5 files changed, 903 insertions, 0 deletions
diff --git a/xpcom/rust/moz_task/Cargo.toml b/xpcom/rust/moz_task/Cargo.toml
new file mode 100644
index 0000000000..ec60bbf156
--- /dev/null
+++ b/xpcom/rust/moz_task/Cargo.toml
@@ -0,0 +1,16 @@
+[package]
+name = "moz_task"
+version = "0.1.0"
+authors = ["Myk Melez <myk@mykzilla.org>"]
+license = "MPL-2.0"
+description = "Rust wrappers around XPCOM threading functions"
+edition = "2018"
+
+[dependencies]
+log = "0.4"
+cstr = "0.2"
+libc = "0.2"
+async-task = { version = "4.0" }
+nserror = { path = "../nserror" }
+nsstring = { path = "../nsstring" }
+xpcom = { path = "../xpcom" }
diff --git a/xpcom/rust/moz_task/src/dispatcher.rs b/xpcom/rust/moz_task/src/dispatcher.rs
new file mode 100644
index 0000000000..17ad9ceb81
--- /dev/null
+++ b/xpcom/rust/moz_task/src/dispatcher.rs
@@ -0,0 +1,153 @@
+/* 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 http://mozilla.org/MPL/2.0/. */
+
+use crate::{
+    dispatch_background_task_runnable, dispatch_runnable, get_current_thread, DispatchOptions,
+};
+use nserror::{nsresult, NS_OK};
+use nsstring::nsACString;
+use std::sync::Mutex;
+use xpcom::interfaces::{nsIEventTarget, nsIRunnablePriority};
+use xpcom::xpcom;
+
+/// Basic wrapper to convert a FnOnce callback into a `nsIRunnable` to be
+/// dispatched using XPCOM.
+#[xpcom(implement(nsIRunnable, nsINamed, nsIRunnablePriority), atomic)]
+struct RunnableFunction<F: FnOnce() + 'static> {
+    name: &'static str,
+    priority: u32,
+    function: Mutex<Option<F>>,
+}
+
+impl<F: FnOnce() + 'static> RunnableFunction<F> {
+    #[allow(non_snake_case)]
+    fn Run(&self) -> nsresult {
+        let function = self.function.lock().unwrap().take();
+        debug_assert!(function.is_some(), "runnable invoked twice?");
+        if let Some(function) = function {
+            function();
+        }
+        NS_OK
+    }
+
+    #[allow(non_snake_case)]
+    unsafe fn GetName(&self, result: *mut nsACString) -> nsresult {
+        (*result).assign(self.name);
+        NS_OK
+    }
+
+    #[allow(non_snake_case)]
+    unsafe fn GetPriority(&self, result: *mut u32) -> nsresult {
+        *result = self.priority;
+        NS_OK
+    }
+}
+
+pub struct RunnableBuilder<F> {
+    name: &'static str,
+    function: F,
+    priority: u32,
+    options: DispatchOptions,
+}
+
+impl<F> RunnableBuilder<F> {
+    pub fn new(name: &'static str, function: F) -> Self {
+        RunnableBuilder {
+            name,
+            function,
+            priority: nsIRunnablePriority::PRIORITY_NORMAL,
+            options: DispatchOptions::default(),
+        }
+    }
+
+    pub fn priority(mut self, priority: u32) -> Self {
+        self.priority = priority;
+        self
+    }
+
+    pub fn options(mut self, options: DispatchOptions) -> Self {
+        self.options = options;
+        self
+    }
+
+    pub fn may_block(mut self, may_block: bool) -> Self {
+        self.options = self.options.may_block(may_block);
+        self
+    }
+
+    pub unsafe fn at_end(mut self, at_end: bool) -> Self {
+        self.options = self.options.at_end(at_end);
+        self
+    }
+}
+
+impl<F> RunnableBuilder<F>
+where
+    F: FnOnce() + Send + 'static,
+{
+    /// Dispatch this Runnable to the specified EventTarget. The runnable function must be `Send`.
+    pub fn dispatch(self, target: &nsIEventTarget) -> Result<(), nsresult> {
+        let runnable = RunnableFunction::allocate(InitRunnableFunction {
+            name: self.name,
+            priority: self.priority,
+            function: Mutex::new(Some(self.function)),
+        });
+        unsafe { dispatch_runnable(runnable.coerce(), target, self.options) }
+    }
+
+    /// Dispatch this Runnable to the specified EventTarget as a background
+    /// task. The runnable function must be `Send`.
