Replace buffer with bufferedReader

bufferedReader is an optimized implementation of io.Reader that behaves
like
```
io.MultiReader(bytes.NewReader(buffer.Bytes()), io.TeeReader(source, buffer))
```
without allocating.

This has a measurable effect on benchmarks:
```
name        old time/op    new time/op    delta
CMuxConn-4    1.09µs ± 4%    0.99µs ±19%   -9.32%  (p=0.000 n=17+19)

name        old alloc/op   new alloc/op   delta
CMuxConn-4      240B ± 0%      260B ± 0%   +8.33%  (p=0.000 n=20+20)

name        old allocs/op  new allocs/op  delta
CMuxConn-4      9.00 ± 0%      5.00 ± 0%  -44.44%  (p=0.000 n=20+20)
```

Note that appropriate test coverage is provided by `TestRead`.

buffer.go

@@ -1,57 +1,35 @@
 package cmux
 
-import "io"
+import (
+	"bytes"
+	"io"
+)
 
-var _ io.ReadWriter = (*buffer)(nil)
+// bufferedReader is an optimized implementation of io.Reader that behaves like
-
+// ```
-type buffer struct {
+// io.MultiReader(bytes.NewReader(buffer.Bytes()), io.TeeReader(source, buffer))
-	read int
+// ```
-	data []byte
+// without allocating.
+type bufferedReader struct {
+	source     io.Reader
+	buffer     *bytes.Buffer
+	bufferRead int
+	bufferSize int
 }
 
-// From the io.Reader documentation:
+func (s *bufferedReader) Read(p []byte) (int, error) {
-//
+	// Functionality of bytes.Reader.
-// When Read encounters an error or end-of-file condition after
+	bn := copy(p, s.buffer.Bytes()[s.bufferRead:s.bufferSize])
-// successfully reading n > 0 bytes, it returns the number of
+	s.bufferRead += bn
-// bytes read.  It may return the (non-nil) error from the same call
-// or return the error (and n == 0) from a subsequent call.
-// An instance of this general case is that a Reader returning
-// a non-zero number of bytes at the end of the input stream may
-// return either err == EOF or err == nil.  The next Read should
-// return 0, EOF.
-//
-// This function implements the latter behaviour, returning the
-// (non-nil) error from the same call.
-func (b *buffer) Read(p []byte) (int, error) {
-	var err error
-	n := copy(p, b.data[b.read:])
-	b.read += n
-	if b.read == len(b.data) {
-		err = io.EOF
-	}
-	return n, err
-}
 
-func (b *buffer) Len() int {
+	p = p[bn:]
-	return len(b.data) - b.read
-}
 
-func (b *buffer) resetRead() {
+	// Funtionality of io.TeeReader.
-	b.read = 0
+	sn, sErr := s.source.Read(p)
+	if sn > 0 {
+		if wn, wErr := s.buffer.Write(p[:sn]); wErr != nil {
+			return bn + wn, wErr
 		}
-
+	}
-// From the io.Writer documentation:
+	return bn + sn, sErr
-//
-// Write writes len(p) bytes from p to the underlying data stream.
-// It returns the number of bytes written from p (0 <= n <= len(p))
-// and any error encountered that caused the write to stop early.
-// Write must return a non-nil error if it returns n < len(p).
-// Write must not modify the slice data, even temporarily.
-//
-// Implementations must not retain p.
-//
-// In a previous incarnation, this implementation retained the incoming slice.
-func (b *buffer) Write(p []byte) (int, error) {
-	b.data = append(b.data, p...)
-	return len(p), nil
 }

