False negative motion collision detection in ST_ChangeEdgeGeom

[strk]

While playing with topology edge simplification I encountered a case in which ST_ChangeEdgeGeom allows to change an edge geometry from A to B but doesn't allow it to move back from B to A.

Looking at the movement visually I can see that the movement should never be allowed.

[strk]

from orange to green is allowed, from green to orange it is not: ERROR: Edge motion collision at POINT(699288 6901218)

The collision point is correctly reported as the island you see near the bottom.

[strk]

I confirm that the result of the bogus allowance is an invalid topology (wrong face linking).

[strk]

Found the culprit: ST_SymDifference(EMPTY, POLY) returns EMPTY -- filed #1776 for this. Once that one is fixed this one should also be. We'll want a testcase for this anyway.

[strk]

r9641 fixes ST_SymDifference and r9642 regress-tests this specific use case.