+    pub fn dispatch_background_task(self) -> Result<(), nsresult> {
+        let runnable = RunnableFunction::allocate(InitRunnableFunction {
+            name: self.name,
+            priority: self.priority,
+            function: Mutex::new(Some(self.function)),
+        });
+        unsafe { dispatch_background_task_runnable(runnable.coerce(), self.options) }
+    }
+}
+
+impl<F> RunnableBuilder<F>
+where
+    F: FnOnce() + 'static,
+{
+    /// Dispatch this Runnable to the current thread.
+    ///
+    /// Unlike `dispatch` and `dispatch_background_task`, the runnable does not
+    /// need to be `Send` to dispatch to the current thread.
+    pub fn dispatch_local(self) -> Result<(), nsresult> {
+        let target = get_current_thread()?;
+        let runnable = RunnableFunction::allocate(InitRunnableFunction {
+            name: self.name,
+            priority: self.priority,
+            function: Mutex::new(Some(self.function)),
+        });
+        unsafe { dispatch_runnable(runnable.coerce(), target.coerce(), self.options) }
+    }
+}
+
+pub fn dispatch_onto<F>(
+    name: &'static str,
+    target: &nsIEventTarget,
+    function: F,
+) -> Result<(), nsresult>
+where
+    F: FnOnce() + Send + 'static,
+{
+    RunnableBuilder::new(name, function).dispatch(target)
+}
+
+pub fn dispatch_background_task<F>(name: &'static str, function: F) -> Result<(), nsresult>
+where
+    F: FnOnce() + Send + 'static,
+{
+    RunnableBuilder::new(name, function).dispatch_background_task()
+}
+
+pub fn dispatch_local<F>(name: &'static str, function: F) -> Result<(), nsresult>
+where
+    F: FnOnce() + 'static,
+{
+    RunnableBuilder::new(name, function).dispatch_local()
+}
diff --git a/xpcom/rust/moz_task/src/event_loop.rs b/xpcom/rust/moz_task/src/event_loop.rs
new file mode 100644
index 0000000000..f8d113ed57
--- /dev/null
+++ b/xpcom/rust/moz_task/src/event_loop.rs
@@ -0,0 +1,66 @@
+/* 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 http://mozilla.org/MPL/2.0/. */
+
+extern crate nsstring;
+
+use cstr::cstr;
+use nserror::{nsresult, NS_ERROR_SERVICE_NOT_AVAILABLE, NS_ERROR_UNEXPECTED, NS_OK};
+use nsstring::*;
+use std::cell::RefCell;
+use std::future::Future;
+use xpcom::{interfaces::nsIThreadManager, xpcom, xpcom_method};
+
+#[xpcom(implement(nsINestedEventLoopCondition), nonatomic)]
+struct FutureCompleteCondition<T: 'static> {
+    value: RefCell<Option<T>>,
+}
+
+impl<T: 'static> FutureCompleteCondition<T> {
+    xpcom_method!(is_done => IsDone() -> bool);
+    fn is_done(&self) -> Result<bool, nsresult> {
+        Ok(self.value.borrow().is_some())
+    }
+}
+
+/// Spin the event loop on the current thread until `future` is resolved.
+///
+/// # Safety
+///
+/// Spinning a nested event loop should always be avoided when possible, as it
+/// can cause hangs, break JS run-to-completion guarantees, and break other C++
+/// code currently on the stack relying on heap invariants. While in a pure-rust
+/// codebase this method would only be ill-advised and not technically "unsafe",
+/// it is marked as unsafe due to the potential for triggering unsafety in
+/// unrelated C++ code.
+pub unsafe fn spin_event_loop_until<F>(
+    reason: &'static str,
+    future: F,
+) -> Result<F::Output, nsresult>
+where
+    F: Future + 'static,
+    F::Output: 'static,
+{
+    let thread_manager =
+        xpcom::get_service::<nsIThreadManager>(cstr!("@mozilla.org/thread-manager;1"))
+            .ok_or(NS_ERROR_SERVICE_NOT_AVAILABLE)?;
+
+    let cond = FutureCompleteCondition::<F::Output>::allocate(InitFutureCompleteCondition {
+        value: RefCell::new(None),
+    });
+
+    // Spawn our future onto the current thread event loop, and record the
+    // completed value as it completes.
