Dynamic config conversion improvements

cmd/tools/gendynamicconfig/main.go

@@ -155,10 +155,11 @@ func generateType(w io.Writer, tp *settingType, prec *settingPrecedence) {
 	writeTemplatedCode(w, `
 {{ if .T.IsGeneric -}}
 type {{.P.Name}}TypedSetting[T any] setting[T, func({{.P.GoArgs}})]
+type {{.P.Name}}TypedConstrainedDefaultSetting[T any] constrainedDefaultSetting[T, func({{.P.GoArgs}})]
 
 // New{{.P.Name}}TypedSetting creates a setting that uses mapstructure to handle complex structured
-// values. The value from dynamic config will be copied over a shallow copy of 'def', which means
-// 'def' must not contain any non-nil slices, maps, or pointers.
+// values. The value from dynamic config will be _merged_ over a deep copy of 'def'. Be very careful
+// when using non-empty maps or slices as defaults, the result may not be what you want.
 func New{{.P.Name}}TypedSetting[T any](key Key, def T, description string) {{.P.Name}}TypedSetting[T] {
 	s := {{.P.Name}}TypedSetting[T]{
 		key:         key,
@@ -183,10 +184,10 @@ func New{{.P.Name}}TypedSettingWithConverter[T any](key Key, convert func(any) (
 }
 
 // New{{.P.Name}}TypedSettingWithConstrainedDefault creates a setting with a compound default value.
-func New{{.P.Name}}TypedSettingWithConstrainedDefault[T any](key Key, convert func(any) (T, error), cdef []TypedConstrainedValue[T], description string) {{.P.Name}}TypedSetting[T] {
-	s := {{.P.Name}}TypedSetting[T]{
+func New{{.P.Name}}TypedSettingWithConstrainedDefault[T any](key Key, convert func(any) (T, error), cdef []TypedConstrainedValue[T], description string) {{.P.Name}}TypedConstrainedDefaultSetting[T] {
+	s := {{.P.Name}}TypedConstrainedDefaultSetting[T]{
 		key:         key,
-		cdef:        &cdef,
+		cdef:        cdef,
 		convert:     convert,
 		description: description,
 	}
@@ -201,6 +202,13 @@ func (s {{.P.Name}}TypedSetting[T]) Validate(v any) error {
 	return err
 }
 
+func (s {{.P.Name}}TypedConstrainedDefaultSetting[T]) Key() Key               { return s.key }
+func (s {{.P.Name}}TypedConstrainedDefaultSetting[T]) Precedence() Precedence { return Precedence{{.P.Name}} }
+func (s {{.P.Name}}TypedConstrainedDefaultSetting[T]) Validate(v any) error {
+	_, err := s.convert(v)
+	return err
+}
+
 func (s {{.P.Name}}TypedSetting[T]) WithDefault(v T) {{.P.Name}}TypedSetting[T] {
 	newS := s
 	newS.def = v
@@ -225,6 +233,22 @@ func (s {{.P.Name}}TypedSetting[T]) Get(c *Collection) TypedPropertyFnWith{{.P.N
 			c,
 			s.key,
 			s.def,
+			s.convert,
+			prec,
+		)
+	}
+}
+
+{{if eq .P.Name "Global" -}}
+func (s {{.P.Name}}TypedConstrainedDefaultSetting[T]) Get(c *Collection) TypedPropertyFn[T] {
+{{- else -}}
+func (s {{.P.Name}}TypedConstrainedDefaultSetting[T]) Get(c *Collection) TypedPropertyFnWith{{.P.Name}}Filter[T] {
+{{- end}}
+	return func({{.P.GoArgs}}) T {
+		prec := {{.P.Expr}}
+		return matchAndConvertWithConstrainedDefault(
+			c,
+			s.key,
 			s.cdef,
 			s.convert,
 			prec,
@@ -246,7 +270,7 @@ func (s {{.P.Name}}TypedSetting[T]) Subscribe(c *Collection) TypedSubscribableWi
 	return func({{.P.GoArgs}}, callback func(T)) (T, func()) {
 {{- end}}
 		prec := {{.P.Expr}}
-		return subscribe(c, s.key, s.def, s.cdef, s.convert, prec, callback)
+		return subscribe(c, s.key, s.def, s.convert, prec, callback)
 	}
 }
 
@@ -260,6 +284,16 @@ func (s {{.P.Name}}TypedSetting[T]) dispatchUpdate(c *Collection, sub any, cvs [
 	)
 }
 
+func (s {{.P.Name}}TypedConstrainedDefaultSetting[T]) dispatchUpdate(c *Collection, sub any, cvs []ConstrainedValue) {
+	dispatchUpdate(
+		c,
+		s.key,
+		s.convert,
+		sub.(*subscription[T]),
+		cvs,
+	)
+}
+
 {{if eq .P.Name "Global" -}}
 func GetTypedPropertyFn[T any](value T) TypedPropertyFn[T] {
 {{- else -}}
@@ -271,12 +305,13 @@ func GetTypedPropertyFnFilteredBy{{.P.Name}}[T any](value T) TypedPropertyFnWith
 }
 {{- else -}}
 type {{.P.Name}}{{.T.Name}}Setting = {{.P.Name}}TypedSetting[{{.T.GoType}}]
+type {{.P.Name}}{{.T.Name}}ConstrainedDefaultSetting = {{.P.Name}}TypedConstrainedDefaultSetting[{{.T.GoType}}]
 
 func New{{.P.Name}}{{.T.Name}}Setting(key Key, def {{.T.GoType}}, description string) {{.P.Name}}{{.T.Name}}Setting {
 	return New{{.P.Name}}TypedSettingWithConverter[{{.T.GoType}}](key, convert{{.T.Name}}, def, description)
 }
 
-func New{{.P.Name}}{{.T.Name}}SettingWithConstrainedDefault(key Key, cdef []TypedConstrainedValue[{{.T.GoType}}], description string) {{.P.Name}}{{.T.Name}}Setting {
+func New{{.P.Name}}{{.T.Name}}SettingWithConstrainedDefault(key Key, cdef []TypedConstrainedValue[{{.T.GoType}}], description string) {{.P.Name}}{{.T.Name}}ConstrainedDefaultSetting {
 	return New{{.P.Name}}TypedSettingWithConstrainedDefault[{{.T.GoType}}](key, convert{{.T.Name}}, cdef, description)
 }
 

common/dynamicconfig/client.go

@@ -47,6 +47,11 @@ type (
 		// Note that GetValue is called very often! You should not synchronously call out to an
 		// external system. Instead you should keep a set of all configured values, refresh it
 		// periodically or when notified, and only do in-memory lookups inside of GetValue.
+		//
+		// Implementations should prefer to return the same slice in response to the same key
+		// as long as the value hasn't changed. Value conversions are cached using weak
+		// pointers into the returned slice, so new slices will result in unnecessary calls to
+		// conversion functions.
 		GetValue(key Key) []ConstrainedValue
 	}