sgx_tstd/src/io/mod.rs

// Licensed to the Apache Software Foundation (ASF) under one
// or more contributor license agreements.  See the NOTICE file
// distributed with this work for additional information
// regarding copyright ownership.  The ASF licenses this file
// to you under the Apache License, Version 2.0 (the
// "License"); you may not use this file except in compliance
// with the License.  You may obtain a copy of the License at
//
//   http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing,
// software distributed under the License is distributed on an
// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
// KIND, either express or implied.  See the License for the
// specific language governing permissions and limitations
// under the License..

use crate::memchr;
use core::cmp;
use core::fmt;
use core::ptr;
use core::mem;
use core::ops::{Deref, DerefMut};
use alloc_crate::slice;
use alloc_crate::vec::Vec;
use alloc_crate::str;
use alloc_crate::string::String;
use crate::sys;

pub use self::buffered::IntoInnerError;
pub use self::buffered::{BufReader, BufWriter, LineWriter};
pub use self::cursor::Cursor;
pub use self::error::{Error, ErrorKind, Result};
pub use self::lazy::{Lazy};
pub use sys::os::{errno, set_errno, error_string};
#[cfg(feature = "stdio")]
pub use self::stdio::{stderr, stdin, stdout, Stderr, Stdin, Stdout};
#[cfg(feature = "stdio")]
pub use self::stdio::{StderrLock, StdinLock, StdoutLock};
#[cfg(feature = "stdio")]
pub use self::stdio::{_eprint, _print};
pub use self::util::{copy, empty, repeat, sink, Empty, Repeat, Sink};

pub mod prelude;
mod buffered;
mod cursor;
mod error;
mod impls;
mod lazy;
#[cfg(feature = "stdio")]
mod stdio;
mod util;

const DEFAULT_BUF_SIZE: usize = crate::sys_common::io::DEFAULT_BUF_SIZE;

struct Guard<'a> {
    buf: &'a mut Vec<u8>,
    len: usize,
}

impl Drop for Guard<'_> {
    fn drop(&mut self) {
        unsafe {
            self.buf.set_len(self.len);
        }
    }
}

// A few methods below (read_to_string, read_line) will append data into a
// `String` buffer, but we need to be pretty careful when doing this. The
// implementation will just call `.as_mut_vec()` and then delegate to a
// byte-oriented reading method, but we must ensure that when returning we never
// leave `buf` in a state such that it contains invalid UTF-8 in its bounds.
//
// To this end, we use an RAII guard (to protect against panics) which updates
// the length of the string when it is dropped. This guard initially truncates
// the string to the prior length and only after we've validated that the
// new contents are valid UTF-8 do we allow it to set a longer length.
//
// The unsafety in this function is twofold:
//
// 1. We're looking at the raw bytes of `buf`, so we take on the burden of UTF-8
//    checks.
// 2. We're passing a raw buffer to the function `f`, and it is expected that
//    the function only *appends* bytes to the buffer. We'll get undefined
//    behavior if existing bytes are overwritten to have non-UTF-8 data.
fn append_to_string<F>(buf: &mut String, f: F) -> Result<usize>
where
    F: FnOnce(&mut Vec<u8>) -> Result<usize>,
{
    unsafe {
        let mut g = Guard { len: buf.len(), buf: buf.as_mut_vec() };
        let ret = f(g.buf);
        if str::from_utf8(&g.buf[g.len..]).is_err() {
            ret.and_then(|_| {
                Err(Error::new(ErrorKind::InvalidData, "stream did not contain valid UTF-8"))
            })
        } else {
            g.len = g.buf.len();
            ret
        }
    }
}

// This uses an adaptive system to extend the vector when it fills. We want to
// avoid paying to allocate and zero a huge chunk of memory if the reader only
// has 4 bytes while still making large reads if the reader does have a ton
// of data to return. Simply tacking on an extra DEFAULT_BUF_SIZE space every
// time is 4,500 times (!) slower than a default reservation size of 32 if the
// reader has a very small amount of data to return.
//
// Because we're extending the buffer with uninitialized data for trusted
// readers, we need to make sure to truncate that if any of this panics.
fn read_to_end<R: Read + ?Sized>(r: &mut R, buf: &mut Vec<u8>) -> Result<usize> {
    read_to_end_with_reservation(r, buf, |_| 32)
}

fn read_to_end_with_reservation<R, F>(
    r: &mut R,
    buf: &mut Vec<u8>,
    mut reservation_size: F,
) -> Result<usize>
where
    R: Read + ?Sized,
    F: FnMut(&R) -> usize,
{
    let start_len = buf.len();
    let mut g = Guard { len: buf.len(), buf };
    let ret;
    loop {
        if g.len == g.buf.len() {
            unsafe {
                // FIXME(danielhenrymantilla): #42788
                //
                //   - This creates a (mut) reference to a slice of
                //     _uninitialized_ integers, which is **undefined behavior**
                //
                //   - Only the standard library gets to soundly "ignore" this,
                //     based on its privileged knowledge of unstable rustc
                //     internals;
                g.buf.reserve(reservation_size(r));
                let capacity = g.buf.capacity();
                g.buf.set_len(capacity);
                r.initializer().initialize(&mut g.buf[g.len..]);
            }
        }

        match r.read(&mut g.buf[g.len..]) {
            Ok(0) => {
                ret = Ok(g.len - start_len);
                break;
            }
            Ok(n) => g.len += n,
            Err(ref e) if e.kind() == ErrorKind::Interrupted => {}
            Err(e) => {
                ret = Err(e);
                break;
            }
        }
    }

    ret
}

pub(crate) fn default_read_vectored<F>(read: F, bufs: &mut [IoSliceMut<'_>]) -> Result<usize>
where
    F: FnOnce(&mut [u8]) -> Result<usize>,
{
    let buf = bufs.iter_mut().find(|b| !b.is_empty()).map_or(&mut [][..], |b| &mut **b);
    read(buf)
}

pub(crate) fn default_write_vectored<F>(write: F, bufs: &[IoSlice<'_>]) -> Result<usize>
where
    F: FnOnce(&[u8]) -> Result<usize>,
{
    let buf = bufs.iter().find(|b| !b.is_empty()).map_or(&[][..], |b| &**b);
    write(buf)
}