+    let cond2 = cond.clone();
+    crate::spawn_local(reason, async move {
+        let rv = future.await;
+        *cond2.value.borrow_mut() = Some(rv);
+    })
+    .detach();
+
+    thread_manager
+        .SpinEventLoopUntil(&*nsCStr::from(reason), cond.coerce())
+        .to_result()?;
+    let rv = cond.value.borrow_mut().take();
+    rv.ok_or(NS_ERROR_UNEXPECTED)
+}
diff --git a/xpcom/rust/moz_task/src/executor.rs b/xpcom/rust/moz_task/src/executor.rs
new file mode 100644
index 0000000000..0016839373
--- /dev/null
+++ b/xpcom/rust/moz_task/src/executor.rs
@@ -0,0 +1,291 @@
+/* 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 http://mozilla.org/MPL/2.0/. */
+
+use crate::{get_current_thread, DispatchOptions, RunnableBuilder};
+use std::{
+    cell::Cell,
+    fmt::Debug,
+    future::Future,
+    pin::Pin,
+    ptr,
+    sync::Arc,
+    task::{Context, Poll},
+};
+use xpcom::interfaces::{nsIEventTarget, nsIRunnablePriority};
+use xpcom::RefPtr;
+
+/// A spawned task.
+///
+/// A [`AsyncTask`] can be awaited to retrieve the output of its future.
+///
+/// Dropping an [`AsyncTask`] cancels it, which means its future won't be polled
+/// again. To drop the [`AsyncTask`] handle without canceling it, use
+/// [`detach()`][`AsyncTask::detach()`] instead. To cancel a task gracefully and
+/// wait until it is fully destroyed, use the [`cancel()`][AsyncTask::cancel()]
+/// method.
+///
+/// A task which is cancelled due to the nsIEventTarget it was dispatched to no
+/// longer accepting events will never be resolved.
+#[derive(Debug)]
+#[must_use = "tasks get canceled when dropped, use `.detach()` to run them in the background"]
+pub struct AsyncTask<T> {
+    task: async_task::FallibleTask<T>,
+}
+
+impl<T> AsyncTask<T> {
+    fn new(task: async_task::Task<T>) -> Self {
+        AsyncTask {
+            task: task.fallible(),
+        }
+    }
+
+    /// Detaches the task to let it keep running in the background.
+    pub fn detach(self) {
+        self.task.detach()
+    }
+
+    /// Cancels the task and waits for it to stop running.
+    ///
+    /// Returns the task's output if it was completed just before it got canceled, or [`None`] if
+    /// it didn't complete.
+    ///
+    /// While it's possible to simply drop the [`Task`] to cancel it, this is a cleaner way of
+    /// canceling because it also waits for the task to stop running.
+    pub async fn cancel(self) -> Option<T> {
+        self.task.cancel().await
+    }
+}
+
+impl<T> Future for AsyncTask<T> {
+    type Output = T;
+
+    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        // Wrap the future produced by `AsyncTask` to never resolve if the
+        // Runnable was dropped, and the task was cancelled.
+        match Pin::new(&mut self.task).poll(cx) {
+            Poll::Ready(Some(t)) => Poll::Ready(t),
+            Poll::Ready(None) | Poll::Pending => Poll::Pending,
+        }
+    }
+}
+
+enum SpawnTarget {
+    BackgroundTask,
+    EventTarget(RefPtr<nsIEventTarget>),
+}
+
+// SAFETY: All XPCOM interfaces are considered !Send + !Sync, however all
+// well-behaved nsIEventTarget instances must be threadsafe.
+unsafe impl Send for SpawnTarget {}
+unsafe impl Sync for SpawnTarget {}
+
+/// Information used by tasks as they are spawned. Stored in an Arc such that
+/// their identity can be used for `POLLING_TASK`.
+struct TaskSpawnConfig {
+    name: &'static str,
+    priority: u32,
+    options: DispatchOptions,
+    target: SpawnTarget,
+}
+
+thread_local! {
+    /// Raw pointer to the TaskSpawnConfig for the currently polling task. Used
+    /// to detect scheduling callbacks for a runnable while it is polled, to set
+    /// `DISPATCH_AT_END` on the notification.
