gracefully shut down the channel on drop instead of discarding events

Previously, if the debouncer's shutdown flag was set while there was
anything in its accumulator, the contents of the accumulator would never
be sent through the mpsc channel, effectively causing the events folded
into the accumulator to be lost. This patch modifies the shutdown
behaviour to send the contents of the accumulator through the channel
before stopping the debouncer and closing the channel.

src/lib.rs

@@ -301,7 +301,15 @@ fn debounce_thread<T>(
         };
     
         match event {
-            DebouncerEvent::Shutdown => break 'debounce,
+            DebouncerEvent::Shutdown => {
+                // Before shutting down, check if there is anything in the accumulator and send it
+                // through the channel if it is still open.
+                if let Some(acc) = guard.swap_acc() {
+                    tx.send(acc).ok();
+                }
+
+                break 'debounce;
+            },
 
             DebouncerEvent::Dirty => {
                 // Wait for the debounce time, or until the `sleep_cvar` is notified and the
@@ -315,16 +323,68 @@ fn debounce_thread<T>(
                         !state.should_shutdown()
                     }).unwrap();
 
-                if guard.should_shutdown() {
-                    break 'debounce;
-                }
-
+                // Once we have finished waiting, send the contents of the accumulator through the
+                // channel.
                 if let Some(acc) = guard.swap_acc() {
                     if tx.send(acc).is_err() {
-                        break 'debounce
+                        // If the other side of the channel has been closed, stop the debouncer.
+                        break 'debounce;
                     }
                 }
+
+                // Check if the shutdown flag was set while we were waiting, and stop the deboucner
+                // if so.
+                if guard.should_shutdown() {
+                    break 'debounce;
+                }
             },
         }
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use std::{time::Duration, thread};
+
+    use super::{Debouncer, fold};
+
+    #[test]
+    fn test_debounce() {
+        let (debouncer, rx) = Debouncer::new(
+            Duration::from_millis(50),
+            fold::fold_vec_push::<u8>
+        ).unwrap();
+
+        for i in 0..3 {
+            for j in 0..10 {
+                debouncer.debounce(i * 10 + j);
+                thread::sleep(Duration::from_millis(4));
+            }
+
+            thread::sleep(Duration::from_millis(20));
+        }
+
+        assert_eq!(rx.recv().unwrap(), &[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]);
+        assert_eq!(rx.recv().unwrap(), &[10, 11, 12, 13, 14, 15, 16, 17, 18, 19]);
+        assert_eq!(rx.recv().unwrap(), &[20, 21, 22, 23, 24, 25, 26, 27, 28, 29]);
+    }
+
+    #[test]
+    fn test_debouncer_shutdown() {
+        let (debouncer, rx) = Debouncer::new(
+            Duration::from_millis(100),
+            fold::fold_vec_push::<u8>
+        ).unwrap();
+
+        debouncer.debounce(1);
+        debouncer.debounce(2);
+        debouncer.debounce(3);
+
+        // Drop the debouncer, shutting it down.
+        drop(debouncer);
+
+        // Test that the events emitted just before the shutdown are not lost.
+        assert_eq!(rx.recv().unwrap(), &[1, 2, 3]);
+        assert!(rx.recv().is_err());
+    }
+}