/// The `Read` trait allows for reading bytes from a source.
///
/// Implementors of the `Read` trait are called 'readers'.
///
/// Readers are defined by one required method, [`read()`]. Each call to [`read()`]
/// will attempt to pull bytes from this source into a provided buffer. A
/// number of other methods are implemented in terms of [`read()`], giving
/// implementors a number of ways to read bytes while only needing to implement
/// a single method.
///
/// Readers are intended to be composable with one another. Many implementors
/// throughout [`std::io`] take and provide types which implement the `Read`
/// trait.
///
/// Please note that each call to [`read()`] may involve a system call, and
/// therefore, using something that implements [`BufRead`], such as
/// [`BufReader`], will be more efficient.
///
pub trait Read {
    /// Pull some bytes from this source into the specified buffer, returning
    /// how many bytes were read.
    ///
    /// This function does not provide any guarantees about whether it blocks
    /// waiting for data, but if an object needs to block for a read and cannot,
    /// it will typically signal this via an [`Err`] return value.
    ///
    /// If the return value of this method is [`Ok(n)`], then it must be
    /// guaranteed that `0 <= n <= buf.len()`. A nonzero `n` value indicates
    /// that the buffer `buf` has been filled in with `n` bytes of data from this
    /// source. If `n` is `0`, then it can indicate one of two scenarios:
    ///
    /// 1. This reader has reached its "end of file" and will likely no longer
    ///    be able to produce bytes. Note that this does not mean that the
    ///    reader will *always* no longer be able to produce bytes.
    /// 2. The buffer specified was 0 bytes in length.
    ///
    /// No guarantees are provided about the contents of `buf` when this
    /// function is called, implementations cannot rely on any property of the
    /// contents of `buf` being true. It is recommended that *implementations*
    /// only write data to `buf` instead of reading its contents.
    ///
    /// Correspondingly, however, *callers* of this method may not assume any guarantees
    /// about how the implementation uses `buf`. The trait is safe to implement,
    /// so it is possible that the code that's supposed to write to the buffer might also read
    /// from it. It is your responsibility to make sure that `buf` is initialized
    /// before calling `read`. Calling `read` with an uninitialized `buf` (of the kind one
    /// obtains via [`MaybeUninit<T>`]) is not safe, and can lead to undefined behavior.
    ///
    /// [`MaybeUninit<T>`]: ../mem/union.MaybeUninit.html
    ///
    /// # Errors
    ///
    /// If this function encounters any form of I/O or other error, an error
    /// variant will be returned. If an error is returned then it must be
    /// guaranteed that no bytes were read.
    ///
    /// An error of the [`ErrorKind::Interrupted`] kind is non-fatal and the read
    /// operation should be retried if there is nothing else to do.
    ///
    fn read(&mut self, buf: &mut [u8]) -> Result<usize>;

    /// Like `read`, except that it reads into a slice of buffers.
    ///
    /// Data is copied to fill each buffer in order, with the final buffer
    /// written to possibly being only partially filled. This method must behave
    /// as a single call to `read` with the buffers concatenated would.
    ///
    /// The default implementation calls `read` with either the first nonempty
    /// buffer provided, or an empty one if none exists.
    fn read_vectored(&mut self, bufs: &mut [IoSliceMut<'_>]) -> Result<usize> {
        default_read_vectored(|b| self.read(b), bufs)
    }

    /// Determines if this `Read`er can work with buffers of uninitialized
    /// memory.
    ///
    /// The default implementation returns an initializer which will zero
    /// buffers.
    ///
    /// If a `Read`er guarantees that it can work properly with uninitialized
    /// memory, it should call [`Initializer::nop()`]. See the documentation for
    /// [`Initializer`] for details.
    ///
    /// The behavior of this method must be independent of the state of the
    /// `Read`er - the method only takes `&self` so that it can be used through
    /// trait objects.
    ///
    /// # Safety
    ///
    /// This method is unsafe because a `Read`er could otherwise return a
    /// non-zeroing `Initializer` from another `Read` type without an `unsafe`
    /// block.
    ///
    /// [`Initializer::nop()`]: ../../std/io/struct.Initializer.html#method.nop
    /// [`Initializer`]: ../../std/io/struct.Initializer.html
    #[inline]
    unsafe fn initializer(&self) -> Initializer {
        Initializer::zeroing()
    }

    /// Read all bytes until EOF in this source, placing them into `buf`.
    ///
    /// All bytes read from this source will be appended to the specified buffer
    /// `buf`. This function will continuously call [`read()`] to append more data to
    /// `buf` until [`read()`] returns either [`Ok(0)`] or an error of
    /// non-[`ErrorKind::Interrupted`] kind.
    ///
    /// If successful, this function will return the total number of bytes read.
    ///
    /// # Errors
    ///
    /// If this function encounters an error of the kind
    /// [`ErrorKind::Interrupted`] then the error is ignored and the operation
    /// will continue.
    ///
    /// If any other read error is encountered then this function immediately
    /// returns. Any bytes which have already been read will be appended to
    /// `buf`.
    ///
    fn read_to_end(&mut self, buf: &mut Vec<u8>) -> Result<usize> {
        read_to_end(self, buf)
    }

