Use Drop impl to send worker idle message (#1718)

## Issue Addressed

NA

## Proposed Changes

Uses a `Drop` implementation to help ensure that `BeaconProcessor` workers are freed. This will help prevent against regression, if someone happens to add an early return and it will also help in the case of a panic.

## Additional Info

NA

beacon_node/network/src/beacon_processor/mod.rs

@@ -674,7 +674,16 @@ impl<T: BeaconChainTypes> BeaconProcessor<T> {
     /// Spawns a blocking worker thread to process some `Work`.
     ///
     /// Sends an message on `idle_tx` when the work is complete and the task is stopping.
-    fn spawn_worker(&mut self, mut idle_tx: mpsc::Sender<()>, work: Work<T::EthSpec>) {
+    fn spawn_worker(&mut self, idle_tx: mpsc::Sender<()>, work: Work<T::EthSpec>) {
+        // Wrap the `idle_tx` in a struct that will fire the idle message whenever it is dropped.
+        //
+        // This helps ensure that the worker is always freed in the case of an early exit or panic.
+        // As such, this instantiation should happen as early in the function as possible.
+        let send_idle_on_drop = SendOnDrop {
+            tx: idle_tx,
+            log: self.log.clone(),
+        };
+
         let work_id = work.str_id();
         let worker_timer =
             metrics::start_timer_vec(&metrics::BEACON_PROCESSOR_WORKER_TIME, &[work_id]);
@@ -804,16 +813,40 @@ impl<T: BeaconChainTypes> BeaconProcessor<T> {
                     "worker" => worker_id,
                 );
 
-                idle_tx.try_send(()).unwrap_or_else(|e| {
-                    crit!(
-                        log,
-                        "Unable to free worker";
-                        "msg" => "failed to send idle_tx message",
-                        "error" => e.to_string()
-                    )
-                });
+                // This explicit `drop` is used to remind the programmer that this variable must
+                // not be dropped until the worker is complete. Dropping it early will cause the
+                // worker to be marked as "free" and cause an over-spawning of workers.
+                drop(send_idle_on_drop);
             },
             WORKER_TASK_NAME,
         );
     }
 }
+
+/// This struct will send a message on `self.tx` when it is dropped. An error will be logged on
+/// `self.log` if the send fails (this happens when the node is shutting down).
+///
+/// ## Purpose
+///
+/// This is useful for ensuring that a worker-freed message is still sent if a worker panics.
+///
+/// The Rust docs for `Drop` state that `Drop` is called during an unwind in a panic:
+///
+/// https://doc.rust-lang.org/std/ops/trait.Drop.html#panics
+pub struct SendOnDrop {
+    tx: mpsc::Sender<()>,
+    log: Logger,
+}
+
+impl Drop for SendOnDrop {
+    fn drop(&mut self) {
+        if let Err(e) = self.tx.try_send(()) {
+            warn!(
+                self.log,
+                "Unable to free worker";
+                "msg" => "did not free worker, shutdown may be underway",
+                "error" => e.to_string()
+            )
+        }
+    }
+}