FIX: line segments that project to same start and end point retain length

lib/cartopy/tests/test_line_string.py

@@ -80,6 +80,21 @@ def test_multi_linestring_return_type(self):
             shapely.MultiLineString(), ccrs.PlateCarree())
         assert isinstance(multi_line_string, shapely.MultiLineString)
 
+    def test_start_end_project_equal(self):
+        # Here the start and end points project to the same point in the target domain,
+        # but we should get a line that encircles the equator.
+        tgt_proj = ccrs.NorthPolarStereo(central_longitude=180)
+        src_proj = ccrs.PlateCarree(central_longitude=180)
+        start = [-180, 0]
+        end = [180, 0]
+
+        # Verify these points project to same point, otherwise the test is invalid
+        assert (tgt_proj.transform_point(*start, src_proj) ==
+                tgt_proj.transform_point(*end, src_proj))
+
+        line_string = shapely.LineString([start, end])
+        assert tgt_proj.project_geometry(line_string, src_proj).length > 7e7
+
 
 class FakeProjection(ccrs.PlateCarree):
     def __init__(self, left_offset=0, right_offset=0):

lib/cartopy/trace.pyx

@@ -327,7 +327,13 @@ cdef bool straightAndDomain(double t_start, const Point &p_start,
         along = (seg_dx*mid_dx + seg_dy*mid_dy) / seg_hypot_sq
 
         if isnan(along):
-            valid = True
+            mid_hypot_sq = mid_dx*mid_dx + mid_dy*mid_dy
+            if mid_hypot_sq > 0.0 and seg_hypot_sq == 0.0:
+               # End points project to the same location, but the midpoint is
+               # elsewhere.
+               valid = False
+            else:
+                valid = True
         else:
             valid = 0.0 < along < 1.0
             if valid:
@@ -476,6 +482,7 @@ cdef void _project_segment(double[:] src_from, double[:] src_to,
             print("   => ", t_min, "to", t_max)
             print("   => (", p_min.x, ", ", p_min.y, ") to (",
                   p_max.x, ", ", p_max.y, ")")
+            print("p_current.x", p_current.x, "p_current.y", p_current.y)
 
         if state == POINT_IN:
             lines.add_point_if_empty(p_current)