From e6ea44c9119b0f5976ea7b335395753833fc382d Mon Sep 17 00:00:00 2001
From: Protobuf Team Bot <protobuf-github-bot@google.com>
Date: Tue, 7 Nov 2023 13:38:16 -0800
Subject: [PATCH] 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          |  2 +-
 upb/test/BUILD                    |  1 +
 upb/test/proto3_test.proto        | 10 ++++++++++
 upb/test/test_mini_table_oneof.cc | 10 ++++++++++
 4 files changed, 22 insertions(+), 1 deletion(-)

diff --git a/upb/mini_table/message.c b/upb/mini_table/message.c
index 56f23014a3..28635cdb0a 100644
--- a/upb/mini_table/message.c
+++ b/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;
     }
diff --git a/upb/test/BUILD b/upb/test/BUILD
index b101237ddf..93661f452e 100644
--- a/upb/test/BUILD
+++ b/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",
diff --git a/upb/test/proto3_test.proto b/upb/test/proto3_test.proto
index 28bc4d347d..774538f42b 100644
--- a/upb/test/proto3_test.proto
+++ b/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;
+}
diff --git a/upb/test/test_mini_table_oneof.cc b/upb/test/test_mini_table_oneof.cc
index 3c0ecae0f5..850d57186e 100644
--- a/upb/test/test_mini_table_oneof.cc
+++ b/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 =