# file.rs - source
Source of the Rust file `/home/runner/.cargo/registry/src/github.com-1ecc6299db9ec823/tokio-1.4.0/src/fs/file.rs`.
Keywords: rust, rustlang, rust-lang

      1
      2
      3
      4
      5
      6
      7
      8
      9
     10
     11
     12
     13
     14
     15
     16
     17
     18
     19
     20
     21
     22
     23
     24
     25
     26
     27
     28
     29
     30
     31
     32
     33
     34
     35
     36
     37
     38
     39
     40
     41
     42
     43
     44
     45
     46
     47
     48
     49
     50
     51
     52
     53
     54
     55
     56
     57
     58
     59
     60
     61
     62
     63
     64
     65
     66
     67
     68
     69
     70
     71
     72
     73
     74
     75
     76
     77
     78
     79
     80
     81
     82
     83
     84
     85
     86
     87
     88
     89
     90
     91
     92
     93
     94
     95
     96
     97
     98
     99
    100
    101
    102
    103
    104
    105
    106
    107
    108
    109
    110
    111
    112
    113
    114
    115
    116
    117
    118
    119
    120
    121
    122
    123
    124
    125
    126
    127
    128
    129
    130
    131
    132
    133
    134
    135
    136
    137
    138
    139
    140
    141
    142
    143
    144
    145
    146
    147
    148
    149
    150
    151
    152
    153
    154
    155
    156
    157
    158
    159
    160
    161
    162
    163
    164
    165
    166
    167
    168
    169
    170
    171
    172
    173
    174
    175
    176
    177
    178
    179
    180
    181
    182
    183
    184
    185
    186
    187
    188
    189
    190
    191
    192
    193
    194
    195
    196
    197
    198
    199
    200
    201
    202
    203
    204
    205
    206
    207
    208
    209
    210
    211
    212
    213
    214
    215
    216
    217
    218
    219
    220
    221
    222
    223
    224
    225
    226
    227
    228
    229
    230
    231
    232
    233
    234
    235
    236
    237
    238
    239
    240
    241
    242
    243
    244
    245
    246
    247
    248
    249
    250
    251
    252
    253
    254
    255
    256
    257
    258
    259
    260
    261
    262
    263
    264
    265
    266
    267
    268
    269
    270
    271
    272
    273
    274
    275
    276
    277
    278
    279
    280
    281
    282
    283
    284
    285
    286
    287
    288
    289
    290
    291
    292
    293
    294
    295
    296
    297
    298
    299
    300
    301
    302
    303
    304
    305
    306
    307
    308
    309
    310
    311
    312
    313
    314
    315
    316
    317
    318
    319
    320
    321
    322
    323
    324
    325
    326
    327
    328
    329
    330
    331
    332
    333
    334
    335
    336
    337
    338
    339
    340
    341
    342
    343
    344
    345
    346
    347
    348
    349
    350
    351
    352
    353
    354
    355
    356
    357
    358
    359
    360
    361
    362
    363
    364
    365
    366
    367
    368
    369
    370
    371
    372
    373
    374
    375
    376
    377
    378
    379
    380
    381
    382
    383
    384
    385
    386
    387
    388
    389
    390
    391
    392
    393
    394
    395
    396
    397
    398
    399
    400
    401
    402
    403
    404
    405
    406
    407
    408
    409
    410
    411
    412
    413
    414
    415
    416
    417
    418
    419
    420
    421
    422
    423
    424
    425
    426
    427
    428
    429
    430
    431
    432
    433
    434
    435
    436
    437
    438
    439
    440
    441
    442
    443
    444
    445
    446
    447
    448
    449
    450
    451
    452
    453
    454
    455
    456
    457
    458
    459
    460
    461
    462
    463
    464
    465
    466
    467
    468
    469
    470
    471
    472
    473
    474
    475
    476
    477
    478
    479
    480
    481
    482
    483
    484
    485
    486
    487
    488
    489
    490
    491
    492
    493
    494
    495
    496
    497
    498
    499
    500
    501
    502
    503
    504
    505
    506
    507
    508
    509
    510
    511
    512
    513
    514
    515
    516
    517
    518
    519
    520
    521
    522
    523
    524
    525
    526
    527
    528
    529
    530
    531
    532
    533
    534
    535
    536
    537
    538
    539
    540
    541
    542
    543
    544
    545
    546
    547
    548
    549
    550
    551
    552
    553
    554
    555
    556
    557
    558
    559
    560
    561
    562
    563
    564
    565
    566
    567
    568
    569
    570
    571
    572
    573
    574
    575
    576
    577
    578
    579
    580
    581
    582
    583
    584
    585
    586
    587
    588
    589
    590
    591
    592
    593
    594
    595
    596
    597
    598
    599
    600
    601
    602
    603
    604
    605
    606
    607
    608
    609
    610
    611
    612
    613
    614
    615
    616
    617
    618
    619
    620
    621
    622
    623
    624
    625
    626
    627
    628
    629
    630
    631
    632
    633
    634
    635
    636
    637
    638
    639
    640
    641
    642
    643
    644
    645
    646
    647
    648
    649
    650
    651
    652
    653
    654
    655
    656
    657
    658
    659
    660
    661
    662
    663
    664
    665
    666
    667
    668
    669
    670
    671
    672
    673
    674
    675
    676
    677
    678
    679
    680
    681
    682
    683
    684
    685
    686
    687
    688
    689
    690
    691
    692
    693
    694
    695
    696
    697
    698
    699
    700
    701
    702
    703
    704
    705
    706
    707
    708
    709
    710
    711
    712
    713
    714
    715
    716
    717
    718
    719
    720
    721
    722
    723
    724
    725
    726
    727
    728
    729
    730
    731
    732
    733
    734
    735
    736
    737
    738
    739
    740
    741
    742
    743
    744
    745
    746
    747
    748
    749
    750
    751
    752
    753
    754
    755
    756
    757
    758

