//! The three pipes of one container: its arguments go in on standard input, its output and errors
//! come back, each on its own thread so no pipe can block another.

use std::io::{Read, Write};
use std::process::Child;
use std::thread::JoinHandle;

/// Step 4: the three threads that feed and read one container, so no pipe can block another.
pub(crate) struct Io {
    writer: Option<JoinHandle<()>>,
    stdout: Option<JoinHandle<(Vec<u8>, bool)>>,
    stderr: Option<JoinHandle<(Vec<u8>, bool)>>,
}

impl Io {
    /// Take `child`'s three pipes (`child.stdin.take()` and so on) and start one thread for each:
    /// write `input` to standard input and then drop it; `read_capped` standard output with `cap`;
    /// `read_capped` standard error with `STDERR_KEPT`. A pipe that is `None` gets no thread.
    pub(crate) fn start(child: &mut Child, input: String, cap: usize) -> Io {
        let writer = child.stdin.take().map(|mut stdin| {
            std::thread::spawn(move || {
                let _ = stdin.write_all(input.as_bytes());
            })
        });
        let stdout = child
            .stdout
            .take()
            .map(|stdout| std::thread::spawn(move || read_capped(stdout, cap)));
        let stderr = child.stderr.take().map(|stderr| {
            std::thread::spawn(move || read_capped(stderr, crate::container::STDERR_KEPT))
        });
        Io {
            writer,
            stdout,
            stderr,
        }
    }

    /// Step 6: join the three threads. A thread that is missing or panicked counts as empty
    /// (`join().ok()`, `unwrap_or_default()`). Returns the kept standard output, whether there was
    /// more, and the kept standard error decoded with `from_utf8_lossy`.
    pub(crate) fn finish(self) -> (Vec<u8>, bool, String) {
        if let Some(handle) = self.writer {
            let _ = handle.join();
        }
        let (out, truncated) = match self.stdout {
            Some(handle) => handle.join().ok().unwrap_or_default(),
            None => (Vec::new(), false),
        };
        let err = match self.stderr {
            Some(handle) => {
                let (bytes, _) = handle.join().ok().unwrap_or_default();
                String::from_utf8_lossy(&bytes).into_owned()
            }
            None => String::new(),
        };
        (out, truncated, err)
    }
}

/// Everything `from` gives, keeping the first `cap` bytes; `true` if there was more. Reads on past
/// the cap, so the writer is never blocked or broken by a closed pipe. No indexing: `buf.get(..n)`.
pub(crate) fn read_capped(from: impl Read, cap: usize) -> (Vec<u8>, bool) {
    let mut from = from;
    let mut kept: Vec<u8> = Vec::new();
    let mut truncated = false;
    let mut chunk = [0u8; 8192];
    loop {
        let n = match from.read(&mut chunk) {
            Ok(0) => break,
            Ok(n) => n,
            Err(_) => break,
        };
        let remaining = cap - kept.len();
        if remaining == 0 {
            truncated = true;
        } else {
            let take = remaining.min(n);
            kept.extend_from_slice(&chunk[..take]);
            if n > take {
                truncated = true;
            }
        }
    }
    (kept, truncated)
}