    /// Read all bytes until EOF in this source, appending them to `buf`.
    ///
    /// If successful, this function returns the number of bytes which were read
    /// and appended to `buf`.
    ///
    /// # Errors
    ///
    /// If the data in this stream is *not* valid UTF-8 then an error is
    /// returned and `buf` is unchanged.
    ///
    /// See [`read_to_end`][readtoend] for other error semantics.
    ///
    /// [readtoend]: #method.read_to_end
    ///
    fn read_to_string(&mut self, buf: &mut String) -> Result<usize> {
        // Note that we do *not* call `.read_to_end()` here. We are passing
        // `&mut Vec<u8>` (the raw contents of `buf`) into the `read_to_end`
        // method to fill it up. An arbitrary implementation could overwrite the
        // entire contents of the vector, not just append to it (which is what
        // we are expecting).
        //
        // To prevent extraneously checking the UTF-8-ness of the entire buffer
        // we pass it to our hardcoded `read_to_end` implementation which we
        // know is guaranteed to only read data into the end of the buffer.
        append_to_string(buf, |b| read_to_end(self, b))
    }

    /// Read the exact number of bytes required to fill `buf`.
    ///
    /// This function reads as many bytes as necessary to completely fill the
    /// specified buffer `buf`.
    ///
    /// No guarantees are provided about the contents of `buf` when this
    /// function is called, implementations cannot rely on any property of the
    /// contents of `buf` being true. It is recommended that implementations
    /// only write data to `buf` instead of reading its contents.
    ///
    /// # Errors
    ///
    /// If this function encounters an error of the kind
    /// [`ErrorKind::Interrupted`] then the error is ignored and the operation
    /// will continue.
    ///
    /// If this function encounters an "end of file" before completely filling
    /// the buffer, it returns an error of the kind [`ErrorKind::UnexpectedEof`].
    /// The contents of `buf` are unspecified in this case.
    ///
    /// If any other read error is encountered then this function immediately
    /// returns. The contents of `buf` are unspecified in this case.
    ///
    /// If this function returns an error, it is unspecified how many bytes it
    /// has read, but it will never read more than would be necessary to
    /// completely fill the buffer.
    ///
    fn read_exact(&mut self, mut buf: &mut [u8]) -> Result<()> {
        while !buf.is_empty() {
            match self.read(buf) {
                Ok(0) => break,
                Ok(n) => {
                    let tmp = buf;
                    buf = &mut tmp[n..];
                }
                Err(ref e) if e.kind() == ErrorKind::Interrupted => {}
                Err(e) => return Err(e),
            }
        }
        if !buf.is_empty() {
            Err(Error::new(ErrorKind::UnexpectedEof, "failed to fill whole buffer"))
        } else {
            Ok(())
        }
    }

    /// Creates a "by reference" adaptor for this instance of `Read`.
    ///
    /// The returned adaptor also implements `Read` and will simply borrow this
    /// current reader.
    ///
    fn by_ref(&mut self) -> &mut Self
    where
        Self: Sized,
    {
        self
    }

    /// Transforms this `Read` instance to an [`Iterator`] over its bytes.
    ///
    /// The returned type implements [`Iterator`] where the `Item` is
    /// [`Result`]`<`[`u8`]`, `[`io::Error`]`>`.
    /// The yielded item is [`Ok`] if a byte was successfully read and [`Err`]
    /// otherwise. EOF is mapped to returning [`None`] from this iterator.
    ///
    fn bytes(self) -> Bytes<Self>
    where
        Self: Sized,
    {
        Bytes { inner: self }
    }

    /// Creates an adaptor which will chain this stream with another.
    ///
    /// The returned `Read` instance will first read all bytes from this object
    /// until EOF is encountered. Afterwards the output is equivalent to the
    /// output of `next`.
    ///
    fn chain<R: Read>(self, next: R) -> Chain<Self, R>
    where
        Self: Sized,
    {
        Chain { first: self, second: next, done_first: false }
    }

    /// Creates an adaptor which will read at most `limit` bytes from it.
    ///
    /// This function returns a new instance of `Read` which will read at most
    /// `limit` bytes, after which it will always return EOF ([`Ok(0)`]). Any
    /// read errors will not count towards the number of bytes read and future
    /// calls to [`read()`] may succeed.
    ///
    fn take(self, limit: u64) -> Take<Self>
    where
        Self: Sized,
    {
        Take { inner: self, limit }
    }
}

/// A buffer type used with `Read::read_vectored`.
///
/// It is semantically a wrapper around an `&mut [u8]`, but is guaranteed to be
/// ABI compatible with the `iovec` type on Unix platforms and `WSABUF` on
/// Windows.
#[repr(transparent)]
pub struct IoSliceMut<'a>(sys::io::IoSliceMut<'a>);

unsafe impl<'a> Send for IoSliceMut<'a> {}

unsafe impl<'a> Sync for IoSliceMut<'a> {}

impl<'a> fmt::Debug for IoSliceMut<'a> {
    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
        fmt::Debug::fmt(self.0.as_slice(), fmt)
    }
}

impl<'a> IoSliceMut<'a> {
    /// Creates a new `IoSliceMut` wrapping a byte slice.
    ///
    /// # Panics
    ///
    /// Panics on Windows if the slice is larger than 4GB.
    #[inline]
    pub fn new(buf: &'a mut [u8]) -> IoSliceMut<'a> {
        IoSliceMut(sys::io::IoSliceMut::new(buf))
    }

    /// Advance the internal cursor of the slice.
    ///
    /// # Notes
    ///
    /// Elements in the slice may be modified if the cursor is not advanced to
    /// the end of the slice. For example if we have a slice of buffers with 2
    /// `IoSliceMut`s, both of length 8, and we advance the cursor by 10 bytes
    /// the first `IoSliceMut` will be untouched however the second will be
    /// modified to remove the first 2 bytes (10 - 8).
    ///
    #[inline]
    pub fn advance<'b>(bufs: &'b mut [IoSliceMut<'a>], n: usize) -> &'b mut [IoSliceMut<'a>] {
        // Number of buffers to remove.
        let mut remove = 0;
        // Total length of all the to be removed buffers.
        let mut accumulated_len = 0;
        for buf in bufs.iter() {
            if accumulated_len + buf.len() > n {
                break;
            } else {
                accumulated_len += buf.len();
                remove += 1;
            }
        }

        let bufs = &mut bufs[remove..];
        if !bufs.is_empty() {
            bufs[0].0.advance(n - accumulated_len)
        }
        bufs
    }
}

impl<'a> Deref for IoSliceMut<'a> {
    type Target = [u8];

