fix: lookup hosts before passing to NewOnAddresses (#5245)

* fix: lookup hosts before passing to NewOnAddresses

* fix: propagate lookupErr

* fix: handle `--host '[::1]'` IPv6 literals

* fix: distinguish between address in use and other errors

```
$ tilt up --port 10350
Tilt started on http://localhost:10350/
v0.23.1-dev, built 2021-12-02
Error: Tilt cannot start because you already have another process on port 10350
If you want to run multiple Tilt instances simultaneously,
use the --port flag or TILT_PORT env variable to set a custom port
Original error: listen tcp 127.0.0.1:10350: bind: address already in use

$ tilt up --host '[::1' --port 10351
Tilt started (without browser UI)
v0.23.1-dev, built 2021-12-02
Error: listen tcp: address [::1:10351: missing ']' in address
```

* fix: filter out addresses that can't be listened

* errors: clean up error handling

internal/hud/server/apiserver.go

@@ -92,10 +92,13 @@ func ProvideConfigAccess(dir *dirs.TiltDevDir) clientcmd.ConfigAccess {
 func ProvideWebListener(host model.WebHost, port model.WebPort) (WebListener, error) {
 	webListener, err := net.Listen("tcp", fmt.Sprintf("%s:%d", string(host), int(port)))
 	if err != nil {
-		return nil, fmt.Errorf("Tilt cannot start because you already have another process on port %d\n"+
-			"If you want to run multiple Tilt instances simultaneously,\n"+
-			"use the --port flag or TILT_PORT env variable to set a custom port\nOriginal error: %v",
-			port, err)
+		if strings.HasSuffix(err.Error(), "address already in use") {
+			return nil, fmt.Errorf("Tilt cannot start because you already have another process on port %d\n"+
+				"If you want to run multiple Tilt instances simultaneously,\n"+
+				"use the --port flag or TILT_PORT env variable to set a custom port\nOriginal error: %v",
+				port, err)
+		}
+		return nil, err
 	}
 	return WebListener(webListener), nil
 }

internal/k8s/portforward.go

@@ -5,7 +5,9 @@ import (
 	"fmt"
 	"net"
 	"net/http"
+	"net/url"
 	"strconv"
+	"strings"
 
 	v1 "k8s.io/client-go/kubernetes/typed/core/v1"
 	_ "k8s.io/client-go/plugin/pkg/client/auth/gcp" // registers gcp auth provider
@@ -130,7 +132,11 @@ func (c portForwardClient) CreatePortForwarder(ctx context.Context, namespace Na
 			ports,
 			readyChan)
 	} else {
-		addresses := []string{host}
+		var addresses []string
+		addresses, err = getListenableAddresses(host)
+		if err != nil {
+			return nil, err
+		}
 		pf, err = portforward.NewOnAddresses(
 			ctx,
 			dialer,
@@ -148,6 +154,35 @@ func (c portForwardClient) CreatePortForwarder(ctx context.Context, namespace Na
 	}, nil
 }
 
+func getListenableAddresses(host string) ([]string, error) {
+	// handle IPv6 literals like `[::1]`
+	url, err := url.Parse(fmt.Sprintf("http://%s/", host))
+	if err != nil {
+		return nil, errors.Wrap(err, fmt.Sprintf("invalid host %s", host))
+	}
+	addresses, err := net.LookupHost(url.Hostname())
+	if err != nil {
+		return nil, errors.Wrap(err, fmt.Sprintf("failed to look up address for %s", host))
+	}
+	listenable := make([]string, 0)
+	for _, addr := range addresses {
+		var l net.Listener
+		if ipv6 := strings.Contains(addr, ":"); ipv6 {
+			l, err = net.Listen("tcp6", fmt.Sprintf("[%s]:0", addr))
+		} else {
+			l, err = net.Listen("tcp4", fmt.Sprintf("%s:0", addr))
+		}
+		if err == nil {
+			l.Close()
+			listenable = append(listenable, addr)
+		}
+	}
+	if len(listenable) == 0 {
+		return nil, errors.Errorf("host %s: cannot listen on any resolved addresses: %v", host, addresses)
+	}
+	return listenable, nil
+}
+
 func getAvailablePort() (int, error) {
 	l, err := net.Listen("tcp", ":0")
 	if err != nil {