fix: Implement flush function on Async

lib.rs

@@ -474,6 +474,21 @@ impl Drain for AsyncCore {
     ) -> AsyncResult<()> {
         self.send(AsyncRecord::from(record, logger_values))
     }
+
+    /// Flush the drain which waits as long (poll interval `interval`) till
+    /// the channel to the async worker thread becomes empty.
+    fn flush(&self, interval: std::time::Duration) -> io::Result<()> {
+        let sender = match self.get_sender() {
+            Ok(s) => s,
+            Err(_) => return Err(io::Error::from(io::ErrorKind::BrokenPipe)),
+        };
+
+        while !sender.is_empty() {
+            thread::sleep(interval);
+        }
+
+        Ok(())
+    }
 }
 
 /// Serialized record.
@@ -796,6 +811,11 @@ impl Drain for Async {
 
         Ok(())
     }
+
+    #[inline]
+    fn flush(&self, interval: std::time::Duration) -> io::Result<()> {
+        self.core.flush(interval)
+    }
 }
 
 impl Drop for Async {
@@ -806,25 +826,37 @@ impl Drop for Async {
 
 // }}}
 
-
 #[cfg(test)]
 mod test {
     use super::*;
-    use std::sync::mpsc;
+    use std::{sync::mpsc, time::Duration};
 
     #[test]
     fn integration_test() {
         let (mock_drain, mock_drain_rx) = MockDrain::new();
-        let async_drain = AsyncBuilder::new(mock_drain)
-            .build();
-        let slog = slog::Logger::root(async_drain.fuse(), o!("field1" => "value1"));
+        let async_drain = AsyncBuilder::new(mock_drain).build();
+
+        let e = async_drain.flush(Duration::from_millis(10));
+        assert_eq!(matches!(e, Ok(())), true);
+
+        let slog =
+            slog::Logger::root(async_drain.fuse(), o!("field1" => "value1"));
 
         info!(slog, "Message 1"; "field2" => "value2");
         warn!(slog, "Message 2"; "field3" => "value3");
-        assert_eq!(mock_drain_rx.recv().unwrap(), r#"INFO Message 1: [("field1", "value1"), ("field2", "value2")]"#);
-        assert_eq!(mock_drain_rx.recv().unwrap(), r#"WARN Message 2: [("field1", "value1"), ("field3", "value3")]"#);
-    }
 
+        let e = slog.flush(Duration::from_millis(10));
+        assert_eq!(matches!(e, Ok(())), true);
+
+        assert_eq!(
+            mock_drain_rx.recv().unwrap(),
+            r#"INFO Message 1: [("field1", "value1"), ("field2", "value2")]"#
+        );
+        assert_eq!(
+            mock_drain_rx.recv().unwrap(),
+            r#"WARN Message 2: [("field1", "value1"), ("field3", "value3")]"#
+        );
+    }
 
     /// Test-helper drain
     #[derive(Debug)]
@@ -843,7 +875,11 @@ mod test {
         type Ok = ();
         type Err = slog::Never;
 
-        fn log(&self, record: &Record, logger_kv: &OwnedKVList) -> Result<Self::Ok, Self::Err> {
+        fn log(
+            &self,
+            record: &Record,
+            logger_kv: &OwnedKVList,
+        ) -> Result<Self::Ok, Self::Err> {
             let mut serializer = MockSerializer::default();
             logger_kv.serialize(record, &mut serializer).unwrap();
             record.kv().serialize(record, &mut serializer).unwrap();
@@ -861,7 +897,11 @@ mod test {
     }
 
     impl slog::Serializer for MockSerializer {
-        fn emit_arguments(&mut self, key: Key, val: &fmt::Arguments) -> Result<(), slog::Error> {
+        fn emit_arguments(
+            &mut self,
+            key: Key,
+            val: &fmt::Arguments,
+        ) -> Result<(), slog::Error> {
             self.kvs.push((key.to_string(), val.to_string()));
             Ok(())
         }