Fix postProcessors having value semantics when unset

Currently, if postProcessors was empty, it would be set to a new
pointer value. But that would only apply for the current Configurable,
so if you passed a Configurable by value to a function and then
that function added a postProcessor, the caller wouldn't see that.

Configurable properties in general have a confusing mix of pointer
and value semantics (thanks go...), this fixes one part of it but in
followup cls I'll try to make it consistent.

Bug: 362579941
Test: m nothing --no-skip-soong-tests
Change-Id: Ib33497768be3af45233cf1a682adffb28ee64bfd

proptools/configurable.go

@@ -502,8 +502,12 @@ func (c *Configurable[T]) AddPostProcessor(p func(T) T) {
 	// See Configurable.evaluate for more details on the postProcessors algorithm
 	// and data structure.
 	num_links := c.inner.numLinks()
-	if c.postProcessors == nil || len(*c.postProcessors) == 0 {
+	if c.postProcessors == nil {
-		c.postProcessors = &[][]postProcessor[T]{{{
+		var nilCases []ConfigurableCase[T]
+		c.initialize(nil, "", nil, nilCases)
+	}
+	if len(*c.postProcessors) == 0 {
+		*c.postProcessors = [][]postProcessor[T]{{{
 			f:     p,
 			start: 0,
 			end:   num_links,

proptools/configurable_test.go

@@ -43,6 +43,24 @@ func TestPostProcessor(t *testing.T) {
 	}
 }
 
+func TestPostProcessorWhenPassedToHelperFunction(t *testing.T) {
+	prop := NewConfigurable[[]string](nil, nil)
+	prop.AppendSimpleValue([]string{"a"})
+	prop.AppendSimpleValue([]string{"b"})
+
+	helper := func(p Configurable[[]string]) {
+		p.AddPostProcessor(addToElements("1"))
+	}
+
+	helper(prop)
+
+	expected := []string{"a1", "b1"}
+	x := prop.Get(&configurableEvalutorForTesting{})
+	if !reflect.DeepEqual(x.Get(), expected) {
+		t.Fatalf("Expected %v, got %v", expected, x.Get())
+	}
+}
+
 func addToElements(s string) func([]string) []string {
 	return func(arr []string) []string {
 		for i := range arr {