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//! Resumable wire-frame reader shared by both replication stacks.
//!
//! The free `read_frame` helpers in `replication::protocol` and
//! `fixed_replication::protocol` issue three sequential `read_exact` calls;
//! on a socket with `SO_RCVTIMEO` a timeout mid-frame silently discards the
//! bytes already consumed and desynchronizes the stream. Any code path that
//! keeps reading from the same connection after a `WouldBlock`/`TimedOut`
//! must therefore use [`FrameReader`], which buffers partial frames across
//! timeouts. Blocking handshake paths keep using the free `read_frame`.
use core::marker::PhantomData;
use std::io::{self, Read};
/// Protocol parameter: type-byte validation plus the payload size cap.
pub trait FrameKind: Sized + Copy {
const MAX_PAYLOAD: usize;
fn from_u8(b: u8) -> Option<Self>;
}
/// Wire frame: `[type:u8][len:u32 LE][payload:len bytes]`.
#[derive(Debug)]
pub struct GenericFrame<M> {
pub msg_type: M,
pub payload: Vec<u8>,
}
/// Stateful, resumable frame reader for non-blocking / read-timeout sockets.
///
/// Accumulates bytes across `WouldBlock`/`TimedOut` and only yields a frame
/// once a full `[type][len][payload]` is present, so a frame split by a
/// timeout is reassembled exactly on the next call.
pub struct FrameReader<M: FrameKind> {
/// Bytes read from the socket but not yet consumed into a completed frame.
pending: Vec<u8>,
_marker: PhantomData<M>,
}
// Manual impl: derive(Default) would add a spurious `M: Default` bound via
// PhantomData.
impl<M: FrameKind> Default for FrameReader<M> {
fn default() -> Self {
Self::new()
}
}
impl<M: FrameKind> FrameReader<M> {
/// How many bytes to request per underlying `read`. Sized so a large
/// frame needs few syscalls without an oversized per-call allocation.
const READ_CHUNK: usize = 64 * 1024;
pub fn new() -> Self {
Self {
pending: Vec::new(),
_marker: PhantomData,
}
}
/// Bytes buffered toward the in-progress frame.
///
/// A growing value across two timeout returns proves the peer made
/// progress (liveness signal). Only meaningful between resumable
/// (`WouldBlock`/`TimedOut`) errors — after a terminal error the reader
/// must be discarded together with the connection.
pub fn buffered(&self) -> usize {
self.pending.len()
}
/// Read one frame, resuming from any partially-buffered bytes.
///
/// On a `WouldBlock`/`TimedOut` mid-frame the error is returned *without*
/// discarding buffered bytes — call again later to continue. After any
/// other error (EOF, `InvalidData`, transport errors) the reader state is
/// unspecified: discard it together with the connection.
pub fn read_frame(&mut self, r: &mut impl Read) -> io::Result<GenericFrame<M>> {
loop {
if let Some(frame) = self.try_parse()? {
return Ok(frame);
}
let old_len = self.pending.len();
self.pending.resize(old_len + Self::READ_CHUNK, 0);
match r.read(&mut self.pending[old_len..]) {
Ok(0) => {
self.pending.truncate(old_len);
return Err(io::Error::new(
io::ErrorKind::UnexpectedEof,
"peer closed connection mid-frame",
));
}
Ok(n) => {
self.pending.truncate(old_len + n);
// Loop back to try_parse with the new bytes.
}
Err(ref e) if e.kind() == io::ErrorKind::Interrupted => {
self.pending.truncate(old_len);
// Retry.
}
Err(e) => {
self.pending.truncate(old_len);
return Err(e);
}
}
}
}
/// Parse a full frame out of `pending` if one is buffered. Returns
/// `Ok(None)` when more bytes are needed. The type byte is validated as
/// soon as it is buffered (parity with the free `read_frame`, which
/// errors after one byte), and an oversized length errors before any
/// payload wait.
fn try_parse(&mut self) -> io::Result<Option<GenericFrame<M>>> {
let Some(&type_byte) = self.pending.first() else {
return Ok(None);
};
let msg_type = M::from_u8(type_byte).ok_or_else(|| {
io::Error::new(
io::ErrorKind::InvalidData,
format!("unknown message type: {type_byte}"),
)
})?;
if self.pending.len() < 5 {
return Ok(None);
}
let len =
u32::from_le_bytes(self.pending[1..5].try_into().expect("5 bytes buffered")) as usize;
if len > M::MAX_PAYLOAD {
return Err(io::Error::new(
io::ErrorKind::InvalidData,
format!("frame payload too large: {len} > {}", M::MAX_PAYLOAD),
));
}
let total = 5 + len;
if self.pending.len() < total {
return Ok(None);
}
let payload = self.pending[5..total].to_vec();
self.pending.drain(..total);
Ok(Some(GenericFrame { msg_type, payload }))
}
}
#[cfg(test)]
mod tests {
use super::*;
use std::collections::VecDeque;
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
enum TestMsg {
Ping,
Data,
}
impl FrameKind for TestMsg {
const MAX_PAYLOAD: usize = 64;
fn from_u8(b: u8) -> Option<Self> {
match b {
1 => Some(Self::Ping),
2 => Some(Self::Data),
_ => None,
}
}
}
/// Scripted reader: each `read` call pops the next item — either bytes
/// (copied into the buffer whole) or an error. An exhausted script reads
/// as EOF (`Ok(0)`).