    #[inline]
    fn deref(&self) -> &[u8] {
        self.0.as_slice()
    }
}

impl<'a> DerefMut for IoSliceMut<'a> {
    #[inline]
    fn deref_mut(&mut self) -> &mut [u8] {
        self.0.as_mut_slice()
    }
}

/// A buffer type used with `Write::write_vectored`.
///
/// It is semantically a wrapper around an `&[u8]`, but is guaranteed to be
/// ABI compatible with the `iovec` type on Unix platforms and `WSABUF` on
/// Windows.
#[derive(Copy, Clone)]
#[repr(transparent)]
pub struct IoSlice<'a>(sys::io::IoSlice<'a>);

unsafe impl<'a> Send for IoSlice<'a> {}

unsafe impl<'a> Sync for IoSlice<'a> {}

impl<'a> fmt::Debug for IoSlice<'a> {
    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
        fmt::Debug::fmt(self.0.as_slice(), fmt)
    }
}

impl<'a> IoSlice<'a> {
    /// Creates a new `IoSlice` wrapping a byte slice.
    ///
    /// # Panics
    ///
    /// Panics on Windows if the slice is larger than 4GB.
    #[inline]
    pub fn new(buf: &'a [u8]) -> IoSlice<'a> {
        IoSlice(sys::io::IoSlice::new(buf))
    }

    /// Advance the internal cursor of the slice.
    ///
    /// # Notes
    ///
    /// Elements in the slice may be modified if the cursor is not advanced to
    /// the end of the slice. For example if we have a slice of buffers with 2
    /// `IoSlice`s, both of length 8, and we advance the cursor by 10 bytes the
    /// first `IoSlice` will be untouched however the second will be modified to
    /// remove the first 2 bytes (10 - 8).
    ///
    #[inline]
    pub fn advance<'b>(bufs: &'b mut [IoSlice<'a>], n: usize) -> &'b mut [IoSlice<'a>] {
        // Number of buffers to remove.
        let mut remove = 0;
        // Total length of all the to be removed buffers.
        let mut accumulated_len = 0;
        for buf in bufs.iter() {
            if accumulated_len + buf.len() > n {
                break;
            } else {
                accumulated_len += buf.len();
                remove += 1;
            }
        }

        let bufs = &mut bufs[remove..];
        if !bufs.is_empty() {
            bufs[0].0.advance(n - accumulated_len)
        }
        bufs
    }
}

impl<'a> Deref for IoSlice<'a> {
    type Target = [u8];

    #[inline]
    fn deref(&self) -> &[u8] {
        self.0.as_slice()
    }
}

/// A type used to conditionally initialize buffers passed to `Read` methods.
#[derive(Debug)]
pub struct Initializer(bool);

impl Initializer {
    /// Returns a new `Initializer` which will zero out buffers.
    #[inline]
    pub fn zeroing() -> Initializer {
        Initializer(true)
    }

    /// Returns a new `Initializer` which will not zero out buffers.
    ///
    /// # Safety
    ///
    /// This may only be called by `Read`ers which guarantee that they will not
    /// read from buffers passed to `Read` methods, and that the return value of
    /// the method accurately reflects the number of bytes that have been
    /// written to the head of the buffer.
    #[inline]
    pub unsafe fn nop() -> Initializer {
        Initializer(false)
    }

    /// Indicates if a buffer should be initialized.
    #[inline]
    pub fn should_initialize(&self) -> bool {
        self.0
    }

    /// Initializes a buffer if necessary.
    #[inline]
    pub fn initialize(&self, buf: &mut [u8]) {
        if self.should_initialize() {
            unsafe { ptr::write_bytes(buf.as_mut_ptr(), 0, buf.len()) }
        }
    }
}

/// A trait for objects which are byte-oriented sinks.
///
/// Implementors of the `Write` trait are sometimes called 'writers'.
///
/// Writers are defined by two required methods, [`write`] and [`flush`]:
///
/// * The [`write`] method will attempt to write some data into the object,
///   returning how many bytes were successfully written.
///
/// * The [`flush`] method is useful for adaptors and explicit buffers
///   themselves for ensuring that all buffered data has been pushed out to the
///   'true sink'.
///
/// Writers are intended to be composable with one another. Many implementors
/// throughout [`std::io`] take and provide types which implement the `Write`
/// trait.
///
/// [`write`]: #tymethod.write
/// [`flush`]: #tymethod.flush
/// [`std::io`]: index.html
///
pub trait Write {
    /// Write a buffer into this writer, returning how many bytes were written.
    ///
    /// This function will attempt to write the entire contents of `buf`, but
    /// the entire write may not succeed, or the write may also generate an
    /// error. A call to `write` represents *at most one* attempt to write to
    /// any wrapped object.
    ///
    /// Calls to `write` are not guaranteed to block waiting for data to be
    /// written, and a write which would otherwise block can be indicated through
    /// an [`Err`] variant.
    ///
    /// If the return value is [`Ok(n)`] then it must be guaranteed that
    /// `n <= buf.len()`. A return value of `0` typically means that the
    /// underlying object is no longer able to accept bytes and will likely not
    /// be able to in the future as well, or that the buffer provided is empty.
    ///
    /// # Errors
    ///
    /// Each call to `write` may generate an I/O error indicating that the
    /// operation could not be completed. If an error is returned then no bytes
    /// in the buffer were written to this writer.
    ///
    /// It is **not** considered an error if the entire buffer could not be
    /// written to this writer.
    ///
    /// An error of the [`ErrorKind::Interrupted`] kind is non-fatal and the
    /// write operation should be retried if there is nothing else to do.
    ///
    /// [`Err`]: ../../std/result/enum.Result.html#variant.Err
    /// [`Ok(n)`]:  ../../std/result/enum.Result.html#variant.Ok
    /// [`ErrorKind::Interrupted`]: ../../std/io/enum.ErrorKind.html#variant.Interrupted
    ///
    fn write(&mut self, buf: &[u8]) -> Result<usize>;

