copy_from_query drops headers if any postgis NOTICE/LOCATION messages produced

when using Connection.copy_from_query I had a statement which eventually resolved to something like:

COPY (
    select 1
    from field_year_crop_variety_layer fycvl
    cross join filters
    join field_meta fm on
        fm.field_id = fycvl.field_id and
        (filters.filter_geometry is null or st_intersects(filters.filter_geometry, fm.centroid))
    order by fycvl.field_id, fycvl.harvest_year, fycvl.layer_id, fycvl.crop, fycvl.variety
    limit '100000'::int
) TO STDOUT (FORMAT 'csv', HEADER True)

where filter_geometry is a geometry, and fm.centroid is a geography.

and no matter what I did asyncpg would not return the headers. It wasn't until I broke it down and ran it from psql that I noticed the output:

field_id,harvest_year,layer_id,crop,variety,is_precision_data,date_period,avg_date,data_is_missing,missing_data_message,crop_variety_layer_area_m2,geometry,computed_at
NOTICE:  00000: Coordinate values were coerced into range [-180 -90, 180 90] for GEOGRAPHY
LOCATION:  geography_from_geometry, geography_inout.c:510
NOTICE:  00000: Coordinate values were coerced into range [-180 -90, 180 90] for GEOGRAPHY
LOCATION:  geography_from_geometry, geography_inout.c:510
143011,2017,0,corn,unknown,t,"[2017-04-25,2017-04-26)",2017-04-24,f,,7240.833,,1752130305.616610

sure enough, if I fixed the bug (casting fm.centroid to a geometry) the headers appeared. This seems like a bug in asyncpg when interpreting the data returned.

Connection.copy_from_query から始め、PostGIS の NOTICE/LOCATION メッセージを有効にした状態で、提供された COPY クエリを PostgreSQL に対して再現します。これらのメッセージが COPY の出力と併せてどのように処理されるかを追跡します。このような通知が生成された場合に、返されるデータが CSV ヘッダーを保持していれば完了です。