postgres-replication: reuse underlying allocation for tuple data

The logical replication parsing routine for tuple data left some
performance on the table by copying the replication data into new
allocations instead of reusing the ones provided by the underlying
`Bytes` buffer.

Signed-off-by: Petros Angelatos <petrosagg@gmail.com>

Author: petrosagg

postgres-replication/src/protocol.rs

@@ -442,9 +442,8 @@ impl TupleData {
             TUPLE_DATA_TOAST_TAG => TupleData::UnchangedToast,
             TUPLE_DATA_TEXT_TAG => {
                 let len = buf.read_i32::<BigEndian>()?;
-                let mut data = vec![0; len as usize];
-                buf.read_exact(&mut data)?;
-                TupleData::Text(data.into())
+                let data = buf.read_buf(len as usize)?;
+                TupleData::Text(data)
             }
             TUPLE_DATA_BINARY_TAG => {
                 let len = buf.read_i32::<BigEndian>()?;
@@ -778,6 +777,20 @@ impl Buffer {
         self.idx = self.bytes.len();
         buf
     }
+
+    #[inline]
+    fn read_buf(&mut self, len: usize) -> io::Result<Bytes> {
+        if self.idx + len <= self.bytes.len() {
+            let buf = self.bytes.slice(self.idx..(self.idx + len));
+            self.idx += len;
+            Ok(buf)
+        } else {
+            Err(io::Error::new(
+                io::ErrorKind::UnexpectedEof,
+                "unexpected EOF",
+            ))
+        }
+    }
 }
 
 impl Read for Buffer {