    /// Like `write`, except that it writes from a slice of buffers.
    ///
    /// Data is copied from each buffer in order, with the final buffer
    /// read from possibly being only partially consumed. This method must
    /// behave as a call to `write` with the buffers concatenated would.
    ///
    /// The default implementation calls `write` with either the first nonempty
    /// buffer provided, or an empty one if none exists.
    fn write_vectored(&mut self, bufs: &[IoSlice<'_>]) -> Result<usize> {
        default_write_vectored(|b| self.write(b), bufs)
    }

    /// Flush this output stream, ensuring that all intermediately buffered
    /// contents reach their destination.
    ///
    /// # Errors
    ///
    /// It is considered an error if not all bytes could be written due to
    /// I/O errors or EOF being reached.
    ///
    fn flush(&mut self) -> Result<()>;

    /// Attempts to write an entire buffer into this writer.
    ///
    /// This method will continuously call [`write`] until there is no more data
    /// to be written or an error of non-[`ErrorKind::Interrupted`] kind is
    /// returned. This method will not return until the entire buffer has been
    /// successfully written or such an error occurs. The first error that is
    /// not of [`ErrorKind::Interrupted`] kind generated from this method will be
    /// returned.
    ///
    /// If the buffer contains no data, this will never call [`write`].
    ///
    /// # Errors
    ///
    /// This function will return the first error of
    /// non-[`ErrorKind::Interrupted`] kind that [`write`] returns.
    ///
    /// [`ErrorKind::Interrupted`]: ../../std/io/enum.ErrorKind.html#variant.Interrupted
    /// [`write`]: #tymethod.write
    ///
    fn write_all(&mut self, mut buf: &[u8]) -> Result<()> {
        while !buf.is_empty() {
            match self.write(buf) {
                Ok(0) => {
                    return Err(Error::new(ErrorKind::WriteZero, "failed to write whole buffer"));
                }
                Ok(n) => buf = &buf[n..],
                Err(ref e) if e.kind() == ErrorKind::Interrupted => {}
                Err(e) => return Err(e),
            }
        }
        Ok(())
    }

    /// Attempts to write multiple buffers into this writer.
    ///
    /// This method will continuously call [`write_vectored`] until there is no
    /// more data to be written or an error of non-[`ErrorKind::Interrupted`]
    /// kind is returned. This method will not return until all buffers have
    /// been successfully written or such an error occurs. The first error that
    /// is not of [`ErrorKind::Interrupted`] kind generated from this method
    /// will be returned.
    ///
    /// If the buffer contains no data, this will never call [`write_vectored`].
    ///
    /// [`write_vectored`]: #method.write_vectored
    /// [`ErrorKind::Interrupted`]: ../../std/io/enum.ErrorKind.html#variant.Interrupted
    ///
    /// # Notes
    ///
    ///
    /// Unlike `io::Write::write_vectored`, this takes a *mutable* reference to
    /// a slice of `IoSlice`s, not an immutable one. That's because we need to
    /// modify the slice to keep track of the bytes already written.
    ///
    /// Once this function returns, the contents of `bufs` are unspecified, as
    /// this depends on how many calls to `write_vectored` were necessary. It is
    /// best to understand this function as taking ownership of `bufs` and to
    /// not use `bufs` afterwards. The underlying buffers, to which the
    /// `IoSlice`s point (but not the `IoSlice`s themselves), are unchanged and
    /// can be reused.
    ///
    fn write_all_vectored(&mut self, mut bufs: &mut [IoSlice<'_>]) -> Result<()> {
        while !bufs.is_empty() {
            match self.write_vectored(bufs) {
                Ok(0) => {
                    return Err(Error::new(ErrorKind::WriteZero, "failed to write whole buffer"));
                }
                Ok(n) => bufs = IoSlice::advance(mem::take(&mut bufs), n),
                Err(ref e) if e.kind() == ErrorKind::Interrupted => {}
                Err(e) => return Err(e),
            }
        }
        Ok(())
    }

    /// Writes a formatted string into this writer, returning any error
    /// encountered.
    ///
    /// This method is primarily used to interface with the
    /// [`format_args!`][formatargs] macro, but it is rare that this should
    /// explicitly be called. The [`write!`][write] macro should be favored to
    /// invoke this method instead.
    ///
    /// [formatargs]: ../macro.format_args.html
    /// [write]: ../macro.write.html
    ///
    /// This function internally uses the [`write_all`][writeall] method on
    /// this trait and hence will continuously write data so long as no errors
    /// are received. This also means that partial writes are not indicated in
    /// this signature.
    ///
    /// [writeall]: #method.write_all
    ///
    /// # Errors
    ///
    /// This function will return any I/O error reported while formatting.
    ///
    fn write_fmt(&mut self, fmt: fmt::Arguments<'_>) -> Result<()> {
        // Create a shim which translates a Write to a fmt::Write and saves
        // off I/O errors. instead of discarding them
        struct Adaptor<'a, T: ?Sized + 'a> {
            inner: &'a mut T,
            error: Result<()>,
        }

        impl<T: Write + ?Sized> fmt::Write for Adaptor<'_, T> {
            fn write_str(&mut self, s: &str) -> fmt::Result {
                match self.inner.write_all(s.as_bytes()) {
                    Ok(()) => Ok(()),
                    Err(e) => {
                        self.error = Err(e);
                        Err(fmt::Error)
                    }
                }
            }
        }

        let mut output = Adaptor { inner: self, error: Ok(()) };
        match fmt::write(&mut output, fmt) {
            Ok(()) => Ok(()),
            Err(..) => {
                // check if the error came from the underlying `Write` or not
                if output.error.is_err() {
                    output.error
                } else {
                    Err(Error::new(ErrorKind::Other, "formatter error"))
                }
            }
        }
    }