+    static POLLING_TASK: Cell<*const TaskSpawnConfig> = Cell::new(ptr::null());
+}
+
+fn schedule(config: Arc<TaskSpawnConfig>, runnable: async_task::Runnable) {
+    // If we're dispatching this task while it is currently running on the same
+    // thread, set the `DISPATCH_AT_END` flag in the dispatch options to tell
+    // our threadpool target to not bother to spin up another thread.
+    let currently_polling = POLLING_TASK.with(|t| t.get() == Arc::as_ptr(&config));
+
+    // SAFETY: We use the POLLING_TASK thread local to check if we meet the
+    // requirements for `at_end`.
+    let options = unsafe { config.options.at_end(currently_polling) };
+
+    // Build the RunnableBuilder for our task to be dispatched.
+    let config2 = config.clone();
+    let builder = RunnableBuilder::new(config.name, move || {
+        // Record the pointer for the currently executing task in the
+        // POLLING_TASK thread-local so that nested dispatches can detect it.
+        POLLING_TASK.with(|t| {
+            let prev = t.get();
+            t.set(Arc::as_ptr(&config2));
+            runnable.run();
+            t.set(prev);
+        });
+    })
+    .priority(config.priority)
+    .options(options);
+
+    let rv = match &config.target {
+        SpawnTarget::BackgroundTask => builder.dispatch_background_task(),
+        SpawnTarget::EventTarget(target) => builder.dispatch(&*target),
+    };
+    if let Err(err) = rv {
+        log::warn!(
+            "dispatch for spawned task '{}' failed: {:?}",
+            config.name,
+            err
+        );
+    }
+}
+
+/// Helper for starting an async task which will run a future to completion.
+#[derive(Debug)]
+pub struct TaskBuilder<F> {
+    name: &'static str,
+    future: F,
+    priority: u32,
+    options: DispatchOptions,
+}
+
+impl<F> TaskBuilder<F> {
+    pub fn new(name: &'static str, future: F) -> TaskBuilder<F> {
+        TaskBuilder {
+            name,
+            future,
+            priority: nsIRunnablePriority::PRIORITY_NORMAL,
+            options: DispatchOptions::default(),
+        }
+    }
+
+    /// Specify the priority of the task's runnables.
+    pub fn priority(mut self, priority: u32) -> Self {
+        self.priority = priority;
+        self
+    }
+
+    /// Specify options to use when dispatching the task.
+    pub fn options(mut self, options: DispatchOptions) -> Self {
+        self.options = options;
+        self
+    }
+
+    /// Set whether or not the event may block, and should be run on the IO
+    /// thread pool.
+    pub fn may_block(mut self, may_block: bool) -> Self {
+        self.options = self.options.may_block(may_block);
+        self
+    }
+}
+
+impl<F> TaskBuilder<F>
+where
+    F: Future + Send + 'static,
+    F::Output: Send + 'static,
+{
+    /// Run the future on the background task pool.
+    pub fn spawn(self) -> AsyncTask<F::Output> {
+        let config = Arc::new(TaskSpawnConfig {
+            name: self.name,
+            priority: self.priority,
+            options: self.options,
+            target: SpawnTarget::BackgroundTask,
+        });
+        let (runnable, task) = async_task::spawn(self.future, move |runnable| {
+            schedule(config.clone(), runnable)
+        });
+        runnable.schedule();
+        AsyncTask::new(task)
+    }
+
+    /// Run the future on the specified nsIEventTarget.
+    pub fn spawn_onto(self, target: &nsIEventTarget) -> AsyncTask<F::Output> {
+        let config = Arc::new(TaskSpawnConfig {
+            name: self.name,
+            priority: self.priority,
+            options: self.options,
+            target: SpawnTarget::EventTarget(RefPtr::new(target)),
+        });
+        let (runnable, task) = async_task::spawn(self.future, move |runnable| {
+            schedule(config.clone(), runnable)
+        });
+        runnable.schedule();
+        AsyncTask::new(task)
+    }
+}
+
+impl<F> TaskBuilder<F>
+where
+    F: Future + 'static,
+    F::Output: 'static,
+{
+    /// Run the future on the current thread.
+    ///
+    /// Unlike the other `spawn` methods, this method supports non-Send futures.
