Re: [PR] GH-51029: [C++] Fix MapArray validation with unknown null count [arrow]

["AnuragRaut08 (via GitHub)" <gi...@apache.org>]

AnuragRaut08 commented on code in PR #51093:
URL: https://github.com/apache/arrow/pull/51093#discussion_r3905873729


##########
cpp/src/arrow/array/array_nested.cc:
##########
@@ -896,13 +896,19 @@ Status MapArray::ValidateChildData(
   if (pair_data->type->id() != Type::STRUCT) {
     return Status::Invalid("Map array child array should have struct type");
   }
-  if (pair_data->MayHaveNulls()) {
+  // Use GetNullCount() rather than MayHaveNulls() so that an all-valid
+  // validity bitmap (null_count == kUnknownNullCount but every bit set)
+  // is not mistakenly treated as containing nulls.  MayHaveNulls() returns
+  // true whenever a bitmap is present and null_count != 0, which includes
+  // kUnknownNullCount (-1); GetNullCount() actually counts the bits and
+  // caches the result.  See GH-51029.
+  if (pair_data->GetNullCount() != 0) {

Review Comment:
   Done, removed the lengthy comment.



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