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diff --git a/components/remutex/lib.rs b/components/remutex/lib.rs
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+/* 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/. */
+
+//! An implementation of re-entrant mutexes.
+//!
+//! Re-entrant mutexes are like mutexes, but where it is expected
+//! that a single thread may own a lock more than once.
+
+//! It provides the same interface as https://github.com/rust-lang/rust/blob/master/src/libstd/sys/common/remutex.rs
+//! so if those types are ever exported, we should be able to replace this implemtation.
+
+#![feature(nonzero)]
+
+extern crate core;
+#[macro_use] extern crate lazy_static;
+#[macro_use] extern crate log;
+
+use core::nonzero::NonZero;
+use std::cell::{Cell, UnsafeCell};
+use std::ops::Deref;
+use std::sync::{LockResult, Mutex, MutexGuard, PoisonError, TryLockError, TryLockResult};
+use std::sync::atomic::{AtomicUsize, Ordering};
+
+/// A type for thread ids.
+
+// TODO: can we use the thread-id crate for this?
+
+#[derive(Copy, Clone, Eq, PartialEq, Ord, PartialOrd, Hash, Debug)]
+pub struct ThreadId(NonZero<usize>);
+
+lazy_static!{ static ref THREAD_COUNT: AtomicUsize = AtomicUsize::new(1); }
+
+impl ThreadId {
+    #[allow(unsafe_code)]
+    fn new() -> ThreadId {
+        let number = THREAD_COUNT.fetch_add(1, Ordering::SeqCst);
+        ThreadId(unsafe { NonZero::new(number) })
+    }
+    pub fn current() -> ThreadId {
+        THREAD_ID.with(|tls| tls.clone())
+    }
+}
+
+thread_local!{ static THREAD_ID: ThreadId = ThreadId::new() }
+
+/// A type for atomic storage of thread ids.
+#[derive(Debug)]
+pub struct AtomicOptThreadId(AtomicUsize);
+
+impl AtomicOptThreadId {
+    pub fn new() -> AtomicOptThreadId {
+        AtomicOptThreadId(AtomicUsize::new(0))
+    }
+    pub fn store(&self, value: Option<ThreadId>, ordering: Ordering) {
+        let number = value.map(|id| *id.0).unwrap_or(0);
+        self.0.store(number, ordering);
+    }
+    #[allow(unsafe_code)]
+    pub fn load(&self, ordering: Ordering) -> Option<ThreadId> {
+        let number = self.0.load(ordering);
+        if number == 0 { None } else { Some(ThreadId(unsafe { NonZero::new(number) })) }
+    }
+    #[allow(unsafe_code)]
+    pub fn swap(&self, value: Option<ThreadId>, ordering: Ordering) -> Option<ThreadId> {
+        let number = value.map(|id| *id.0).unwrap_or(0);
+        let number = self.0.swap(number, ordering);
+        if number == 0 { None } else { Some(ThreadId(unsafe { NonZero::new(number) })) }
+    }
+}
+
+/// A type for hand-over-hand mutexes.
+///
+/// These support `lock` and `unlock` functions. `lock` blocks waiting to become the
+/// mutex owner. `unlock` can only be called by the lock owner, and panics otherwise.
+/// They have the same happens-before and poisoning semantics as `Mutex`.
+
+// TODO: Can we use `raw_lock` and `raw_unlock` from `parking_lot`'s `Mutex` for this?
+
+pub struct HandOverHandMutex {
+    mutex: Mutex<()>,
+    owner: AtomicOptThreadId,
+    guard: UnsafeCell<Option<MutexGuard<'static, ()>>>,
+}
+
+impl HandOverHandMutex {
+    pub fn new() -> HandOverHandMutex {
+        HandOverHandMutex {
+            mutex: Mutex::new(()),
+            owner: AtomicOptThreadId::new(),
+            guard: UnsafeCell::new(None),
+        }
+    }
+    #[allow(unsafe_code)]
+    unsafe fn set_guard_and_owner<'a>(&'a self, guard: MutexGuard<'a, ()>) {
+        // The following two lines allow us to unsafely store
+        // Some(guard): Option<MutexGuard<'a, ()>
+        // in self.guard, even though its contents are Option<MutexGuard<'static, ()>>,
+        // that is the lifetime is 'a not 'static.
+        let guard_ptr = &mut *(self.guard.get() as *mut u8 as *mut Option<MutexGuard<'a, ()>>);
+        *guard_ptr = Some(guard);
+        self.owner.store(Some(ThreadId::current()), Ordering::Relaxed);
+    }
+    #[allow(unsafe_code)]
+    unsafe fn unset_guard_and_owner(&self) {
+        let guard_ptr = &mut *self.guard.get();
+        let old_owner = self.owner();
+        self.owner.store(None, Ordering::Relaxed);
+        // Make sure we release the lock before checking the assertions.
+        // We protect logging by a re-entrant lock, so we don't want
+        // to do any incidental logging while we the lock is held.
+        drop(guard_ptr.take());
+        // Now we have released the lock, it's okay to use logging.
