chore: move InProcNet to testutil (#18563)

Moves `InProcNet` to `testutil` so that it can be reused by X11 forwarding tests (see up stack PRs).

testutil/net.go

@@ -0,0 +1,105 @@
+package testutil
+
+import (
+	"context"
+	"net"
+	"sync"
+
+	"golang.org/x/xerrors"
+)
+
+type Addr struct {
+	network string
+	addr    string
+}
+
+func NewAddr(network, addr string) Addr {
+	return Addr{network, addr}
+}
+
+func (a Addr) Network() string {
+	return a.network
+}
+
+func (a Addr) Address() string {
+	return a.addr
+}
+
+func (a Addr) String() string {
+	return a.network + "|" + a.addr
+}
+
+type InProcNet struct {
+	sync.Mutex
+
+	listeners map[Addr]*inProcListener
+}
+
+type inProcListener struct {
+	c chan net.Conn
+	n *InProcNet
+	a Addr
+	o sync.Once
+}
+
+func NewInProcNet() *InProcNet {
+	return &InProcNet{listeners: make(map[Addr]*inProcListener)}
+}
+
+func (n *InProcNet) Listen(network, address string) (net.Listener, error) {
+	a := Addr{network, address}
+	n.Lock()
+	defer n.Unlock()
+	if _, ok := n.listeners[a]; ok {
+		return nil, xerrors.New("busy")
+	}
+	l := newInProcListener(n, a)
+	n.listeners[a] = l
+	return l, nil
+}
+
+func (n *InProcNet) Dial(ctx context.Context, a Addr) (net.Conn, error) {
+	n.Lock()
+	defer n.Unlock()
+	l, ok := n.listeners[a]
+	if !ok {
+		return nil, xerrors.Errorf("nothing listening on %s", a)
+	}
+	x, y := net.Pipe()
+	select {
+	case <-ctx.Done():
+		return nil, ctx.Err()
+	case l.c <- x:
+		return y, nil
+	}
+}
+
+func newInProcListener(n *InProcNet, a Addr) *inProcListener {
+	return &inProcListener{
+		c: make(chan net.Conn),
+		n: n,
+		a: a,
+	}
+}
+
+func (l *inProcListener) Accept() (net.Conn, error) {
+	c, ok := <-l.c
+	if !ok {
+		return nil, net.ErrClosed
+	}
+	return c, nil
+}
+
+func (l *inProcListener) Close() error {
+	l.o.Do(func() {
+		l.n.Lock()
+		defer l.n.Unlock()
+		delete(l.n.listeners, l.a)
+		close(l.c)
+	})
+	return nil
+}
+
+func (l *inProcListener) Addr() net.Addr {
+	return l.a
+}