//! Connection wrapper around a Unix socket stream.
//!
//! [`Connection`] pairs a buffered reader with a raw writer and exposes
//! `send` / `receive` for framed JSON messages.

use crate::frame::{read_frame, write_frame};
use anyhow::{Context, Result};
use serde::de::DeserializeOwned;
use serde::Serialize;
use std::io::BufReader;
use std::os::unix::net::UnixStream;
use tracing;

/// A framed JSON connection over a Unix socket.
///
/// Wraps the raw [`UnixStream`] with a [`BufReader`] on the read side and
/// direct writes (with explicit flushing) on the write side.
pub struct Connection {
    /// Buffered reader for receiving frames.
    reader: BufReader<UnixStream>,
    /// Unbuffered writer (flushed after every frame).
    writer: UnixStream,
}

impl Connection {
    /// Create a new `Connection` from an already-connected [`UnixStream`].
    ///
    /// # Panics
    ///
    /// Panics if `UnixStream::try_clone` fails — this should never happen on
    /// Linux (it calls `dup(2)`).
    #[must_use]
    pub fn new(stream: UnixStream) -> Self {
        // Clone the stream so that reader and writer can reference separate
        // file-descriptor handles.  `UnixStream::try_clone` is infallible on
        // Unix (it calls `dup(2)`).
        let reader = BufReader::new(
            stream
                .try_clone()
                .expect("UnixStream::try_clone should never fail on Linux"),
        );
        let writer = stream;
        Self { reader, writer }
    }

    /// Serialise `msg` to JSON and send it as a length-prefixed frame.
    ///
    /// # Errors
    ///
    /// Delegates to [`serde_json::to_vec`] for serialisation and
    /// [`write_frame`] for writing.
    pub fn send<T: Serialize>(&mut self, msg: &T) -> Result<()> {
        let json = serde_json::to_vec(msg).context("failed to serialise message to JSON")?;
        tracing::trace!("sending frame ({} byte(s))", json.len());
        write_frame(&mut self.writer, &json).context("failed to write frame to connection")
    }

    /// Read one framed JSON message and deserialise it.
    ///
    /// Returns `Ok(None)` when the remote end has closed the connection
    /// cleanly (EOF).  Returns `Ok(Some(msg))` on a successful read.
    ///
    /// # Errors
    ///
    /// Delegates to [`read_frame`] for reading and
    /// [`serde_json::from_slice`] for deserialisation.
    pub fn receive<T: DeserializeOwned>(&mut self) -> Result<Option<T>> {
        let raw = read_frame(&mut self.reader).context("failed to read frame from connection")?;

        match raw {
            None => {
                tracing::trace!("connection closed (EOF)");
                Ok(None)
            }
            Some(bytes) => {
                tracing::trace!("received frame ({} byte(s))", bytes.len());
                let msg: T = serde_json::from_slice(&bytes).with_context(|| {
                    format!("failed to deserialise frame ({} byte(s))", bytes.len())
                })?;
                Ok(Some(msg))
            }
        }
    }
}

// Safety: `UnixStream` is `Send` but not `Sync`.  Wrapping `Connection` in
// a `Mutex` (as done in `IpcClient`) provides the `Sync` guarantee.
// The type itself is `Send` because both fields are `Send`.
//
// We explicitly assert Send here for clarity:
fn _assert_send()
where
    Connection: Send,
{
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::test_utils::Ping;

    /// Helper: create a pair of connected `Connection` values via a
    /// Unix socket pair.
    fn pair() -> (Connection, Connection) {
        let (a, b) = UnixStream::pair().expect("UnixStream::pair failed");
        (Connection::new(a), Connection::new(b))
    }

    #[test]
    fn round_trip() {
        let (mut left, mut right) = pair();

        left.send(&Ping { seq: 42 }).unwrap();
        let received: Ping = right.receive().unwrap().expect("expected a frame");
        assert_eq!(received, Ping { seq: 42 });
    }

    #[test]
    fn eof_detection() {
        let (left, right) = pair();
        drop(right); // close remote end

        // Send something first so we can read past it... actually let's
        // just drop the peer and check that receive returns None.
        // Since we dropped right, left's reads should eventually get EOF.
        // But with a socket pair, dropping one end signals EOF on the other.
        drop(left); // drop left too — we'll test EOF on a fresh pair
        let (mut a, _) = pair();
        let result: Option<Ping> = a.receive().unwrap();
        assert!(result.is_none());
    }
}