fs: add deprecated fs::File::seek fn (#1991)

This fixes an API compatibility regression when `AsyncSeek` was added.

Fixes: #1989

ci/azure-clippy.yml

@@ -12,5 +12,5 @@ jobs:
         cargo clippy --version
       displayName: Install clippy
     - script: |
-        cargo clippy --all --all-features -- -A clippy::mutex-atomic
+        cargo clippy --all --all-features -- -A clippy::mutex-atomic -A clippy::needless-doctest-main
       displayName: cargo clippy --all

tokio/src/fs/file.rs

@@ -176,6 +176,63 @@ impl File {
         }
     }
 
+    /// Seek to an offset, in bytes, in a stream.
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// use tokio::fs::File;
+    /// use tokio::prelude::*;
+    ///
+    /// use std::io::SeekFrom;
+    ///
+    /// # async fn dox() -> std::io::Result<()> {
+    /// let mut file = File::open("foo.txt").await?;
+    /// file.seek(SeekFrom::Start(6)).await?;
+    ///
+    /// let mut contents = vec![0u8; 10];
+    /// file.read_exact(&mut contents).await?;
+    /// # Ok(())
+    /// # }
+    /// ```
+    pub async fn seek(&mut self, mut pos: SeekFrom) -> io::Result<u64> {
+        self.complete_inflight().await;
+
+        let mut buf = match self.state {
+            Idle(ref mut buf_cell) => buf_cell.take().unwrap(),
+            _ => unreachable!(),
+        };
+
+        // 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 = self.std.clone();
+
+        // Start the operation
+        self.state = Busy(sys::run(move || {
+            let res = (&*std).seek(pos);
+            (Operation::Seek(res), buf)
+        }));
+
+        let (op, buf) = match self.state {
+            Idle(_) => unreachable!(),
+            Busy(ref mut rx) => rx.await.unwrap(),
+        };
+
+        self.state = Idle(Some(buf));
+
+        match op {
+            Operation::Seek(res) => res,
+            _ => unreachable!(),
+        }
+    }
+
     /// Attempts to sync all OS-internal metadata to disk.
     ///
     /// This function will attempt to ensure that all in-core data reaches the