net/dns: deal with Windows wsl.exe hangs

Despite the fact that WSL configuration is still disabled by default, we
continue to log the machine's list of WSL distros as a diagnostic measure.

Unfortunately I have seen the "wsl.exe -l" command hang indefinitely. This patch
adds a (more than reasonable) 10s timeout to ensure that tailscaled does not get
stuck while executing this operation.

I also modified the Windows implementation of NewOSConfigurator to do the
logging asynchronously, since that information is not required in order to
continue starting up.

Fixes #7476

Signed-off-by: Aaron Klotz <aaron@tailscale.com>

net/dns/manager_windows.go

@@ -51,12 +51,14 @@ func NewOSConfigurator(logf logger.Logf, interfaceName string) (OSConfigurator,
 		ret.nrptDB = newNRPTRuleDatabase(logf)
 	}
 
-	// Log WSL status once at startup.
-	if distros, err := wslDistros(); err != nil {
-		logf("WSL: could not list distributions: %v", err)
-	} else {
-		logf("WSL: found %d distributions", len(distros))
-	}
+	go func() {
+		// Log WSL status once at startup.
+		if distros, err := wslDistros(); err != nil {
+			logf("WSL: could not list distributions: %v", err)
+		} else {
+			logf("WSL: found %d distributions", len(distros))
+		}
+	}()
 
 	return ret, nil
 }

net/dns/wsl_windows.go

@@ -5,6 +5,7 @@ package dns
 
 import (
 	"bytes"
+	"context"
 	"errors"
 	"fmt"
 	"os"
@@ -12,6 +13,7 @@ import (
 	"os/user"
 	"strings"
 	"syscall"
+	"time"
 
 	"golang.org/x/sys/windows"
 	"tailscale.com/types/logger"
@@ -20,7 +22,13 @@ import (
 
 // wslDistros reports the names of the installed WSL2 linux distributions.
 func wslDistros() ([]string, error) {
-	b, err := wslCombinedOutput(exec.Command("wsl.exe", "-l"))
+	// There is a bug in some builds of wsl.exe that causes it to block
+	// indefinitely while executing this operation. Set a timeout so that we don't
+	// get wedged! (Issue #7476)
+	ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
+	defer cancel()
+
+	b, err := wslCombinedOutput(exec.CommandContext(ctx, "wsl.exe", "-l"))
 	if err != nil {
 		return nil, fmt.Errorf("%v: %q", err, string(b))
 	}