sketch of changes for https://github.com/github/go-liveshare/pull/13

cmd/ghcs/logs.go

@@ -3,6 +3,7 @@ package main
 import (
 	"context"
 	"fmt"
+	"net"
 	"os"
 
 	"github.com/github/ghcs/api"
@@ -60,10 +61,13 @@ func logs(ctx context.Context, tail bool, codespaceName string) error {
 		return fmt.Errorf("connecting to Live Share: %v", err)
 	}
 
-	localSSHPort, err := codespaces.UnusedPort()
+	// Ensure local port is listening before client (getPostCreateOutput) connects.
+	listen, err := liveshare.Listen(0) // zero => arbitrary
 	if err != nil {
 		return err
 	}
+	defer listen.Close()
+	localPort := listen.Addr().(*net.TCPAddr).Port
 
 	remoteSSHServerPort, sshUser, err := codespaces.StartSSHServer(ctx, session, log)
 	if err != nil {
@@ -77,7 +81,7 @@ func logs(ctx context.Context, tail bool, codespaceName string) error {
 
 	dst := fmt.Sprintf("%s@localhost", sshUser)
 	cmd := codespaces.NewRemoteCommand(
-		ctx, localSSHPort, dst, fmt.Sprintf("%s /workspaces/.codespaces/.persistedshare/creation.log", cmdType),
+		ctx, localPort, dst, fmt.Sprintf("%s /workspaces/.codespaces/.persistedshare/creation.log", cmdType),
 	)
 
 	// Error channels are buffered so that neither sending goroutine gets stuck.
@@ -85,7 +89,7 @@ func logs(ctx context.Context, tail bool, codespaceName string) error {
 	tunnelClosed := make(chan error, 1)
 	go func() {
 		fwd := liveshare.NewPortForwarder(session, "sshd", remoteSSHServerPort)
-		tunnelClosed <- fwd.ForwardToLocalPort(ctx, localSSHPort) // error is non-nil
+		tunnelClosed <- fwd.ForwardToLocalPort(ctx, listen) // error is non-nil
 	}()
 
 	cmdDone := make(chan error, 1)

cmd/ghcs/ports.go

@@ -277,11 +277,18 @@ func forwardPorts(log *output.Logger, codespaceName string, ports []string) erro
 	defer cancel()
 	for _, pair := range portPairs {
 		pair := pair
-		log.Printf("Forwarding ports: remote %d <=> local %d\n", pair.remote, pair.local)
-		name := fmt.Sprintf("share-%d", pair.remote)
+
 		go func() {
+			listen, err := liveshare.Listen(pair.local)
+			if err != nil {
+				errc <- err
+				return
+			}
+			defer listen.Close()
+			log.Printf("Forwarding ports: remote %d <=> local %d\n", pair.remote, pair.local)
+			name := fmt.Sprintf("share-%d", pair.remote)
 			fwd := liveshare.NewPortForwarder(session, name, pair.remote)
-			errc <- fwd.ForwardToLocalPort(ctx, pair.local) // error always non-nil
+			errc <- fwd.ForwardToLocalPort(ctx, listen) // error always non-nil
 		}()
 	}
 

cmd/ghcs/ssh.go

@@ -4,6 +4,7 @@ import (
 	"bufio"
 	"context"
 	"fmt"
+	"net"
 	"os"
 	"strings"
 
@@ -81,14 +82,15 @@ func ssh(ctx context.Context, sshProfile, codespaceName string, localSSHServerPo
 	}
 	log.Print("\n")
 
