238 lines
10 KiB
Rust
238 lines
10 KiB
Rust
use crate::frame::{self, Frame};
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use bytes::{Buf, BytesMut};
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use std::io::{self, Cursor};
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use tokio::io::{AsyncReadExt, AsyncWriteExt, BufWriter};
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use tokio::net::TcpStream;
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/// Send and receive `Frame` values from a remote peer.
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///
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/// When implementing networking protocols, a message on that protocol is
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/// often composed of several smaller messages known as frames. The purpose of
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/// `Connection` is to read and write frames on the underlying `TcpStream`.
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///
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/// To read frames, the `Connection` uses an internal buffer, which is filled
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/// up until there are enough bytes to create a full frame. Once this happens,
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/// the `Connection` creates the frame and returns it to the caller.
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///
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/// When sending frames, the frame is first encoded into the write buffer.
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/// The contents of the write buffer are then written to the socket.
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#[derive(Debug)]
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pub struct Connection {
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// The `TcpStream`. It is decorated with a `BufWriter`, which provides write
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// level buffering. The `BufWriter` implementation provided by Tokio is
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// sufficient for our needs.
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stream: BufWriter<TcpStream>,
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// The buffer for reading frames.
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buffer: BytesMut,
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}
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impl Connection {
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/// Create a new `Connection`, backed by `socket`. Read and write buffers
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/// are initialized.
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pub fn new(socket: TcpStream) -> Connection {
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Connection {
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stream: BufWriter::new(socket),
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// Default to a 4KB read buffer. For the use case of mini redis,
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// this is fine. However, real applications will want to tune this
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// value to their specific use case. There is a high likelihood that
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// a larger read buffer will work better.
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buffer: BytesMut::with_capacity(4 * 1024),
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}
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}
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/// Read a single `Frame` value from the underlying stream.
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///
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/// The function waits until it has retrieved enough data to parse a frame.
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/// Any data remaining in the read buffer after the frame has been parsed is
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/// kept there for the next call to `read_frame`.
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///
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/// # Returns
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///
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/// On success, the received frame is returned. If the `TcpStream`
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/// is closed in a way that doesn't break a frame in half, it returns
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/// `None`. Otherwise, an error is returned.
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pub async fn read_frame(&mut self) -> crate::Result<Option<Frame>> {
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loop {
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// Attempt to parse a frame from the buffered data. If enough data
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// has been buffered, the frame is returned.
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if let Some(frame) = self.parse_frame()? {
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return Ok(Some(frame));
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}
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// There is not enough buffered data to read a frame. Attempt to
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// read more data from the socket.
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//
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// On success, the number of bytes is returned. `0` indicates "end
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// of stream".
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if 0 == self.stream.read_buf(&mut self.buffer).await? {
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// The remote closed the connection. For this to be a clean
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// shutdown, there should be no data in the read buffer. If
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// there is, this means that the peer closed the socket while
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// sending a frame.
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if self.buffer.is_empty() {
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return Ok(None);
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} else {
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let s = "connection reset by peer".into();
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return Err(s);
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}
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}
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}
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}
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/// Tries to parse a frame from the buffer. If the buffer contains enough
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/// data, the frame is returned and the data removed from the buffer. If not
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/// enough data has been buffered yet, `Ok(None)` is returned. If the
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/// buffered data does not represent a valid frame, `Err` is returned.
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fn parse_frame(&mut self) -> crate::Result<Option<Frame>> {
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use frame::Error::Incomplete;
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// Cursor is used to track the "current" location in the
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// buffer. Cursor also implements `Buf` from the `bytes` crate
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// which provides a number of helpful utilities for working
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// with bytes.
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let mut buf = Cursor::new(&self.buffer[..]);
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// The first step is to check if enough data has been buffered to parse
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// a single frame. This step is usually much faster than doing a full
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// parse of the frame, and allows us to skip allocating data structures
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// to hold the frame data unless we know the full frame has been
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// received.
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match Frame::check(&mut buf) {
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Ok(_) => {
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// The `check` function will have advanced the cursor until the
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// end of the frame. Since the cursor had position set to zero
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// before `Frame::check` was called, we obtain the length of the
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// frame by checking the cursor position.
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let len = buf.position() as usize;
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// Reset the position to zero before passing the cursor to
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// `Frame::parse`.