+    ///
+    /// # Panics
+    ///
+    /// This method may panic if run on a thread which cannot run local futures
+    /// (e.g. due to it is not being an XPCOM thread, or if we are very late
+    /// during shutdown).
+    pub fn spawn_local(self) -> AsyncTask<F::Output> {
+        let current_thread = get_current_thread().expect("cannot get current thread");
+        let config = Arc::new(TaskSpawnConfig {
+            name: self.name,
+            priority: self.priority,
+            options: self.options,
+            target: SpawnTarget::EventTarget(RefPtr::new(current_thread.coerce())),
+        });
+        let (runnable, task) = async_task::spawn_local(self.future, move |runnable| {
+            schedule(config.clone(), runnable)
+        });
+        runnable.schedule();
+        AsyncTask::new(task)
+    }
+}
+
+/// Spawn a future onto the background task pool. The future will not be run on
+/// the main thread.
+pub fn spawn<F>(name: &'static str, future: F) -> AsyncTask<F::Output>
+where
+    F: Future + Send + 'static,
+    F::Output: Send + 'static,
+{
+    TaskBuilder::new(name, future).spawn()
+}
+
+/// Spawn a potentially-blocking future onto the background task pool. The
+/// future will not be run on the main thread.
+pub fn spawn_blocking<F>(name: &'static str, future: F) -> AsyncTask<F::Output>
+where
+    F: Future + Send + 'static,
+    F::Output: Send + 'static,
+{
+    TaskBuilder::new(name, future).may_block(true).spawn()
+}
+
+/// Spawn a local future onto the current thread.
+pub fn spawn_local<F>(name: &'static str, future: F) -> AsyncTask<F::Output>
+where
+    F: Future + 'static,
+    F::Output: 'static,
+{
+    TaskBuilder::new(name, future).spawn_local()
+}
+
+pub fn spawn_onto<F>(name: &'static str, target: &nsIEventTarget, future: F) -> AsyncTask<F::Output>
+where
+    F: Future + Send + 'static,
+    F::Output: Send + 'static,
+{
+    TaskBuilder::new(name, future).spawn_onto(target)
+}
+
+pub fn spawn_onto_blocking<F>(
+    name: &'static str,
+    target: &nsIEventTarget,
+    future: F,
+) -> AsyncTask<F::Output>
+where
+    F: Future + Send + 'static,
+    F::Output: Send + 'static,
+{
+    TaskBuilder::new(name, future)
+        .may_block(true)
+        .spawn_onto(target)
+}
diff --git a/xpcom/rust/moz_task/src/lib.rs b/xpcom/rust/moz_task/src/lib.rs
new file mode 100644
index 0000000000..7c1ea3f970
--- /dev/null
+++ b/xpcom/rust/moz_task/src/lib.rs
@@ -0,0 +1,377 @@
+/* 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 http://mozilla.org/MPL/2.0/. */
+
+//! This module wraps XPCOM threading functions with Rust functions
+//! to make it safer and more convenient to call the XPCOM functions.
+//! It also provides the Task trait and TaskRunnable struct,
+//! which make it easier to dispatch tasks to threads.
+
+mod dispatcher;
+pub use dispatcher::{dispatch_background_task, dispatch_local, dispatch_onto, RunnableBuilder};
+mod event_loop;
+mod executor;
+pub use executor::{
+    spawn, spawn_blocking, spawn_local, spawn_onto, spawn_onto_blocking, AsyncTask, TaskBuilder,
+};
+
+// Expose functions intended to be used only in gtest via this module.
+// We don't use a feature gate here to stop the need to compile all crates that
+// depend upon `moz_task` twice.
