TODO: M gets dropped from pickle serialization of LinearRing types

[mwtoews]

On shapely main with #1808 to enable M and ZM in WKB outputs, one "TODO" item needs to be resolved for test_pickle_m. To demonstrate the issue:

import pickle
import shapely

assert shapely.geos_version[0:2] >= (3, 12), shapely.geos_version_string

geom = shapely.from_wkt("LINEARRING M (0 0 1, 1 0 2, 1 1 3, 0 1 2, 0 0 1)")
pickled = pickle.dumps(geom)
actual = pickle.loads(pickled)

print(f"{geom!r} != {actual!r}")
# <LINEARRING M (0 0 1, 1 0 2, 1 1 3, 0 1 2, 0 0 1)> != <LINEARRING (0 0, 1 0, 1 1, 0 1, 0 0)>

I recall there is special handling for LinearRing types in shapely's src/* C code, as there is no WKB equivalent for LinearRing. It will need to be updated to handle M and ZM coordinate types.

[jorisvandenbossche]

The special handling is currently not in the C code but here:

shapely/shapely/geometry/polygon.py

Lines 16 to 22 in e157995

	def _unpickle_linearring(wkb):
	linestring = shapely.from_wkb(wkb)
	srid = shapely.get_srid(linestring)
	linearring = shapely.linearrings(shapely.get_coordinates(linestring))
	if srid:
	linearring = shapely.set_srid(linearring, srid)
	return linearring

shapely/shapely/geometry/polygon.py

Lines 113 to 116 in e157995

	def __reduce__(self):
	"""WKB doesn't differentiate between LineString and LinearRing so we
	need to move the coordinate sequence into the correct geometry type"""
	return (_unpickle_linearring, (shapely.to_wkb(self, include_srid=True),))

We should probably add a helper function in C that converts a LineString to a LinearRing by just getting the CoordSequence from the line and put that in a LinearRing, so that it preserves the full coord seq and its dimensionality.