    //! Types for working with [`File`].
    //!
    //! [`File`]: File

    use self::State::*;
    use crate::fs::{asyncify, sys};
    use crate::io::blocking::Buf;
    use crate::io::{AsyncRead, AsyncSeek, AsyncWrite, ReadBuf};
    use crate::sync::Mutex;

    use std::fmt;
    use std::fs::{Metadata, Permissions};
    use std::future::Future;
    use std::io::{self, Seek, SeekFrom};
    use std::path::Path;
    use std::pin::Pin;
    use std::sync::Arc;
    use std::task::Context;
    use std::task::Poll;
    use std::task::Poll::*;

    /// A reference to an open file on the filesystem.
    ///
    /// This is a specialized version of [`std::fs::File`][std] for usage from the
    /// Tokio runtime.
    ///
    /// An instance of a `File` can be read and/or written depending on what options
    /// it was opened with. Files also implement [`AsyncSeek`] to alter the logical
    /// cursor that the file contains internally.
    ///
    /// A file will not be closed immediately when it goes out of scope if there
    /// are any IO operations that have not yet completed. To ensure that a file is
    /// closed immediately when it is dropped, you should call [`flush`] before
    /// dropping it. Note that this does not ensure that the file has been fully
    /// written to disk; the operating system might keep the changes around in an
    /// in-memory buffer. See the [`sync_all`] method for telling the OS to write
    /// the data to disk.
    ///
    /// Reading and writing to a `File` is usually done using the convenience
    /// methods found on the [`AsyncReadExt`] and [`AsyncWriteExt`] traits.
    ///
    /// [std]: struct@std::fs::File
    /// [`AsyncSeek`]: trait@crate::io::AsyncSeek
    /// [`flush`]: fn@crate::io::AsyncWriteExt::flush
    /// [`sync_all`]: fn@crate::fs::File::sync_all
    /// [`AsyncReadExt`]: trait@crate::io::AsyncReadExt
    /// [`AsyncWriteExt`]: trait@crate::io::AsyncWriteExt
    ///
    /// # Examples
    ///
    /// Create a new file and asynchronously write bytes to it:
    ///
    /// ```no_run
    /// use tokio::fs::File;
    /// use tokio::io::AsyncWriteExt; // for write_all()
    ///
    /// # async fn dox() -> std::io::Result<()> {
    /// let mut file = File::create("foo.txt").await?;
    /// file.write_all(b"hello, world!").await?;
    /// # Ok(())
    /// # }
    /// ```
    ///
    /// Read the contents of a file into a buffer
    ///
    /// ```no_run
    /// use tokio::fs::File;
    /// use tokio::io::AsyncReadExt; // for read_to_end()
    ///
    /// # async fn dox() -> std::io::Result<()> {
    /// let mut file = File::open("foo.txt").await?;
    ///
    /// let mut contents = vec![];
    /// file.read_to_end(&mut contents).await?;
    ///
    /// println!("len = {}", contents.len());
    /// # Ok(())
    /// # }
    /// ```
    pub struct File {
        std: Arc<sys::File>,
        inner: Mutex<Inner>,
    }

