ST_Distance doesn't return the minimum distance of two geometries.

[cuteDen-ECNU]

According to the crash libgeos/geos#977, the function related to the geometries with EMPTY seems not to be explored by the test suites.
Consider this statement:

geos/build/bin/geosop -a 'MULTIPOINT((-2 0), EMPTY)' -b ' GEOMETRYCOLLECTION(POINT(1 0),LINESTRING(0 0,1 0))' distance

# Segmentation fault (core dumped)

According to the following definition, ST_Distance returns the minimum distance between two geometries :

returns the minimum 2D Cartesian (planar) distance between two geometries

But the result of ST_Distance(a2, a1) is 3, which is not the minimum distance.
Meanwhile, the result of ST_Distance(a2, a1) is not same as ST_Distance( a1, a2).
So I believe it is a functional issue that the current test suites haven't explored.

[muchang]

Consider this statement:

SELECT ST_Distance(a2, a1), ST_Distance(a1, a2)
FROM ST_GeomFromText('MULTIPOINT((-2 0), EMPTY)') As a1
,ST_GeomFromText(' GEOMETRYCOLLECTION(POINT(1 0),LINESTRING(0 0,1 0))') As a2;
--actual{3,2}
--expected{2,2}

According to the following definition, ST_Distance returns the minimum distance between two geometries :

returns the minimum 2D Cartesian (planar) distance between two geometries

Therefore, the result of ST_Distance(a2, a1) should be the same as ST_Distance(a1, a2) which is 2.
However, ST_Distance(a2, a1) returns 3, which seems to be a bug.

If we replace the EMPTY from the MULTIPOINT with a concrete point, such as (5, 0), ST_Distance(a2, a1) behaviors correctly.
Thus, this bug might be related to the EMPTY point.

[cuteDen-ECNU]

bugtracker link: libgeos/geos#979
fix link: 13c25a8
reproduce: a7a12f5