Support empty srcs for configurable attrs

Test: mixed build linkerconfig
Change-Id: I1ee56f7fa46ce2971ecae6d2ca9258631a32f46a

bp2build/configurability.go

@@ -80,32 +80,55 @@ func getLabelListValues(list bazel.LabelListAttribute) (reflect.Value, []selects
 	if !list.HasConfigurableValues() {
 		return value, []selects{}
 	}
+	var ret []selects
 
 	archSelects := map[string]reflect.Value{}
 	for arch, selectKey := range bazel.PlatformArchMap {
-		archSelects[selectKey] = reflect.ValueOf(list.GetValueForArch(arch).Includes)
+		if use, value := labelListSelectValue(selectKey, list.GetValueForArch(arch)); use {
+			archSelects[selectKey] = value
+		}
+	}
+	if len(archSelects) > 0 {
+		ret = append(ret, archSelects)
 	}
 
 	osSelects := map[string]reflect.Value{}
-	osArchSelects := make([]selects, 0)
+	osArchSelects := []selects{}
 	for _, os := range android.SortedStringKeys(bazel.PlatformOsMap) {
 		selectKey := bazel.PlatformOsMap[os]
-		osSelects[selectKey] = reflect.ValueOf(list.GetOsValueForTarget(os).Includes)
-		archSelects := make(map[string]reflect.Value)
+		if use, value := labelListSelectValue(selectKey, list.GetOsValueForTarget(os)); use {
+			osSelects[selectKey] = value
+		}
+		selects := make(map[string]reflect.Value)
 		// TODO(b/187530594): Should we also check arch=CONDITIOSN_DEFAULT? (not in AllArches)
 		for _, arch := range bazel.AllArches {
 			target := os + "_" + arch
 			selectKey := bazel.PlatformTargetMap[target]
-			archSelects[selectKey] = reflect.ValueOf(list.GetOsArchValueForTarget(os, arch).Includes)
+			if use, value := labelListSelectValue(selectKey, list.GetOsArchValueForTarget(os, arch)); use {
+				selects[selectKey] = value
+			}
+		}
+		if len(selects) > 0 {
+			osArchSelects = append(osArchSelects, selects)
 		}
-		osArchSelects = append(osArchSelects, archSelects)
 	}
+	if len(osSelects) > 0 {
+		ret = append(ret, osSelects)
+	}
+	ret = append(ret, osArchSelects...)
 
-	var selects []selects
-	selects = append(selects, archSelects)
-	selects = append(selects, osSelects)
-	selects = append(selects, osArchSelects...)
-	return value, selects
+	return value, ret
+}
+
+func labelListSelectValue(selectKey string, list bazel.LabelList) (bool, reflect.Value) {
+	if selectKey == bazel.ConditionsDefaultSelectKey || len(list.Includes) > 0 {
+		return true, reflect.ValueOf(list.Includes)
+	} else if len(list.Excludes) > 0 {
+		// if there is still an excludes -- we need to have an empty list for this select & use the
+		// value in conditions default Includes
+		return true, reflect.ValueOf([]string{})
+	}
+	return false, reflect.Zero(reflect.TypeOf([]string{}))
 }
 
 // prettyPrintAttribute converts an Attribute to its Bazel syntax. May contain