Assert a partitioned bundle directory holds exactly one file per partition

Anonymous LIST is denied on the bucket, so a static client cannot discover parts: it constructs tile={t}/data_0.parquet and reads whatever is there. A second part is therefore not a performance detail, it is missing data that nobody is told about — and a gate that globs **/*.parquet reads all six parts and sees nothing wrong. Every partitioned thing the bundle writes goes through this.

Usage

app_one_file_per_partition(dir, name = basename(dir))

Arguments

dir

a partitioned directory (tile={t}/ beneath it)

name

what to call it in the error

Value

the number of partitions, invisibly; errors on any partition with a file count other than one