    struct Inner {
        state: State,

        /// Errors from writes/flushes are returned in write/flush calls. If a write
        /// 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)]
    enum State {
        Idle(Option<Buf>),
        Busy(sys::Blocking<(Operation, Buf)>),
    }

    #[derive(Debug)]
    enum Operation {
        Read(io::Result<usize>),
        Write(io::Result<()>),
        Seek(io::Result<u64>),
    }

    impl File {
        /// Attempts to open a file in read-only mode.
        ///
        /// See [`OpenOptions`] for more details.
        ///
        /// [`OpenOptions`]: super::OpenOptions
        ///
        /// # Errors
        ///
        /// This function will return an error if called from outside of the Tokio
        /// runtime or if path does not already exist. Other errors may also be
        /// returned according to OpenOptions::open.
        ///
        /// # Examples
        ///
        /// ```no_run
        /// use tokio::fs::File;
        /// use tokio::io::AsyncReadExt;
        ///
        /// # async fn dox() -> std::io::Result<()> {
        /// let mut file = File::open("foo.txt").await?;
        ///
        /// let mut contents = vec![];
        /// file.read_to_end(&mut contents).await?;
        ///
        /// println!("len = {}", contents.len());
        /// # Ok(())
        /// # }
        /// ```
        ///
        /// The [`read_to_end`] method is defined on the [`AsyncReadExt`] trait.
        ///
        /// [`read_to_end`]: fn@crate::io::AsyncReadExt::read_to_end
        /// [`AsyncReadExt`]: trait@crate::io::AsyncReadExt
        pub async fn open(path: impl AsRef<Path>) -> io::Result<File> {
            let path = path.as_ref().to_owned();
            let std = asyncify(|| sys::File::open(path)).await?;

            Ok(File::from_std(std))
        }

        /// Opens a file in write-only mode.
        ///
        /// This function will create a file if it does not exist, and will truncate
        /// it if it does.
        ///
        /// See [`OpenOptions`] for more details.
        ///
        /// [`OpenOptions`]: super::OpenOptions
        ///
        /// # Errors
        ///
        /// Results in an error if called from outside of the Tokio runtime or if
        /// the underlying [`create`] call results in an error.
        ///
        /// [`create`]: std::fs::File::create
        ///
        /// # Examples
        ///
        /// ```no_run
        /// use tokio::fs::File;
        /// use tokio::io::AsyncWriteExt;
        ///
        /// # async fn dox() -> std::io::Result<()> {
        /// let mut file = File::create("foo.txt").await?;
        /// file.write_all(b"hello, world!").await?;
        /// # Ok(())
        /// # }
        /// ```
        ///
        /// The [`write_all`] method is defined on the [`AsyncWriteExt`] trait.
        ///
        /// [`write_all`]: fn@crate::io::AsyncWriteExt::write_all
        /// [`AsyncWriteExt`]: trait@crate::io::AsyncWriteExt
        pub async fn create(path: impl AsRef<Path>) -> io::Result<File> {
            let path = path.as_ref().to_owned();
            let std_file = asyncify(move || sys::File::create(path)).await?;
            Ok(File::from_std(std_file))
        }

