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    //! An implementation of asynchronous process management for Tokio.
    //!
    //! This module provides a [`Command`] struct that imitates the interface of the
    //! [`std::process::Command`] type in the standard library, but provides asynchronous versions of
    //! functions that create processes. These functions (`spawn`, `status`, `output` and their
    //! variants) return "future aware" types that interoperate with Tokio. The asynchronous process
    //! support is provided through signal handling on Unix and system APIs on Windows.
    //!
    //! [`std::process::Command`]: std::process::Command
    //!
    //! # Examples
    //!
    //! Here's an example program which will spawn `echo hello world` and then wait
    //! for it complete.
    //!
    //! ```no_run
    //! use tokio::process::Command;
    //!
    //! #[tokio::main]
    //! async fn main() -> Result<(), Box<dyn std::error::Error>> {
    //!     // The usage is similar as with the standard library's `Command` type
    //!     let mut child = Command::new("echo")
    //!         .arg("hello")
    //!         .arg("world")
    //!         .spawn()
    //!         .expect("failed to spawn");
    //!
    //!     // Await until the command completes
    //!     let status = child.wait().await?;
    //!     println!("the command exited with: {}", status);
    //!     Ok(())
    //! }
    //! ```
    //!
    //! Next, let's take a look at an example where we not only spawn `echo hello
    //! world` but we also capture its output.
    //!
    //! ```no_run
    //! use tokio::process::Command;
    //!
    //! #[tokio::main]
    //! async fn main() -> Result<(), Box<dyn std::error::Error>> {
    //!     // Like above, but use `output` which returns a future instead of
    //!     // immediately returning the `Child`.
    //!     let output = Command::new("echo").arg("hello").arg("world")
    //!                         .output();
    //!
    //!     let output = output.await?;
    //!
    //!     assert!(output.status.success());
    //!     assert_eq!(output.stdout, b"hello world\n");
    //!     Ok(())
    //! }
    //! ```
    //!
    //! We can also read input line by line.
    //!
    //! ```no_run
    //! use tokio::io::{BufReader, AsyncBufReadExt};
    //! use tokio::process::Command;
    //!
    //! use std::process::Stdio;
    //!
    //! #[tokio::main]
    //! async fn main() -> Result<(), Box<dyn std::error::Error>> {
    //!     let mut cmd = Command::new("cat");
    //!
    //!     // Specify that we want the command's standard output piped back to us.
    //!     // By default, standard input/output/error will be inherited from the
    //!     // current process (for example, this means that standard input will
    //!     // come from the keyboard and standard output/error will go directly to
    //!     // the terminal if this process is invoked from the command line).
    //!     cmd.stdout(Stdio::piped());
    //!
    //!     let mut child = cmd.spawn()
    //!         .expect("failed to spawn command");
    //!
    //!     let stdout = child.stdout.take()
    //!         .expect("child did not have a handle to stdout");
    //!
    //!     let mut reader = BufReader::new(stdout).lines();
    //!
    //!     // Ensure the child process is spawned in the runtime so it can
    //!     // make progress on its own while we await for any output.
    //!     tokio::spawn(async move {
    //!         let status = child.wait().await
    //!             .expect("child process encountered an error");
    //!
    //!         println!("child status was: {}", status);
    //!     });
    //!
    //!     while let Some(line) = reader.next_line().await? {
    //!         println!("Line: {}", line);
    //!     }
    //!
    //!     Ok(())
    //! }
    //! ```
    //!
    //! With some coordination, we can also pipe the output of one command into
    //! another.
    //!
    //! ```no_run
    //! use tokio::join;
    //! use tokio::process::Command;
    //! use std::convert::TryInto;
    //! use std::process::Stdio;
    //!
    //! #[tokio::main]
    //! async fn main() -> Result<(), Box<dyn std::error::Error>> {
    //!     let mut echo = Command::new("echo")
    //!         .arg("hello world!")
    //!         .stdout(Stdio::piped())
    //!         .spawn()
    //!         .expect("failed to spawn echo");
    //!
    //!     let tr_stdin: Stdio = echo
    //!         .stdout
    //!         .take()
    //!         .unwrap()
    //!         .try_into()
    //!         .expect("failed to convert to Stdio");
    //!
    //!     let tr = Command::new("tr")
    //!         .arg("a-z")
    //!         .arg("A-Z")
    //!         .stdin(tr_stdin)
    //!         .stdout(Stdio::piped())
    //!         .spawn()
    //!         .expect("failed to spawn tr");
    //!
    //!     let (echo_result, tr_output) = join!(echo.wait(), tr.wait_with_output());
    //!
    //!     assert!(echo_result.unwrap().success());
    //!
    //!     let tr_output = tr_output.expect("failed to await tr");
    //!     assert!(tr_output.status.success());
    //!
    //!     assert_eq!(tr_output.stdout, b"HELLO WORLD!\n");
    //!
    //!     Ok(())
    //! }
    //! ```
    //!
    //! # Caveats
    //!
    //! ## Dropping/Cancellation
    //!
    //! Similar to the behavior to the standard library, and unlike the futures
    //! paradigm of dropping-implies-cancellation, a spawned process will, by
    //! default, continue to execute even after the `Child` handle has been dropped.
    //!
    //! The [`Command::kill_on_drop`] method can be used to modify this behavior
    //! and kill the child process if the `Child` wrapper is dropped before it
    //! has exited.
    //!
    //! ## Unix Processes
    //!
    //! On Unix platforms processes must be "reaped" by their parent process after
    //! they have exited in order to release all OS resources. A child process which
    //! has exited, but has not yet been reaped by its parent is considered a "zombie"
    //! process. Such processes continue to count against limits imposed by the system,
    //! and having too many zombie processes present can prevent additional processes
    //! from being spawned.
    //!
    //! The tokio runtime will, on a best-effort basis, attempt to reap and clean up
    //! any process which it has spawned. No additional guarantees are made with regards
    //! how quickly or how often this procedure will take place.
    //!
    //! It is recommended to avoid dropping a [`Child`] process handle before it has been
    //! fully `await`ed if stricter cleanup guarantees are required.
    //!
    //! [`Command`]: crate::process::Command
    //! [`Command::kill_on_drop`]: crate::process::Command::kill_on_drop
    //! [`Child`]: crate::process::Child

    #[path = "unix/mod.rs"]
    #[cfg(unix)]
    mod imp;

    #[cfg(unix)]
    pub(crate) mod unix {
        pub(crate) use super::imp::*;
    }

    #[path = "windows.rs"]
    #[cfg(windows)]
    mod imp;

    mod kill;

    use crate::io::{AsyncRead, AsyncWrite, ReadBuf};
    use crate::process::kill::Kill;

    use std::convert::TryInto;
    use std::ffi::OsStr;
    use std::future::Future;
    use std::io;
    #[cfg(unix)]
    use std::os::unix::process::CommandExt;
    #[cfg(windows)]
    use std::os::windows::process::CommandExt;
    use std::path::Path;
    use std::pin::Pin;
    use std::process::{Command as StdCommand, ExitStatus, Output, Stdio};
    use std::task::Context;
    use std::task::Poll;

    /// This structure mimics the API of [`std::process::Command`] found in the standard library, but
    /// replaces functions that create a process with an asynchronous variant. The main provided
    /// asynchronous functions are [spawn](Command::spawn), [status](Command::status), and
    /// [output](Command::output).
    ///
    /// `Command` uses asynchronous versions of some `std` types (for example [`Child`]).
    ///
    /// [`std::process::Command`]: std::process::Command
    /// [`Child`]: struct@Child
    #[derive(Debug)]
    pub struct Command {
        std: StdCommand,
        kill_on_drop: bool,
    }

    pub(crate) struct SpawnedChild {
        child: imp::Child,
        stdin: Option<imp::ChildStdin>,
        stdout: Option<imp::ChildStdout>,
        stderr: Option<imp::ChildStderr>,
    }

    impl Command {
        /// Constructs a new `Command` for launching the program at
        /// path `program`, with the following default configuration:
        ///
        /// * No arguments to the program
        /// * Inherit the current process's environment
        /// * Inherit the current process's working directory
        /// * Inherit stdin/stdout/stderr for `spawn` or `status`, but create pipes for `output`
        ///
        /// Builder methods are provided to change these defaults and
        /// otherwise configure the process.
        ///
        /// If `program` is not an absolute path, the `PATH` will be searched in
        /// an OS-defined way.
        ///
        /// The search path to be used may be controlled by setting the
        /// `PATH` environment variable on the Command,
        /// but this has some implementation limitations on Windows
        /// (see issue [rust-lang/rust#37519]).
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```no_run
        /// use tokio::process::Command;
        /// let command = Command::new("sh");
        /// ```
        ///
        /// [rust-lang/rust#37519]: https://github.com/rust-lang/rust/issues/37519
        pub fn new<S: AsRef<OsStr>>(program: S) -> Command {
            Self::from(StdCommand::new(program))
        }

        /// Adds an argument to pass to the program.
        ///
        /// Only one argument can be passed per use. So instead of:
        ///
        /// ```no_run
        /// tokio::process::Command::new("sh")
        ///   .arg("-C /path/to/repo");
        /// ```
        ///
        /// usage would be:
        ///
        /// ```no_run
        /// tokio::process::Command::new("sh")
        ///   .arg("-C")
        ///   .arg("/path/to/repo");
        /// ```
        ///
        /// To pass multiple arguments see [`args`].
        ///
        /// [`args`]: method@Self::args
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```no_run
        /// use tokio::process::Command;
        ///
        /// let command = Command::new("ls")
        ///         .arg("-l")
        ///         .arg("-a");
        /// ```
        pub fn arg<S: AsRef<OsStr>>(&mut self, arg: S) -> &mut Command {
            self.std.arg(arg);
            self
        }

        /// Adds multiple arguments to pass to the program.
        ///
        /// To pass a single argument see [`arg`].
        ///
        /// [`arg`]: method@Self::arg
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```no_run
        /// use tokio::process::Command;
        ///
        /// let command = Command::new("ls")
        ///         .args(&["-l", "-a"]);
        /// ```
        pub fn args<I, S>(&mut self, args: I) -> &mut Command
        where
            I: IntoIterator<Item = S>,
            S: AsRef<OsStr>,
        {
            self.std.args(args);
            self
        }

        /// Inserts or updates an environment variable mapping.
        ///
        /// Note that environment variable names are case-insensitive (but case-preserving) on Windows,
        /// and case-sensitive on all other platforms.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```no_run
        /// use tokio::process::Command;
        ///
        /// let command = Command::new("ls")
        ///         .env("PATH", "/bin");
        /// ```
        pub fn env<K, V>(&mut self, key: K, val: V) -> &mut Command
        where
            K: AsRef<OsStr>,
            V: AsRef<OsStr>,
        {
            self.std.env(key, val);
            self
        }

        /// Adds or updates multiple environment variable mappings.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```no_run
        /// use tokio::process::Command;
        /// use std::process::{Stdio};
        /// use std::env;
        /// use std::collections::HashMap;
        ///
        /// let filtered_env : HashMap<String, String> =
        ///     env::vars().filter(|&(ref k, _)|
        ///         k == "TERM" || k == "TZ" || k == "LANG" || k == "PATH"
        ///     ).collect();
        ///
        /// let command = Command::new("printenv")
        ///         .stdin(Stdio::null())
        ///         .stdout(Stdio::inherit())
        ///         .env_clear()
        ///         .envs(&filtered_env);
        /// ```
        pub fn envs<I, K, V>(&mut self, vars: I) -> &mut Command
        where
            I: IntoIterator<Item = (K, V)>,
            K: AsRef<OsStr>,
            V: AsRef<OsStr>,
        {
            self.std.envs(vars);
            self
        }

        /// Removes an environment variable mapping.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```no_run
        /// use tokio::process::Command;
        ///
        /// let command = Command::new("ls")
        ///         .env_remove("PATH");
        /// ```
        pub fn env_remove<K: AsRef<OsStr>>(&mut self, key: K) -> &mut Command {
            self.std.env_remove(key);
            self
        }

        /// Clears the entire environment map for the child process.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```no_run
        /// use tokio::process::Command;
        ///
        /// let command = Command::new("ls")
        ///         .env_clear();
        /// ```
        pub fn env_clear(&mut self) -> &mut Command {
            self.std.env_clear();
            self
        }

        /// Sets the working directory for the child process.
        ///
        /// # Platform-specific behavior
        ///
        /// If the program path is relative (e.g., `"./script.sh"`), it's ambiguous
        /// whether it should be interpreted relative to the parent's working
        /// directory or relative to `current_dir`. The behavior in this case is
        /// platform specific and unstable, and it's recommended to use
        /// [`canonicalize`] to get an absolute program path instead.
        ///
        /// [`canonicalize`]: crate::fs::canonicalize()
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```no_run
        /// use tokio::process::Command;
        ///
        /// let command = Command::new("ls")
        ///         .current_dir("/bin");
        /// ```
        pub fn current_dir<P: AsRef<Path>>(&mut self, dir: P) -> &mut Command {
            self.std.current_dir(dir);
            self
        }

        /// Sets configuration for the child process's standard input (stdin) handle.
        ///
        /// Defaults to [`inherit`] when used with `spawn` or `status`, and
        /// defaults to [`piped`] when used with `output`.
        ///
        /// [`inherit`]: std::process::Stdio::inherit
        /// [`piped`]: std::process::Stdio::piped
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```no_run
        /// use std::process::{Stdio};
        /// use tokio::process::Command;
        ///
        /// let command = Command::new("ls")
        ///         .stdin(Stdio::null());
        /// ```
        pub fn stdin<T: Into<Stdio>>(&mut self, cfg: T) -> &mut Command {
            self.std.stdin(cfg);
            self
        }

        /// Sets configuration for the child process's standard output (stdout) handle.
        ///
        /// Defaults to [`inherit`] when used with `spawn` or `status`, and
        /// defaults to [`piped`] when used with `output`.
        ///
        /// [`inherit`]: std::process::Stdio::inherit
        /// [`piped`]: std::process::Stdio::piped
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```no_run
        /// use tokio::process::Command;;
        /// use std::process::Stdio;
        ///
        /// let command = Command::new("ls")
        ///         .stdout(Stdio::null());
        /// ```
        pub fn stdout<T: Into<Stdio>>(&mut self, cfg: T) -> &mut Command {
            self.std.stdout(cfg);
            self
        }

        /// Sets configuration for the child process's standard error (stderr) handle.
        ///
        /// Defaults to [`inherit`] when used with `spawn` or `status`, and
        /// defaults to [`piped`] when used with `output`.
        ///
        /// [`inherit`]: std::process::Stdio::inherit
        /// [`piped`]: std::process::Stdio::piped
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```no_run
        /// use tokio::process::Command;;
        /// use std::process::{Stdio};
        ///
        /// let command = Command::new("ls")
        ///         .stderr(Stdio::null());
        /// ```
        pub fn stderr<T: Into<Stdio>>(&mut self, cfg: T) -> &mut Command {
            self.std.stderr(cfg);
            self
        }

        /// Controls whether a `kill` operation should be invoked on a spawned child
        /// process when its corresponding `Child` handle is dropped.
        ///
        /// By default, this value is assumed to be `false`, meaning the next spawned
        /// process will not be killed on drop, similar to the behavior of the standard
        /// library.
        ///
        /// # Caveats
        ///
        /// On Unix platforms processes must be "reaped" by their parent process after
        /// they have exited in order to release all OS resources. A child process which
        /// has exited, but has not yet been reaped by its parent is considered a "zombie"
        /// process. Such processes continue to count against limits imposed by the system,
        /// and having too many zombie processes present can prevent additional processes
        /// from being spawned.
        ///
        /// Although issuing a `kill` signal to the child process is a synchronous
        /// operation, the resulting zombie process cannot be `.await`ed inside of the
        /// destructor to avoid blocking other tasks. The tokio runtime will, on a
        /// best-effort basis, attempt to reap and clean up such processes in the
        /// background, but makes no additional guarantees are made with regards
        /// how quickly or how often this procedure will take place.
        ///
        /// If stronger guarantees are required, it is recommended to avoid dropping
        /// a [`Child`] handle where possible, and instead utilize `child.wait().await`
        /// or `child.kill().await` where possible.
        pub fn kill_on_drop(&mut self, kill_on_drop: bool) -> &mut Command {
            self.kill_on_drop = kill_on_drop;
            self
        }

        /// Sets the [process creation flags][1] to be passed to `CreateProcess`.
        ///
        /// These will always be ORed with `CREATE_UNICODE_ENVIRONMENT`.
        ///
        /// [1]: https://msdn.microsoft.com/en-us/library/windows/desktop/ms684863(v=vs.85).aspx
        #[cfg(windows)]
        pub fn creation_flags(&mut self, flags: u32) -> &mut Command {
            self.std.creation_flags(flags);
            self
        }

        /// Sets the child process's user ID. This translates to a
        /// `setuid` call in the child process. Failure in the `setuid`
        /// call will cause the spawn to fail.
        #[cfg(unix)]
        pub fn uid(&mut self, id: u32) -> &mut Command {
            self.std.uid(id);
            self
        }

        /// Similar to `uid` but sets the group ID of the child process. This has
        /// the same semantics as the `uid` field.
        #[cfg(unix)]
        pub fn gid(&mut self, id: u32) -> &mut Command {
            self.std.gid(id);
            self
        }

        /// Schedules a closure to be run just before the `exec` function is
        /// invoked.
        ///
        /// The closure is allowed to return an I/O error whose OS error code will
        /// be communicated back to the parent and returned as an error from when
        /// the spawn was requested.
        ///
        /// Multiple closures can be registered and they will be called in order of
        /// their registration. If a closure returns `Err` then no further closures
        /// will be called and the spawn operation will immediately return with a
        /// failure.
        ///
        /// # Safety
        ///
        /// This closure will be run in the context of the child process after a
        /// `fork`. This primarily means that any modifications made to memory on
        /// behalf of this closure will **not** be visible to the parent process.
        /// This is often a very constrained environment where normal operations
        /// like `malloc` or acquiring a mutex are not guaranteed to work (due to
        /// other threads perhaps still running when the `fork` was run).
        ///
        /// This also means that all resources such as file descriptors and
        /// memory-mapped regions got duplicated. It is your responsibility to make
        /// sure that the closure does not violate library invariants by making
        /// invalid use of these duplicates.
        ///
        /// When this closure is run, aspects such as the stdio file descriptors and
        /// working directory have successfully been changed, so output to these
        /// locations may not appear where intended.
        #[cfg(unix)]
        pub unsafe fn pre_exec<F>(&mut self, f: F) -> &mut Command
        where
            F: FnMut() -> io::Result<()> + Send + Sync + 'static,
        {
            self.std.pre_exec(f);
            self
        }

        /// Executes the command as a child process, returning a handle to it.
        ///
        /// By default, stdin, stdout and stderr are inherited from the parent.
        ///
        /// This method will spawn the child process synchronously and return a
        /// handle to a future-aware child process. The `Child` returned implements
        /// `Future` itself to acquire the `ExitStatus` of the child, and otherwise
        /// the `Child` has methods to acquire handles to the stdin, stdout, and
        /// stderr streams.
        ///
        /// All I/O this child does will be associated with the current default
        /// event loop.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```no_run
        /// use tokio::process::Command;
        ///
        /// async fn run_ls() -> std::process::ExitStatus {
        ///     Command::new("ls")
        ///         .spawn()
        ///         .expect("ls command failed to start")
        ///         .wait()
        ///         .await
        ///         .expect("ls command failed to run")
        /// }
        /// ```
        ///
        /// # Caveats
        ///
        /// ## Dropping/Cancellation
        ///
        /// Similar to the behavior to the standard library, and unlike the futures
        /// paradigm of dropping-implies-cancellation, a spawned process will, by
        /// default, continue to execute even after the `Child` handle has been dropped.
        ///
        /// The [`Command::kill_on_drop`] method can be used to modify this behavior
        /// and kill the child process if the `Child` wrapper is dropped before it
        /// has exited.
        ///
        /// ## Unix Processes
        ///
        /// On Unix platforms processes must be "reaped" by their parent process after
        /// they have exited in order to release all OS resources. A child process which
        /// has exited, but has not yet been reaped by its parent is considered a "zombie"
        /// process. Such processes continue to count against limits imposed by the system,
        /// and having too many zombie processes present can prevent additional processes
        /// from being spawned.
        ///
        /// The tokio runtime will, on a best-effort basis, attempt to reap and clean up
        /// any process which it has spawned. No additional guarantees are made with regards
        /// how quickly or how often this procedure will take place.
        ///
        /// It is recommended to avoid dropping a [`Child`] process handle before it has been
        /// fully `await`ed if stricter cleanup guarantees are required.
        ///
        /// [`Command`]: crate::process::Command
        /// [`Command::kill_on_drop`]: crate::process::Command::kill_on_drop
        /// [`Child`]: crate::process::Child
        ///
        /// # Errors
        ///
        /// On Unix platforms this method will fail with `std::io::ErrorKind::WouldBlock`
        /// if the system process limit is reached (which includes other applications
        /// running on the system).
        pub fn spawn(&mut self) -> io::Result<Child> {
            imp::spawn_child(&mut self.std).map(|spawned_child| Child {
                child: FusedChild::Child(ChildDropGuard {
                    inner: spawned_child.child,
                    kill_on_drop: self.kill_on_drop,
                }),
                stdin: spawned_child.stdin.map(|inner| ChildStdin { inner }),
                stdout: spawned_child.stdout.map(|inner| ChildStdout { inner }),
                stderr: spawned_child.stderr.map(|inner| ChildStderr { inner }),
            })
        }

        /// Executes the command as a child process, waiting for it to finish and
        /// collecting its exit status.
        ///
        /// By default, stdin, stdout and stderr are inherited from the parent.
        /// If any input/output handles are set to a pipe then they will be immediately
        /// closed after the child is spawned.
        ///
        /// All I/O this child does will be associated with the current default
        /// event loop.
        ///
        /// The destructor of the future returned by this function will kill
        /// the child if [`kill_on_drop`] is set to true.
        ///
        /// [`kill_on_drop`]: fn@Self::kill_on_drop
        ///
        /// # Errors
        ///
        /// This future will return an error if the child process cannot be spawned
        /// or if there is an error while awaiting its status.
        ///
        /// On Unix platforms this method will fail with `std::io::ErrorKind::WouldBlock`
        /// if the system process limit is reached (which includes other applications
        /// running on the system).
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```no_run
        /// use tokio::process::Command;
        ///
        /// async fn run_ls() -> std::process::ExitStatus {
        ///     Command::new("ls")
        ///         .status()
        ///         .await
        ///         .expect("ls command failed to run")
        /// }
        /// ```
        pub fn status(&mut self) -> impl Future<Output = io::Result<ExitStatus>> {
            let child = self.spawn();

            async {
                let mut child = child?;

                // Ensure we close any stdio handles so we can't deadlock
                // waiting on the child which may be waiting to read/write
                // to a pipe we're holding.
                child.stdin.take();
                child.stdout.take();
                child.stderr.take();

                child.wait().await
            }
        }

        /// Executes the command as a child process, waiting for it to finish and
        /// collecting all of its output.
        ///
        /// > **Note**: this method, unlike the standard library, will
        /// > unconditionally configure the stdout/stderr handles to be pipes, even
        /// > if they have been previously configured. If this is not desired then
        /// > the `spawn` method should be used in combination with the
        /// > `wait_with_output` method on child.
        ///
        /// This method will return a future representing the collection of the
        /// child process's stdout/stderr. It will resolve to
        /// the `Output` type in the standard library, containing `stdout` and
        /// `stderr` as `Vec<u8>` along with an `ExitStatus` representing how the
        /// process exited.
        ///
        /// All I/O this child does will be associated with the current default
        /// event loop.
        ///
        /// The destructor of the future returned by this function will kill
        /// the child if [`kill_on_drop`] is set to true.
        ///
        /// [`kill_on_drop`]: fn@Self::kill_on_drop
        ///
        /// # Errors
        ///
        /// This future will return an error if the child process cannot be spawned
        /// or if there is an error while awaiting its status.
        ///
        /// On Unix platforms this method will fail with `std::io::ErrorKind::WouldBlock`
        /// if the system process limit is reached (which includes other applications
        /// running on the system).
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```no_run
        /// use tokio::process::Command;
        ///
        /// async fn run_ls() {
        ///     let output: std::process::Output = Command::new("ls")
        ///         .output()
        ///         .await
        ///         .expect("ls command failed to run");
        ///     println!("stderr of ls: {:?}", output.stderr);
        /// }
        /// ```
        pub fn output(&mut self) -> impl Future<Output = io::Result<Output>> {
            self.std.stdout(Stdio::piped());
            self.std.stderr(Stdio::piped());

            let child = self.spawn();

            async { child?.wait_with_output().await }
        }
    }

    impl From<StdCommand> for Command {
        fn from(std: StdCommand) -> Command {
            Command {
                std,
                kill_on_drop: false,
            }
        }
    }

    /// A drop guard which can ensure the child process is killed on drop if specified.
    #[derive(Debug)]
    struct ChildDropGuard<T: Kill> {
        inner: T,
        kill_on_drop: bool,
    }

    impl<T: Kill> Kill for ChildDropGuard<T> {
        fn kill(&mut self) -> io::Result<()> {
            let ret = self.inner.kill();

            if ret.is_ok() {
                self.kill_on_drop = false;
            }

            ret
        }
    }

    impl<T: Kill> Drop for ChildDropGuard<T> {
        fn drop(&mut self) {
            if self.kill_on_drop {
                drop(self.kill());
            }
        }
    }

    impl<T, E, F> Future for ChildDropGuard<F>
    where
        F: Future<Output = Result<T, E>> + Kill + Unpin,
    {
        type Output = Result<T, E>;

        fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
            // Keep track of task budget
            let coop = ready!(crate::coop::poll_proceed(cx));

            let ret = Pin::new(&mut self.inner).poll(cx);

            if let Poll::Ready(Ok(_)) = ret {
                // Avoid the overhead of trying to kill a reaped process
                self.kill_on_drop = false;
            }

            if ret.is_ready() {
                coop.made_progress();
            }

            ret
        }
    }

    /// Keeps track of the exit status of a child process without worrying about
    /// polling the underlying futures even after they have completed.
    #[derive(Debug)]
    enum FusedChild {
        Child(ChildDropGuard<imp::Child>),
        Done(ExitStatus),
    }

    /// Representation of a child process spawned onto an event loop.
    ///
    /// # Caveats
    /// Similar to the behavior to the standard library, and unlike the futures
    /// paradigm of dropping-implies-cancellation, a spawned process will, by
    /// default, continue to execute even after the `Child` handle has been dropped.
    ///
    /// The `Command::kill_on_drop` method can be used to modify this behavior
    /// and kill the child process if the `Child` wrapper is dropped before it
    /// has exited.
    #[derive(Debug)]
    pub struct Child {
        child: FusedChild,

        /// The handle for writing to the child's standard input (stdin), if it has
        /// been captured. To avoid partially moving the `child` and thus blocking
        /// yourself from calling functions on `child` while using `stdin`, you might
        /// find it helpful to do:
        ///
        /// ```no_run
        /// # let mut child = tokio::process::Command::new("echo").spawn().unwrap();
        /// let stdin = child.stdin.take().unwrap();
        /// ```
        pub stdin: Option<ChildStdin>,

        /// The handle for reading from the child's standard output (stdout), if it
        /// has been captured. You might find it helpful to do
        ///
        /// ```no_run
        /// # let mut child = tokio::process::Command::new("echo").spawn().unwrap();
        /// let stdout = child.stdout.take().unwrap();
        /// ```
        ///
        /// to avoid partially moving the `child` and thus blocking yourself from calling
        /// functions on `child` while using `stdout`.
        pub stdout: Option<ChildStdout>,

        /// The handle for reading from the child's standard error (stderr), if it
        /// has been captured. You might find it helpful to do
        ///
        /// ```no_run
        /// # let mut child = tokio::process::Command::new("echo").spawn().unwrap();
        /// let stderr = child.stderr.take().unwrap();
        /// ```
        ///
        /// to avoid partially moving the `child` and thus blocking yourself from calling
        /// functions on `child` while using `stderr`.
        pub stderr: Option<ChildStderr>,
    }

    impl Child {
        /// Returns the OS-assigned process identifier associated with this child
        /// while it is still running.
        ///
        /// Once the child has been polled to completion this will return `None`.
        /// This is done to avoid confusion on platforms like Unix where the OS
        /// identifier could be reused once the process has completed.
        pub fn id(&self) -> Option<u32> {
            match &self.child {
                FusedChild::Child(child) => Some(child.inner.id()),
                FusedChild::Done(_) => None,
            }
        }

        /// Attempts to force the child to exit, but does not wait for the request
        /// to take effect.
        ///
        /// On Unix platforms, this is the equivalent to sending a SIGKILL. Note
        /// that on Unix platforms it is possible for a zombie process to remain
        /// after a kill is sent; to avoid this, the caller should ensure that either
        /// `child.wait().await` or `child.try_wait()` is invoked successfully.
        pub fn start_kill(&mut self) -> io::Result<()> {
            match &mut self.child {
                FusedChild::Child(child) => child.kill(),
                FusedChild::Done(_) => Err(io::Error::new(
                    io::ErrorKind::InvalidInput,
                    "invalid argument: can't kill an exited process",
                )),
            }
        }

        /// Forces the child to exit.
        ///
        /// This is equivalent to sending a SIGKILL on unix platforms.
        ///
        /// If the child has to be killed remotely, it is possible to do it using
        /// a combination of the select! macro and a oneshot channel. In the following
        /// example, the child will run until completion unless a message is sent on
        /// the oneshot channel. If that happens, the child is killed immediately
        /// using the `.kill()` method.
        ///
        /// ```no_run
        /// use tokio::process::Command;
        /// use tokio::sync::oneshot::channel;
        ///
        /// #[tokio::main]
        /// async fn main() {
        ///     let (send, recv) = channel::<()>();
        ///     let mut child = Command::new("sleep").arg("1").spawn().unwrap();
        ///     tokio::spawn(async move { send.send(()) });
        ///     tokio::select! {
        ///         _ = child.wait() => {}
        ///         _ = recv => child.kill().await.expect("kill failed"),
        ///     }
        /// }
        /// ```
        pub async fn kill(&mut self) -> io::Result<()> {
            self.start_kill()?;
            self.wait().await?;
            Ok(())
        }

        /// Waits for the child to exit completely, returning the status that it
        /// exited with. This function will continue to have the same return value
        /// after it has been called at least once.
        ///
        /// The stdin handle to the child process, if any, will be closed
        /// before waiting. This helps avoid deadlock: it ensures that the
        /// child does not block waiting for input from the parent, while
        /// the parent waits for the child to exit.
        ///
        /// If the caller wishes to explicitly control when the child's stdin
        /// handle is closed, they may `.take()` it before calling `.wait()`:
        ///
        /// ```no_run
        /// use tokio::io::AsyncWriteExt;
        /// use tokio::process::Command;
        ///
        /// #[tokio::main]
        /// async fn main() {
        ///     let mut child = Command::new("cat").spawn().unwrap();
        ///
        ///     let mut stdin = child.stdin.take().unwrap();
        ///     tokio::spawn(async move {
        ///         // do something with stdin here...
        ///         stdin.write_all(b"hello world\n").await.unwrap();
        ///
        ///         // then drop when finished
        ///         drop(stdin);
        ///     });
        ///
        ///     // wait for the process to complete
        ///     let _ = child.wait().await;
        /// }
        /// ```
        pub async fn wait(&mut self) -> io::Result<ExitStatus> {
            // Ensure stdin is closed so the child isn't stuck waiting on
            // input while the parent is waiting for it to exit.
            drop(self.stdin.take());

            match &mut self.child {
                FusedChild::Done(exit) => Ok(*exit),
                FusedChild::Child(child) => {
                    let ret = child.await;

                    if let Ok(exit) = ret {
                        self.child = FusedChild::Done(exit);
                    }

                    ret
                }
            }
        }

        /// Attempts to collect the exit status of the child if it has already
        /// exited.
        ///
        /// This function will not block the calling thread and will only
        /// check to see if the child process has exited or not. If the child has
        /// exited then on Unix the process ID is reaped. This function is
        /// guaranteed to repeatedly return a successful exit status so long as the
        /// child has already exited.
        ///
        /// If the child has exited, then `Ok(Some(status))` is returned. If the
        /// exit status is not available at this time then `Ok(None)` is returned.
        /// If an error occurs, then that error is returned.
        ///
        /// Note that unlike `wait`, this function will not attempt to drop stdin,
        /// nor will it wake the current task if the child exits.
        pub fn try_wait(&mut self) -> io::Result<Option<ExitStatus>> {
            match &mut self.child {
                FusedChild::Done(exit) => Ok(Some(*exit)),
                FusedChild::Child(guard) => {
                    let ret = guard.inner.try_wait();

                    if let Ok(Some(exit)) = ret {
                        // Avoid the overhead of trying to kill a reaped process
                        guard.kill_on_drop = false;
                        self.child = FusedChild::Done(exit);
                    }

                    ret
                }
            }
        }

        /// Returns a future that will resolve to an `Output`, containing the exit
        /// status, stdout, and stderr of the child process.
        ///
        /// The returned future will simultaneously waits for the child to exit and
        /// collect all remaining output on the stdout/stderr handles, returning an
        /// `Output` instance.
        ///
        /// The stdin handle to the child process, if any, will be closed before
        /// waiting. This helps avoid deadlock: it ensures that the child does not
        /// block waiting for input from the parent, while the parent waits for the
        /// child to exit.
        ///
        /// By default, stdin, stdout and stderr are inherited from the parent. In
        /// order to capture the output into this `Output` it is necessary to create
        /// new pipes between parent and child. Use `stdout(Stdio::piped())` or
        /// `stderr(Stdio::piped())`, respectively, when creating a `Command`.
        pub async fn wait_with_output(mut self) -> io::Result<Output> {
            use crate::future::try_join3;

            async fn read_to_end<A: AsyncRead + Unpin>(io: Option<A>) -> io::Result<Vec<u8>> {
                let mut vec = Vec::new();
                if let Some(mut io) = io {
                    crate::io::util::read_to_end(&mut io, &mut vec).await?;
                }
                Ok(vec)
            }

            let stdout_fut = read_to_end(self.stdout.take());
            let stderr_fut = read_to_end(self.stderr.take());

            let (status, stdout, stderr) = try_join3(self.wait(), stdout_fut, stderr_fut).await?;

            Ok(Output {
                status,
                stdout,
                stderr,
            })
        }
    }

    /// The standard input stream for spawned children.
    ///
    /// This type implements the `AsyncWrite` trait to pass data to the stdin handle of
    /// handle of a child process asynchronously.
    #[derive(Debug)]
    pub struct ChildStdin {
        inner: imp::ChildStdin,
    }

    /// The standard output stream for spawned children.
    ///
    /// This type implements the `AsyncRead` trait to read data from the stdout
    /// handle of a child process asynchronously.
    #[derive(Debug)]
    pub struct ChildStdout {
        inner: imp::ChildStdout,
    }

    /// The standard error stream for spawned children.
    ///
    /// This type implements the `AsyncRead` trait to read data from the stderr
    /// handle of a child process asynchronously.
    #[derive(Debug)]
    pub struct ChildStderr {
        inner: imp::ChildStderr,
    }

    impl AsyncWrite for ChildStdin {
        fn poll_write(
            self: Pin<&mut Self>,
            cx: &mut Context<'_>,
            buf: &[u8],
        ) -> Poll<io::Result<usize>> {
            self.inner.poll_write(cx, buf)
        }

        fn poll_flush(self: Pin<&mut Self>, _cx: &mut Context<'_>) -> Poll<io::Result<()>> {
            Poll::Ready(Ok(()))
        }

        fn poll_shutdown(self: Pin<&mut Self>, _cx: &mut Context<'_>) -> Poll<io::Result<()>> {
            Poll::Ready(Ok(()))
        }
    }

    impl AsyncRead for ChildStdout {
        fn poll_read(
            self: Pin<&mut Self>,
            cx: &mut Context<'_>,
            buf: &mut ReadBuf<'_>,
        ) -> Poll<io::Result<()>> {
            // Safety: pipes support reading into uninitialized memory
            unsafe { self.inner.poll_read(cx, buf) }
        }
    }

    impl AsyncRead for ChildStderr {
        fn poll_read(
            self: Pin<&mut Self>,
            cx: &mut Context<'_>,
            buf: &mut ReadBuf<'_>,
        ) -> Poll<io::Result<()>> {
            // Safety: pipes support reading into uninitialized memory
            unsafe { self.inner.poll_read(cx, buf) }
        }
    }

    impl TryInto<Stdio> for ChildStdin {
        type Error = io::Error;

        fn try_into(self) -> Result<Stdio, Self::Error> {
            imp::convert_to_stdio(self.inner)
        }
    }

    impl TryInto<Stdio> for ChildStdout {
        type Error = io::Error;

        fn try_into(self) -> Result<Stdio, Self::Error> {
            imp::convert_to_stdio(self.inner)
        }
    }

    impl TryInto<Stdio> for ChildStderr {
        type Error = io::Error;

        fn try_into(self) -> Result<Stdio, Self::Error> {
            imp::convert_to_stdio(self.inner)
        }
    }

    #[cfg(unix)]
    mod sys {
        use std::os::unix::io::{AsRawFd, RawFd};

        use super::{ChildStderr, ChildStdin, ChildStdout};

        impl AsRawFd for ChildStdin {
            fn as_raw_fd(&self) -> RawFd {
                self.inner.as_raw_fd()
            }
        }

        impl AsRawFd for ChildStdout {
            fn as_raw_fd(&self) -> RawFd {
                self.inner.as_raw_fd()
            }
        }

        impl AsRawFd for ChildStderr {
            fn as_raw_fd(&self) -> RawFd {
                self.inner.as_raw_fd()
            }
        }
    }

    #[cfg(windows)]
    mod sys {
        use std::os::windows::io::{AsRawHandle, RawHandle};

        use super::{ChildStderr, ChildStdin, ChildStdout};

        impl AsRawHandle for ChildStdin {
            fn as_raw_handle(&self) -> RawHandle {
                self.inner.as_raw_handle()
            }
        }

        impl AsRawHandle for ChildStdout {
            fn as_raw_handle(&self) -> RawHandle {
                self.inner.as_raw_handle()
            }
        }

        impl AsRawHandle for ChildStderr {
            fn as_raw_handle(&self) -> RawHandle {
                self.inner.as_raw_handle()
            }
        }
    }

    #[cfg(all(test, not(loom)))]
    mod test {
        use super::kill::Kill;
        use super::ChildDropGuard;

        use futures::future::FutureExt;
        use std::future::Future;
        use std::io;
        use std::pin::Pin;
        use std::task::{Context, Poll};

        struct Mock {
            num_kills: usize,
            num_polls: usize,
            poll_result: Poll<Result<(), ()>>,
        }

        impl Mock {
            fn new() -> Self {
                Self::with_result(Poll::Pending)
            }

            fn with_result(result: Poll<Result<(), ()>>) -> Self {
                Self {
                    num_kills: 0,
                    num_polls: 0,
                    poll_result: result,
                }
            }
        }

        impl Kill for Mock {
            fn kill(&mut self) -> io::Result<()> {
                self.num_kills += 1;
                Ok(())
            }
        }

        impl Future for Mock {
            type Output = Result<(), ()>;

            fn poll(self: Pin<&mut Self>, _cx: &mut Context<'_>) -> Poll<Self::Output> {
                let inner = Pin::get_mut(self);
                inner.num_polls += 1;
                inner.poll_result
            }
        }

        #[test]
        fn kills_on_drop_if_specified() {
            let mut mock = Mock::new();

            {
                let guard = ChildDropGuard {
                    inner: &mut mock,
                    kill_on_drop: true,
                };
                drop(guard);
            }

            assert_eq!(1, mock.num_kills);
            assert_eq!(0, mock.num_polls);
        }

        #[test]
        fn no_kill_on_drop_by_default() {
            let mut mock = Mock::new();

            {
                let guard = ChildDropGuard {
                    inner: &mut mock,
                    kill_on_drop: false,
                };
                drop(guard);
            }

            assert_eq!(0, mock.num_kills);
            assert_eq!(0, mock.num_polls);
        }

        #[test]
        fn no_kill_if_already_killed() {
            let mut mock = Mock::new();

            {
                let mut guard = ChildDropGuard {
                    inner: &mut mock,
                    kill_on_drop: true,
                };
                let _ = guard.kill();
                drop(guard);
            }

            assert_eq!(1, mock.num_kills);
            assert_eq!(0, mock.num_polls);
        }

        #[test]
        fn no_kill_if_reaped() {
            let mut mock_pending = Mock::with_result(Poll::Pending);
            let mut mock_reaped = Mock::with_result(Poll::Ready(Ok(())));
            let mut mock_err = Mock::with_result(Poll::Ready(Err(())));

            let waker = futures::task::noop_waker();
            let mut context = Context::from_waker(&waker);
            {
                let mut guard = ChildDropGuard {
                    inner: &mut mock_pending,
                    kill_on_drop: true,
                };
                let _ = guard.poll_unpin(&mut context);

                let mut guard = ChildDropGuard {
                    inner: &mut mock_reaped,
                    kill_on_drop: true,
                };
                let _ = guard.poll_unpin(&mut context);

                let mut guard = ChildDropGuard {
                    inner: &mut mock_err,
                    kill_on_drop: true,
                };
                let _ = guard.poll_unpin(&mut context);
            }

            assert_eq!(1, mock_pending.num_kills);
            assert_eq!(1, mock_pending.num_polls);

            assert_eq!(0, mock_reaped.num_kills);
            assert_eq!(1, mock_reaped.num_polls);

            assert_eq!(1, mock_err.num_kills);
            assert_eq!(1, mock_err.num_polls);
        }
    }