net: implement WriterTo for pipes

Implements the WriterTo interface for net pipes to avoid intermediate buffers
when copying with `io.Copy`.

Fixes golang#34624

src/net/pipe.go

@@ -175,6 +175,34 @@ func (p *pipe) read(b []byte) (n int, err error) {
 	}
 }
 
+func (p *pipe) WriteTo(w io.Writer) (n int64, err error) {
+	for {
+		switch {
+		case isClosedChan(p.localDone):
+			return n, io.ErrClosedPipe
+		case isClosedChan(p.remoteDone):
+			return n, nil
+		case isClosedChan(p.readDeadline.wait()):
+			return n, timeoutError{}
+		}
+		select {
+		case bw := <-p.rdRx:
+			nr, err := w.Write(bw)
+			p.rdTx <- nr
+			n += int64(nr)
+			if err != nil {
+				return n, err
+			}
+		case <-p.localDone:
+			return n, io.ErrClosedPipe
+		case <-p.remoteDone:
+			return n, nil
+		case <-p.readDeadline.wait():
+			return n, timeoutError{}
+		}
+	}
+}
+
 func (p *pipe) Write(b []byte) (int, error) {
 	n, err := p.write(b)
 	if err != nil && err != io.ErrClosedPipe {

src/net/pipe_test.go

@@ -5,6 +5,8 @@
 package net_test
 
 import (
+	"bytes"
+	"fmt"
 	"io"
 	"net"
 	"testing"
@@ -47,3 +49,44 @@ func TestPipeCloseError(t *testing.T) {
 		t.Errorf("c2.SetDeadline() = %v, want io.ErrClosedPipe", err)
 	}
 }
+
+// Test WriteTo
+func TestPipeCopy(t *testing.T) {
+	done := make(chan struct{})
+	r, w := net.Pipe()
+
+	var expected bytes.Buffer
+	go func() {
+		defer close(done)
+		defer w.Close()
+		for i := 0; i < 10; i++ {
+			n1, _ := fmt.Fprintf(&expected, "message: %d\n", i)
+			n2, err := fmt.Fprintf(w, "message: %d\n", i)
+			if err != nil {
+				t.Errorf("write: %v", err)
+				return
+			}
+			if n1 != n2 {
+				t.Errorf("expected to write %d bytes, wrote %d", n1, n2)
+			}
+		}
+	}()
+
+	var actual bytes.Buffer
+	n, err := io.Copy(&actual, r)
+	if err != nil {
+		t.Errorf("read: %v", err)
+	}
+
+	// cleanup
+	r.Close()
+	<-done
+
+	if int(n) != actual.Len() {
+		t.Errorf("amount copied doesn't match amount read: %d != %d", n, actual.Len())
+	}
+
+	if !bytes.Equal(actual.Bytes(), expected.Bytes()) {
+		t.Errorf("did not read expected data")
+	}
+}