Add upb_Message_NextSerializableField and unify field presence checks.

PiperOrigin-RevId: 968809631

Author: adazh

upb/message/internal/accessors.h

@@ -547,6 +547,44 @@ upb_Message_GetOrCreateMutableMessage(struct upb_Message* msg,
   return sub_message;
 }
 
+UPB_INLINE bool UPB_PRIVATE(_upb_Message_FieldIsSet)(
+    const struct upb_Message* msg, const upb_MiniTableField* f) {
+  if (f->presence == 0) {
+    // Proto3 presence or map/array.
+    const void* mem = UPB_PTR_AT(msg, f->UPB_PRIVATE(offset), void);
+    switch (UPB_PRIVATE(_upb_MiniTableField_GetRep)(f)) {
+      case kUpb_FieldRep_1Byte: {
+        char ch;
+        memcpy(&ch, mem, 1);
+        return ch != 0;
+      }
+      case kUpb_FieldRep_4Byte: {
+        uint32_t u32;
+        memcpy(&u32, mem, 4);
+        return u32 != 0;
+      }
+      case kUpb_FieldRep_8Byte: {
+        uint64_t u64;
+        memcpy(&u64, mem, 8);
+        return u64 != 0;
+      }
+      case kUpb_FieldRep_StringView: {
+        const upb_StringView* str = (const upb_StringView*)mem;
+        return str->size != 0;
+      }
+      default:
+        UPB_UNREACHABLE();
+    }
+  } else if (UPB_PRIVATE(_upb_MiniTableField_HasHasbit)(f)) {
+    // Proto2 presence: hasbit.
+    return UPB_PRIVATE(_upb_Message_GetHasbit)(msg, f);
+  } else {
+    // Field is in a oneof.
+    return UPB_PRIVATE(_upb_Message_GetOneofCase)(msg, f) ==
+           upb_MiniTableField_Number(f);
+  }
+}
+
 UPB_API_INLINE upb_StringView
 upb_Message_GetString(const struct upb_Message* msg,
                       const upb_MiniTableField* f, upb_StringView default_val) {

upb/message/message.c

@@ -257,3 +257,22 @@ void upb_Message_Freeze(upb_Message* msg, const upb_MiniTable* m) {
     }
   }
 }
+
+bool upb_Message_NextSerializableField(const upb_Message* msg,
+                                       const upb_MiniTable* mt,
+                                       const upb_MiniTableField** f,
+                                       uintptr_t* iter) {
+  const size_t field_count = upb_MiniTable_FieldCount(mt);
+  uintptr_t i = *iter;
+  while (i < field_count) {
+    const upb_MiniTableField* field = upb_MiniTable_GetFieldByIndex(mt, i);
+    if (UPB_PRIVATE(_upb_Message_FieldIsSet)(msg, field)) {
+      *f = field;
+      *iter = i + 1;
+      return true;
+    }
+    i++;
+  }
+  *iter = i;
+  return false;
+}

upb/message/message.h

@@ -21,6 +21,7 @@
 #include "upb/message/internal/message.h"
 #include "upb/message/internal/types.h"
 #include "upb/mini_table/extension.h"
+#include "upb/mini_table/field.h"
 #include "upb/mini_table/message.h"
 
 // Must be last.
@@ -83,6 +84,29 @@ UPB_INLINE bool UPB_PRIVATE(_upb_Message_NextExtensionReverse)(
     const struct upb_Message* msg, const upb_MiniTableExtension** out_e,
     upb_MessageValue* out_v, uintptr_t* iter);
 
+#define kUpb_Message_SerializableFieldBegin 0
+
+// Iterates over all fields in the message that are set/present.
+//
+// NOTE: Unset/NULL repeated fields and maps are not considered set and will be
+// ignored. However, allocated repeated fields and maps (non-NULL pointer) with
+// zero elements are considered set/present and will be returned by this
+// iterator. Callers that need to skip empty collections should check their
+// sizes explicitly.
+//
+// To start iterating, set `iter = kUpb_Message_SerializableFieldBegin`.
+// Returns true if a serializable field was found, sets `*f` to that field,
+// and updates `*iter`. Returns false when iteration is complete.
+//
+//   const upb_MiniTableField* f;
+//   uintptr_t iter = kUpb_Message_SerializableFieldBegin;
+//   while (upb_Message_NextSerializableField(msg, mt, &f, &iter)) {
+//     // ...
+//   }
+UPB_NODISCARD UPB_API bool upb_Message_NextSerializableField(
+    const upb_Message* msg, const upb_MiniTable* mt,
+    const upb_MiniTableField** f, uintptr_t* iter);
+
 // Mark a message and all of its descendents as frozen/immutable.
 UPB_API void upb_Message_Freeze(upb_Message* msg, const upb_MiniTable* m);