+pub mod gtest_only {
+    pub use crate::event_loop::spin_event_loop_until;
+}
+
+use nserror::nsresult;
+use nsstring::{nsACString, nsCString};
+use std::{ffi::CStr, marker::PhantomData, mem, ptr};
+use xpcom::{
+    getter_addrefs,
+    interfaces::{nsIEventTarget, nsIRunnable, nsISerialEventTarget, nsISupports, nsIThread},
+    AtomicRefcnt, RefCounted, RefPtr, XpCom,
+};
+
+extern "C" {
+    fn NS_GetCurrentThreadRust(result: *mut *const nsIThread) -> nsresult;
+    fn NS_GetMainThreadRust(result: *mut *const nsIThread) -> nsresult;
+    fn NS_IsMainThread() -> bool;
+    fn NS_NewNamedThreadWithDefaultStackSize(
+        name: *const nsACString,
+        result: *mut *const nsIThread,
+        event: *const nsIRunnable,
+    ) -> nsresult;
+    fn NS_IsOnCurrentThread(target: *const nsIEventTarget) -> bool;
+    fn NS_ProxyReleaseISupports(
+        name: *const libc::c_char,
+        target: *const nsIEventTarget,
+        doomed: *const nsISupports,
+        always_proxy: bool,
+    );
+    fn NS_CreateBackgroundTaskQueue(
+        name: *const libc::c_char,
+        target: *mut *const nsISerialEventTarget,
+    ) -> nsresult;
+    fn NS_DispatchBackgroundTask(event: *const nsIRunnable, flags: u32) -> nsresult;
+}
+
+pub fn get_current_thread() -> Result<RefPtr<nsIThread>, nsresult> {
+    getter_addrefs(|p| unsafe { NS_GetCurrentThreadRust(p) })
+}
+
+pub fn get_main_thread() -> Result<RefPtr<nsIThread>, nsresult> {
+    getter_addrefs(|p| unsafe { NS_GetMainThreadRust(p) })
+}
+
+pub fn is_main_thread() -> bool {
+    unsafe { NS_IsMainThread() }
+}
+
+pub fn create_thread(name: &str) -> Result<RefPtr<nsIThread>, nsresult> {
+    getter_addrefs(|p| unsafe {
+        NS_NewNamedThreadWithDefaultStackSize(&*nsCString::from(name), p, ptr::null())
+    })
+}
+
+pub fn is_on_current_thread(target: &nsIEventTarget) -> bool {
+    unsafe { NS_IsOnCurrentThread(target) }
+}
+
+/// Creates a queue that runs tasks on the background thread pool. The tasks
+/// will run in the order they're dispatched, one after the other.
+pub fn create_background_task_queue(
+    name: &'static CStr,
+) -> Result<RefPtr<nsISerialEventTarget>, nsresult> {
+    getter_addrefs(|p| unsafe { NS_CreateBackgroundTaskQueue(name.as_ptr(), p) })
+}
+
+/// Dispatches a one-shot runnable to an event target, like a thread or a
+/// task queue, with the given options.
+///
+/// This function leaks the runnable if dispatch fails.
+///
+/// # Safety
+///
+/// As there is no guarantee that the runnable is actually `Send + Sync`, we
+/// can't know that it's safe to dispatch an `nsIRunnable` to any
+/// `nsIEventTarget`.
+pub unsafe fn dispatch_runnable(
+    runnable: &nsIRunnable,
+    target: &nsIEventTarget,
+    options: DispatchOptions,
+) -> Result<(), nsresult> {
+    // NOTE: DispatchFromScript performs an AddRef on `runnable` which is
+    // why this function leaks on failure.
+    target
+        .DispatchFromScript(runnable, options.flags())
+        .to_result()
+}
+
+/// Dispatches a one-shot task runnable to the background thread pool with the
+/// given options. The task may run concurrently with other background tasks.
+/// If you need tasks to run in a specific order, please create a background
+/// task queue using `create_background_task_queue`, and dispatch tasks to it
+/// instead.
+///
+/// This function leaks the runnable if dispatch fails. This avoids a race where
+/// a runnable can be destroyed on either the original or target thread, which
+/// is important if the runnable holds thread-unsafe members.
+///
+/// ### Safety
+///
+/// As there is no guarantee that the runnable is actually `Send + Sync`, we
+/// can't know that it's safe to dispatch an `nsIRunnable` to any
+/// `nsIEventTarget`.
+pub unsafe fn dispatch_background_task_runnable(
+    runnable: &nsIRunnable,
+    options: DispatchOptions,
+) -> Result<(), nsresult> {
+    // This eventually calls the non-`already_AddRefed<nsIRunnable>` overload of
+    // `nsIEventTarget::Dispatch` (see xpcom/threads/nsIEventTarget.idl#20-25),
+    // which adds an owning reference and leaks if dispatch fails.
+    NS_DispatchBackgroundTask(runnable, options.flags()).to_result()
+}
+
+/// Options to control how task runnables are dispatched.
