geomfn: prevent smaller densifyFrac for ST_{Frechet,Hausdorff}Distance

Release justification: low-risk change to existing functionality

Release note (sql change): Prevent densifyFracs < 1e-6 for
ST_FrechetDistance and ST_HausdorffDistance to protect panics and out of
memory errors.

pkg/geo/geomfn/distance.go

@@ -739,14 +739,8 @@ func FrechetDistanceDensify(a, b geo.Geometry, densifyFrac float64) (*float64, e
 	if a.SRID() != b.SRID() {
 		return nil, geo.NewMismatchingSRIDsError(a.SpatialObject(), b.SpatialObject())
 	}
-	// GEOS throws SIGFPE due to division by zero if densifyFraq is too small,
-	// so we explicitly error instead. The threshold was empirically found to be
-	// 1e-20, while 1e-19 results in "geos error: vector".
-	//
-	// Note that small values of densifyFrac are prohibitively expensive and
-	// likely to cause out-of-memory conditions.
-	if densifyFrac < 1e-19 {
-		return nil, errors.New("densifyFrac is too small")
+	if err := verifyDensifyFrac(densifyFrac); err != nil {
+		return nil, err
 	}
 	distance, err := geos.FrechetDistanceDensify(a.EWKB(), b.EWKB(), densifyFrac)
 	if err != nil {
@@ -778,6 +772,10 @@ func HausdorffDistanceDensify(a, b geo.Geometry, densifyFrac float64) (*float64,
 	if a.SRID() != b.SRID() {
 		return nil, geo.NewMismatchingSRIDsError(a.SpatialObject(), b.SpatialObject())
 	}
+	if err := verifyDensifyFrac(densifyFrac); err != nil {
+		return nil, err
+	}
+
 	distance, err := geos.HausdorffDistanceDensify(a.EWKB(), b.EWKB(), densifyFrac)
 	if err != nil {
 		return nil, err
@@ -803,3 +801,16 @@ func ClosestPoint(a, b geo.Geometry) (geo.Geometry, error) {
 	}
 	return closestPoint, nil
 }
+
+func verifyDensifyFrac(f float64) error {
+	if f < 0 || f > 1 {
+		return errors.Newf("fraction must be in range [0, 1], got %f", f)
+	}
+	// Very small densifyFrac potentially causes a SIGFPE or generate a large
+	// amount of memory. Guard against this.
+	const fracTooSmall = 1e-6
+	if f > 0 && f < fracTooSmall {
+		return errors.Newf("fraction %f is too small, must be at least %f", f, fracTooSmall)
+	}
+	return nil
+}

pkg/geo/geomfn/distance_test.go

@@ -790,11 +790,8 @@ func TestFrechetDistanceDensify(t *testing.T) {
 		densifyFrac float64
 	}{
 		// Very small densifyFrac causes a SIGFPE in GEOS due to division-by-zero.
-		// The threshold was empirically found to be at 1e-20, while at 1e-19
-		// GEOS instead returns the error "geos error: vector". We explicitly
-		// disallow <1e-19 in the code, and test that both of these error. We do
-		// not test larger values, since very small densify values consume a
-		// large amount of memory causing out-of-memory errors.
+		// We explicitly disallow <1e-6 in the code, and test that both of these error.
+		{"LINESTRING (130 0, 0 0, 0 150)", "LINESTRING (10 10, 10 150, 130 10)", 1e-7},
 		{"LINESTRING (130 0, 0 0, 0 150)", "LINESTRING (10 10, 10 150, 130 10)", 1e-19},
 		{"LINESTRING (130 0, 0 0, 0 150)", "LINESTRING (10 10, 10 150, 130 10)", 1e-20},
 		{"LINESTRING (130 0, 0 0, 0 150)", "LINESTRING (10 10, 10 150, 130 10)", 1e-100},
@@ -867,8 +864,6 @@ func TestHausdorffDistanceDensify(t *testing.T) {
 		{"LINESTRING EMPTY", "LINESTRING EMPTY", 0.5, nil},
 		{"LINESTRING (130 0, 0 0, 0 150)", "LINESTRING EMPTY", 0.5, nil},
 		{"LINESTRING EMPTY", "LINESTRING (10 10, 10 150, 130 10)", 0.5, nil},
-		{"LINESTRING (130 0, 0 0, 0 150)", "LINESTRING (10 10, 10 150, 130 10)", 1e-100, pf(14.142135623730951)},
-		{"LINESTRING (130 0, 0 0, 0 150)", "LINESTRING (10 10, 10 150, 130 10)", 1e-20, pf(14.142135623730951)},
 		{"LINESTRING (130 0, 0 0, 0 150)", "LINESTRING (10 10, 10 150, 130 10)", 0.2, pf(66)},
 		{"LINESTRING (130 0, 0 0, 0 150)", "LINESTRING (10 10, 10 150, 130 10)", 0.4, pf(56.66666666666667)},
 		{"LINESTRING (130 0, 0 0, 0 150)", "LINESTRING (10 10, 10 150, 130 10)", 0.6, pf(70)},
@@ -902,6 +897,7 @@ func TestHausdorffDistanceDensify(t *testing.T) {
 		{"LINESTRING (130 0, 0 0, 0 150)", "LINESTRING (10 10, 10 150, 130 10)", -1},
 		{"LINESTRING (130 0, 0 0, 0 150)", "LINESTRING (10 10, 10 150, 130 10)", -0.1},
 		{"LINESTRING (130 0, 0 0, 0 150)", "LINESTRING (10 10, 10 150, 130 10)", 0.0},
+		{"LINESTRING (130 0, 0 0, 0 150)", "LINESTRING (10 10, 10 150, 130 10)", 0.0000001},
 		{"LINESTRING (130 0, 0 0, 0 150)", "LINESTRING (10 10, 10 150, 130 10)", 1.1},
 	}