implement task locals

dignifiedquire · dignifiedquire · commit 4f4d36bb8abd · 2020-04-26T18:23:21.000+02:00
diff --git a/src/task/block_on.rs b/src/task/block_on.rs
@@ -1,8 +1,6 @@
 use std::future::Future;
 
-use kv_log_macro::trace;
-
-use crate::task::Task;
+use crate::task::Builder;
 
 /// Spawns a task and blocks the current thread on its result.
 ///
@@ -31,33 +29,5 @@ pub fn block_on<F, T>(future: F) -> T
 where
     F: Future<Output = T>,
 {
-    // Create a new task handle.
-    let task = Task::new(None);
-
-    // Log this `block_on` operation.
-    trace!("block_on", {
-        task_id: task.id().0,
-        parent_task_id: Task::get_current(|t| t.id().0).unwrap_or(0),
-    });
-
-    let wrapped_future = async move {
-        // Drop task-locals on exit.
-        defer! {
-            Task::get_current(|t| unsafe { t.drop_locals() });
-        }
-
-        // Log completion on exit.
-        defer! {
-            trace!("completed", {
-                task_id: Task::get_current(|t| t.id().0),
-            });
-        }
-
-        future.await
-    };
-
-    once_cell::sync::Lazy::force(&crate::rt::RUNTIME);
-
-    // Run the future as a task.
-    unsafe { Task::set_current(&task, || smol::block_on(wrapped_future)) }
+    Builder::new().blocking(future)
 }
diff --git a/src/task/builder.rs b/src/task/builder.rs
@@ -1,9 +1,12 @@
 use std::future::Future;
+use std::pin::Pin;
+use std::sync::Arc;
+use std::task::{Context, Poll};
 
 use kv_log_macro::trace;
 
 use crate::io;
-use crate::task::{JoinHandle, Task};
+use crate::task::{JoinHandle, Task, TaskLocalsWrapper};
 
 /// Task builder that configures the settings of a new task.
 #[derive(Debug, Default)]
@@ -25,41 +28,83 @@ impl Builder {
         self
     }
 
+    fn build<F, T>(self, future: F) -> SupportTaskLocals<F>
+    where
+        F: Future<Output = T>,
+    {
+        let name = self.name.map(Arc::new);
+
+        // Create a new task handle.
+        let task = Task::new(name);
+
+        once_cell::sync::Lazy::force(&crate::rt::RUNTIME);
+
+        let tag = TaskLocalsWrapper::new(task.clone());
+
+        // FIXME: do not require all futures to be boxed.
+        SupportTaskLocals {
+            tag,
+            future: Box::pin(future),
+        }
+    }
+
     /// Spawns a task with the configured settings.
     pub fn spawn<F, T>(self, future: F) -> io::Result<JoinHandle<T>>
     where
         F: Future<Output = T> + Send + 'static,
         T: Send + 'static,
     {
-        // Create a new task handle.
-        let task = Task::new(self.name);
+        let wrapped = self.build(future);
 
         // Log this `spawn` operation.
         trace!("spawn", {
-            task_id: task.id().0,
-            parent_task_id: Task::get_current(|t| t.id().0).unwrap_or(0),
+            task_id: wrapped.tag.id().0,
+            parent_task_id: TaskLocalsWrapper::get_current(|t| t.id().0).unwrap_or(0),
         });
 
-        let wrapped_future = async move {
-            // Drop task-locals on exit.
-            defer! {
-                Task::get_current(|t| unsafe { t.drop_locals() });
-            }
-
-            // Log completion on exit.
-            defer! {
-                trace!("completed", {
-                    task_id: Task::get_current(|t| t.id().0),
-                });
-            }
-            future.await
-        };
+        let task = wrapped.tag.task().clone();
+        let smol_task = smol::Task::spawn(wrapped).detach();
 
-        once_cell::sync::Lazy::force(&crate::rt::RUNTIME);
+        Ok(JoinHandle::new(smol_task, task))
+    }
 
-        // FIXME: figure out how to set the current task.
+    /// Spawns a task with the configured settings, blocking on its execution.
+    pub fn blocking<F, T>(self, future: F) -> T
+    where
+        F: Future<Output = T>,
+    {
+        let wrapped = self.build(future);
 
