Initialize mini table subs, so that upb_MiniTable_Build creates stable unlinked table.

PiperOrigin-RevId: 479630135

upb/mini_table.c

@@ -768,7 +768,9 @@ static const char* upb_MtDecoder_ParseModifier(upb_MtDecoder* d,
 
 static void upb_MtDecoder_AllocateSubs(upb_MtDecoder* d, uint32_t sub_count) {
   size_t subs_bytes = sizeof(*d->table->subs) * sub_count;
-  d->table->subs = upb_Arena_Malloc(d->arena, subs_bytes);
+  void* subs = upb_Arena_Malloc(d->arena, subs_bytes);
+  memset(subs, 0, subs_bytes);
+  d->table->subs = subs;
   upb_MtDecoder_CheckOutOfMemory(d, d->table->subs);
 }
 

upb/mini_table_test.cc

@@ -236,6 +236,22 @@ TEST(MiniTableEnumTest, Enum) {
   }
 }
 
+TEST_P(MiniTableTest, SubsInitializedToNull) {
+  upb::Arena arena;
+  upb::MtDataEncoder e;
+  // Create mini table with 2 message fields.
+  ASSERT_TRUE(e.StartMessage(0));
+  ASSERT_TRUE(e.PutField(kUpb_FieldType_Message, 15, 0));
+  ASSERT_TRUE(e.PutField(kUpb_FieldType_Message, 16, 0));
+  upb::Status status;
+  upb_MiniTable* table = upb_MiniTable_Build(
+      e.data().data(), e.data().size(), GetParam(), arena.ptr(), status.ptr());
+  ASSERT_NE(nullptr, table);
+  EXPECT_EQ(table->field_count, 2);
+  EXPECT_EQ(table->subs[0].submsg, nullptr);
+  EXPECT_EQ(table->subs[1].submsg, nullptr);
+}
+
 TEST(MiniTableEnumTest, PositiveAndNegative) {
   upb::Arena arena;
   upb::MtDataEncoder e;