Change panic on seeking busy file to error.

tokio/src/fs/file.rs

@@ -565,29 +565,30 @@ impl AsyncSeek for File {
         let me = self.get_mut();
         let inner = me.inner.get_mut();
 
-        loop {
-            match inner.state {
-                Busy(_) => panic!("must wait for poll_complete before calling start_seek"),
-                Idle(ref mut buf_cell) => {
-                    let mut buf = buf_cell.take().unwrap();
-
-                    // Factor in any unread data from the buf
-                    if !buf.is_empty() {
-                        let n = buf.discard_read();
-
-                        if let SeekFrom::Current(ref mut offset) = pos {
-                            *offset += n;
-                        }
+        match inner.state {
+            Busy(_) => Err(io::Error::new(
+                io::ErrorKind::Other,
+                "other file operation is pending, call poll_complete before start_seek",
+            )),
+            Idle(ref mut buf_cell) => {
+                let mut buf = buf_cell.take().unwrap();
+
+                // Factor in any unread data from the buf
+                if !buf.is_empty() {
+                    let n = buf.discard_read();
+
+                    if let SeekFrom::Current(ref mut offset) = pos {
+                        *offset += n;
                     }
+                }
 
-                    let std = me.std.clone();
+                let std = me.std.clone();
 
-                    inner.state = Busy(spawn_blocking(move || {
-                        let res = (&*std).seek(pos);
-                        (Operation::Seek(res), buf)
-                    }));
-                    return Ok(());
-                }
+                inner.state = Busy(spawn_blocking(move || {
+                    let res = (&*std).seek(pos);
+                    (Operation::Seek(res), buf)
+                }));
+                Ok(())
             }
         }
     }

tokio/src/fs/file/tests.rs

@@ -955,3 +955,24 @@ fn partial_read_set_len_ok() {
     assert_eq!(n, FOO.len());
     assert_eq!(&buf[..n], FOO);
 }
+
+#[test]
+fn busy_file_seek_error() {
+    let mut file = MockFile::default();
+    let mut seq = Sequence::new();
+    file.expect_inner_write()
+        .once()
+        .in_sequence(&mut seq)
+        .returning(|_| Err(io::ErrorKind::Other.into()));
+
+    let mut file = crate::io::BufReader::new(File::from_std(file));
+    {
+        let mut t = task::spawn(file.write(HELLO));
+        assert_ready_ok!(t.poll());
+    }
+
+    pool::run_one();
+
+    let mut t = task::spawn(file.seek(SeekFrom::Start(0)));
+    assert_ready_err!(t.poll());
+}