    /// Creates a "by reference" adaptor for this instance of `Write`.
    ///
    /// The returned adaptor also implements `Write` and will simply borrow this
    /// current writer.
    ///
    fn by_ref(&mut self) -> &mut Self
    where
        Self: Sized,
    {
        self
    }
}

/// The `Seek` trait provides a cursor which can be moved within a stream of
/// bytes.
///
/// The stream typically has a fixed size, allowing seeking relative to either
/// end or the current offset.
///
pub trait Seek {
    /// Seek to an offset, in bytes, in a stream.
    ///
    /// A seek beyond the end of a stream is allowed, but behavior is defined
    /// by the implementation.
    ///
    /// 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`].
    ///
    /// # Errors
    ///
    /// Seeking to a negative offset is considered an error.
    ///
    /// [`SeekFrom::Start`]: enum.SeekFrom.html#variant.Start
    fn seek(&mut self, pos: SeekFrom) -> Result<u64>;

    /// Returns the length of this stream (in bytes).
    ///
    /// This method is implemented using up to three seek operations. If this
    /// method returns successfully, the seek position is unchanged (i.e. the
    /// position before calling this method is the same as afterwards).
    /// However, if this method returns an error, the seek position is
    /// unspecified.
    ///
    /// If you need to obtain the length of *many* streams and you don't care
    /// about the seek position afterwards, you can reduce the number of seek
    /// operations by simply calling `seek(SeekFrom::End(0))` and using its
    /// return value (it is also the stream length).
    ///
    /// Note that length of a stream can change over time (for example, when
    /// data is appended to a file). So calling this method multiple times does
    /// not necessarily return the same length each time.
    ///
    fn stream_len(&mut self) -> Result<u64> {
        let old_pos = self.stream_position()?;
        let len = self.seek(SeekFrom::End(0))?;

        // Avoid seeking a third time when we were already at the end of the
        // stream. The branch is usually way cheaper than a seek operation.
        if old_pos != len {
            self.seek(SeekFrom::Start(old_pos))?;
        }

        Ok(len)
    }

    /// Returns the current seek position from the start of the stream.
    ///
    /// This is equivalent to `self.seek(SeekFrom::Current(0))`.
    ///
    fn stream_position(&mut self) -> Result<u64> {
        self.seek(SeekFrom::Current(0))
    }
}

/// Enumeration of possible methods to seek within an I/O object.
///
/// It is used by the [`Seek`] trait.
///
/// [`Seek`]: trait.Seek.html
#[derive(Copy, PartialEq, Eq, Clone, Debug)]
pub enum SeekFrom {
    /// Sets the offset to the provided number of bytes.
    Start(u64),

    /// Sets the offset to the size of this object plus the specified number of
    /// bytes.
    ///
    /// It is possible to seek beyond the end of an object, but it's an error to
    /// seek before byte 0.
    End(i64),

    /// Sets the offset to the current position plus the specified number of
    /// bytes.
    ///
    /// It is possible to seek beyond the end of an object, but it's an error to
    /// seek before byte 0.
    Current(i64),
}

fn read_until<R: BufRead + ?Sized>(r: &mut R, delim: u8, buf: &mut Vec<u8>) -> Result<usize> {
    let mut read = 0;
    loop {
        let (done, used) = {
            let available = match r.fill_buf() {
                Ok(n) => n,
                Err(ref e) if e.kind() == ErrorKind::Interrupted => continue,
                Err(e) => return Err(e),
            };
            match memchr::memchr(delim, available) {
                Some(i) => {
                    buf.extend_from_slice(&available[..=i]);
                    (true, i + 1)
                }
                None => {
                    buf.extend_from_slice(available);
                    (false, available.len())
                }
            }
        };
        r.consume(used);
        read += used;
        if done || used == 0 {
            return Ok(read);
        }
    }
}

/// A `BufRead` is a type of `Read`er which has an internal buffer, allowing it
/// to perform extra ways of reading.
///
/// For example, reading line-by-line is inefficient without using a buffer, so
/// if you want to read by line, you'll need `BufRead`, which includes a
/// [`read_line`] method as well as a [`lines`] iterator.
///
pub trait BufRead: Read {
    /// Returns the contents of the internal buffer, filling it with more data
    /// from the inner reader if it is empty.
    ///
    /// This function is a lower-level call. It needs to be paired with the
    /// [`consume`] method to function properly. When calling this
    /// method, none of the contents will be "read" in the sense that later
    /// calling `read` may return the same contents. As such, [`consume`] must
    /// be called with the number of bytes that are consumed from this buffer to
    /// ensure that the bytes are never returned twice.
    ///
    /// [`consume`]: #tymethod.consume
    ///
    /// An empty buffer returned indicates that the stream has reached EOF.
    ///
    /// # Errors
    ///
    /// This function will return an I/O error if the underlying reader was
    /// read, but returned an error.
    ///
    fn fill_buf(&mut self) -> Result<&[u8]>;

    /// Tells this buffer that `amt` bytes have been consumed from the buffer,
    /// so they should no longer be returned in calls to `read`.
    ///
    /// This function is a lower-level call. It needs to be paired with the
    /// [`fill_buf`] method to function properly. This function does
    /// not perform any I/O, it simply informs this object that some amount of
    /// its buffer, returned from [`fill_buf`], has been consumed and should
    /// no longer be returned. As such, this function may do odd things if
    /// [`fill_buf`] isn't called before calling it.
    ///
    /// The `amt` must be `<=` the number of bytes in the buffer returned by
    /// [`fill_buf`].
    ///
    fn consume(&mut self, amt: usize);

