clientupdate, util/osshare, util/winutil, version: improve Windows GUI filename resolution and WinUI build awareness

On Windows arm64 we are going to need to ship two different GUI builds;
one for Win10 (GOARCH=386) and one for Win11 (GOARCH=amd64, tags +=
winui). Due to quirks in MSI packaging, they cannot both share the
same filename. This requires some fixes in places where we have
hardcoded "tailscale-ipn" as the GUI filename.

We also do some cleanup in clientupdate to ensure that autoupdates
will continue to work correctly with the temporary "-winui" package
variant.

Fixes #17480
Updates https://github.com/tailscale/corp/issues/29940

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

util/osshare/filesharingstatus_windows.go

@@ -9,30 +9,31 @@ import (
 	"fmt"
 	"os"
 	"path/filepath"
-	"sync"
+	"runtime"
 
 	"golang.org/x/sys/windows/registry"
+	"tailscale.com/types/lazy"
 	"tailscale.com/types/logger"
+	"tailscale.com/util/winutil"
 )
 
 const (
 	sendFileShellKey = `*\shell\tailscale`
 )
 
-var ipnExePath struct {
-	sync.Mutex
-	cache string // absolute path of tailscale-ipn.exe, populated lazily on first use
-}
+var ipnExePath lazy.SyncValue[string] // absolute path of the GUI executable
 
 func getIpnExePath(logf logger.Logf) string {
-	ipnExePath.Lock()
-	defer ipnExePath.Unlock()
-
-	if ipnExePath.cache != "" {
-		return ipnExePath.cache
+	exe, err := winutil.GUIPathFromReg()
+	if err == nil {
+		return exe
 	}
 
-	// Find the absolute path of tailscale-ipn.exe assuming that it's in the same
+	return findGUIInSameDirAsThisExe(logf)
+}
+
+func findGUIInSameDirAsThisExe(logf logger.Logf) string {
+	// Find the absolute path of the GUI, assuming that it's in the same
 	// directory as this executable (tailscaled.exe).
 	p, err := os.Executable()
 	if err != nil {
@@ -43,14 +44,23 @@ func getIpnExePath(logf logger.Logf) string {
 		logf("filepath.EvalSymlinks error: %v", err)
 		return ""
 	}
-	p = filepath.Join(filepath.Dir(p), "tailscale-ipn.exe")
 	if p, err = filepath.Abs(p); err != nil {
 		logf("filepath.Abs error: %v", err)
 		return ""
 	}
-	ipnExePath.cache = p
-
-	return p
+	d := filepath.Dir(p)
+	candidates := []string{"tailscale-ipn.exe"}
+	if runtime.GOARCH == "arm64" {
+		// This name may be used on Windows 10 ARM64.
+		candidates = append(candidates, "tailscale-gui-386.exe")
+	}
+	for _, c := range candidates {
+		testPath := filepath.Join(d, c)
+		if _, err := os.Stat(testPath); err == nil {
+			return testPath
+		}
+	}
+	return ""
 }
 
 // SetFileSharingEnabled adds/removes "Send with Tailscale" from the Windows shell menu.
@@ -64,7 +74,9 @@ func SetFileSharingEnabled(enabled bool, logf logger.Logf) {
 }
 
 func enableFileSharing(logf logger.Logf) {
-	path := getIpnExePath(logf)
+	path := ipnExePath.Get(func() string {
+		return getIpnExePath(logf)
+	})
 	if path == "" {
 		return
 	}
@@ -79,7 +91,7 @@ func enableFileSharing(logf logger.Logf) {
 		logf("k.SetStringValue error: %v", err)
 		return
 	}
-	if err := k.SetStringValue("Icon", path+",0"); err != nil {
+	if err := k.SetStringValue("Icon", path+",1"); err != nil {
 		logf("k.SetStringValue error: %v", err)
 		return
 	}

util/winutil/winutil_windows.go

@@ -8,8 +8,10 @@ import (
 	"fmt"
 	"log"
 	"math"
+	"os"
 	"os/exec"
 	"os/user"
+	"path/filepath"
 	"reflect"
 	"runtime"
 	"strings"
@@ -33,6 +35,10 @@ var ErrNoShell = errors.New("no Shell process is present")
 // ErrNoValue is returned when the value doesn't exist in the registry.
 var ErrNoValue = registry.ErrNotExist
 
+// ErrBadRegValueFormat is returned when a string value does not match the
+// expected format.
+var ErrBadRegValueFormat = errors.New("registry value formatted incorrectly")
+
 // GetDesktopPID searches the PID of the process that's running the
 // currently active desktop. Returns ErrNoShell if the shell is not present.
 // Usually the PID will be for explorer.exe.
@@ -947,3 +953,22 @@ func IsDomainName(name string) (bool, error) {
 
 	return isDomainName(name16)
 }
+
+// GUIPathFromReg obtains the path to the client GUI executable from the
+// registry value that was written during installation.
+func GUIPathFromReg() (string, error) {
+	regPath, err := GetRegString("GUIPath")
+	if err != nil {
+		return "", err
+	}
+
+	if !filepath.IsAbs(regPath) {
+		return "", ErrBadRegValueFormat
+	}
+
+	if _, err := os.Stat(regPath); err != nil {
+		return "", err
+	}
+
+	return regPath, nil
+}