Reuse connection read buffer

To avoid extra copies and small allocations previously large
read buffers were allocated and never reused. However, the down side of
this was greater total memory allocation and the possibility that a
reference to a single byte could pin an entire buffer.

Now the buffer is reused.

pgproto3/chunkreader.go

@@ -6,11 +6,9 @@ import (
 
 // chunkReader is a io.Reader wrapper that minimizes IO reads and memory allocations. It allocates memory in chunks and
 // will read as much as will fit in the current buffer in a single call regardless of how large a read is actually
-// requested. The memory returned via Next is owned by the caller. This avoids the need for an additional copy.
+// requested. The memory returned via Next is only valid until the next call to Next.
 //
-// The downside of this approach is that a large buffer can be pinned in memory even if only a small slice is
-// referenced. For example, an entire 4096 byte block could be pinned in memory by even a 1 byte slice. In these rare
-// cases it would be advantageous to copy the bytes to another slice.
+// This is roughly equivalent to a bufio.Reader that only uses Peek and Discard to never copy bytes.
 type chunkReader struct {
 	r io.Reader
 
@@ -38,13 +36,14 @@ func newChunkReader(r io.Reader, minBufSize int) *chunkReader {
 	}
 }
 
-// Next returns buf filled with the next n bytes. The caller gains ownership of buf. It is not necessary to make a copy
-// of buf. If an error occurs, buf will be nil.
+// Next returns buf filled with the next n bytes. buf is only valid until next call of Next. If an error occurs, buf
+// will be nil.
 func (r *chunkReader) Next(n int) (buf []byte, err error) {
 	// n bytes already in buf
 	if (r.wp - r.rp) >= n {
 		buf = r.buf[r.rp : r.rp+n]
 		r.rp += n
+		r.resetBufIfEmpty()
 		return buf, err
 	}
 
@@ -66,6 +65,7 @@ func (r *chunkReader) Next(n int) (buf []byte, err error) {
 
 	buf = r.buf[r.rp : r.rp+n]
 	r.rp += n
+	r.resetBufIfEmpty()
 	return buf, nil
 }
 
@@ -87,3 +87,13 @@ func (r *chunkReader) copyBufContents(dest []byte) {
 	r.rp = 0
 	r.buf = dest
 }
+
+func (r *chunkReader) resetBufIfEmpty() {
+	if r.rp == r.wp {
+		if len(r.buf) > r.minBufLen {
+			r.buf = make([]byte, r.minBufLen)
+		}
+		r.rp = 0
+		r.wp = 0
+	}
+}