io: remove Poll from the AsyncSeek::start_seek return value (#2885)

4 files changed, 49 insertions, 66 deletions

diff --git a/tokio-util/src/either.rs b/tokio-util/src/either.rs
index 3c749c1b..f5246af2 100644
--- a/tokio-util/src/either.rs
+++ b/tokio-util/src/either.rs
@@ -125,12 +125,8 @@ where
     L: AsyncSeek,
     R: AsyncSeek,
 {
-    fn start_seek(
-        self: Pin<&mut Self>,
-        cx: &mut Context<'_>,
-        position: SeekFrom,
-    ) -> Poll<Result<()>> {
-        delegate_call!(self.start_seek(cx, position))
+    fn start_seek(self: Pin<&mut Self>, position: SeekFrom) -> Result<()> {
+        delegate_call!(self.start_seek(position))
     }
 
     fn poll_complete(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<u64>> {
diff --git a/tokio/src/fs/file.rs b/tokio/src/fs/file.rs
index 319c2c7f..9556a22f 100644
--- a/tokio/src/fs/file.rs
+++ b/tokio/src/fs/file.rs
@@ -86,6 +86,8 @@ pub struct File {
     /// error is observed while performing a read, it is saved until the next
     /// write / flush call.
     last_write_err: Option<io::ErrorKind>,
+
+    pos: u64,
 }
 
 #[derive(Debug)]
@@ -199,6 +201,7 @@ impl File {
             std: Arc::new(std),
             state: State::Idle(Some(Buf::with_capacity(0))),
             last_write_err: None,
+            pos: 0,
         }
     }
 
@@ -332,7 +335,9 @@ impl File {
         self.state = Idle(Some(buf));
 
         match op {
-            Operation::Seek(res) => res.map(|_| ()),
+            Operation::Seek(res) => res.map(|pos| {
+                self.pos = pos;
+            }),
             _ => unreachable!(),
         }
     }
@@ -524,9 +529,12 @@ impl AsyncRead for File {
                             self.last_write_err = Some(e.kind());
                             self.state = Idle(Some(buf));
                         }
-                        Operation::Seek(_) => {
+                        Operation::Seek(result) => {
                             assert!(buf.is_empty());
                             self.state = Idle(Some(buf));
+                            if let Ok(pos) = result {
+                                self.pos = pos;
+                            }
                             continue;
                         }
                     }
@@ -537,13 +545,10 @@ impl AsyncRead for File {
 }
 
 impl AsyncSeek for File {
-    fn start_seek(
-        mut self: Pin<&mut Self>,
-        cx: &mut Context<'_>,
-        mut pos: SeekFrom,
-    ) -> Poll<io::Result<()>> {
+    fn start_seek(mut self: Pin<&mut Self>, mut pos: SeekFrom) -> io::Result<()> {
         loop {
             match self.state {
+                Busy(_) => panic!("must wait for poll_complete before calling start_seek"),
                 Idle(ref mut buf_cell) => {
                     let mut buf = buf_cell.take().unwrap();
 
@@ -562,22 +567,7 @@ impl AsyncSeek for File {
                         let res = (&*std).seek(pos);
                         (Operation::Seek(res), buf)
                     }));
-
-                    return Ready(Ok(()));
-                }
-                Busy(ref mut rx) => {
-                    let (op, buf) = ready!(Pin::new(rx).poll(cx))?;
-                    self.state = Idle(Some(buf));
-
-                    match op {
-                        Operation::Read(_) => {}
-                        Operation::Write(Err(e)) => {
-                            assert!(self.last_write_err.is_none());
-                            self.last_write_err = Some(e.kind());
-                        }
-                        Operation::Write(_) => {}
-                        Operation::Seek(_) => {}
-                    }
+                    return Ok(());
                 }
             }
         }
@@ -586,7 +576,7 @@ impl AsyncSeek for File {
     fn poll_complete(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<u64>> {
         loop {
             match self.state {
-                Idle(_) => panic!("must call start_seek before calling poll_complete"),
+                Idle(_) => return Poll::Ready(Ok(self.pos)),
                 Busy(ref mut rx) => {
                     let (op, buf) = ready!(Pin::new(rx).poll(cx))?;
                     self.state = Idle(Some(buf));
@@ -598,7 +588,12 @@ impl AsyncSeek for File {
                             self.last_write_err = Some(e.kind());
                         }
                         Operation::Write(_) => {}
-                        Operation::Seek(res) => return Ready(res),
+                        Operation::Seek(res) => {
+                            if let Ok(pos) = res {
+                                self.pos = pos;
+                            }
+                            return Ready(res);
+                        }
                     }
                 }
             }
diff --git a/tokio/src/io/async_seek.rs b/tokio/src/io/async_seek.rs
index 32ed0a22..bd7a992e 100644
--- a/tokio/src/io/async_seek.rs
+++ b/tokio/src/io/async_seek.rs
@@ -23,36 +23,33 @@ pub trait AsyncSeek {
     ///
     /// If this function returns successfully, then the job has been submitted.
     /// To find out when it completes, call `poll_complete`.
-    fn start_seek(
-        self: Pin<&mut Self>,
-        cx: &mut Context<'_>,
-        position: SeekFrom,
-    ) -> Poll<io::Result<()>>;
+    ///
+    /// # Errors
+    ///
+    /// This function can return [`io::ErrorKind::Other`] in case there is
+    /// another seek in progress. To avoid this, it is advisable that any call
+    /// to `start_seek` is preceded by a call to `poll_complete` to ensure all
+    /// pending seeks have completed.
+    fn start_seek(self: Pin<&mut Self>, position: SeekFrom) -> io::Result<()>;
 
