rt: add a Handle::current() (#2040)

Adds `Handle::current()` for accessing a handle to the runtime
associated with the current thread. This handle can then be
passed to other threads in order to spawn or perform other
runtime related tasks.

tokio/src/runtime/blocking/pool.rs

@@ -8,7 +8,6 @@ use crate::runtime::blocking::task::BlockingTask;
 use crate::runtime::{self, context::ThreadContext, io, time, Builder, Callback};
 use crate::task::{self, JoinHandle};
 
-use std::cell::Cell;
 use std::collections::VecDeque;
 use std::fmt;
 use std::time::Duration;
@@ -68,11 +67,6 @@ struct Shared {
 
 type Task = task::Task<NoopSchedule>;
 
-thread_local! {
-    /// Thread-local tracking the current executor
-    static BLOCKING: Cell<Option<*const Spawner>> = Cell::new(None)
-}
-
 const KEEP_ALIVE: Duration = Duration::from_secs(10);
 
 /// Run the provided function on an executor dedicated to blocking operations.
@@ -80,16 +74,14 @@ pub(crate) fn spawn_blocking<F, R>(func: F) -> JoinHandle<R>
 where
     F: FnOnce() -> R + Send + 'static,
 {
-    BLOCKING.with(|cell| {
-        let schedule = match cell.get() {
-            Some(ptr) => unsafe { &*ptr },
-            None => panic!("not currently running on the Tokio runtime."),
-        };
+    use crate::runtime::context::ThreadContext;
 
-        let (task, handle) = task::joinable(BlockingTask::new(func));
-        schedule.schedule(task);
-        handle
-    })
+    let schedule =
+        ThreadContext::blocking_spawner().expect("not currently running on the Tokio runtime.");
+
+    let (task, handle) = task::joinable(BlockingTask::new(func));
+    schedule.schedule(task);
+    handle
 }
 
 // ===== impl BlockingPool =====
@@ -168,30 +160,10 @@ impl Spawner {
     where
         F: FnOnce() -> R,
     {
-        // While scary, this is safe. The function takes a `&BlockingPool`,
-        // which guarantees that the reference lives for the duration of
-        // `with_pool`.
-        //
-        // Because we are always clearing the TLS value at the end of the
-        // function, we can cast the reference to 'static which thread-local
-        // cells require.
-        BLOCKING.with(|cell| {
-            let was = cell.replace(None);
+        let ctx = crate::runtime::context::ThreadContext::clone_current();
+        let _e = ctx.with_blocking_spawner(self.clone()).enter();
 
-            // Ensure that the pool is removed from the thread-local context
-            // when leaving the scope. This handles cases that involve panicking.
-            struct Reset<'a>(&'a Cell<Option<*const Spawner>>, Option<*const Spawner>);
-
-            impl Drop for Reset<'_> {
-                fn drop(&mut self) {
-                    self.0.set(self.1);
-                }
-            }
-
-            let _reset = Reset(cell, was);
-            cell.set(Some(self as *const Spawner));
-            f()
-        })
+        f()
     }
 
     fn schedule(&self, task: Task) {
@@ -248,6 +220,7 @@ impl Spawner {
             self.inner.io_handle.clone(),
             self.inner.time_handle.clone(),
             Some(self.inner.clock.clone()),
+            Some(self.clone()),
         );
         let spawner = self.clone();
         builder