buffer_test.go

@@ -1,113 +0,0 @@
-package cmux
-
-import (
-	"bytes"
-	"io"
-	"testing"
-)
-
-func TestWriteNoModify(t *testing.T) {
-	var b buffer
-
-	const origWriteByte = 0
-	const postWriteByte = 1
-
-	writeBytes := []byte{origWriteByte}
-	if _, err := b.Write(writeBytes); err != nil {
-		t.Fatal(err)
-	}
-	writeBytes[0] = postWriteByte
-	readBytes := make([]byte, 1)
-	if _, err := b.Read(readBytes); err != io.EOF {
-		t.Fatal(err)
-	}
-
-	if readBytes[0] != origWriteByte {
-		t.Fatalf("expected to read %x, but read %x; buffer retained passed-in slice", origWriteByte, postWriteByte)
-	}
-}
-
-const writeString = "deadbeef"
-
-func TestBuffer(t *testing.T) {
-	writeBytes := []byte(writeString)
-
-	const numWrites = 10
-
-	var b buffer
-	for i := 0; i < numWrites; i++ {
-		n, err := b.Write(writeBytes)
-		if err != nil && err != io.EOF {
-			t.Fatal(err)
-		}
-		if n != len(writeBytes) {
-			t.Fatalf("cannot write all the bytes: want=%d got=%d", len(writeBytes), n)
-		}
-	}
-
-	for j := 0; j < 2; j++ {
-		readBytes := make([]byte, len(writeBytes))
-		for i := 0; i < numWrites; i++ {
-			n, err := b.Read(readBytes)
-			if i == numWrites-1 {
-				// The last read should report EOF.
-				if err != io.EOF {
-					t.Fatal(err)
-				}
-			} else if err != nil {
-				t.Fatal(err)
-			}
-			if n != len(readBytes) {
-				t.Fatalf("cannot read all the bytes: want=%d got=%d", len(readBytes), n)
-			}
-			if !bytes.Equal(writeBytes, readBytes) {
-				t.Errorf("different bytes read: want=%d got=%d", writeBytes, readBytes)
-			}
-		}
-		n, err := b.Read(readBytes)
-		if err != io.EOF {
-			t.Errorf("expected EOF")
-		}
-		if n != 0 {
-			t.Errorf("expected buffer to be empty, but got %d bytes", n)
-		}
-
-		b.resetRead()
-	}
-}
-
-func TestBufferOffset(t *testing.T) {
-	writeBytes := []byte(writeString)
-
-	var b buffer
-	n, err := b.Write(writeBytes)
-	if err != nil {
-		t.Fatal(err)
-	}
-	if n != len(writeBytes) {
-		t.Fatalf("cannot write all the bytes: want=%d got=%d", len(writeBytes), n)
-	}
-
-	const readSize = 2
-
-	numReads := len(writeBytes) / readSize
-
-	for i := 0; i < numReads; i++ {
-		readBytes := make([]byte, readSize)
-		n, err := b.Read(readBytes)
-		if i == numReads-1 {
-			// The last read should report EOF.
-			if err != io.EOF {
-				t.Fatal(err)
-			}
-		} else if err != nil {
-			t.Fatal(err)
-		}
-		if n != readSize {
-			t.Fatalf("cannot read the bytes: want=%d got=%d", readSize, n)
-		}
-		if got := writeBytes[i*readSize : i*readSize+readSize]; !bytes.Equal(got, readBytes) {
-			t.Fatalf("different bytes read: want=%s got=%s", readBytes, got)
-		}
-	}
-}

cmux.go

@@ -1,6 +1,7 @@
 package cmux
 
 import (
+	"bytes"
 	"fmt"
 	"io"
 	"net"
@@ -125,8 +126,7 @@ func (m *cMux) serve(c net.Conn, donec <-chan struct{}, wg *sync.WaitGroup) {
 	muc := newMuxConn(c)
 	for _, sl := range m.sls {
 		for _, s := range sl.ss {
-			matched := s(muc.sniffer())
+			matched := s(muc.getSniffer())
-			muc.reset()
 			if matched {
 				select {
 				case sl.l.connc <- muc:
@@ -177,7 +177,8 @@ func (l muxListener) Accept() (net.Conn, error) {
 // MuxConn wraps a net.Conn and provides transparent sniffing of connection data.
 type MuxConn struct {
 	net.Conn
-	buf buffer
+	buf     bytes.Buffer
+	sniffer bufferedReader
 }
 
 func newMuxConn(c net.Conn) *MuxConn {
@@ -201,17 +202,17 @@ func newMuxConn(c net.Conn) *MuxConn {
 // (non-nil) error from the same call.
 func (m *MuxConn) Read(p []byte) (int, error) {
 	n1, err := m.buf.Read(p)
-	if err != io.EOF {
+	if err == nil && m.buf.Len() == 0 {
+		err = io.EOF
+	}
+	if n1 == len(p) || err != io.EOF {
 		return n1, err
 	}
 	n2, err := m.Conn.Read(p[n1:])
 	return n1 + n2, err
 }
 
-func (m *MuxConn) sniffer() io.Reader {
+func (m *MuxConn) getSniffer() io.Reader {
-	return io.MultiReader(&m.buf, io.TeeReader(m.Conn, &m.buf))
+	m.sniffer = bufferedReader{source: m.Conn, buffer: &m.buf, bufferSize: m.buf.Len()}
-}
+	return &m.sniffer
-
-func (m *MuxConn) reset() {
-	m.buf.resetRead()
 }