Implement :serverlist command

- Add serverlist command to list SQL Server instances via SQL Browser service
- Move server listing logic from cmd/sqlcmd to pkg/sqlcmd for reuse
- Both -L flag and :serverlist command now use shared ListLocalServers function
- Add comprehensive tests for serverlist functionality

Author: dlevy-msft-sql

cmd/sqlcmd/sqlcmd.go

@@ -5,20 +5,16 @@
 package sqlcmd
 
 import (
-	"context"
 	"errors"
 	"fmt"
-	"net"
 	"os"
 	"regexp"
 	"runtime/trace"
 	"strconv"
 	"strings"
-	"time"
 
 	mssql "github.com/microsoft/go-mssqldb"
 	"github.com/microsoft/go-mssqldb/azuread"
-	"github.com/microsoft/go-mssqldb/msdsn"
 	"github.com/microsoft/go-sqlcmd/internal/localizer"
 	"github.com/microsoft/go-sqlcmd/pkg/console"
 	"github.com/microsoft/go-sqlcmd/pkg/sqlcmd"
@@ -236,7 +232,7 @@ func Execute(version string) {
 					fmt.Println()
 					fmt.Println(localizer.Sprintf("Servers:"))
 				}
-				listLocalServers()
+				sqlcmd.ListLocalServers(os.Stdout)
 				os.Exit(0)
 			}
 			if len(argss) > 0 {
@@ -911,76 +907,3 @@ func run(vars *sqlcmd.Variables, args *SQLCmdArguments) (int, error) {
 	s.SetError(nil)
 	return s.Exitcode, err
 }
-
-func listLocalServers() {
-	bmsg := []byte{byte(msdsn.BrowserAllInstances)}
-	resp := make([]byte, 16*1024-1)
-	dialer := &net.Dialer{}
-	ctx, cancel := context.WithTimeout(context.Background(), time.Second*30)
-	defer cancel()
-	conn, err := dialer.DialContext(ctx, "udp", ":1434")
-	// silently ignore failures to connect, same as ODBC
-	if err != nil {
-		return
-	}
-	defer conn.Close()
-	dl, _ := ctx.Deadline()
-	_ = conn.SetDeadline(dl)
-	_, err = conn.Write(bmsg)
-	if err != nil {
-		if !errors.Is(err, os.ErrDeadlineExceeded) {
-			fmt.Println(err)
-		}
-		return
-	}
-	read, err := conn.Read(resp)
-	if err != nil {
-		if !errors.Is(err, os.ErrDeadlineExceeded) {
-			fmt.Println(err)
-		}
-		return
-	}
-
-	data := parseInstances(resp[:read])
-	instances := make([]string, 0, len(data))
-	for s := range data {
-		if s == "MSSQLSERVER" {
-
-			instances = append(instances, "(local)", data[s]["ServerName"])
-		} else {
-			instances = append(instances, fmt.Sprintf(`%s\%s`, data[s]["ServerName"], s))
-		}
-	}
-	for _, s := range instances {
-		fmt.Println("  ", s)
-	}
-}
-
-func parseInstances(msg []byte) msdsn.BrowserData {
-	results := msdsn.BrowserData{}
-	if len(msg) > 3 && msg[0] == 5 {
-		out_s := string(msg[3:])
-		tokens := strings.Split(out_s, ";")
-		instdict := map[string]string{}
-		got_name := false
-		var name string
-		for _, token := range tokens {
-			if got_name {
-				instdict[name] = token
-				got_name = false
-			} else {
-				name = token
-				if len(name) == 0 {
-					if len(instdict) == 0 {
-						break
-					}
-					results[strings.ToUpper(instdict["InstanceName"])] = instdict
-					instdict = map[string]string{}
-					continue
-				}
-				got_name = true
-			}
-		}
-	}
-	return results
-}