winutil: refactor methods to get values from registry to also return (#9536)

errors
Updates tailscale/corp#14879

Signed-off-by: Claire Wang <claire@tailscale.com>

util/winutil/policy/policy_windows.go

@@ -49,7 +49,10 @@ func (p PreferenceOptionPolicy) ShouldEnable(userChoice bool) bool {
 // "always" and "never" remove the user's ability to make a selection. If not
 // present or set to a different value, "user-decides" is the default.
 func GetPreferenceOptionPolicy(name string) PreferenceOptionPolicy {
-	opt := winutil.GetPolicyString(name, "user-decides")
+	opt, err := winutil.GetPolicyString(name)
+	if opt == "" || err != nil {
+		return showChoiceByPolicy
+	}
 	switch opt {
 	case "always":
 		return alwaysByPolicy
@@ -81,7 +84,10 @@ func (p VisibilityPolicy) Show() bool {
 // true) or "hide" (return true). If not present or set to a different value,
 // "show" (return false) is the default.
 func GetVisibilityPolicy(name string) VisibilityPolicy {
-	opt := winutil.GetPolicyString(name, "show")
+	opt, err := winutil.GetPolicyString(name)
+	if opt == "" || err != nil {
+		return visibleByPolicy
+	}
 	switch opt {
 	case "hide":
 		return hiddenByPolicy
@@ -96,8 +102,8 @@ func GetVisibilityPolicy(name string) VisibilityPolicy {
 // understands. If the registry value is "" or can not be processed,
 // defaultValue is returned instead.
 func GetDurationPolicy(name string, defaultValue time.Duration) time.Duration {
-	opt := winutil.GetPolicyString(name, "")
-	if opt == "" {
+	opt, err := winutil.GetPolicyString(name)
+	if opt == "" || err != nil {
 		return defaultValue
 	}
 	v, err := time.ParseDuration(opt)

util/winutil/winutil.go

@@ -13,45 +13,49 @@ import (
 const RegBase = regBase
 
 // GetPolicyString looks up a registry value in the local machine's path for
-// system policies, or returns the given default if it can't.
+// system policies, or returns empty string and the error.
 // Use this function to read values that may be set by sysadmins via the MSI
 // installer or via GPO. For registry settings that you do *not* want to be
 // visible to sysadmin tools, use GetRegString instead.
 //
 // This function will only work on GOOS=windows. Trying to run it on any other
-// OS will always return the default value.
-func GetPolicyString(name, defval string) string {
-	return getPolicyString(name, defval)
+// OS will always return an empty string and ErrNoValue.
+// If value does not exist or another error happens, returns empty string and error.
+func GetPolicyString(name string) (string, error) {
+	return getPolicyString(name)
 }
 
 // GetPolicyInteger looks up a registry value in the local machine's path for
-// system policies, or returns the given default if it can't.
+// system policies, or returns 0 and the associated error.
 // Use this function to read values that may be set by sysadmins via the MSI
 // installer or via GPO. For registry settings that you do *not* want to be
 // visible to sysadmin tools, use GetRegInteger instead.
 //
 // This function will only work on GOOS=windows. Trying to run it on any other
-// OS will always return the default value.
-func GetPolicyInteger(name string, defval uint64) uint64 {
-	return getPolicyInteger(name, defval)
+// OS will always return 0 and ErrNoValue.
+// If value does not exist or another error happens, returns 0 and error.
+func GetPolicyInteger(name string) (uint64, error) {
+	return getPolicyInteger(name)
 }
 
 // GetRegString looks up a registry path in the local machine path, or returns
-// the given default if it can't.
+// an empty string and error.
 //
 // This function will only work on GOOS=windows. Trying to run it on any other
-// OS will always return the default value.
-func GetRegString(name, defval string) string {
-	return getRegString(name, defval)
+// OS will always return an empty string and ErrNoValue.
+// If value does not exist or another error happens, returns empty string and error.
+func GetRegString(name string) (string, error) {
+	return getRegString(name)
 }
 
 // GetRegInteger looks up a registry path in the local machine path, or returns
-// the given default if it can't.
+// 0 and the error.
 //
 // This function will only work on GOOS=windows. Trying to run it on any other
-// OS will always return the default value.
-func GetRegInteger(name string, defval uint64) uint64 {
-	return getRegInteger(name, defval)
+// OS will always return 0 and ErrNoValue.
+// If value does not exist or another error happens, returns 0 and error.
+func GetRegInteger(name string) (uint64, error) {
+	return getRegInteger(name)
 }
 
 // IsSIDValidPrincipal determines whether the SID contained in uid represents a

util/winutil/winutil_notwindows.go

@@ -6,6 +6,7 @@
 package winutil
 
 import (
+	"errors"
 	"fmt"
 	"os/user"
 	"runtime"
@@ -13,13 +14,15 @@ import (
 
 const regBase = ``
 
-func getPolicyString(name, defval string) string { return defval }
+var ErrNoValue = errors.New("no value because registry is unavailable on this OS")
 
-func getPolicyInteger(name string, defval uint64) uint64 { return defval }
+func getPolicyString(name string) (string, error) { return "", ErrNoValue }
 