    /// Read all bytes into `buf` until the delimiter `byte` or EOF is reached.
    ///
    /// This function will read bytes from the underlying stream until the
    /// delimiter or EOF is found. Once found, all bytes up to, and including,
    /// the delimiter (if found) will be appended to `buf`.
    ///
    /// If successful, this function will return the total number of bytes read.
    ///
    /// # Errors
    ///
    /// This function will ignore all instances of [`ErrorKind::Interrupted`] and
    /// will otherwise return any errors returned by [`fill_buf`].
    ///
    /// If an I/O error is encountered then all bytes read so far will be
    /// present in `buf` and its length will have been adjusted appropriately.
    ///
    /// [`fill_buf`]: #tymethod.fill_buf
    /// [`ErrorKind::Interrupted`]: enum.ErrorKind.html#variant.Interrupted
    ///
    fn read_until(&mut self, byte: u8, buf: &mut Vec<u8>) -> Result<usize> {
        read_until(self, byte, buf)
    }

    /// Read all bytes until a newline (the 0xA byte) is reached, and append
    /// them to the provided buffer.
    ///
    /// This function will read bytes from the underlying stream until the
    /// newline delimiter (the 0xA byte) or EOF is found. Once found, all bytes
    /// up to, and including, the delimiter (if found) will be appended to
    /// `buf`.
    ///
    /// If successful, this function will return the total number of bytes read.
    ///
    /// If this function returns `Ok(0)`, the stream has reached EOF.
    ///
    /// # Errors
    ///
    /// This function has the same error semantics as [`read_until`] and will
    /// also return an error if the read bytes are not valid UTF-8. If an I/O
    /// error is encountered then `buf` may contain some bytes already read in
    /// the event that all data read so far was valid UTF-8.
    ///
    /// [`read_until`]: #method.read_until
    ///
    fn read_line(&mut self, buf: &mut String) -> Result<usize> {
        // Note that we are not calling the `.read_until` method here, but
        // rather our hardcoded implementation. For more details as to why, see
        // the comments in `read_to_end`.
        append_to_string(buf, |b| read_until(self, b'\n', b))
    }

    /// Returns an iterator over the contents of this reader split on the byte
    /// `byte`.
    ///
    /// The iterator returned from this function will return instances of
    /// [`io::Result`]`<`[`Vec<u8>`]`>`. Each vector returned will *not* have
    /// the delimiter byte at the end.
    ///
    /// This function will yield errors whenever [`read_until`] would have
    /// also yielded an error.
    ///
    /// [`io::Result`]: type.Result.html
    /// [`Vec<u8>`]: ../vec/struct.Vec.html
    /// [`read_until`]: #method.read_until
    ///
    fn split(self, byte: u8) -> Split<Self>
    where
        Self: Sized,
    {
        Split { buf: self, delim: byte }
    }

    /// Returns an iterator over the lines of this reader.
    ///
    /// The iterator returned from this function will yield instances of
    /// [`io::Result`]`<`[`String`]`>`. Each string returned will *not* have a newline
    /// byte (the 0xA byte) or CRLF (0xD, 0xA bytes) at the end.
    ///
    /// [`io::Result`]: type.Result.html
    /// [`String`]: ../string/struct.String.html
    ///
    /// # Errors
    ///
    /// Each line of the iterator has the same error semantics as [`BufRead::read_line`].
    ///
    /// [`BufRead::read_line`]: trait.BufRead.html#method.read_line
    fn lines(self) -> Lines<Self>
    where
        Self: Sized,
    {
        Lines { buf: self }
    }
}

/// Adaptor to chain together two readers.
///
/// This struct is generally created by calling [`chain`] on a reader.
/// Please see the documentation of [`chain`] for more details.
///
/// [`chain`]: trait.Read.html#method.chain
pub struct Chain<T, U> {
    first: T,
    second: U,
    done_first: bool,
}

impl<T, U> Chain<T, U> {
    /// Consumes the `Chain`, returning the wrapped readers.
    ///
    pub fn into_inner(self) -> (T, U) {
        (self.first, self.second)
    }

    /// Gets references to the underlying readers in this `Chain`.
    ///
    pub fn get_ref(&self) -> (&T, &U) {
        (&self.first, &self.second)
    }

    /// Gets mutable references to the underlying readers in this `Chain`.
    ///
    /// Care should be taken to avoid modifying the internal I/O state of the
    /// underlying readers as doing so may corrupt the internal state of this
    /// `Chain`.
    ///
    pub fn get_mut(&mut self) -> (&mut T, &mut U) {
        (&mut self.first, &mut self.second)
    }
}

impl<T: fmt::Debug, U: fmt::Debug> fmt::Debug for Chain<T, U> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("Chain").field("t", &self.first).field("u", &self.second).finish()
    }
}

impl<T: Read, U: Read> Read for Chain<T, U> {
    fn read(&mut self, buf: &mut [u8]) -> Result<usize> {
        if !self.done_first {
            match self.first.read(buf)? {
                0 if !buf.is_empty() => self.done_first = true,
                n => return Ok(n),
            }
        }
        self.second.read(buf)
    }

    fn read_vectored(&mut self, bufs: &mut [IoSliceMut<'_>]) -> Result<usize> {
        if !self.done_first {
            match self.first.read_vectored(bufs)? {
                0 if bufs.iter().any(|b| !b.is_empty()) => self.done_first = true,
                n => return Ok(n),
            }
        }
        self.second.read_vectored(bufs)
    }

    unsafe fn initializer(&self) -> Initializer {
        let initializer = self.first.initializer();
        if initializer.should_initialize() { initializer } else { self.second.initializer() }
    }
}

impl<T: BufRead, U: BufRead> BufRead for Chain<T, U> {
    fn fill_buf(&mut self) -> Result<&[u8]> {
        if !self.done_first {
            match self.first.fill_buf()? {
                buf if buf.is_empty() => {
                    self.done_first = true;
                }
                buf => return Ok(buf),
            }
        }
        self.second.fill_buf()
    }

    fn consume(&mut self, amt: usize) {
        if !self.done_first { self.first.consume(amt) } else { self.second.consume(amt) }
    }
}

