client/tailscale, cmd/tailscale, localapi: add 'tailscale nc' (actually)

Adds missing file from fc12cbfcd3.

GitHub was having issues earlier and it was all green because the
checks never actually ran, but the DCO non-Actions check at least did,
so "green" and I merged, not realizing it hadn't really run anything.

Updates #3802

Change-Id: I29f605eebe5336f1f3ca28ebb78b092dd99d9fd8
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

cmd/tailscale/cli/nc.go

@@ -0,0 +1,63 @@
+// Copyright (c) 2022 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package cli
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"io"
+	"os"
+	"strconv"
+
+	"github.com/peterbourgon/ff/v3/ffcli"
+	"tailscale.com/client/tailscale"
+)
+
+var ncCmd = &ffcli.Command{
+	Name:       "nc",
+	ShortUsage: "nc <hostname-or-IP> <port>",
+	ShortHelp:  "Connect to a port on a host, connected to stdin/stdout",
+	Exec:       runNC,
+}
+
+func runNC(ctx context.Context, args []string) error {
+	st, err := tailscale.Status(ctx)
+	if err != nil {
+		return fixTailscaledConnectError(err)
+	}
+	description, ok := isRunningOrStarting(st)
+	if !ok {
+		printf("%s\n", description)
+		os.Exit(1)
+	}
+
+	if len(args) != 2 {
+		return errors.New("usage: nc <hostname-or-IP> <port>")
+	}
+
+	hostOrIP, portStr := args[0], args[1]
+	port, err := strconv.ParseUint(portStr, 10, 16)
+	if err != nil {
+		return fmt.Errorf("invalid port number %q", portStr)
+	}
+
+	// TODO(bradfitz): also add UDP too, via flag?
+	c, err := tailscale.DialTCP(ctx, hostOrIP, uint16(port))
+	if err != nil {
+		return fmt.Errorf("Dial(%q, %v): %w", hostOrIP, port, err)
+	}
+	defer c.Close()
+	errc := make(chan error, 1)
+	go func() {
+		_, err := io.Copy(os.Stdout, c)
+		errc <- err
+	}()
+	go func() {
+		_, err := io.Copy(c, os.Stdin)
+		errc <- err
+	}()
+	return <-errc
+}