+///
+/// NOTE: The `DISPATCH_SYNC` flag is intentionally not supported by this type.
+#[derive(Copy, Clone, Debug, Eq, Hash, PartialEq)]
+pub struct DispatchOptions(u32);
+
+impl Default for DispatchOptions {
+    #[inline]
+    fn default() -> Self {
+        DispatchOptions(nsIEventTarget::DISPATCH_NORMAL)
+    }
+}
+
+impl DispatchOptions {
+    /// Creates a blank set of options. The runnable will be dispatched using
+    /// the default mode.
+    #[inline]
+    pub fn new() -> Self {
+        DispatchOptions::default()
+    }
+
+    /// Indicates whether or not the dispatched runnable may block its target
+    /// thread by waiting on I/O. If `true`, the runnable may be dispatched to a
+    /// dedicated thread pool, leaving the main pool free for CPU-bound tasks.
+    #[inline]
+    pub fn may_block(self, may_block: bool) -> DispatchOptions {
+        const FLAG: u32 = nsIEventTarget::DISPATCH_EVENT_MAY_BLOCK;
+        if may_block {
+            DispatchOptions(self.flags() | FLAG)
+        } else {
+            DispatchOptions(self.flags() & !FLAG)
+        }
+    }
+
+    /// Specifies that the dispatch is occurring from a running event that was
+    /// dispatched to the same event target, and that event is about to finish.
+    ///
+    /// A thread pool can use this as an optimization hint to not spin up
+    /// another thread, since the current thread is about to become idle.
+    ///
+    /// Setting this flag is unsafe, as it may only be used from the target
+    /// event target when the event is about to finish.
+    #[inline]
+    pub unsafe fn at_end(self, may_block: bool) -> DispatchOptions {
+        const FLAG: u32 = nsIEventTarget::DISPATCH_AT_END;
+        if may_block {
+            DispatchOptions(self.flags() | FLAG)
+        } else {
+            DispatchOptions(self.flags() & !FLAG)
+        }
+    }
+
+    /// Returns the set of bitflags to pass to `DispatchFromScript`.
+    #[inline]
+    fn flags(self) -> u32 {
+        self.0
+    }
+}
+
+/// A task represents an operation that asynchronously executes on a target
+/// thread, and returns its result to the original thread.
+///
+/// # Alternatives
+///
+/// This trait is no longer necessary for basic tasks to be dispatched to
+/// another thread with a callback on the originating thread. `moz_task` now has
+/// a series of more rust-like primitives which can be used instead. For
+/// example, it may be preferable to use the async executor over `Task`:
+///
+/// ```ignore
+/// // Spawn a task onto the background task pool, and capture the result of its
+/// // execution.
+/// let bg_task = moz_task::spawn("Example", async move {
+///     do_background_work(captured_state)
+/// });
+///
+/// // Spawn another task on the calling thread which will await on the result
+/// // of the async operation, and invoke a non-Send callback. This task won't
+/// // be awaited on, so needs to be `detach`-ed.
+/// moz_task::spawn_local("Example", async move {
+///     callback.completed(bg_task.await);
+/// })
+/// .detach();
+/// ```
+///
+/// If no result is needed, the task returned from `spawn` may be also detached
+/// directly.
+pub trait Task {
+    // FIXME: These could accept `&mut`.
+    fn run(&self);
+    fn done(&self) -> Result<(), nsresult>;
+}
+
+pub struct TaskRunnable {
+    name: &'static str,
+    task: Box<dyn Task + Send + Sync>,
+}
+
+impl TaskRunnable {
+    // XXX: Fixme: clean up this old API. (bug 1744312)
+    pub fn new(
+        name: &'static str,
+        task: Box<dyn Task + Send + Sync>,
+    ) -> Result<TaskRunnable, nsresult> {
+        Ok(TaskRunnable { name, task })
+    }
+
+    pub fn dispatch(self, target: &nsIEventTarget) -> Result<(), nsresult> {
+        self.dispatch_with_options(target, DispatchOptions::default())
+    }
+
+    pub fn dispatch_with_options(
+        self,
+        target: &nsIEventTarget,
+        options: DispatchOptions,
+    ) -> Result<(), nsresult> {
+        // Perform `task.run()` on a background thread.