-	usingCustomPort := true
-	if localSSHServerPort == 0 {
-		usingCustomPort = false // suppress log of command line in Shell
-		localSSHServerPort, err = codespaces.UnusedPort()
-		if err != nil {
-			return err
-		}
+	usingCustomPort := localSSHServerPort != 0 // suppress log of command line in Shell
+
+	// Ensure local port is listening before client (Shell) connects.
+	listen, err := liveshare.Listen(localSSHServerPort)
+	if err != nil {
+		return err
 	}
+	defer listen.Close()
+	localSSHServerPort = listen.Addr().(*net.TCPAddr).Port
 
 	connectDestination := sshProfile
 	if connectDestination == "" {
@@ -98,7 +100,7 @@ func ssh(ctx context.Context, sshProfile, codespaceName string, localSSHServerPo
 	tunnelClosed := make(chan error)
 	go func() {
 		fwd := liveshare.NewPortForwarder(session, "sshd", remoteSSHServerPort)
-		tunnelClosed <- fwd.ForwardToLocalPort(ctx, localSSHServerPort) // error is always non-nil
+		tunnelClosed <- fwd.ForwardToLocalPort(ctx, listen) // error is always non-nil
 	}()
 
 	shellClosed := make(chan error)

internal/codespaces/ssh.go

@@ -4,7 +4,6 @@ import (
 	"context"
 	"errors"
 	"fmt"
-	"net"
 	"os"
 	"os/exec"
 	"strconv"
@@ -13,25 +12,6 @@ import (
 	"github.com/github/go-liveshare"
 )
 
-// UnusedPort returns the number of a local TCP port that is currently
-// unbound, or an error if none was available.
-//
-// Use of this function carries an inherent risk of a time-of-check to
-// time-of-use race against other processes.
-func UnusedPort() (int, error) {
-	addr, err := net.ResolveTCPAddr("tcp", "localhost:0")
-	if err != nil {
-		return 0, fmt.Errorf("internal error while choosing port: %v", err)
-	}
-
-	l, err := net.ListenTCP("tcp", addr)
-	if err != nil {
-		return 0, fmt.Errorf("choosing available port: %v", err)
-	}
-	defer l.Close()
-	return l.Addr().(*net.TCPAddr).Port, nil
-}
-
 // StartSSHServer installs (if necessary) and starts the SSH in the codespace.
 // It returns the remote port where it is running, the user to log in with, or an error if something failed.
 func StartSSHServer(ctx context.Context, session *liveshare.Session, log logger) (serverPort int, user string, err error) {

internal/codespaces/states.go

@@ -5,6 +5,7 @@ import (
 	"context"
 	"encoding/json"
 	"fmt"
+	"net"
 	"strings"
 	"time"
 
@@ -46,10 +47,12 @@ func PollPostCreateStates(ctx context.Context, log logger, apiClient *api.API, u
 		return fmt.Errorf("connect to Live Share: %v", err)
 	}
 
-	localSSHPort, err := UnusedPort()
+	// Ensure local port is listening before client (getPostCreateOutput) connects.
+	listen, err := liveshare.Listen(0)
 	if err != nil {
 		return err
 	}
+	localPort := listen.Addr().(*net.TCPAddr).Port
 
 	remoteSSHServerPort, sshUser, err := StartSSHServer(ctx, session, log)
 	if err != nil {
@@ -59,7 +62,7 @@ func PollPostCreateStates(ctx context.Context, log logger, apiClient *api.API, u
 	tunnelClosed := make(chan error, 1) // buffered to avoid sender stuckness
 	go func() {
 		fwd := liveshare.NewPortForwarder(session, "sshd", remoteSSHServerPort)
-		tunnelClosed <- fwd.ForwardToLocalPort(ctx, localSSHPort) // error is non-nil
+		tunnelClosed <- fwd.ForwardToLocalPort(ctx, listen) // error is non-nil
 	}()
 
 	t := time.NewTicker(1 * time.Second)
@@ -74,7 +77,7 @@ func PollPostCreateStates(ctx context.Context, log logger, apiClient *api.API, u
 			return fmt.Errorf("connection failed: %v", err)
 
 		case <-t.C:
-			states, err := getPostCreateOutput(ctx, localSSHPort, codespace, sshUser)
+			states, err := getPostCreateOutput(ctx, localPort, codespace, sshUser)
 			if err != nil {
 				return fmt.Errorf("get post create output: %v", err)
 			}