Fix upb_MiniTable_GetOneof(miniTable, field) to work correctly if `field` is the very first field in the proto.

PiperOrigin-RevId: 580285127

upb/mini_table/message.c

@@ -55,7 +55,7 @@ const upb_MiniTableField* upb_MiniTable_GetOneof(const upb_MiniTable* m,
   }
   const upb_MiniTableField* ptr = &m->fields[0];
   const upb_MiniTableField* end = &m->fields[m->field_count];
-  while (++ptr < end) {
+  for (; ptr < end; ptr++) {
     if (ptr->presence == (*f).presence) {
       return ptr;
     }

upb/test/BUILD

@@ -239,6 +239,7 @@ cc_test(
     ],
     deps = [
         ":empty_upb_proto_reflection",
+        ":proto3_test_upb_proto",
         ":test_messages_proto2_upb_minitable",
         ":test_upb_proto",
         "@com_google_googletest//:gtest_main",

upb/test/proto3_test.proto

@@ -27,3 +27,13 @@ message TestMessage3 {
   optional TestMessage3 msg = 5;
   repeated TestMessage3 r_msg = 6;
 }
+
+// See the InitialFieldOneOf test in test_mini_table_oneof.cc.
+message TestOneOfInitialField {
+  oneof oneof_field {
+    int32 a = 1;
+    uint32 b = 2;
+  }
+
+  float c = 3;
+}

upb/test/test_mini_table_oneof.cc

@@ -9,6 +9,7 @@
 #include "google/protobuf/test_messages_proto2.upb_minitable.h"
 #include "upb/mini_table/field.h"
 #include "upb/mini_table/message.h"
+#include "upb/test/proto3_test.upb.h"
 
 // Must be last.
 #include "upb/port/def.inc"
@@ -29,6 +30,15 @@ TEST(MiniTableOneofTest, OneOfIteratorProto2) {
   } while (upb_MiniTable_NextOneofField(google_protobuf_table, &ptr));
 }
 
+TEST(MiniTableOneofTest, InitialFieldOneOf) {
+  const upb_MiniTable* table = &upb__test__TestOneOfInitialField_msg_init;
+  const upb_MiniTableField* field = upb_MiniTable_FindFieldByNumber(table, 1);
+  ASSERT_TRUE(field != nullptr);
+
+  const upb_MiniTableField* ptr = upb_MiniTable_GetOneof(table, field);
+  EXPECT_TRUE(ptr == field);
+}
+
 TEST(MiniTableOneofTest, InitialFieldNotOneOf) {
   constexpr int test_field_number = 1;  // optional int that is not a oneof
   const upb_MiniTable* google_protobuf_table =