into a new event that is otherwise
        /// a clone.
        #[async_trait]
        pub trait TransferEvent {
            async fn transfer(&self) -> Self;
        }

        impl fmt::Display for EventType {
            fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
                write!(f, "{}", match self {
                    $(
                        EventType::$name => stringify!($string_name),
                    )*
                }
                .to_string())
            }
        }

        impl TryFrom<String> for EventType {
            type Error = anyhow::Error;

            fn try_from(string: String) -> Result<EventType, Self::Error> {
                match string.as_str() {
                    $(
                        stringify!($string_name) => Ok(EventType::$name),
                    )*
                    other => Err(format_err!("invalid string for event type: {:?}", other))
                }
            }
        }

        impl Into<fsys::EventType> for EventType {
            fn into(self) -> fsys::EventType {
                match self {
                    $(
                        EventType::$name => fsys::EventType::$name,
                    )*
                }
            }
        }

        impl From<fsys::EventType> for EventType {
            fn from(fidl_event_type: fsys::EventType) -> Self {
                match fidl_event_type {
                    $(
                        fsys::EventType::$name => EventType::$name,
                    )*
                }
            }
        }

        impl EventType {
            /// Returns all available event types.
            pub fn values() -> Vec<EventType> {
                vec![
                    $(EventType::$name,)*
                ]
            }
        }

        impl HasEventType for EventPayload {
            fn event_type(&self) -> EventType {
                match self {
                    $(
                        EventPayload::$name { .. } => EventType::$name,
                    )*
                }
            }
        }

        impl HasEventType for EventErrorPayload {
            fn event_type(&self) -> EventType {
                match self {
                    $(
                        EventErrorPayload::$name { .. } => EventType::$name,
                    )*
                }
            }
        }

        impl fmt::Display for EventErrorPayload {
            fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
                write!(f, "{}", match self {
                    $(
                        EventErrorPayload::$name { .. } => stringify!($string_name),
                    )*
                }
                .to_string())
            }
        }
    };
}

impl EventType {
    pub fn synthesized_only() -> Vec<Self> {
        vec![EventType::Running]
    }
}

// Keep the event types listed below in alphabetical order!
events!([
    /// A capability exposed to the framework by a component is available.
    (CapabilityReady, capability_ready),
    /// After a CapabilityProvider has been selected through the CapabilityRouted event,
    /// the CapabilityRequested event is dispatched with the ServerEnd of the channel
    /// for the capability.
    (CapabilityRequested, capability_requested),
    /// A capability is being requested by a component and requires routing.
    /// The event propagation system is used to supply the capability being requested.
    (CapabilityRouted, capability_routed),
    /// An instance was destroyed successfully. The instance is stopped and no longer
    /// exists in the parent's realm.
    (Destroyed, destroyed),
    /// A component instance was discovered.
    (Discovered, discovered),
    /// Destruction of an instance has begun. The instance may/may not be stopped by this point.
    /// The instance still exists in the parent's realm but will soon be removed.
    /// TODO(fxbug.dev/39417): Ensure the instance is stopped before this event.
    (MarkedForDestruction, marked_for_destruction),
    /// An instance's declaration was resolved successfully for the first time.
    (Resolved, resolved),
    /// An instance is about to be started.
    (Started, started),
    /// An instance was stopped successfully.
    /// This event must occur before Destroyed.
    (Stopped, stopped),
    /// A component is running.
    (Running, running),
]);

impl Into<CapabilityName> for EventType {
    fn into(self) -> CapabilityName {
        self.to_string().into()
    }
}

/// Holds the `source` error that caused the transition corresponding to `event_type`
/// to fail.
#[derive(Debug, Clone)]
pub struct EventError {
    pub source: ModelError,
    pub event_error_payload: EventErrorPayload,
}

impl EventError {
    pub fn new(err: &ModelError, event_error_payload: EventErrorPayload) -> Self {
        Self { source: err.clone(), event_error_payload }
    }
}

