Fix begin iterator for empty arrays.

If the underlying array for a repeated field is a nullptr (which is possible for const array access on a message field that hasn't been set) the begin iterator will currently contain garbage data, which can lead to an illegal access. This CL adds a nullptr check to `begin()`, similar to what already exists for `size()`.

PiperOrigin-RevId: 595217999

protos/repeated_field.h

@@ -136,9 +136,11 @@ class RepeatedFieldProxy
   }
 
   iterator begin() const {
-    return iterator({static_cast<upb_Message**>(
-                         const_cast<void*>(upb_Array_DataPtr(this->arr_))),
-                     this->arena_});
+    return iterator(
+        {static_cast<upb_Message**>(
+             this->arr_ ? const_cast<void*>(upb_Array_DataPtr(this->arr_))
+                        : nullptr),
+         this->arena_});
   }
   iterator end() const { return begin() + this->size(); }
   reverse_iterator rbegin() const { return reverse_iterator(end()); }

protos_generator/tests/test_generated.cc

@@ -5,6 +5,7 @@
 // license that can be found in the LICENSE file or at
 // https://developers.google.com/open-source/licenses/bsd
 
+#include <iterator>
 #include <limits>
 #include <memory>
 #include <string>
@@ -571,6 +572,19 @@ TEST(CppGeneratedCode, RepeatedFieldProxyForMessages) {
   EXPECT_EQ(test_model.mutable_child_models()->size(), 0);
 }
 
+TEST(CppGeneratedCode, EmptyRepeatedFieldProxyForMessages) {
+  ::protos::Arena arena;
+  auto test_model = ::protos::CreateMessage<TestModel>(arena);
+  EXPECT_EQ(0, test_model.child_models().size());
+  ChildModel1 child1;
+  child1.set_child_str1(kTestStr1);
+
+  EXPECT_EQ(test_model.child_models().size(), 0);
+  EXPECT_EQ(std::distance(test_model.child_models().begin(),
+                          test_model.child_models().end()),
+            0);
+}
+
 TEST(CppGeneratedCode, RepeatedFieldProxyForMessagesIndexOperator) {
   ::protos::Arena arena;
   auto test_model = ::protos::CreateMessage<TestModel>(arena);