struct ScriptReader {
script: VecDeque<io::Result<Vec<u8>>>,
}
impl ScriptReader {
fn new(script: Vec<io::Result<Vec<u8>>>) -> Self {
Self {
script: script.into(),
}
}
}
impl Read for ScriptReader {
fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
match self.script.pop_front() {
Some(Ok(bytes)) => {
assert!(bytes.len() <= buf.len(), "script chunk exceeds read buffer");
buf[..bytes.len()].copy_from_slice(&bytes);
Ok(bytes.len())
}
Some(Err(e)) => Err(e),
None => Ok(0),
}
}
}
fn would_block() -> io::Error {
io::Error::new(io::ErrorKind::WouldBlock, "wb")
}
fn frame_bytes(t: u8, payload: &[u8]) -> Vec<u8> {
let mut v = vec![t];
v.extend_from_slice(&(payload.len() as u32).to_le_bytes());
v.extend_from_slice(payload);
v
}
#[test]
fn resumes_across_timeouts_at_every_boundary() {
let wire = frame_bytes(2, b"hello"); // 10 bytes: 1 + 4 + 5
// Split points: after the type byte, inside the length, inside payload.
for split in [1usize, 3, 7] {
let (a, b) = wire.split_at(split);
let mut r = ScriptReader::new(vec![Ok(a.to_vec()), Err(would_block()), Ok(b.to_vec())]);
let mut fr = FrameReader::<TestMsg>::new();
let err = fr.read_frame(&mut r).unwrap_err();
assert_eq!(err.kind(), io::ErrorKind::WouldBlock, "split={split}");
assert_eq!(fr.buffered(), split, "split={split}");
let frame = fr.read_frame(&mut r).unwrap();
assert_eq!(frame.msg_type, TestMsg::Data);
assert_eq!(frame.payload, b"hello");
assert_eq!(fr.buffered(), 0);
}
}
#[test]
fn empty_payload_frame() {
let mut r = ScriptReader::new(vec![Ok(frame_bytes(1, b""))]);
let mut fr = FrameReader::<TestMsg>::new();
let frame = fr.read_frame(&mut r).unwrap();
assert_eq!(frame.msg_type, TestMsg::Ping);
assert!(frame.payload.is_empty());
assert_eq!(fr.buffered(), 0);
}
#[test]
fn two_frames_in_one_chunk() {
let mut wire = frame_bytes(1, b"");
wire.extend_from_slice(&frame_bytes(2, b"xy"));
let mut r = ScriptReader::new(vec![Ok(wire)]);
let mut fr = FrameReader::<TestMsg>::new();
let f1 = fr.read_frame(&mut r).unwrap();
assert_eq!(f1.msg_type, TestMsg::Ping);
// The second frame is already fully buffered in pending — it is
// read without any new syscalls.
let f2 = fr.read_frame(&mut r).unwrap();
assert_eq!(f2.msg_type, TestMsg::Data);
assert_eq!(f2.payload, b"xy");
assert_eq!(fr.buffered(), 0);
}
#[test]
fn garbage_type_byte_fails_immediately() {
// Only ONE byte delivered, then silence: early validation (R3) must
// reject it without waiting for a full 5-byte header.
let mut r = ScriptReader::new(vec![Ok(vec![0xEE])]);
let mut fr = FrameReader::<TestMsg>::new();
let err = fr.read_frame(&mut r).unwrap_err();
assert_eq!(err.kind(), io::ErrorKind::InvalidData);
assert!(err.to_string().contains("unknown message type"));
}
#[test]
fn oversized_len_fails_before_payload() {
let mut header = vec![1u8];
header.extend_from_slice(&65u32.to_le_bytes()); // MAX_PAYLOAD = 64
let mut r = ScriptReader::new(vec![Ok(header)]);
let mut fr = FrameReader::<TestMsg>::new();
let err = fr.read_frame(&mut r).unwrap_err();
assert_eq!(err.kind(), io::ErrorKind::InvalidData);
assert!(err.to_string().contains("frame payload too large"));
}
#[test]
fn eof_mid_frame() {
// Valid type byte, then EOF (exhausted script → Ok(0)).
let mut r = ScriptReader::new(vec![Ok(vec![1u8])]);
let mut fr = FrameReader::<TestMsg>::new();
let err = fr.read_frame(&mut r).unwrap_err();
assert_eq!(err.kind(), io::ErrorKind::UnexpectedEof);
}
#[test]
fn interrupted_is_retried_internally() {
let mut r = ScriptReader::new(vec![
Err(io::Error::new(io::ErrorKind::Interrupted, "eintr")),
Ok(frame_bytes(1, b"")),
]);
let mut fr = FrameReader::<TestMsg>::new();
let frame = fr.read_frame(&mut r).unwrap();
assert_eq!(frame.msg_type, TestMsg::Ping);
}
#[test]
fn default_and_debug_contracts() {
// R5: Default (parity with the old fixed FrameReader's
// derive(Default)) and Debug on the frame (parity with the old
// Frame structs' derive(Debug)).
let fr = FrameReader::<TestMsg>::default();
assert_eq!(fr.buffered(), 0);
let frame = GenericFrame {
msg_type: TestMsg::Ping,
payload: vec![1],
};
let dbg = format!("{frame:?}");
assert!(dbg.contains("Ping"));
}
}