#[derive(Clone)]
pub enum EventErrorPayload {
    // Keep the events listed below in alphabetical order!
    CapabilityReady { name: String },
    CapabilityRequested { source_moniker: AbsoluteMoniker, name: String },
    CapabilityRouted,
    Destroyed,
    Discovered,
    MarkedForDestruction,
    Resolved,
    Started,
    Stopped,
    Running { started_timestamp: zx::Time },
}

impl HasEventType for EventError {
    fn event_type(&self) -> EventType {
        self.event_error_payload.event_type()
    }
}

impl fmt::Debug for EventErrorPayload {
    fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
        let mut formatter = fmt.debug_struct("EventErrorPayload");
        formatter.field("type", &self.event_type());
        match self {
            EventErrorPayload::CapabilityReady { name } => formatter.field("name", &name).finish(),
            EventErrorPayload::CapabilityRequested { source_moniker, name } => {
                formatter.field("source_moniker", &source_moniker);
                formatter.field("name", &name).finish()
            }
            EventErrorPayload::CapabilityRouted
            | EventErrorPayload::Destroyed
            | EventErrorPayload::Discovered
            | EventErrorPayload::MarkedForDestruction
            | EventErrorPayload::Resolved
            | EventErrorPayload::Started
            | EventErrorPayload::Stopped
            | EventErrorPayload::Running { .. } => formatter.finish(),
        }
    }
}

/// The component manager calls out to objects that implement the `Hook` trait on registered
/// component manager events. Hooks block the flow of a task, and can mutate, decorate and replace
/// capabilities. This permits `Hook` to serve as a point of extensibility for the component
/// manager.
#[async_trait]
pub trait Hook: Send + Sync {
    async fn on(self: Arc<Self>, event: &Event) -> Result<(), ModelError>;
}

/// An object registers a hook into a component manager event via a `HooksRegistration` object.
/// A single object may register for multiple events through a vector of `EventType`. `Hooks`
/// does not retain the callback. The hook is lazily removed when the callback object loses
/// strong references.
pub struct HooksRegistration {
    name: &'static str,
    events: Vec<EventType>,
    callback: Weak<dyn Hook>,
}

impl HooksRegistration {
    pub fn new(
        name: &'static str,
        events: Vec<EventType>,
        callback: Weak<dyn Hook>,
    ) -> HooksRegistration {
        Self { name, events, callback }
    }
}

#[derive(Clone)]
pub enum EventPayload {
    // Keep the events listed below in alphabetical order!
    CapabilityReady {
        name: String,
        node: NodeProxy,
    },
    CapabilityRequested {
        source_moniker: AbsoluteMoniker,
        name: String,
        capability: Arc<Mutex<Option<zx::Channel>>>,
    },
    CapabilityRouted {
        source: CapabilitySource,
        // Events are passed to hooks as immutable borrows. In order to mutate,
        // a field within an Event, interior mutability is employed here with
        // a Mutex.
        capability_provider: Arc<Mutex<Option<Box<dyn CapabilityProvider>>>>,
    },
    Destroyed,
    Discovered,
    MarkedForDestruction,
    Resolved {
        component: WeakComponentInstance,
        resolved_url: String,
        decl: ComponentDecl,
    },
    Started {
        component: WeakComponentInstance,
        runtime: RuntimeInfo,
        component_decl: ComponentDecl,
        bind_reason: BindReason,
    },
    Stopped {
        status: zx::Status,
    },
    Running {
        started_timestamp: zx::Time,
    },
}

/// Information about a component's runtime provided to `Started`.
#[derive(Clone)]
pub struct RuntimeInfo {
    pub resolved_url: String,
    pub package_dir: Option<DirectoryProxy>,
    pub outgoing_dir: Option<DirectoryProxy>,
    pub runtime_dir: Option<DirectoryProxy>,
    pub diagnostics_receiver:
        Arc<Mutex<Option<oneshot::Receiver<fdiagnostics::ComponentDiagnostics>>>>,
}

impl RuntimeInfo {
    pub fn from_runtime(runtime: &mut Runtime, resolved_url: String) -> Self {
        let diagnostics_receiver = Arc::new(Mutex::new(
            runtime
                .controller
                .as_mut()
                .and_then(|controller| controller.take_diagnostics_receiver()),
        ));
        Self {
            resolved_url,
            package_dir: runtime
                .namespace
                .as_ref()
                .and_then(|n| clone_dir(n.package_dir.as_ref().map(|d| d as &DirectoryProxy))),
            outgoing_dir: clone_dir(runtime.outgoing_dir.as_ref()),
            runtime_dir: clone_dir(runtime.runtime_dir.as_ref()),
            diagnostics_receiver,
        }
    }
}