+        assert_eq!(old_owner, Some(ThreadId::current()));
+    }
+    #[allow(unsafe_code)]
+    pub fn lock<'a>(&'a self) -> LockResult<()> {
+        let (guard, result) = match self.mutex.lock() {
+            Ok(guard) => (guard, Ok(())),
+            Err(err) => (err.into_inner(), Err(PoisonError::new(()))),
+        };
+        unsafe { self.set_guard_and_owner(guard); }
+        result
+    }
+    #[allow(unsafe_code)]
+    pub fn try_lock(&self) -> TryLockResult<()> {
+        let (guard, result) = match self.mutex.try_lock() {
+            Ok(guard) => (guard, Ok(())),
+            Err(TryLockError::WouldBlock) => return Err(TryLockError::WouldBlock),
+            Err(TryLockError::Poisoned(err)) => (err.into_inner(), Err(TryLockError::Poisoned(PoisonError::new(())))),
+        };
+        unsafe { self.set_guard_and_owner(guard); }
+        result
+    }
+    #[allow(unsafe_code)]
+    pub fn unlock(&self) {
+        unsafe { self.unset_guard_and_owner(); }
+    }
+    pub fn owner(&self) -> Option<ThreadId> {
+        self.owner.load(Ordering::Relaxed)
+    }
+}
+
+#[allow(unsafe_code)]
+unsafe impl Send for HandOverHandMutex {}
+
+/// A type for re-entrant mutexes.
+///
+/// It provides the same interface as https://github.com/rust-lang/rust/blob/master/src/libstd/sys/common/remutex.rs
+
+pub struct ReentrantMutex<T> {
+    mutex: HandOverHandMutex,
+    count: Cell<usize>,
+    data: T,
+}
+
+#[allow(unsafe_code)]
+unsafe impl<T> Sync for ReentrantMutex<T> where T: Send {}
+
+impl<T> ReentrantMutex<T> {
+    pub fn new(data: T) -> ReentrantMutex<T> {
+        debug!("{:?} Creating new lock.", ThreadId::current());
+        ReentrantMutex {
+            mutex: HandOverHandMutex::new(),
+            count: Cell::new(0),
+            data: data,
+        }
+    }
+
+    pub fn lock(&self) -> LockResult<ReentrantMutexGuard<T>> {
+        debug!("{:?} Locking.", ThreadId::current());
+        if self.mutex.owner() != Some(ThreadId::current()) {
+            debug!("{:?} Becoming owner.", ThreadId::current());
+            if let Err(_) = self.mutex.lock() {
+                debug!("{:?} Poison!", ThreadId::current());
+                return Err(PoisonError::new(self.mk_guard()));
+            }
+            debug!("{:?} Became owner.", ThreadId::current());
+        }
+        Ok(self.mk_guard())
+    }
+
+    pub fn try_lock(&self) -> TryLockResult<ReentrantMutexGuard<T>> {
+        debug!("{:?} Try locking.", ThreadId::current());
+        if self.mutex.owner() != Some(ThreadId::current()) {
+            debug!("{:?} Becoming owner?", ThreadId::current());
+            if let Err(err) = self.mutex.try_lock() {
+                match err {
+                    TryLockError::WouldBlock => {
+                        debug!("{:?} Would block.", ThreadId::current());
+                        return Err(TryLockError::WouldBlock)
+                    },
+                    TryLockError::Poisoned(_) => {
+                        debug!("{:?} Poison!", ThreadId::current());
+                        return Err(TryLockError::Poisoned(PoisonError::new(self.mk_guard())));
+                    },
+                }
+            }
+            debug!("{:?} Became owner.", ThreadId::current());
+        }
+        Ok(self.mk_guard())
+    }
+
+    fn unlock(&self) {
+        debug!("{:?} Unlocking.", ThreadId::current());
+        let count = self.count.get().checked_sub(1).expect("Underflowed lock count.");
+        debug!("{:?} Decrementing count to {}.", ThreadId::current(), count);
+        self.count.set(count);
+        if count == 0 {
+            debug!("{:?} Releasing mutex.", ThreadId::current());
+            self.mutex.unlock();
+        }
+    }
+
+    fn mk_guard(&self) -> ReentrantMutexGuard<T> {
+        let count = self.count.get().checked_add(1).expect("Overflowed lock count.");
+        debug!("{:?} Incrementing count to {}.", ThreadId::current(), count);
+        self.count.set(count);
+        ReentrantMutexGuard { mutex: self }
+    }
+}
+
+#[must_use]
+pub struct ReentrantMutexGuard<'a, T> where T: 'static {
+    mutex: &'a ReentrantMutex<T>,
+}
+
+impl<'a, T> Drop for ReentrantMutexGuard<'a, T> {
+    #[allow(unsafe_code)]
+    fn drop(&mut self) {
+        self.mutex.unlock()
+    }
+}
+
+impl<'a, T> Deref for ReentrantMutexGuard<'a, T> {
+    type Target = T;
+    fn deref(&self) -> &T {
+        &self.mutex.data
+    }
+}