     /// Waits for a seek operation to complete.
     ///
     /// If the seek operation completed successfully,
     /// this method returns the new position from the start of the stream.
-    /// That position can be used later with [`SeekFrom::Start`].
+    /// That position can be used later with [`SeekFrom::Start`]. Repeatedly
+    /// calling this function without calling `start_seek` might return the
+    /// same result.
     ///
     /// # Errors
     ///
     /// Seeking to a negative offset is considered an error.
-    ///
-    /// # Panics
-    ///
-    /// Calling this method without calling `start_seek` first is an error.
     fn poll_complete(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<u64>>;
 }
 
 macro_rules! deref_async_seek {
     () => {
-        fn start_seek(
-            mut self: Pin<&mut Self>,
-            cx: &mut Context<'_>,
-            pos: SeekFrom,
-        ) -> Poll<io::Result<()>> {
-            Pin::new(&mut **self).start_seek(cx, pos)
+        fn start_seek(mut self: Pin<&mut Self>, pos: SeekFrom) -> io::Result<()> {
+            Pin::new(&mut **self).start_seek(pos)
         }
 
         fn poll_complete(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<u64>> {
@@ -74,12 +71,8 @@ where
     P: DerefMut + Unpin,
     P::Target: AsyncSeek,
 {
-    fn start_seek(
-        self: Pin<&mut Self>,
-        cx: &mut Context<'_>,
-        pos: SeekFrom,
-    ) -> Poll<io::Result<()>> {
-        self.get_mut().as_mut().start_seek(cx, pos)
+    fn start_seek(self: Pin<&mut Self>, pos: SeekFrom) -> io::Result<()> {
+        self.get_mut().as_mut().start_seek(pos)
     }
 
     fn poll_complete(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<u64>> {
@@ -88,12 +81,8 @@ where
 }
 
 impl<T: AsRef<[u8]> + Unpin> AsyncSeek for io::Cursor<T> {
-    fn start_seek(
-        mut self: Pin<&mut Self>,
-        _: &mut Context<'_>,
-        pos: SeekFrom,
-    ) -> Poll<io::Result<()>> {
-        Poll::Ready(io::Seek::seek(&mut *self, pos).map(drop))
+    fn start_seek(mut self: Pin<&mut Self>, pos: SeekFrom) -> io::Result<()> {
+        io::Seek::seek(&mut *self, pos).map(drop)
     }
     fn poll_complete(self: Pin<&mut Self>, _: &mut Context<'_>) -> Poll<io::Result<u64>> {
         Poll::Ready(Ok(self.get_mut().position()))
diff --git a/tokio/src/io/seek.rs b/tokio/src/io/seek.rs
index c90330a5..e64205d9 100644
--- a/tokio/src/io/seek.rs
+++ b/tokio/src/io/seek.rs
@@ -40,14 +40,17 @@ where
     fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
         let me = self.project();
         match me.pos {
-            Some(pos) => match Pin::new(&mut *me.seek).start_seek(cx, *pos) {
-                Poll::Ready(Ok(())) => {
-                    *me.pos = None;
-                    Pin::new(&mut *me.seek).poll_complete(cx)
+            Some(pos) => {
+                // ensure no seek in progress
+                ready!(Pin::new(&mut *me.seek).poll_complete(cx))?;
+                match Pin::new(&mut *me.seek).start_seek(*pos) {
+                    Ok(()) => {
+                        *me.pos = None;
+                        Pin::new(&mut *me.seek).poll_complete(cx)
+                    }
+                    Err(e) => Poll::Ready(Err(e)),
                 }
-                Poll::Ready(Err(e)) => Poll::Ready(Err(e)),
-                Poll::Pending => Poll::Pending,
-            },
+            }
             None => Pin::new(&mut *me.seek).poll_complete(cx),
         }
     }