// TODO(fsamuel): We should probably preserve the original error messages
// instead of dropping them.
fn clone_dir(dir: Option<&DirectoryProxy>) -> Option<DirectoryProxy> {
    dir.and_then(|d| io_util::clone_directory(d, fio::CLONE_FLAG_SAME_RIGHTS).ok())
}

impl fmt::Debug for EventPayload {
    fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
        let mut formatter = fmt.debug_struct("EventPayload");
        formatter.field("type", &self.event_type());
        match self {
            EventPayload::CapabilityReady { name, .. } => {
                formatter.field("name", &name.as_str()).finish()
            }
            EventPayload::CapabilityRequested { name, .. } => {
                formatter.field("name", &name).finish()
            }
            EventPayload::CapabilityRouted { source: capability, .. } => {
                formatter.field("capability", &capability).finish()
            }
            EventPayload::Started { component_decl, .. } => {
                formatter.field("component_decl", &component_decl).finish()
            }
            EventPayload::Resolved { component: _, resolved_url, decl } => {
                formatter.field("resolved_url", resolved_url);
                formatter.field("decl", decl).finish()
            }
            EventPayload::Stopped { status } => formatter.field("status", status).finish(),
            EventPayload::Destroyed
            | EventPayload::Discovered
            | EventPayload::MarkedForDestruction
            | EventPayload::Running { .. } => formatter.finish(),
        }
    }
}

pub type EventResult = Result<EventPayload, EventError>;

#[derive(Clone, Debug)]
pub struct Event {
    /// Each event has a unique 64-bit integer assigned to it
    pub id: u64,

    /// Moniker of component that this event applies to
    pub target_moniker: ExtendedMoniker,

    /// Component url of the component that this event applies to
    pub component_url: String,

    /// Result of the event
    pub result: EventResult,

    /// Time when this event was created
    pub timestamp: zx::Time,
}

impl Event {
    pub fn new(component: &Arc<ComponentInstance>, result: EventResult) -> Self {
        let timestamp = zx::Time::get_monotonic();
        Self::new_internal(
            component.abs_moniker.clone().into(),
            component.component_url.clone(),
            timestamp,
            result,
        )
    }

    pub fn new_builtin(result: EventResult) -> Self {
        let timestamp = zx::Time::get_monotonic();
        Self::new_internal(
            ExtendedMoniker::ComponentManager,
            "bin/component_manager".to_string(),
            timestamp,
            result,
        )
    }

    pub fn new_with_timestamp(
        component: &Arc<ComponentInstance>,
        result: EventResult,
        timestamp: zx::Time,
    ) -> Self {
        Self::new_internal(
            component.abs_moniker.clone().into(),
            component.component_url.clone(),
            timestamp,
            result,
        )
    }

    pub fn child_discovered(
        target_moniker: AbsoluteMoniker,
        component_url: impl Into<String>,
    ) -> Self {
        let timestamp = zx::Time::get_monotonic();
        Self::new_internal(
            target_moniker.into(),
            component_url.into(),
            timestamp,
            Ok(EventPayload::Discovered),
        )
    }

    #[cfg(test)]
    pub fn new_for_test(
        target_moniker: AbsoluteMoniker,
        component_url: impl Into<String>,
        result: EventResult,
    ) -> Self {
        let timestamp = zx::Time::get_monotonic();
        Self::new_internal(
            ExtendedMoniker::ComponentInstance(target_moniker),
            component_url.into(),
            timestamp,
            result,
        )
    }

    fn new_internal(
        target_moniker: ExtendedMoniker,
        component_url: String,
        timestamp: zx::Time,
        result: EventResult,
    ) -> Self {
        // Generate a random 64-bit integer to identify this event
        let id = random::<u64>();
        Self { id, target_moniker, component_url, timestamp, result }
    }
}

impl HasEventType for Result<EventPayload, EventError> {
    fn event_type(&self) -> EventType {
        match self {
            Ok(payload) => payload.event_type(),
            Err(error) => error.event_type(),
        }
    }
}

#[async_trait]
impl TransferEvent for Result<EventPayload, EventError> {
    async fn transfer(&self) -> Self {
        match self {
            Ok(EventPayload::CapabilityRequested { source_moniker, name, capability }) => {
                let capability = capability.lock().await.take();
                match capability {
                    Some(capability) => Ok(EventPayload::CapabilityRequested {
                        source_moniker: source_moniker.clone(),
                        name: name.to_string(),
                        capability: Arc::new(Mutex::new(Some(capability))),
                    }),
                    None => Err(EventError {
                        source: EventsError::cannot_transfer(EventType::CapabilityRequested).into(),
                        event_error_payload: EventErrorPayload::CapabilityRequested {
                            source_moniker: source_moniker.clone(),
                            name: name.to_string(),
                        },
                    }),
                }
            }
            result => result.clone(),
        }
    }
}

impl HasEventType for Event {
    fn event_type(&self) -> EventType {
        self.result.event_type()
    }
}

