[C++] ListBuilder.Finish fails if underlying value builder is empty and .Reserve'd

[asfimport]

Here's a reproduction:

#include <arrow/builder.h>
#include <arrow/status.h>
#include <iostream>

int main() {
        arrow::ListBuilder lb(arrow::default_memory_pool(), std::unique_ptr<arrow::ArrayBuilder>(new arrow::Int32Builder()));
        lb.value_builder()->Reserve(100); // bug
        lb.Append();
        std::shared_ptr<arrow::Array> ar;
        arrow::Status st = lb.Finish(&ar);
        if (!st.ok()) {
                std::cerr << st << '\n';
                return 1;
        }
}

The output is

Invalid: Resize cannot downsize

The Resize call is made at builder_nested.h, line 115. There's a note there about ARROW-2744. Perhaps the fix is to look at capacity rather than length?

Reporter: John Jenkins / @JohnPJenkins
Assignee: Antoine Pitrou / @pitrou

PRs and other links:

• GitHub Pull Request #6595

_{Note: This issue was originally created as ARROW-7985. Please see the migration documentation for further details.}

[asfimport]

Antoine Pitrou / @pitrou:
Thanks for the report! Your suggestion is correct, I'll submit a PR.

[asfimport]

Ben Kietzman / @bkietz:
Issue resolved by pull request 6595
#6595