lib/weakhash, lib/model, cmd/syncthing: Decide if to use weakhash on startup (fixes #3938)

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3949

lib/util/utils.go

@@ -25,6 +25,17 @@ func SetDefaults(data interface{}) error {
 
 		v := tag.Get("default")
 		if len(v) > 0 {
+			if parser, ok := f.Interface().(interface {
+				ParseDefault(string) (interface{}, error)
+			}); ok {
+				val, err := parser.ParseDefault(v)
+				if err != nil {
+					panic(err)
+				}
+				f.Set(reflect.ValueOf(val))
+				continue
+			}
+
 			switch f.Interface().(type) {
 			case string:
 				f.SetString(v)

lib/util/utils_test.go

@@ -8,12 +8,21 @@ package util
 
 import "testing"
 
+type Defaulter struct {
+	Value string
+}
+
+func (Defaulter) ParseDefault(v string) (interface{}, error) {
+	return Defaulter{Value: v}, nil
+}
+
 func TestSetDefaults(t *testing.T) {
 	x := &struct {
-		A string  `default:"string"`
-		B int     `default:"2"`
-		C float64 `default:"2.2"`
-		D bool    `default:"true"`
+		A string    `default:"string"`
+		B int       `default:"2"`
+		C float64   `default:"2.2"`
+		D bool      `default:"true"`
+		E Defaulter `default:"defaulter"`
 	}{}
 
 	if x.A != "" {
@@ -24,6 +33,8 @@ func TestSetDefaults(t *testing.T) {
 		t.Errorf("float failed")
 	} else if x.D {
 		t.Errorf("bool failed")
+	} else if x.E.Value != "" {
+		t.Errorf("defaulter failed")
 	}
 
 	if err := SetDefaults(x); err != nil {
@@ -38,6 +49,8 @@ func TestSetDefaults(t *testing.T) {
 		t.Errorf("float failed")
 	} else if !x.D {
 		t.Errorf("bool failed")
+	} else if x.E.Value != "defaulter" {
+		t.Errorf("defaulter failed")
 	}
 }