#[async_trait]
impl TransferEvent for Event {
    async fn transfer(&self) -> Self {
        Self {
            id: self.id,
            target_moniker: self.target_moniker.clone(),
            component_url: self.component_url.clone(),
            result: self.result.transfer().await,
            timestamp: self.timestamp,
        }
    }
}

impl fmt::Display for Event {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        let output = match &self.result {
            Ok(payload) => {
                let payload = match payload {
                    EventPayload::CapabilityReady { name, .. } => format!("serving {}", name),
                    EventPayload::CapabilityRequested { source_moniker, name, .. } => {
                        format!("requested '{}' from '{}'", name.to_string(), source_moniker)
                    }
                    EventPayload::CapabilityRouted { source, .. } => {
                        format!("routed {}", source.to_string())
                    }
                    EventPayload::Started { bind_reason, .. } => {
                        format!("because {}", bind_reason.to_string())
                    }
                    EventPayload::Stopped { status } => {
                        format!("with status: {}", status.to_string())
                    }
                    EventPayload::Destroyed
                    | EventPayload::Discovered
                    | EventPayload::MarkedForDestruction
                    | EventPayload::Resolved { .. }
                    | EventPayload::Running { .. } => "".to_string(),
                };
                format!("[{}] '{}' {}", self.event_type().to_string(), self.target_moniker, payload)
            }
            Err(error) => format!(
                "[{} error] {} {}",
                self.event_type().to_string(),
                self.target_moniker,
                error.source.to_string()
            ),
        };
        write!(f, "{}", output)
    }
}

/// This is a collection of hooks to component manager events.
pub struct Hooks {
    parent: Option<Arc<Hooks>>,
    hooks_map: Mutex<HashMap<EventType, Vec<HookEntry>>>,
}

impl Hooks {
    pub fn new(parent: Option<Arc<Hooks>>) -> Self {
        Self { parent, hooks_map: Mutex::new(HashMap::new()) }
    }

    pub async fn install(&self, hooks: Vec<HooksRegistration>) {
        let mut hooks_map = self.hooks_map.lock().await;
        for hook in hooks {
            'event_type: for event in hook.events {
                let existing_hooks = &mut hooks_map.entry(event).or_insert(vec![]);

                for existing_hook in existing_hooks.iter() {
                    // If this hook has already been installed, skip to next event type.
                    if existing_hook.callback.ptr_eq(&hook.callback) {
                        break 'event_type;
                    }
                }

                existing_hooks.push(HookEntry { name: hook.name, callback: hook.callback.clone() });
            }
        }
    }

    pub fn dispatch<'a>(&'a self, event: &'a Event) -> BoxFuture<Result<(), ModelError>> {
        Box::pin(async move {
            // Trace event dispatch
            let event_type = format!("{:?}", event.event_type());
            let target_moniker = event.target_moniker.to_string();
            trace::duration!(
                "component_manager",
                "hooks:dispatch",
                "event_type" => event_type.as_str(),
                "target_moniker" => target_moniker.as_str()
            );

            let hooks = {
                trace::duration!(
                   "component_manager",
                   "hooks:upgrade_dedup",
                   "event_type" => event_type.as_ref(),
                   "target_moniker" => target_moniker.as_ref()
                );
                // We must upgrade our weak references to hooks to strong ones before we can
                // call out to them. Since hooks are deduped at install time, we do not need
                // to worry about that here.
                let mut strong_hooks = vec![];
                let mut hooks_map = self.hooks_map.lock().await;
                if let Some(hooks) = hooks_map.get_mut(&event.event_type()) {
                    hooks.retain(|hook| {
                        if let Some(callback) = hook.callback.upgrade() {
                            strong_hooks
                                .push(StrongHookEntry { name: hook.name.clone(), callback });
                            true
                        } else {
                            false
                        }
                    });
                }
                strong_hooks
            };
            for hook in hooks.into_iter() {
                trace::duration!(
                    "component_manager",
                    "hooks:on",
                    "event_type" => event_type.as_ref(),
                    "target_moniker" => target_moniker.as_ref(),
                    "name" => (*hook.name).as_ref()
                );
                hook.callback.on(event).await?;
            }

            if let Some(parent) = &self.parent {
                trace::duration!(
                    "component_manager",
                    "hooks:parent_dispatch",
                    "event_type" => event_type.as_ref(),
                    "target_moniker" => target_moniker.as_ref()
                );
                parent.dispatch(event).await?;
            }