        /// Converts a [`std::fs::File`][std] to a [`tokio::fs::File`][file].
        ///
        /// [std]: std::fs::File
        /// [file]: File
        ///
        /// # Examples
        ///
        /// ```no_run
        /// // This line could block. It is not recommended to do this on the Tokio
        /// // runtime.
        /// let std_file = std::fs::File::open("foo.txt").unwrap();
        /// let file = tokio::fs::File::from_std(std_file);
        /// ```
        pub fn from_std(std: sys::File) -> File {
            File {
                std: Arc::new(std),
                inner: Mutex::new(Inner {
                    state: State::Idle(Some(Buf::with_capacity(0))),
                    last_write_err: None,
                    pos: 0,
                }),
            }
        }

        /// Attempts to sync all OS-internal metadata to disk.
        ///
        /// This function will attempt to ensure that all in-core data reaches the
        /// filesystem before returning.
        ///
        /// # Examples
        ///
        /// ```no_run
        /// use tokio::fs::File;
        /// use tokio::io::AsyncWriteExt;
        ///
        /// # async fn dox() -> std::io::Result<()> {
        /// let mut file = File::create("foo.txt").await?;
        /// file.write_all(b"hello, world!").await?;
        /// file.sync_all().await?;
        /// # Ok(())
        /// # }
        /// ```
        ///
        /// The [`write_all`] method is defined on the [`AsyncWriteExt`] trait.
        ///
        /// [`write_all`]: fn@crate::io::AsyncWriteExt::write_all
        /// [`AsyncWriteExt`]: trait@crate::io::AsyncWriteExt
        pub async fn sync_all(&self) -> io::Result<()> {
            let mut inner = self.inner.lock().await;
            inner.complete_inflight().await;

            let std = self.std.clone();
            asyncify(move || std.sync_all()).await
        }

        /// This function is similar to `sync_all`, except that it may not
        /// synchronize file metadata to the filesystem.
        ///
        /// This is intended for use cases that must synchronize content, but don't
        /// need the metadata on disk. The goal of this method is to reduce disk
        /// operations.
        ///
        /// Note that some platforms may simply implement this in terms of `sync_all`.
        ///
        /// # Examples
        ///
        /// ```no_run
        /// use tokio::fs::File;
        /// use tokio::io::AsyncWriteExt;
        ///
        /// # async fn dox() -> std::io::Result<()> {
        /// let mut file = File::create("foo.txt").await?;
        /// file.write_all(b"hello, world!").await?;
        /// file.sync_data().await?;
        /// # Ok(())
        /// # }
        /// ```
        ///
        /// The [`write_all`] method is defined on the [`AsyncWriteExt`] trait.
        ///
        /// [`write_all`]: fn@crate::io::AsyncWriteExt::write_all
        /// [`AsyncWriteExt`]: trait@crate::io::AsyncWriteExt
        pub async fn sync_data(&self) -> io::Result<()> {
            let mut inner = self.inner.lock().await;
            inner.complete_inflight().await;

            let std = self.std.clone();
            asyncify(move || std.sync_data()).await
        }

        /// Truncates or extends the underlying file, updating the size of this file to become size.
        ///
        /// If the size is less than the current file's size, then the file will be
        /// shrunk. If it is greater than the current file's size, then the file
        /// will be extended to size and have all of the intermediate data filled in
        /// with 0s.
        ///
        /// # Errors
        ///
        /// This function will return an error if the file is not opened for
        /// writing.
        ///
        /// # Examples
        ///
        /// ```no_run
        /// use tokio::fs::File;
        /// use tokio::io::AsyncWriteExt;
        ///
        /// # async fn dox() -> std::io::Result<()> {
        /// let mut file = File::create("foo.txt").await?;
        /// file.write_all(b"hello, world!").await?;
        /// file.set_len(10).await?;
        /// # Ok(())
        /// # }
        /// ```
        ///
        /// The [`write_all`] method is defined on the [`AsyncWriteExt`] trait.
        ///
        /// [`write_all`]: fn@crate::io::AsyncWriteExt::write_all
        /// [`AsyncWriteExt`]: trait@crate::io::AsyncWriteExt
        pub async fn set_len(&self, size: u64) -> io::Result<()> {
            let mut inner = self.inner.lock().await;
            inner.complete_inflight().await;

            let mut buf = match inner.state {
                Idle(ref mut buf_cell) => buf_cell.take().unwrap(),
                _ => unreachable!(),
            };

            let seek = if !buf.is_empty() {
                Some(SeekFrom::Current(buf.discard_read()))
            } else {
                None
            };

            let std = self.std.clone();

            inner.state = Busy(sys::run(move || {
                let res = if let Some(seek) = seek {
                    (&*std).seek(seek).and_then(|_| std.set_len(size))
                } else {
                    std.set_len(size)
                }
                .map(|_| 0); // the value is discarded later