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buf.set_position(0);
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// Parse the frame from the buffer. This allocates the necessary
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// structures to represent the frame and returns the frame
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// value.
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//
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// If the encoded frame representation is invalid, an error is
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// returned. This should terminate the **current** connection
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// but should not impact any other connected client.
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let frame = Frame::parse(&mut buf)?;
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// Discard the parsed data from the read buffer.
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//
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// When `advance` is called on the read buffer, all of the data
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// up to `len` is discarded. The details of how this works is
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// left to `BytesMut`. This is often done by moving an internal
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// cursor, but it may be done by reallocating and copying data.
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self.buffer.advance(len);
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// Return the parsed frame to the caller.
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Ok(Some(frame))
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}
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// There is not enough data present in the read buffer to parse a
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// single frame. We must wait for more data to be received from the
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// socket. Reading from the socket will be done in the statement
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// after this `match`.
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//
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// We do not want to return `Err` from here as this "error" is an
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// expected runtime condition.
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Err(Incomplete) => Ok(None),
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// An error was encountered while parsing the frame. The connection
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// is now in an invalid state. Returning `Err` from here will result
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// in the connection being closed.
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Err(e) => Err(e.into()),
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}
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}
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/// Write a single `Frame` value to the underlying stream.
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///
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/// The `Frame` value is written to the socket using the various `write_*`
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/// functions provided by `AsyncWrite`. Calling these functions directly on
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/// a `TcpStream` is **not** advised, as this will result in a large number of
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/// syscalls. However, it is fine to call these functions on a *buffered*
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/// write stream. The data will be written to the buffer. Once the buffer is
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/// full, it is flushed to the underlying socket.
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pub async fn write_frame(&mut self, frame: &Frame) -> io::Result<()> {
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// Arrays are encoded by encoding each entry. All other frame types are
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// considered literals. For now, mini-redis is not able to encode
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// recursive frame structures. See below for more details.
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match frame {
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Frame::Array(val) => {
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// Encode the frame type prefix. For an array, it is `*`.
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self.stream.write_u8(b'*').await?;
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// Encode the length of the array.
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self.write_decimal(val.len() as u64).await?;
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// Iterate and encode each entry in the array.
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for entry in &**val {
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self.write_value(entry).await?;
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}
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}
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// The frame type is a literal. Encode the value directly.
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_ => self.write_value(frame).await?,
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}
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// Ensure the encoded frame is written to the socket. The calls above
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// are to the buffered stream and writes. Calling `flush` writes the
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// remaining contents of the buffer to the socket.
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self.stream.flush().await
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}
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/// Write a frame literal to the stream
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async fn write_value(&mut self, frame: &Frame) -> io::Result<()> {
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match frame {
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Frame::Simple(val) => {
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self.stream.write_u8(b'+').await?;
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self.stream.write_all(val.as_bytes()).await?;
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self.stream.write_all(b"\r\n").await?;
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}
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Frame::Error(val) => {
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self.stream.write_u8(b'-').await?;
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self.stream.write_all(val.as_bytes()).await?;
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self.stream.write_all(b"\r\n").await?;
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}
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Frame::Integer(val) => {
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self.stream.write_u8(b':').await?;
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self.write_decimal(*val).await?;
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}
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Frame::Null => {
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self.stream.write_all(b"$-1\r\n").await?;
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}
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Frame::Bulk(val) => {
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let len = val.len();
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self.stream.write_u8(b'$').await?;
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self.write_decimal(len as u64).await?;
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self.stream.write_all(val).await?;
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self.stream.write_all(b"\r\n").await?;
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}
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// Encoding an `Array` from within a value cannot be done using a
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// recursive strategy. In general, async fns do not support
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// recursion. Mini-redis has not needed to encode nested arrays yet,
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// so for now it is skipped.
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Frame::Array(_val) => unreachable!(),
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}
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Ok(())
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}
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/// Write a decimal frame to the stream
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async fn write_decimal(&mut self, val: u64) -> io::Result<()> {
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use std::io::Write;
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// Convert the value to a string
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let mut buf = [0u8; 20];
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let mut buf = Cursor::new(&mut buf[..]);
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write!(&mut buf, "{}", val)?;
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let pos = buf.position() as usize;
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self.stream.write_all(&buf.get_ref()[..pos]).await?;
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self.stream.write_all(b"\r\n").await?;
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Ok(())
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}
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}
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