-        let smol_task = smol::Task::spawn(wrapped_future).detach();
-        Ok(JoinHandle::new(smol_task, task))
+        // Log this `block_on` operation.
+        trace!("block_on", {
+            task_id: wrapped.tag.id().0,
+            parent_task_id: TaskLocalsWrapper::get_current(|t| t.id().0).unwrap_or(0),
+        });
+
+        // Run the future as a task.
+        unsafe { TaskLocalsWrapper::set_current(&wrapped.tag, || smol::block_on(wrapped)) }
+    }
+}
+
+/// Wrapper to add support for task locals.
+struct SupportTaskLocals<F> {
+    tag: TaskLocalsWrapper,
+    future: Pin<Box<F>>,
+}
+
+impl<F> Drop for SupportTaskLocals<F> {
+    fn drop(&mut self) {
+        // Log completion on exit.
+        trace!("completed", {
+            task_id: self.tag.id().0,
+        });
+    }
+}
+
+impl<F: Future> Future for SupportTaskLocals<F> {
+    type Output = F::Output;
+
+    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        unsafe { TaskLocalsWrapper::set_current(&self.tag, || Pin::new(&mut self.future).poll(cx)) }
     }
 }
diff --git a/src/task/current.rs b/src/task/current.rs
@@ -1,4 +1,4 @@
-use crate::task::Task;
+use crate::task::{Task, TaskLocalsWrapper};
 
 /// Returns a handle to the current task.
 ///
@@ -23,6 +23,6 @@ use crate::task::Task;
 /// # })
 /// ```
 pub fn current() -> Task {
-    Task::get_current(|t| t.clone())
+    TaskLocalsWrapper::get_current(|t| t.task().clone())
         .expect("`task::current()` called outside the context of a task")
 }
diff --git a/src/task/mod.rs b/src/task/mod.rs
@@ -142,6 +142,7 @@ cfg_default! {
     pub use task_local::{AccessError, LocalKey};
 
     pub(crate) use task_local::LocalsMap;
+    pub(crate) use task_locals_wrapper::TaskLocalsWrapper;
 
     mod block_on;
     mod builder;
@@ -153,6 +154,7 @@ cfg_default! {
     mod task;
     mod task_id;
     mod task_local;
+    mod task_locals_wrapper;
 
     #[cfg(any(feature = "unstable", test))]
     pub use spawn_blocking::spawn_blocking;
diff --git a/src/task/task.rs b/src/task/task.rs
@@ -1,74 +1,32 @@
-use std::cell::Cell;
 use std::fmt;
-use std::mem::ManuallyDrop;
-use std::ptr;
-use std::sync::atomic::{AtomicPtr, Ordering};
 use std::sync::Arc;
 
-use crate::task::{LocalsMap, TaskId};
-use crate::utils::abort_on_panic;
+use crate::task::TaskId;
 
-thread_local! {
-    /// A pointer to the currently running task.
-    static CURRENT: Cell<*const Task> = Cell::new(ptr::null_mut());
-}
-
-/// The inner representation of a task handle.
-struct Inner {
+/// A handle to a task.
+#[derive(Clone)]
+pub struct Task {
     /// The task ID.
     id: TaskId,
 
     /// The optional task name.
-    name: Option<Box<str>>,
-
-    /// The map holding task-local values.
-    locals: LocalsMap,
-}
-
-impl Inner {
-    #[inline]
-    fn new(name: Option<String>) -> Inner {
-        Inner {
-            id: TaskId::generate(),
-            name: name.map(String::into_boxed_str),
-            locals: LocalsMap::new(),
-        }
-    }
+    name: Option<Arc<String>>,
 }
 
-/// A handle to a task.
-pub struct Task {
-    /// The inner representation.
-    ///
-    /// This pointer is lazily initialized on first use. In most cases, the inner representation is
-    /// never touched and therefore we don't allocate it unless it's really needed.
-    inner: AtomicPtr<Inner>,
-}
-
-unsafe impl Send for Task {}
-unsafe impl Sync for Task {}
-
 impl Task {
     /// Creates a new task handle.
-    ///
-    /// If the task is unnamed, the inner representation of the task will be lazily allocated on
-    /// demand.
     #[inline]
-    pub(crate) fn new(name: Option<String>) -> Task {
-        let inner = match name {
-            None => AtomicPtr::default(),
-            Some(name) => {
-                let raw = Arc::into_raw(Arc::new(Inner::new(Some(name))));
-                AtomicPtr::new(raw as *mut Inner)
-            }
-        };
-        Task { inner }
+    pub(crate) fn new(name: Option<Arc<String>>) -> Task {
+        Task {
+            id: TaskId::generate(),
+            name,
+        }
     }
 