                // Return the result as a seek
                (Operation::Seek(res), buf)
            }));

            let (op, buf) = match inner.state {
                Idle(_) => unreachable!(),
                Busy(ref mut rx) => rx.await?,
            };

            inner.state = Idle(Some(buf));

            match op {
                Operation::Seek(res) => res.map(|pos| {
                    inner.pos = pos;
                }),
                _ => unreachable!(),
            }
        }

        /// Queries metadata about the underlying file.
        ///
        /// # Examples
        ///
        /// ```no_run
        /// use tokio::fs::File;
        ///
        /// # async fn dox() -> std::io::Result<()> {
        /// let file = File::open("foo.txt").await?;
        /// let metadata = file.metadata().await?;
        ///
        /// println!("{:?}", metadata);
        /// # Ok(())
        /// # }
        /// ```
        pub async fn metadata(&self) -> io::Result<Metadata> {
            let std = self.std.clone();
            asyncify(move || std.metadata()).await
        }

        /// Create a new `File` instance that shares the same underlying file handle
        /// as the existing `File` instance. Reads, writes, and seeks will affect both
        /// File instances simultaneously.
        ///
        /// # Examples
        ///
        /// ```no_run
        /// use tokio::fs::File;
        ///
        /// # async fn dox() -> std::io::Result<()> {
        /// let file = File::open("foo.txt").await?;
        /// let file_clone = file.try_clone().await?;
        /// # Ok(())
        /// # }
        /// ```
        pub async fn try_clone(&self) -> io::Result<File> {
            let std = self.std.clone();
            let std_file = asyncify(move || std.try_clone()).await?;
            Ok(File::from_std(std_file))
        }

        /// Destructures `File` into a [`std::fs::File`][std]. This function is
        /// async to allow any in-flight operations to complete.
        ///
        /// Use `File::try_into_std` to attempt conversion immediately.
        ///
        /// [std]: std::fs::File
        ///
        /// # Examples
        ///
        /// ```no_run
        /// use tokio::fs::File;
        ///
        /// # async fn dox() -> std::io::Result<()> {
        /// let tokio_file = File::open("foo.txt").await?;
        /// let std_file = tokio_file.into_std().await;
        /// # Ok(())
        /// # }
        /// ```
        pub async fn into_std(mut self) -> sys::File {
            self.inner.get_mut().complete_inflight().await;
            Arc::try_unwrap(self.std).expect("Arc::try_unwrap failed")
        }

        /// Tries to immediately destructure `File` into a [`std::fs::File`][std].
        ///
        /// [std]: std::fs::File
        ///
        /// # Errors
        ///
        /// This function will return an error containing the file if some
        /// operation is in-flight.
        ///
        /// # Examples
        ///
        /// ```no_run
        /// use tokio::fs::File;
        ///
        /// # async fn dox() -> std::io::Result<()> {
        /// let tokio_file = File::open("foo.txt").await?;
        /// let std_file = tokio_file.try_into_std().unwrap();
        /// # Ok(())
        /// # }
        /// ```
        pub fn try_into_std(mut self) -> Result<sys::File, Self> {
            match Arc::try_unwrap(self.std) {
                Ok(file) => Ok(file),
                Err(std_file_arc) => {
                    self.std = std_file_arc;
                    Err(self)
                }
            }
        }