+        let task = self.task;
+        let handle = TaskBuilder::new(self.name, async move {
+            task.run();
+            task
+        })
+        .options(options)
+        .spawn_onto(target);
+
+        // Run `task.done()` on the starting thread once the background thread
+        // is done with the task.
+        spawn_local(self.name, async move {
+            let task = handle.await;
+            let _ = task.done();
+        })
+        .detach();
+        Ok(())
+    }
+
+    pub fn dispatch_background_task_with_options(
+        self,
+        options: DispatchOptions,
+    ) -> Result<(), nsresult> {
+        // Perform `task.run()` on a background thread.
+        let task = self.task;
+        let handle = TaskBuilder::new(self.name, async move {
+            task.run();
+            task
+        })
+        .options(options)
+        .spawn();
+
+        // Run `task.done()` on the starting thread once the background thread
+        // is done with the task.
+        spawn_local(self.name, async move {
+            let task = handle.await;
+            let _ = task.done();
+        })
+        .detach();
+        Ok(())
+    }
+}
+
+pub type ThreadPtrHandle<T> = RefPtr<ThreadPtrHolder<T>>;
+
+/// A Rust analog to `nsMainThreadPtrHolder` that wraps an `nsISupports` object
+/// with thread-safe refcounting. The holder keeps one reference to the wrapped
+/// object that's released when the holder's refcount reaches zero.
+pub struct ThreadPtrHolder<T: XpCom + 'static> {
+    ptr: *const T,
+    marker: PhantomData<T>,
+    name: &'static CStr,
+    owning_thread: RefPtr<nsIThread>,
+    refcnt: AtomicRefcnt,
+}
+
+unsafe impl<T: XpCom + 'static> Send for ThreadPtrHolder<T> {}
+unsafe impl<T: XpCom + 'static> Sync for ThreadPtrHolder<T> {}
+
+unsafe impl<T: XpCom + 'static> RefCounted for ThreadPtrHolder<T> {
+    unsafe fn addref(&self) {
+        self.refcnt.inc();
+    }
+
+    unsafe fn release(&self) {
+        let rc = self.refcnt.dec();
+        if rc == 0 {
+            // Once the holder's count reaches zero, release the wrapped
+            // object...
+            if !self.ptr.is_null() {
+                // The holder can be released on any thread. If we're on the
+                // owning thread, we can release the object directly. Otherwise,
+                // we need to post a proxy release event to release the object
+                // on the owning thread.
+                if is_on_current_thread(&self.owning_thread) {
+                    (*self.ptr).release()
+                } else {
+                    NS_ProxyReleaseISupports(
+                        self.name.as_ptr(),
+                        self.owning_thread.coerce(),
+                        self.ptr as *const T as *const nsISupports,
+                        false,
+                    );
+                }
+            }
+            // ...And deallocate the holder.
+            mem::drop(Box::from_raw(self as *const Self as *mut Self));
+        }
+    }
+}
+
+impl<T: XpCom + 'static> ThreadPtrHolder<T> {
+    /// Creates a new owning thread pointer holder. Returns an error if the
+    /// thread manager has shut down. Panics if `name` isn't a valid C string.
+    pub fn new(name: &'static CStr, ptr: RefPtr<T>) -> Result<RefPtr<Self>, nsresult> {
+        let owning_thread = get_current_thread()?;
+        // Take ownership of the `RefPtr`. This does _not_ decrement its
+        // refcount, which is what we want. Once we've released all references
+        // to the holder, we'll release the wrapped `RefPtr`.
+        let raw: *const T = &*ptr;
+        mem::forget(ptr);
+        unsafe {
+            let boxed = Box::new(ThreadPtrHolder {
+                name,
+                ptr: raw,
+                marker: PhantomData,
+                owning_thread,
+                refcnt: AtomicRefcnt::new(),
+            });
+            Ok(RefPtr::from_raw(Box::into_raw(boxed)).unwrap())
+        }
+    }
+
+    /// Returns the wrapped object's owning thread.
+    pub fn owning_thread(&self) -> &nsIThread {
+        &self.owning_thread
+    }
+
+    /// Returns the wrapped object if called from the owning thread, or
+    /// `None` if called from any other thread.
+    pub fn get(&self) -> Option<&T> {
+        if is_on_current_thread(&self.owning_thread) && !self.ptr.is_null() {
+            unsafe { Some(&*self.ptr) }
+        } else {
+            None
+        }
+    }
+}