A tiny optimisation to not memcpy the buffer as often if we have many messages already buffered

crates/librqbit/src/read_buf.rs

@@ -10,58 +10,72 @@ use tokio::io::AsyncReadExt;
 
 pub struct ReadBuf {
     buf: Vec<u8>,
-    read_so_far: usize,
-    last_size: usize,
+    // How many bytes into the buffer we have read from the connection.
+    // New reads should go past this.
+    filled: usize,
+    // How many bytes have we successfully deserialized.
+    processed: usize,
 }
 
 impl ReadBuf {
     pub fn new() -> Self {
         Self {
             buf: vec![0; PIECE_MESSAGE_DEFAULT_LEN * 2],
-            read_so_far: 0,
-            last_size: 0,
+            filled: 0,
+            processed: 0,
         }
     }
 
-    fn prepare_for_read(&mut self) {
-        if self.read_so_far > self.last_size {
-            self.buf.copy_within(self.last_size..self.read_so_far, 0);
+    fn prepare_for_read(&mut self, need_additional_bytes: usize) {
+        // Ensure the buffer starts from the to-be-deserialized message.
+        if self.processed > 0 {
+            if self.filled > self.processed {
+                self.buf.copy_within(self.processed..self.filled, 0);
+            }
+            self.filled -= self.processed;
+            self.processed = 0;
+        }
+
+        // Ensure we have enough capacity to deserialize the message.
+        if self.buf.len() < self.filled + need_additional_bytes {
+            self.buf.reserve(need_additional_bytes);
+            self.buf.resize(self.buf.capacity(), 0);
         }
-        self.read_so_far -= self.last_size;
-        self.last_size = 0;
     }
 
+    // Read the BT handshake.
     // This MUST be run as the first operation on the buffer.
     pub async fn read_handshake(
         &mut self,
         mut conn: impl AsyncReadExt + Unpin,
         timeout: Duration,
     ) -> anyhow::Result<Handshake<ByteBuf<'_>>> {
-        self.read_so_far = with_timeout(timeout, conn.read(&mut self.buf))
+        self.filled = with_timeout(timeout, conn.read(&mut self.buf))
             .await
             .context("error reading handshake")?;
-        if self.read_so_far == 0 {
-            anyhow::bail!("bad handshake");
+        if self.filled == 0 {
+            anyhow::bail!("peer disconnected while reading handshake");
         }
-        let (h, size) = Handshake::deserialize(&self.buf[..self.read_so_far])
+        let (h, size) = Handshake::deserialize(&self.buf[..self.filled])
             .map_err(|e| anyhow::anyhow!("error deserializing handshake: {:?}", e))?;
-        self.last_size = size;
+        self.processed = size;
         Ok(h)
     }
 
+    // Read a message into the buffer, try to deserialize it and call the callback on it.
+    // We can't return the message because of a borrow checker issue.
     pub async fn read_message(
         &mut self,
         mut conn: impl AsyncReadExt + Unpin,
         timeout: Duration,
         on_message: impl for<'a> FnOnce(MessageBorrowed<'a>) -> anyhow::Result<()>,
     ) -> anyhow::Result<()> {
-        self.prepare_for_read();
         loop {
             let need_additional_bytes =
-                match MessageBorrowed::deserialize(&self.buf[..self.read_so_far]) {
+                match MessageBorrowed::deserialize(&self.buf[self.processed..self.filled]) {
                     Err(MessageDeserializeError::NotEnoughData(d, _)) => d,
                     Ok((msg, size)) => {
-                        self.last_size = size;
+                        self.processed += size;
                         // Rust's borrow checker can't do this early return. So we are using a callback instead.
                         // return Ok(msg);
                         on_message(msg)?;
@@ -69,20 +83,14 @@ impl ReadBuf {
                     }
                     Err(e) => return Err(e.into()),
                 };
-            if self.buf.len() < self.read_so_far + need_additional_bytes {
-                self.buf.reserve(need_additional_bytes);
-                self.buf.resize(self.buf.capacity(), 0);
-            }
-            let size = with_timeout(timeout, conn.read(&mut self.buf[self.read_so_far..]))
+            self.prepare_for_read(need_additional_bytes);
+            let size = with_timeout(timeout, conn.read(&mut self.buf[self.filled..]))
                 .await
                 .context("error reading from peer")?;
             if size == 0 {
-                anyhow::bail!(
-                    "disconnected while reading, read so far: {}",
-                    self.read_so_far
-                )
+                anyhow::bail!("disconnected while reading, read so far: {}", self.filled)
             }
-            self.read_so_far += size;
+            self.filled += size;
         }
     }
 }