        /// Changes the permissions on the underlying file.
        ///
        /// # Platform-specific behavior
        ///
        /// This function currently corresponds to the `fchmod` function on Unix and
        /// the `SetFileInformationByHandle` function on Windows. Note that, this
        /// [may change in the future][changes].
        ///
        /// [changes]: https://doc.rust-lang.org/std/io/index.html#platform-specific-behavior
        ///
        /// # Errors
        ///
        /// This function will return an error if the user lacks permission change
        /// attributes on the underlying file. It may also return an error in other
        /// os-specific unspecified cases.
        ///
        /// # Examples
        ///
        /// ```no_run
        /// use tokio::fs::File;
        ///
        /// # async fn dox() -> std::io::Result<()> {
        /// let file = File::open("foo.txt").await?;
        /// let mut perms = file.metadata().await?.permissions();
        /// perms.set_readonly(true);
        /// file.set_permissions(perms).await?;
        /// # Ok(())
        /// # }
        /// ```
        pub async fn set_permissions(&self, perm: Permissions) -> io::Result<()> {
            let std = self.std.clone();
            asyncify(move || std.set_permissions(perm)).await
        }
    }

    impl AsyncRead for File {
        fn poll_read(
            self: Pin<&mut Self>,
            cx: &mut Context<'_>,
            dst: &mut ReadBuf<'_>,
        ) -> Poll<io::Result<()>> {
            let me = self.get_mut();
            let inner = me.inner.get_mut();

            loop {
                match inner.state {
                    Idle(ref mut buf_cell) => {
                        let mut buf = buf_cell.take().unwrap();

                        if !buf.is_empty() {
                            buf.copy_to(dst);
                            *buf_cell = Some(buf);
                            return Ready(Ok(()));
                        }

                        buf.ensure_capacity_for(dst);
                        let std = me.std.clone();

                        inner.state = Busy(sys::run(move || {
                            let res = buf.read_from(&mut &*std);
                            (Operation::Read(res), buf)
                        }));
                    }
                    Busy(ref mut rx) => {
                        let (op, mut buf) = ready!(Pin::new(rx).poll(cx))?;

                        match op {
                            Operation::Read(Ok(_)) => {
                                buf.copy_to(dst);
                                inner.state = Idle(Some(buf));
                                return Ready(Ok(()));
                            }
                            Operation::Read(Err(e)) => {
                                assert!(buf.is_empty());

                                inner.state = Idle(Some(buf));
                                return Ready(Err(e));
                            }
                            Operation::Write(Ok(_)) => {
                                assert!(buf.is_empty());
                                inner.state = Idle(Some(buf));
                                continue;
                            }
                            Operation::Write(Err(e)) => {
                                assert!(inner.last_write_err.is_none());
                                inner.last_write_err = Some(e.kind());
                                inner.state = Idle(Some(buf));
                            }
                            Operation::Seek(result) => {
                                assert!(buf.is_empty());
                                inner.state = Idle(Some(buf));
                                if let Ok(pos) = result {
                                    inner.pos = pos;
                                }
                                continue;
                            }
                        }
                    }
                }
            }
        }
    }

    impl AsyncSeek for File {
        fn start_seek(self: Pin<&mut Self>, mut pos: SeekFrom) -> io::Result<()> {
            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;
                            }
                        }

                        let std = me.std.clone();

                        inner.state = Busy(sys::run(move || {
                            let res = (&*std).seek(pos);
                            (Operation::Seek(res), buf)
                        }));
                        return Ok(());
                    }
                }
            }
        }

        fn poll_complete(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<u64>> {
            let inner = self.inner.get_mut();

            loop {
                match inner.state {
                    Idle(_) => return Poll::Ready(Ok(inner.pos)),
                    Busy(ref mut rx) => {
                        let (op, buf) = ready!(Pin::new(rx).poll(cx))?;
                        inner.state = Idle(Some(buf));

