tools: add AsyncMutex::new_locked

Allows creating a pre-locked mutex, returning the mutex and
a guard.

Signed-off-by: Wolfgang Bumiller <w.bumiller@proxmox.com>

src/tools/async_mutex.rs

@@ -1,6 +1,7 @@
 use std::marker::PhantomData;
 
-use futures::Poll;
+use failure::{bail, Error};
+use futures::{Async, Poll};
 use futures::future::Future;
 use tokio::sync::lock::Lock as TokioLock;
 pub use tokio::sync::lock::LockGuard as AsyncLockGuard;
@@ -21,6 +22,15 @@ impl<T> AsyncMutex<T> {
             _error: PhantomData,
         }
     }
+
+    pub fn new_locked(value: T) -> Result<(Self, AsyncLockGuard<T>), Error> {
+        let mut this = Self::new(value);
+        let guard = match this.0.poll_lock() {
+            Async::Ready(guard) => guard,
+            _ => bail!("failed to create locked mutex"),
+        };
+        Ok((this, guard))
+    }
 }
 
 /// Represents a lock to be held in the future: