#[macro_use]
extern crate crossbeam_channel as channel;
extern crate docopt;
extern crate libc;
extern crate serde;
#[macro_use]
extern crate serde_derive;
extern crate signal_hook;

use std::env;
use std::error::Error;
use std::io::{self, BufRead, Read, Write};
use std::process::{self, Command, Stdio};
use std::thread;
use std::time::Duration;

use channel::{Receiver, Sender};
use docopt::Docopt;
use libc::c_int;

static USAGE: &'static str = "
Usage: cmail [options] [<args> ...]

Options:
    -h, --help             Display this help message.
    -p ARG, --period ARG   Data is emailed at this period in seconds.
                           Set to 0 to disable and send only one email
                           when the command completes.
                           [default: 900]
    -s, --silent           Don't pass the command's stdout/stderr to the
                           terminal. Instead, only send stdout/stderr
                           in email.
    -a, --send-all         Send the entire command's output on each email.
                           N.B. This saves all output in memory.
    -t ARG, --to ARG       The email address to send to. By default, this
                           is set to $EMAIL. If neither $EMAIL nor --to
                           are set, then an error is returned.
";

#[derive(Debug, Deserialize)]
struct Args {
    arg_args: Vec<String>,
    flag_period: u64,
    flag_silent: bool,
    flag_send_all: bool,
    flag_to: Option<String>,
}

// A poor man's error type. See: http://goo.gl/BLfXQe
type Result<T> = ::std::result::Result<T, Box<Error + Send + Sync>>;

fn main() {
    // We must start our signal notifier before spawning any threads!
    let signal = signal_notify(&[libc::SIGINT, libc::SIGTERM]).unwrap();
    let args: Args = Docopt::new(USAGE)
        .and_then(|d| d.options_first(true).deserialize())
        .unwrap_or_else(|e| e.exit());
    match run(&args, signal) {
        Ok(code) => process::exit(code),
        Err(err) => {
            writeln!(&mut io::stderr(), "ERROR: {}", err).unwrap();
            process::exit(1);
        }
    }
}

/// The main starting point for cmail.
///
/// High level overview:
///
/// 1. Spawn the command given or read from stdin if no command is given.
/// 2. Start the email sender.
/// 3. Create a channel that ticks every N seconds.
/// 4. Start the main loop which waits on three channels: OS signals, the
///    ticker and lines read from the spawned command (or stdin).
fn run(args: &Args, signal: Receiver<c_int>) -> Result<i32> {
    // When we don't have any arguments, cmail sends email containing stdin.
    let (mut cmd, lines, cmd_argv) =
        if args.arg_args.is_empty() {
            let passthru = Passthru::stdout(!args.flag_silent);
            let stdin = passthru.gobble(io::stdin());
            (None, stdin, "<stdin>".to_owned())
        } else {
            let (cmd, lines) = Cmd::run(&args.arg_args, !args.flag_silent)?;
            (Some(cmd), lines, args.arg_args.join(" "))
        };

    let email = match args.flag_to {
        None => env::var("EMAIL").unwrap_or("".to_owned()),
        Some(ref s) => s.to_owned(),
    };
    if email.is_empty() {
        return Err("Please specify an email address with --to or by \
                    setting the EMAIL environment variable.".into());
    }
    let emailer = EmailSender::run(cmd_argv, email, args.flag_send_all);

    // If period is zero, then ticker never ticks.
    let ticker = channel::tick(Duration::from_secs(args.flag_period));
    // Set to true if either the spawned process or a `sendmail` command
    // is interrupted. Setting this to `true` means we've initiated a graceful
    // shutdown of cmail that will culminate in one last email send.
    let mut killed = false;
    // Contains the next batch of lines to email. If the ticker is enabled,
    // then this is emptied at every tick.
    let mut outlines = Vec::with_capacity(1024);
    loop {
        select! {
            // Respond to an OS signal. Currently, we just listen for
            // INT (^C) and TERM (kill).
            recv(signal) => {
                killed = true;
                if let Some(ref mut cmd) = cmd {
                    // If we're running a command and receive an interrupt,
                    // then we don't quit right away and send an email.
                    // Instead, we *ask* the child process to quit and we'll
                    // continue reading from its stdout/stderr until EOF.
                    //
                    // (Once EOF is hit, the `lines` channel is closed.)
                    cmd.kill()?;
                } else {
                    // .. on the other hand, if we're reading from stdin,
                    // then there's really nothing else we can do other than
                    // send what we've got and quit.
                    return emailer.last_send(cmd, outlines, killed);
                }
            },
            // When a tick happens, we just want to send the lines we've
            // accumulated so far and start over again.
            //
            // If, during the tick, the `sendmail` process is interrupted,
            // then we take this as a sign that we should quit.
            //
            // Finally, don't respond to ticks if we're shutting down.
            recv(ticker) => {
                if !killed {
                    killed = emailer.send(outlines)?;
                    outlines = Vec::with_capacity(1024);
                    match cmd {
                        Some(ref mut cmd) if killed => { cmd.kill()?; }
                        _ => {}
                    }
                }
            },
            // Receive a single line read from the spawned process (or stdin).
            // This simply adds the line to our `outlines` buffer.
            // Something interesting only happens when the channel is closed:
            // we send one last email with the lines we've accumulated.
            //
            // N.B. This is the main exit point of cmail under normal
            // operation. In the absence of ticks, this is usually the only
            // channel that gets any activity!
            recv(lines, line) => match line {
                Some(line) => outlines.push(line?),
                None => return emailer.last_send(cmd, outlines, killed),
            },
        }
    }
}

/// An email sender collects groups of lines and sends emails concurrently.
struct EmailSender {
    /// The email sender listens on this channel for sequences of lines.
    send_lines: Sender<Vec<String>>,
    /// When a sequence of lines has been emailed (either successfully or
    /// unsuccessfully), the result is sent on this channel.
    ///
    /// In particular, the next email is not attempted until a consumer
    /// receives the corresponding result on this channel.
    recv_result: Receiver<io::Result<bool>>,
    /// Closed when the email sender shuts down.
    recv_done: Receiver<()>,
}

impl EmailSender {
    /// Creates a new email sender.
    ///
    /// This spawns a thread responsible for sending lines read from the
    /// running command to the email address provided. The value returned
    /// contains several channels that can be used to interact with this
    /// thread. Instead of using the channels explicitly, you should prefer
    /// to use the methods defined below.
    fn run(cmd: String, email: String, send_all: bool) -> EmailSender {
        let mut to_send: Vec<String> = Vec::with_capacity(1024);
        let (send_lines, recv_lines) = channel::bounded::<Vec<String>>(0);
        let (send_result, recv_result) = channel::bounded(0);
        let (send_done, recv_done) = channel::bounded(1);
        thread::spawn(move || {
            let mut interrupted = false;
            for lines in recv_lines {
                if send_all {
                    to_send.extend(lines);
                } else {
                    if lines.len() == 0 {
                        to_send = vec!["No output.".to_owned()];
                    } else {
                        to_send = lines;
                    }
                }
                let result =
                    if interrupted {
                        email_lines(
                            &cmd, &email, &to_send,
                        ).map(|_| interrupted)
                    } else {
                        let r = email_lines_retry(&cmd, &email, &to_send);
                        interrupted = *r.as_ref().unwrap_or(&false);
                        r
                    };
                send_result.send(result);
            }
            // unblock recv_done
            drop(send_done);
        });
        EmailSender {
            send_lines: send_lines,
            recv_result: recv_result,
            recv_done: recv_done,
        }
    }

    /// Consume the email sender and send one last batch of lines.
    ///
    /// If this method completes successfully, then the email sender thread
    /// will have shut down, the last email will have been sent and the spawned
    /// child process (if one exists) will be reaped.
    ///
    /// If cmail was run with a command, then `cmd` should be that command.
    /// Otherwise, it should be `None` when cmail reads from stdin.
    ///
    /// `killed` is a bool indicating whether any of the child processes
    /// spawned by cmail were killed by a signal. When `killed` is true,
    /// a non-zero exit code is returned in the result. Otherwise, a zero
    /// exit code is returned.
    fn last_send(
        self,
        cmd: Option<Cmd>,
        mut lines: Vec<String>,
        killed: bool,
    ) -> Result<i32> {
        let int = match cmd {
            None => killed,
            Some(mut cmd) => !cmd.wait()?.success() || killed,
        };
        let msg =
            if int {
                "Program interrupted."
            } else {
                "Program completed successfully."
            };
        lines.extend(vec!["", "", msg].into_iter().map(str::to_owned));
        self.send(lines)?;
        self.done();
        Ok(if killed { 1 } else { 0 })
    }

    /// Sends a sequence of lines.
    ///
    /// If this method completes successfully, then an email will have been
    /// sent containing the lines given.
    ///
    /// If an interrupt occurred when trying to send mail, then `true` is
    /// returned in the result. Otherwise, `false` is returned. (This
    /// corresponds to the `killed` parameter in `last_send`. It should also
    /// be used to start a graceful shutdown of cmail.)
    fn send(&self, lines: Vec<String>) -> Result<bool> {
        self.send_lines.send(lines);
        Ok(self.recv_result.recv().unwrap()?)
    }

    /// Start a graceful shutdown of the emailing thread and wait for all
    /// remaining lines to be sent.
    fn done(self) {
        // Shut down the thread responsible for sending emails.
        drop(self.send_lines);
        // Wait for it to finished.
        self.recv_done.recv();
    }
}

/// A simple convenience for handling the command that cmail is watching.
struct Cmd {
    child: process::Child,
}

impl Cmd {
    /// Run the given command (where each item in `cmd` is a single argument).
    ///
    /// If `passthru` is true, then the stdout/stderr of the command is printed
    /// on the stdout/stderr of cmail.
    ///
    /// This returns a tuple. The first value is the `Cmd` abstraction, which
    /// can be killed and reaped. The second value is a channel that receives
    /// line results from the corresponding child process. The channel is
    /// closed when the child's stdout and stderr emit EOF.
    fn run(cmd: &[String], passthru: bool)
          -> Result<(Cmd, Receiver<io::Result<String>>)> {
        let mut command = Command::new("sh");
        command
            .arg("-c")
            .arg(cmd.join(" "))
            .stdout(Stdio::piped())
            .stderr(Stdio::piped());
        let mut child = command.spawn()?;

        let stdout = child.stdout.take().unwrap();
        let stderr = child.stderr.take().unwrap();
        let stdout = Passthru::stdout(passthru).gobble(stdout);
        let stderr = Passthru::stderr(passthru).gobble(stderr);
        Ok((Cmd { child: child }, muxer(vec![stdout, stderr])))
    }

    /// Kill this command and wait to reap it.
    fn kill(&mut self) -> Result<process::ExitStatus> {
        // Ignore the error here, in case the child has already died.
        // We simply do not care if `kill` fails.
        let _ = self.child.kill();
        self.wait()
    }

    /// Block until the child is reaped.
    fn wait(&mut self) -> Result<process::ExitStatus> {
        Ok(self.child.wait()?)
    }
}

/// Given a sequence of receiving channels, multiplex them into one.
///
/// This spawns a thread for each element in `inps` and sends them all on to
/// a single channel.
///
/// The resulting channel is closed only when all given channels in `inps`
/// have been closed.
fn muxer<T: Send + 'static>(inps: Vec<Receiver<T>>) -> Receiver<T> {
    // If a command sends a lot of output to stdout/stderr in a short time
    // period, then setting a large buffer here on the channel gives us a
    // little wiggle room to keep up with it.
    let (s, r) = channel::bounded(5000);
    for inp in inps {
        let s = s.clone();
        thread::spawn(move || {
            for item in inp {
                s.send(item);
            }
        });
    }
    r
}

/// Passthru describes how to pass the command's output through the cmail
/// process.
#[derive(Clone, Copy, Debug)]
enum Passthru { No, Stdout, Stderr }

impl Passthru {
    /// Pass through on stdout if `yes` is true.
    fn stdout(yes: bool) -> Passthru {
        if yes { Passthru::Stdout } else { Passthru::No }
    }

    /// Pass through on stderr if `yes` is true.
    fn stderr(yes: bool) -> Passthru {
        if yes { Passthru::Stderr } else { Passthru::No }
    }

    /// Create a writer corresponding to the pass through settings.
    ///
    /// If there's no pass through, then a /dev/null-like writer is returned.
    fn wtr(self) -> Box<io::Write> {
        match self {
            Passthru::No => Box::new(io::sink()),
            Passthru::Stdout => Box::new(io::stdout()),
            Passthru::Stderr => Box::new(io::stderr()),
        }
    }

    /// Read lines on `rdr` and send the *result* along the channel returned,
    ///
    /// This will also apply the pass through settings in `self`.
    fn gobble<R>(self, rdr: R) -> Receiver<io::Result<String>>
            where R: Read + Send + 'static {
        let (s, r) = channel::bounded(0);
        thread::spawn(move || {
            let mut wtr = self.wtr();
            for line in io::BufReader::new(rdr).lines() {
                if let Ok(ref line) = line {
                    writeln!(&mut wtr, "{}", line).unwrap();
                }
                s.send(line);
            }
        });
        r
    }
}

/// Sends an email containing `lines` to `email` for the command `cmd`.
///
/// If the child `sendmail` process was interrupted, then sending the email
/// is retried exactly once.
///
/// If a retry occurred, then `true` is returned inside the result. Otherwise,
/// `false` is returned.
fn email_lines_retry(
    cmd: &str,
    email: &str,
    to_send: &[String],
) -> io::Result<bool> {
    // If the first call to email_lines fails because of an
    // interruption, then we try to send once more.
    // This is to permit the use of ^C in the terminal. The
    // intended effect is to stop the running process and email
    // whatever has been accumulated. But if `sendmail` is running
    // when ^C is sent, then the command fails and no mail is sent.
    // So we try once more: if that produces an error, we give up.
    match email_lines(cmd, email, to_send) {
        Ok(()) => Ok(false),
        Err(ref e) if e.kind() == io::ErrorKind::Interrupted => {
            // If we fail again for any reason, bubble up the
            // error and notify the receiver that we should quit.
            // This lets the user slam on ^C twice in a row
            // to really quit. :]
            email_lines(cmd, email, to_send).map(|_| true)
        }
        Err(e) => Err(e),
    }
}

/// Sends an email containing `lines` to `email` for the command `cmd`.
fn email_lines(cmd: &str, email: &str, lines: &[String]) -> io::Result<()> {
    let mut child = Command::new("sendmail")
        .arg("-t")
        .stdin(Stdio::piped())
        .spawn()?;
    let subject: String = cmd.chars().take(50).collect();
    let sep: String = ::std::iter::repeat('-').take(79).collect();
    {
        // Open a new scope here since `buf` borrows `child.stdin` mutably.
        // We need to drop this borrow before calling `child.wait()`, which
        // also borrows `child` mutably.
        let mut buf = io::BufWriter::new(child.stdin.as_mut().unwrap());
        writeln!(&mut buf, "\
Subject: [cmail] {subject}
From: {email}
To: {email}
", subject=subject, email=email)?;
        // Add some extra fluff to make it clear what command is being run.
        writeln!(&mut buf, "{sep}\n{cmd}\n{sep}", sep=sep, cmd=cmd)?;
        for line in lines {
            writeln!(&mut buf, "{}", line)?;
        }
    }
    let status = child.wait()?;
    if status.success() {
        Ok(())
    } else {
        // If the exit code is `None`, then we infer that the `sendmail`
        // process was killed by a signal.
        // In typical usage, this means the user pressed ^C on their terminal,
        // rather than it being indicative of some other reason why sendmail
        // won't work.
        // We use this information to retry the email send (but are careful
        // to only retry once).
        Err(match status.code() {
            None => {
                io::Error::new(
                    io::ErrorKind::Interrupted,
                    "email send interrupted",
                )
            }
            Some(_) => {
                io::Error::new(io::ErrorKind::Other, status.to_string())
            }
        })
    }
}

fn signal_notify(signals: &[c_int]) -> Result<Receiver<c_int>> {
    let (s, r) = channel::bounded(100);
    let signals = signal_hook::iterator::Signals::new(signals)?;
    thread::spawn(move || {
        for signal in signals.forever() {
            s.send(signal);
        }
    });
    Ok(r)
}