-func getRegString(name, defval string) string { return defval }
+func getPolicyInteger(name string) (uint64, error) { return 0, ErrNoValue }
 
-func getRegInteger(name string, defval uint64) uint64 { return defval }
+func getRegString(name string) (string, error) { return "", ErrNoValue }
+
+func getRegInteger(name string) (uint64, error) { return 0, ErrNoValue }
 
 func isSIDValidPrincipal(uid string) bool { return false }
 

util/winutil/winutil_windows.go

@@ -29,6 +29,9 @@ const (
 // ErrNoShell is returned when the shell process is not found.
 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
+
 // 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.
@@ -47,44 +50,44 @@ func GetDesktopPID() (uint32, error) {
 	return pid, nil
 }
 
-func getPolicyString(name, defval string) string {
+func getPolicyString(name string) (string, error) {
 	s, err := getRegStringInternal(regPolicyBase, name)
 	if err != nil {
 		// Fall back to the legacy path
-		return getRegString(name, defval)
+		return getRegString(name)
 	}
-	return s
+	return s, err
 }
 
-func getPolicyInteger(name string, defval uint64) uint64 {
+func getRegString(name string) (string, error) {
+	s, err := getRegStringInternal(regBase, name)
+	if err != nil {
+		return "", err
+	}
+	return s, err
+}
+
+func getPolicyInteger(name string) (uint64, error) {
 	i, err := getRegIntegerInternal(regPolicyBase, name)
 	if err != nil {
 		// Fall back to the legacy path
-		return getRegInteger(name, defval)
+		return getRegInteger(name)
 	}
-	return i
+	return i, err
 }
 
-func getRegString(name, defval string) string {
-	s, err := getRegStringInternal(regBase, name)
-	if err != nil {
-		return defval
-	}
-	return s
-}
-
-func getRegInteger(name string, defval uint64) uint64 {
+func getRegInteger(name string) (uint64, error) {
 	i, err := getRegIntegerInternal(regBase, name)
 	if err != nil {
-		return defval
+		return 0, err
 	}
-	return i
+	return i, err
 }
 
 func getRegStringInternal(subKey, name string) (string, error) {
 	key, err := registry.OpenKey(registry.LOCAL_MACHINE, subKey, registry.READ)
 	if err != nil {
-		if err != registry.ErrNotExist {
+		if err != ErrNoValue {
 			log.Printf("registry.OpenKey(%v): %v", subKey, err)
 		}
 		return "", err
@@ -93,7 +96,7 @@ func getRegStringInternal(subKey, name string) (string, error) {
 
 	val, _, err := key.GetStringValue(name)
 	if err != nil {
-		if err != registry.ErrNotExist {
+		if err != ErrNoValue {
 			log.Printf("registry.GetStringValue(%v): %v", name, err)
 		}
 		return "", err
@@ -114,7 +117,7 @@ func GetRegStrings(name string, defval []string) []string {
 func getRegStringsInternal(subKey, name string) ([]string, error) {
 	key, err := registry.OpenKey(registry.LOCAL_MACHINE, subKey, registry.READ)
 	if err != nil {
-		if err != registry.ErrNotExist {
+		if err != ErrNoValue {
 			log.Printf("registry.OpenKey(%v): %v", subKey, err)
 		}
 		return nil, err
@@ -123,7 +126,7 @@ func getRegStringsInternal(subKey, name string) ([]string, error) {
 
 	val, _, err := key.GetStringsValue(name)
 	if err != nil {
-		if err != registry.ErrNotExist {
+		if err != ErrNoValue {
 			log.Printf("registry.GetStringValue(%v): %v", name, err)
 		}
 		return nil, err
@@ -154,7 +157,7 @@ func DeleteRegValue(name string) error {
 
 func deleteRegValueInternal(subKey, name string) error {
 	key, err := registry.OpenKey(registry.LOCAL_MACHINE, subKey, registry.SET_VALUE)
-	if err == registry.ErrNotExist {
+	if err == ErrNoValue {
 		return nil
 	}
 	if err != nil {
@@ -164,7 +167,7 @@ func deleteRegValueInternal(subKey, name string) error {
 	defer key.Close()
 
 	err = key.DeleteValue(name)
-	if err == registry.ErrNotExist {
+	if err == ErrNoValue {
 		err = nil
 	}
 	return err
@@ -173,7 +176,7 @@ func deleteRegValueInternal(subKey, name string) error {
 func getRegIntegerInternal(subKey, name string) (uint64, error) {
 	key, err := registry.OpenKey(registry.LOCAL_MACHINE, subKey, registry.READ)
 	if err != nil {
-		if err != registry.ErrNotExist {
+		if err != ErrNoValue {
 			log.Printf("registry.OpenKey(%v): %v", subKey, err)
 		}
 		return 0, err
@@ -182,7 +185,7 @@ func getRegIntegerInternal(subKey, name string) (uint64, error) {
 
 	val, _, err := key.GetIntegerValue(name)
 	if err != nil {
-		if err != registry.ErrNotExist {
+		if err != ErrNoValue {
 			log.Printf("registry.GetIntegerValue(%v): %v", name, err)
 		}
 		return 0, err