Support nullable string return type in reverse C interop. (#2967)

backend.native/tests/build.gradle

@@ -3285,12 +3285,12 @@ task produce_dynamic(type: DynamicKonanTest) {
     cSource = "$projectDir/produce_dynamic/simple/main.c"
     goldValue = "Hello, dynamic!\n" +
                 "Base.foo\n" +
-                "Base.fooParam: a 1\n" +
-                "Child.fooParam: b 2\n" +
-                "Child.fooParam: c 3\n" +
+                "Base.fooParam: a 1 q\n" +
+                "Child.fooParam: b 2 null\n" +
+                "Child.fooParam: c 3 null\n" +
                 "Impl1.I: d 4 Impl1\n" +
                 "Impl2.I: e 5 Impl2\n" +
-                "String is Kotlin/Native\n" +
+                "String is Kotlin/Native nullable is Hi null is OK\n" +
                 "RO property is 42\n" +
                 "RW property is 239\n" +
                 "enum100 = 100\n" +

backend.native/tests/produce_dynamic/simple/hello.kt

@@ -23,7 +23,8 @@ fun getMutable() = Data("foo")
 open class Base {
     open fun foo() = println("Base.foo")
 
-    open fun fooParam(arg0: String, arg1: Int) = println("Base.fooParam: $arg0 $arg1")
+    open fun fooParam(arg0: String, arg1: Int, arg2: String?) =
+            println("Base.fooParam: $arg0 $arg1 ${arg2 ?: "null"}")
 
     @CName(externName = "", shortName = "strangeName") fun странноеИмя() = 111
 
@@ -60,7 +61,8 @@ object Singleton {
 }
 
 class Child : Base() {
-    override fun fooParam(arg0: String, arg1: Int) = println("Child.fooParam: $arg0 $arg1")
+    override fun fooParam(arg0: String, arg1: Int, arg2: String?) =
+            println("Child.fooParam: $arg0 $arg1 ${arg2 ?: "null"}")
 
     val roProperty: Int
         get() = 42
@@ -97,7 +99,6 @@ fun useInlineClasses(ic1: IC1, ic2: IC2, ic3: IC3) {
     assert(ic2.value == "bar")
     assert(ic3.value is Base)
 }
-
 fun setCErrorHandler(callback: CPointer<CFunction<(CPointer<ByteVar>) -> Unit>>?) {
     setUnhandledExceptionHook({
         throwable: Throwable ->
@@ -111,3 +112,11 @@ fun setCErrorHandler(callback: CPointer<CFunction<(CPointer<ByteVar>) -> Unit>>?
 fun throwException() {
     throw Error("Expected error")
 }
+
+fun getNullableString(param: Int) : String? {
+    if (param == 0) {
+        return "Hi"
+    } else {
+        return null
+    }
+}

backend.native/tests/produce_dynamic/simple/main.c

@@ -25,16 +25,19 @@ int main(void) {
     const char* string1 = __ kotlin.root.getString();
     const char* string2 = __ kotlin.root.Singleton.toString(singleton);
     const char* string3 = __ kotlin.root.Data.get_string(data);
+    const char* string4 = __ kotlin.root.getNullableString(0);
+    const char* string5 = __ kotlin.root.getNullableString(1);
 
     __ kotlin.root.hello();
     __ kotlin.root.Base.foo(base);
-    __ kotlin.root.Base.fooParam(base, "a", 1);
-    __ kotlin.root.Child.fooParam(child, "b", 2);
-    __ kotlin.root.Base.fooParam(casted_child, "c", 3);
+    __ kotlin.root.Base.fooParam(base, "a", 1, "q");
+    __ kotlin.root.Child.fooParam(child, "b", 2, (char*)0);
+    __ kotlin.root.Base.fooParam(casted_child, "c", 3, (char*)0);
     __ kotlin.root.I.foo(casted_impl1, "d", 4, casted_impl1);
     __ kotlin.root.I.foo(casted_impl2, "e", 5, casted_impl2);
 
-    printf("String is %s\n", string1);
+    printf("String is %s nullable is %s null is %s\n", string1, string4,
+        string5 ? "BAD" : "OK");
 
     printf("RO property is %d\n", __ kotlin.root.Child.get_roProperty(child));
      __ kotlin.root.Child.set_rwProperty(child, 238);
@@ -58,6 +61,8 @@ int main(void) {
     __ DisposeString(string1);
     __ DisposeString(string2);
     __ DisposeString(string3);
+    __ DisposeString(string4);
+    __ DisposeString(string5);
     __ DisposeStablePointer(base.pinned);
     __ DisposeStablePointer(child.pinned);
     __ DisposeStablePointer(impl1.pinned);

runtime/src/main/cpp/KString.cpp

@@ -41,6 +41,7 @@ KStdStringInserter utf16toUtf8OrThrow(const KChar* start, const KChar* end, KStd
 
 template<utf8to16 conversion>
 OBJ_GETTER(utf8ToUtf16Impl, const char* rawString, const char* end, uint32_t charCount) {
+  if (rawString == nullptr) RETURN_OBJ(nullptr);
   ArrayHeader* result = AllocArrayInstance(theStringTypeInfo, charCount, OBJ_RESULT)->array();
   KChar* rawResult = CharArrayAddressOfElementAt(result, 0);
   auto convertResult = conversion(rawString, end, rawResult);
@@ -712,6 +713,7 @@ OBJ_GETTER(CreateStringFromUtf8, const char* utf8, uint32_t lengthBytes) {
 }
 
 char* CreateCStringFromString(KConstRef kref) {
+  if (kref == nullptr) return nullptr;
   KString kstring = kref->array();
   const KChar* utf16 = CharArrayAddressOfElementAt(kstring, 0);
   KStdString utf8;