jorisvandenbossche commented on code in PR #37467:
URL: https://github.com/apache/arrow/pull/37467#discussion_r1310391926


##########
cpp/src/arrow/array/validate.cc:
##########
@@ -713,8 +713,10 @@ struct ValidateArrayImpl {
     }
 
     // An empty list array can have 0 offsets
-    const auto required_offsets = (data.length > 0) ? data.length + 
data.offset + 1 : 0;
     const auto offsets_byte_size = data.buffers[1]->size();
+    const auto required_offsets = ((data.length > 0) || (offsets_byte_size > 
0))
+                                      ? data.length + data.offset + 1
+                                      : 0;
     if (offsets_byte_size / static_cast<int32_t>(sizeof(offset_type)) <
         required_offsets) {
       return Status::Invalid("Offsets buffer size (bytes): ", 
offsets_byte_size,

Review Comment:
   @pitrou do you know if this "empty list array can have 0 offsets" also 
applies to other types that have offsets, i.e. string types?
   
   In the current version of the PR, I am relying on ValidateFull to catch the 
error of creating a length-0 LargeString array with an offsets buffer of size 
4, while it should be 8. 
   At the moment the offsets buffer size is not checked if the length of the 
array is 0, but I changed this to actually check it if the offset buffer has 
_some_ length (so assuming that _if_ there is an offsets buffer with a non-zero 
legth, it should have a correct length, also if the array itself has length 0)
   
   But if it's difficult to make guarantees about this / to change 
ValidateFull, I can also write a dedicated test for this specific case that 
doesn't rely on calling ValidateFull.



##########
cpp/src/arrow/array/validate.cc:
##########
@@ -713,8 +713,10 @@ struct ValidateArrayImpl {
     }
 
     // An empty list array can have 0 offsets
-    const auto required_offsets = (data.length > 0) ? data.length + 
data.offset + 1 : 0;
     const auto offsets_byte_size = data.buffers[1]->size();
+    const auto required_offsets = ((data.length > 0) || (offsets_byte_size > 
0))
+                                      ? data.length + data.offset + 1
+                                      : 0;
     if (offsets_byte_size / static_cast<int32_t>(sizeof(offset_type)) <
         required_offsets) {
       return Status::Invalid("Offsets buffer size (bytes): ", 
offsets_byte_size,

Review Comment:
   @pitrou do you know if this "empty list array can have 0 offsets" also 
applies to other types that have offsets, i.e. string types?
   
   In the current version of the PR, I am relying on ValidateFull to catch the 
error of creating a length-0 LargeString array with an offsets buffer of size 
4, while it should be 8. 
   At the moment the offsets buffer size is not checked if the length of the 
array is 0, but I changed this to actually check it if the offset buffer has 
_some_ length (so assuming that _if_ there is an offsets buffer with a non-zero 
length, it should have a correct length, also if the array itself has length 0)
   
   But if it's difficult to make guarantees about this / to change 
ValidateFull, I can also write a dedicated test for this specific case that 
doesn't rely on calling ValidateFull.



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