/// Reader adaptor which limits the bytes read from an underlying reader.
///
/// This struct is generally created by calling [`take`] on a reader.
/// Please see the documentation of [`take`] for more details.
///
/// [`take`]: trait.Read.html#method.take
#[derive(Debug)]
pub struct Take<T> {
    inner: T,
    limit: u64,
}

impl<T> Take<T> {
    /// Returns the number of bytes that can be read before this instance will
    /// return EOF.
    ///
    /// # Note
    ///
    /// This instance may reach `EOF` after reading fewer bytes than indicated by
    /// this method if the underlying [`Read`] instance reaches EOF.
    ///
    /// [`Read`]: ../../std/io/trait.Read.html
    ///
    pub fn limit(&self) -> u64 {
        self.limit
    }

    /// Sets the number of bytes that can be read before this instance will
    /// return EOF. This is the same as constructing a new `Take` instance, so
    /// the amount of bytes read and the previous limit value don't matter when
    /// calling this method.
    ///
    pub fn set_limit(&mut self, limit: u64) {
        self.limit = limit;
    }

    /// Consumes the `Take`, returning the wrapped reader.
    ///
    pub fn into_inner(self) -> T {
        self.inner
    }

    /// Gets a reference to the underlying reader.
    ///
    pub fn get_ref(&self) -> &T {
        &self.inner
    }

    /// Gets a mutable reference to the underlying reader.
    ///
    /// Care should be taken to avoid modifying the internal I/O state of the
    /// underlying reader as doing so may corrupt the internal limit of this
    /// `Take`.
    ///
    pub fn get_mut(&mut self) -> &mut T {
        &mut self.inner
    }
}

impl<T: Read> Read for Take<T> {
    fn read(&mut self, buf: &mut [u8]) -> Result<usize> {
        // Don't call into inner reader at all at EOF because it may still block
        if self.limit == 0 {
            return Ok(0);
        }

        let max = cmp::min(buf.len() as u64, self.limit) as usize;
        let n = self.inner.read(&mut buf[..max])?;
        self.limit -= n as u64;
        Ok(n)
    }

    unsafe fn initializer(&self) -> Initializer {
        self.inner.initializer()
    }

    fn read_to_end(&mut self, buf: &mut Vec<u8>) -> Result<usize> {
        // Pass in a reservation_size closure that respects the current value
        // of limit for each read. If we hit the read limit, this prevents the
        // final zero-byte read from allocating again.
        read_to_end_with_reservation(self, buf, |self_| cmp::min(self_.limit, 32) as usize)
    }
}

impl<T: BufRead> BufRead for Take<T> {
    fn fill_buf(&mut self) -> Result<&[u8]> {
        // Don't call into inner reader at all at EOF because it may still block
        if self.limit == 0 {
            return Ok(&[]);
        }

        let buf = self.inner.fill_buf()?;
        let cap = cmp::min(buf.len() as u64, self.limit) as usize;
        Ok(&buf[..cap])
    }

    fn consume(&mut self, amt: usize) {
        // Don't let callers reset the limit by passing an overlarge value
        let amt = cmp::min(amt as u64, self.limit) as usize;
        self.limit -= amt as u64;
        self.inner.consume(amt);
    }
}

/// An iterator over `u8` values of a reader.
///
/// This struct is generally created by calling [`bytes`] on a reader.
/// Please see the documentation of [`bytes`] for more details.
///
/// [`bytes`]: trait.Read.html#method.bytes
#[derive(Debug)]
pub struct Bytes<R> {
    inner: R,
}

impl<R: Read> Iterator for Bytes<R> {
    type Item = Result<u8>;

    fn next(&mut self) -> Option<Result<u8>> {
        let mut byte = 0;
        loop {
            return match self.inner.read(slice::from_mut(&mut byte)) {
                Ok(0) => None,
                Ok(..) => Some(Ok(byte)),
                Err(ref e) if e.kind() == ErrorKind::Interrupted => continue,
                Err(e) => Some(Err(e)),
            };
        }
    }
}

/// An iterator over the contents of an instance of `BufRead` split on a
/// particular byte.
///
/// This struct is generally created by calling [`split`] on a `BufRead`.
/// Please see the documentation of [`split`] for more details.
///
/// [`split`]: trait.BufRead.html#method.split
#[derive(Debug)]
pub struct Split<B> {
    buf: B,
    delim: u8,
}

impl<B: BufRead> Iterator for Split<B> {
    type Item = Result<Vec<u8>>;

    fn next(&mut self) -> Option<Result<Vec<u8>>> {
        let mut buf = Vec::new();
        match self.buf.read_until(self.delim, &mut buf) {
            Ok(0) => None,
            Ok(_n) => {
                if buf[buf.len() - 1] == self.delim {
                    buf.pop();
                }
                Some(Ok(buf))
            }
            Err(e) => Some(Err(e)),
        }
    }
}

/// An iterator over the lines of an instance of `BufRead`.
///
/// This struct is generally created by calling [`lines`] on a `BufRead`.
/// Please see the documentation of [`lines`] for more details.
///
/// [`lines`]: trait.BufRead.html#method.lines
#[derive(Debug)]
pub struct Lines<B> {
    buf: B,
}

impl<B: BufRead> Iterator for Lines<B> {
    type Item = Result<String>;

    fn next(&mut self) -> Option<Result<String>> {
        let mut buf = String::new();
        match self.buf.read_line(&mut buf) {
            Ok(0) => None,
            Ok(_n) => {
                if buf.ends_with('\n') {
                    buf.pop();
                    if buf.ends_with('\r') {
                        buf.pop();
                    }
                }
                Some(Ok(buf))
            }
            Err(e) => Some(Err(e)),
        }
    }
}