            Ok(())
        })
    }
}

// Holds a Weak pointer to the Hook.
struct HookEntry {
    pub name: &'static str,
    pub callback: Weak<dyn Hook>,
}

// Holds a Strong pointer to the Hook. This is produced on dispatch when upgrading
// Weak pointers.
struct StrongHookEntry {
    pub name: &'static str,
    pub callback: Arc<dyn Hook>,
}

#[cfg(test)]
mod tests {
    use {
        super::*, ::routing::error::ComponentInstanceError, matches::assert_matches, std::sync::Arc,
    };

    #[derive(Clone)]
    struct EventLog {
        log: Arc<Mutex<Vec<String>>>,
    }

    impl EventLog {
        pub fn new() -> Self {
            Self { log: Arc::new(Mutex::new(Vec::new())) }
        }

        pub async fn append(&self, item: String) {
            let mut log = self.log.lock().await;
            log.push(item);
        }

        pub async fn get(&self) -> Vec<String> {
            self.log.lock().await.clone()
        }
    }

    struct CallCounter {
        name: String,
        logger: Option<EventLog>,
        call_count: Mutex<i32>,
        last_error: Mutex<Option<ModelError>>,
    }

    impl CallCounter {
        pub fn new(name: &str, logger: Option<EventLog>) -> Arc<Self> {
            Arc::new(Self {
                name: name.to_string(),
                logger,
                call_count: Mutex::new(0),
                last_error: Mutex::new(None),
            })
        }

        pub async fn count(&self) -> i32 {
            *self.call_count.lock().await
        }

        pub async fn last_error(&self) -> Option<ModelError> {
            let last_error = self.last_error.lock().await;
            last_error.as_ref().map(|err| err.clone())
        }

        async fn on_event(&self, event: &Event) -> Result<(), ModelError> {
            let mut call_count = self.call_count.lock().await;
            *call_count += 1;
            if let Some(logger) = &self.logger {
                match &event.result {
                    Ok(_) => {
                        logger
                            .append(format!(
                                "[{}] Ok: {}",
                                self.name,
                                event.event_type().to_string()
                            ))
                            .await;
                    }
                    Err(error) => {
                        logger
                            .append(format!(
                                "[{}] Err: {}",
                                self.name,
                                event.event_type().to_string()
                            ))
                            .await;
                        let mut last_error = self.last_error.lock().await;
                        *last_error = Some(error.source.clone());
                    }
                }
            }
            Ok(())
        }
    }

    #[async_trait]
    impl Hook for CallCounter {
        async fn on(self: Arc<Self>, event: &Event) -> Result<(), ModelError> {
            self.on_event(event).await
        }
    }

    fn log(v: Vec<&str>) -> Vec<String> {
        v.iter().map(|s| s.to_string()).collect()
    }

    // This test verifies that a hook cannot be installed twice.
    #[fuchsia::test]
    async fn install_hook_twice() {
        // CallCounter counts the number of DynamicChildAdded events it receives.
        // It should only ever receive one.
        let call_counter = CallCounter::new("CallCounter", None);
        let hooks = Hooks::new(None);

        // Attempt to install CallCounter twice.
        hooks
            .install(vec![HooksRegistration::new(
                "FirstInstall",
                vec![EventType::Discovered],
                Arc::downgrade(&call_counter) as Weak<dyn Hook>,
            )])
            .await;
        hooks
            .install(vec![HooksRegistration::new(
                "SecondInstall",
                vec![EventType::Discovered],
                Arc::downgrade(&call_counter) as Weak<dyn Hook>,
            )])
            .await;

        let event = Event::new_for_test(
            AbsoluteMoniker::root(),
            "fuchsia-pkg://root",
            Ok(EventPayload::Discovered),
        );
        hooks.dispatch(&event).await.expect("Unable to call hooks.");
        assert_eq!(1, call_counter.count().await);
    }