                        match op {
                            Operation::Read(_) => {}
                            Operation::Write(Err(e)) => {
                                assert!(inner.last_write_err.is_none());
                                inner.last_write_err = Some(e.kind());
                            }
                            Operation::Write(_) => {}
                            Operation::Seek(res) => {
                                if let Ok(pos) = res {
                                    inner.pos = pos;
                                }
                                return Ready(res);
                            }
                        }
                    }
                }
            }
        }
    }

    impl AsyncWrite for File {
        fn poll_write(
            self: Pin<&mut Self>,
            cx: &mut Context<'_>,
            src: &[u8],
        ) -> Poll<io::Result<usize>> {
            let me = self.get_mut();
            let inner = me.inner.get_mut();

            if let Some(e) = inner.last_write_err.take() {
                return Ready(Err(e.into()));
            }

            loop {
                match inner.state {
                    Idle(ref mut buf_cell) => {
                        let mut buf = buf_cell.take().unwrap();

                        let seek = if !buf.is_empty() {
                            Some(SeekFrom::Current(buf.discard_read()))
                        } else {
                            None
                        };

                        let n = buf.copy_from(src);
                        let std = me.std.clone();

                        inner.state = Busy(sys::run(move || {
                            let res = if let Some(seek) = seek {
                                (&*std).seek(seek).and_then(|_| buf.write_to(&mut &*std))
                            } else {
                                buf.write_to(&mut &*std)
                            };

                            (Operation::Write(res), buf)
                        }));

                        return Ready(Ok(n));
                    }
                    Busy(ref mut rx) => {
                        let (op, buf) = ready!(Pin::new(rx).poll(cx))?;
                        inner.state = Idle(Some(buf));

                        match op {
                            Operation::Read(_) => {
                                // We don't care about the result here. The fact
                                // that the cursor has advanced will be reflected in
                                // the next iteration of the loop
                                continue;
                            }
                            Operation::Write(res) => {
                                // If the previous write was successful, continue.
                                // Otherwise, error.
                                res?;
                                continue;
                            }
                            Operation::Seek(_) => {
                                // Ignore the seek
                                continue;
                            }
                        }
                    }
                }
            }
        }

        fn poll_flush(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<(), io::Error>> {
            let inner = self.inner.get_mut();
            inner.poll_flush(cx)
        }

        fn poll_shutdown(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<(), io::Error>> {
            self.poll_flush(cx)
        }
    }

    impl From<sys::File> for File {
        fn from(std: sys::File) -> Self {
            Self::from_std(std)
        }
    }

    impl fmt::Debug for File {
        fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
            fmt.debug_struct("tokio::fs::File")
                .field("std", &self.std)
                .finish()
        }
    }

    #[cfg(unix)]
    impl std::os::unix::io::AsRawFd for File {
        fn as_raw_fd(&self) -> std::os::unix::io::RawFd {
            self.std.as_raw_fd()
        }
    }

    #[cfg(unix)]
    impl std::os::unix::io::FromRawFd for File {
        unsafe fn from_raw_fd(fd: std::os::unix::io::RawFd) -> Self {
            sys::File::from_raw_fd(fd).into()
        }
    }

    #[cfg(windows)]
    impl std::os::windows::io::AsRawHandle for File {
        fn as_raw_handle(&self) -> std::os::windows::io::RawHandle {
            self.std.as_raw_handle()
        }
    }

    #[cfg(windows)]
    impl std::os::windows::io::FromRawHandle for File {
        unsafe fn from_raw_handle(handle: std::os::windows::io::RawHandle) -> Self {
            sys::File::from_raw_handle(handle).into()
        }
    }

    impl Inner {
        async fn complete_inflight(&mut self) {
            use crate::future::poll_fn;

            if let Err(e) = poll_fn(|cx| Pin::new(&mut *self).poll_flush(cx)).await {
                self.last_write_err = Some(e.kind());
            }
        }

        fn poll_flush(&mut self, cx: &mut Context<'_>) -> Poll<Result<(), io::Error>> {
            if let Some(e) = self.last_write_err.take() {
                return Ready(Err(e.into()));
            }

            let (op, buf) = match self.state {
                Idle(_) => return Ready(Ok(())),
                Busy(ref mut rx) => ready!(Pin::new(rx).poll(cx))?,
            };

            // The buffer is not used here
            self.state = Idle(Some(buf));

            match op {
                Operation::Read(_) => Ready(Ok(())),
                Operation::Write(res) => Ready(res),
                Operation::Seek(_) => Ready(Ok(())),
            }
        }
    }