scovich commented on code in PR #7987:
URL: https://github.com/apache/arrow-rs/pull/7987#discussion_r2232860635
##########
parquet-variant/src/builder.rs:
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@@ -1216,24 +1211,45 @@ impl<'a> ListBuilder<'a> {
/// Finalizes this list and appends it to its parent, which otherwise
remains unmodified.
pub fn finish(mut self) {
- let data_size = self.buffer.offset();
+ let buffer = self.parent_state.buffer();
+
+ let data_size = buffer.offset() - self.parent_value_offset_base;
+
let num_elements = self.offsets.len();
let is_large = num_elements > u8::MAX as usize;
let offset_size = int_size(data_size);
- // Get parent's buffer
- let parent_buffer = self.parent_state.buffer();
- let starting_offset = parent_buffer.offset();
+ let starting_offset = self.parent_value_offset_base;
+
+ let header_size = 1 + // header
+ if is_large { 4 } else { 1 } + // is_large
+ (self.offsets.len() + 1) * offset_size as usize; // offsets and
data size
+ // Calculated header size becomes a hint; being wrong only risks extra
allocations.
+ // Make sure to reserve enough capacity to handle the extra bytes
we'll truncate.
Review Comment:
> When `header_size` will be incorrect?
The size if calculated separately, and then the actual bytes are appended.
That opens up a bug surface -- any time the two disagree, `header_size` will be
wrong. If the code directly relied on the size being correct, e.g. because we
allocate that many bytes and then index them, we could have produce a bad
variant value (either because there's an extra run of inserted bytes, or
because of a buffer overflow while indexing. But because the calculated size is
only a capacity hint for the vec, the cost of being wrong is very low.
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