Fixed reflection setting string not changing size field.

Change-Id: I3c3e88a79667d1733b6c47e43b55d8b2604ca8e2
Tested: on Linux.

src/reflection.cpp

@@ -287,14 +287,17 @@ void SetString(const reflection::Schema &schema, const std::string &val,
                       const String *str, std::vector<uint8_t> *flatbuf,
                       const reflection::Object *root_table) {
   auto delta = static_cast<int>(val.size()) - static_cast<int>(str->Length());
-  auto start = static_cast<uoffset_t>(reinterpret_cast<const uint8_t *>(str) -
-                                      flatbuf->data() +
-                                      sizeof(uoffset_t));
+  auto str_start = static_cast<uoffset_t>(
+                     reinterpret_cast<const uint8_t *>(str) - flatbuf->data());
+  auto start = str_start + sizeof(uoffset_t);
   if (delta) {
     // Clear the old string, since we don't want parts of it remaining.
     memset(flatbuf->data() + start, 0, str->Length());
     // Different size, we must expand (or contract).
     ResizeContext(schema, start, delta, flatbuf, root_table);
+    // Set the new length.
+    WriteScalar(flatbuf->data() + str_start,
+                static_cast<uoffset_t>(val.size()));
   }
   // Copy new data. Safe because we created the right amount of space.
   memcpy(flatbuf->data() + start, val.c_str(), val.size() + 1);

tests/test.cpp

@@ -404,6 +404,11 @@ void ReflectionTest(uint8_t *flatbuf, size_t length) {
   rtestarrayofstring->MutateOffset(2, string_ptr);
   TEST_EQ_STR(rtestarrayofstring->Get(0)->c_str(), "bob");
   TEST_EQ_STR(rtestarrayofstring->Get(2)->c_str(), "hank");
+  // Test integrity of all resize operations above.
+  flatbuffers::Verifier resize_verifier(
+        reinterpret_cast<const uint8_t *>(resizingbuf.data()),
+        resizingbuf.size());
+  TEST_EQ(VerifyMonsterBuffer(resize_verifier), true);
   // As an additional test, also set it on the name field.
   // Note: unlike the name change above, this just overwrites the offset,
   // rather than changing the string in-place.