runtime: add Handle::try_current (#2118)

* runtime: add Handle::try_current

Makes it possible to get a Handle only if a Runtime has been started, without panicing if that isn't the case

* Use an error instead

tokio/src/runtime/handle.rs

@@ -1,4 +1,5 @@
 use crate::runtime::{blocking, context, io, time, Spawner};
+use std::{error, fmt};
 
 cfg_rt_core! {
     use crate::task::JoinHandle;
@@ -61,6 +62,15 @@ impl Handle {
     pub fn current() -> Self {
         context::current().expect("not currently running on the Tokio runtime.")
     }
+
+    /// Returns a Handle view over the currently running Runtime
+    ///
+    /// Returns an error if no Runtime has been started
+    ///
+    /// Contrary to `current`, this never panics
+    pub fn try_current() -> Result<Self, TryCurrentError> {
+        context::current().ok_or(TryCurrentError(()))
+    }
 }
 
 cfg_rt_core! {
@@ -105,3 +115,20 @@ cfg_rt_core! {
         }
     }
 }
+
+/// Error returned by `try_current` when no Runtime has been started
+pub struct TryCurrentError(());
+
+impl fmt::Debug for TryCurrentError {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("TryCurrentError").finish()
+    }
+}
+
+impl fmt::Display for TryCurrentError {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.write_str("no tokio Runtime has been initialized")
+    }
+}
+
+impl error::Error for TryCurrentError {}

tokio/src/runtime/mod.rs

@@ -208,7 +208,7 @@ pub(crate) mod enter;
 use self::enter::enter;
 
 mod handle;
-pub use self::handle::Handle;
+pub use self::handle::{Handle, TryCurrentError};
 
 mod io;