    // This test verifies that events propagate from child_hooks to parent_hooks.
    #[fuchsia::test]
    async fn event_propagation() {
        let parent_hooks = Arc::new(Hooks::new(None));
        let child_hooks = Hooks::new(Some(parent_hooks.clone()));

        let event_log = EventLog::new();
        let parent_call_counter = CallCounter::new("ParentCallCounter", Some(event_log.clone()));
        parent_hooks
            .install(vec![HooksRegistration::new(
                "ParentHook",
                vec![EventType::Discovered],
                Arc::downgrade(&parent_call_counter) as Weak<dyn Hook>,
            )])
            .await;

        let child_call_counter = CallCounter::new("ChildCallCounter", Some(event_log.clone()));
        child_hooks
            .install(vec![HooksRegistration::new(
                "ChildHook",
                vec![EventType::Discovered],
                Arc::downgrade(&child_call_counter) as Weak<dyn Hook>,
            )])
            .await;

        assert_eq!(1, Arc::strong_count(&parent_call_counter));
        assert_eq!(1, Arc::strong_count(&child_call_counter));

        let event = Event::new_for_test(
            AbsoluteMoniker::root(),
            "fuchsia-pkg://root",
            Ok(EventPayload::Discovered),
        );
        child_hooks.dispatch(&event).await.expect("Unable to call hooks.");
        // parent_call_counter gets informed of the event on child_hooks even though it has
        // been installed on parent_hooks.
        assert_eq!(1, parent_call_counter.count().await);
        // child_call_counter should be called only once.
        assert_eq!(1, child_call_counter.count().await);

        // Dropping the child_call_counter should drop the weak pointer to it in hooks
        // as well.
        drop(child_call_counter);

        // Dispatching an event on child_hooks will not call out to child_call_counter
        // because it has been destroyed by the call to drop above.
        child_hooks.dispatch(&event).await.expect("Unable to call hooks.");

        // ChildCallCounter should be called before ParentCallCounter.
        assert_eq!(
            log(vec![
                "[ChildCallCounter] Ok: discovered",
                "[ParentCallCounter] Ok: discovered",
                "[ParentCallCounter] Ok: discovered",
            ]),
            event_log.get().await
        );
    }

    // This test verifies that a hook can receive errors.
    #[fuchsia::test]
    async fn error_event() {
        // CallCounter counts the number of DynamicChildAdded events it receives.
        // It should only ever receive one.
        let event_log = EventLog::new();
        let call_counter = CallCounter::new("CallCounter", Some(event_log.clone()));
        let hooks = Hooks::new(None);

        // Attempt to install CallCounter twice.
        hooks
            .install(vec![HooksRegistration::new(
                "CallCounter",
                vec![EventType::Resolved],
                Arc::downgrade(&call_counter) as Weak<dyn Hook>,
            )])
            .await;

        let root = AbsoluteMoniker::root();
        let event = Event::new_for_test(
            root.clone(),
            "fuchsia-pkg://root",
            Err(EventError::new(
                &ModelError::instance_not_found(root.clone()),
                EventErrorPayload::Resolved,
            )),
        );
        hooks.dispatch(&event).await.expect("Unable to call hooks.");
        assert_eq!(1, call_counter.count().await);

        assert_eq!(log(vec!["[CallCounter] Err: resolved",]), event_log.get().await);

        assert_matches!(
            call_counter.last_error().await,
            Some(ModelError::ComponentInstanceError {
                err: ComponentInstanceError::InstanceNotFound { .. }
            })
        );
    }

    // This test verifies that the payload of the CapabilityRequested event will be transferred.
    #[fuchsia::test]
    async fn capability_requested_transfer() {
        let (_, capability_server_end) = zx::Channel::create().unwrap();
        let capability_server_end = Arc::new(Mutex::new(Some(capability_server_end)));
        let event = Event::new_for_test(
            AbsoluteMoniker::root(),
            "fuchsia-pkg://root",
            Ok(EventPayload::CapabilityRequested {
                source_moniker: AbsoluteMoniker::root(),
                name: "foo".to_string(),
                capability: capability_server_end,
            }),
        );

        // Verify the transferred event carries the capability.
        let transferred_event = event.transfer().await;
        match transferred_event.result {
            Ok(EventPayload::CapabilityRequested { capability, .. }) => {
                let capability = capability.lock().await;
                assert!(capability.is_some());
            }
            _ => panic!("Event type unexpected"),
        }

        // Verify that the original event no longer carries the capability.
        match &event.result {
            Ok(EventPayload::CapabilityRequested { capability, .. }) => {
                let capability = capability.lock().await;
                assert!(capability.is_none());
            }
            _ => panic!("Event type unexpected"),
        }

        // Transferring the original event again should produce an error event.
        let second_transferred_event = event.transfer().await;
        match second_transferred_event.result {
            Err(EventError {
                event_error_payload: EventErrorPayload::CapabilityRequested { name, .. },
                ..
            }) => {
                assert_eq!("foo".to_string(), name);
            }
            _ => panic!("Event type unexpected"),
        }
    }
}