     /// Gets the task's unique identifier.
     #[inline]
     pub fn id(&self) -> TaskId {
-        self.inner().id
+        self.id
     }
 
     /// Returns the name of this task.
@@ -77,93 +35,7 @@ impl Task {
     ///
     /// [`Builder::name`]: struct.Builder.html#method.name
     pub fn name(&self) -> Option<&str> {
-        self.inner().name.as_ref().map(|s| &**s)
-    }
-
-    /// Returns the map holding task-local values.
-    pub(crate) fn locals(&self) -> &LocalsMap {
-        &self.inner().locals
-    }
-
-    /// Drops all task-local values.
-    ///
-    /// This method is only safe to call at the end of the task.
-    #[inline]
-    pub(crate) unsafe fn drop_locals(&self) {
-        let raw = self.inner.load(Ordering::Acquire);
-        if let Some(inner) = raw.as_mut() {
-            // Abort the process if dropping task-locals panics.
-            abort_on_panic(|| {
-                inner.locals.clear();
-            });
-        }
-    }
-
-    /// Returns the inner representation, initializing it on first use.
-    fn inner(&self) -> &Inner {
-        loop {
-            let raw = self.inner.load(Ordering::Acquire);
-            if !raw.is_null() {
-                return unsafe { &*raw };
-            }
-
-            let new = Arc::into_raw(Arc::new(Inner::new(None))) as *mut Inner;
-            if self.inner.compare_and_swap(raw, new, Ordering::AcqRel) != raw {
-                unsafe {
-                    drop(Arc::from_raw(new));
-                }
-            }
-        }
-    }
-
-    /// Set a reference to the current task.
-    pub(crate) unsafe fn set_current<F, R>(task: *const Task, f: F) -> R
-    where
-        F: FnOnce() -> R,
-    {
-        CURRENT.with(|current| {
-            let old_task = current.replace(task);
-            defer! {
-                current.set(old_task);
-            }
-            f()
-        })
-    }
-
-    /// Gets a reference to the current task.
-    pub(crate) fn get_current<F, R>(f: F) -> Option<R>
-    where
-        F: FnOnce(&Task) -> R,
-    {
-        let res = CURRENT.try_with(|current| unsafe { current.get().as_ref().map(f) });
-        match res {
-            Ok(Some(val)) => Some(val),
-            Ok(None) | Err(_) => None,
-        }
-    }
-}
-
-impl Drop for Task {
-    fn drop(&mut self) {
-        // Deallocate the inner representation if it was initialized.
-        let raw = *self.inner.get_mut();
-        if !raw.is_null() {
-            unsafe {
-                drop(Arc::from_raw(raw));
-            }
-        }
-    }
-}
-
-impl Clone for Task {
-    fn clone(&self) -> Task {
-        // We need to make sure the inner representation is initialized now so that this instance
-        // and the clone have raw pointers that point to the same `Arc<Inner>`.
-        let arc = unsafe { ManuallyDrop::new(Arc::from_raw(self.inner())) };
-        let raw = Arc::into_raw(Arc::clone(&arc));
-        Task {
-            inner: AtomicPtr::new(raw as *mut Inner),
-        }
+        self.name.as_ref().map(|s| s.as_str())
     }
 }
 
diff --git a/src/task/task_local.rs b/src/task/task_local.rs
@@ -3,7 +3,7 @@ use std::error::Error;
 use std::fmt;
 use std::sync::atomic::{AtomicU32, Ordering};
 
-use crate::task::Task;
+use crate::task::TaskLocalsWrapper;
 
 /// The key for accessing a task-local value.
 ///
@@ -98,7 +98,7 @@ impl<T: Send + 'static> LocalKey<T> {
     where
         F: FnOnce(&T) -> R,
     {
-        Task::get_current(|task| unsafe {
+        TaskLocalsWrapper::get_current(|task| unsafe {
             // Prepare the numeric key, initialization function, and the map of task-locals.
             let key = self.key();
             let init = || Box::new((self.__init)()) as Box<dyn Send>;
diff --git a/src/task/task_locals_wrapper.rs b/src/task/task_locals_wrapper.rs
