Prevent degenerate ways caused by deleting a corner of a triangle

re: #1219
re: openstreetmap/iD#10003

modules/modes/DrawAreaMode.js

@@ -150,8 +150,7 @@ export class DrawAreaMode extends AbstractMode {
     // If any issues, revert back to how things were before we started.
     const graph = editor.stable.graph;
     const drawWay = this.drawWayID && graph.hasEntity(this.drawWayID);
-    const length = drawWay?.nodes?.length || 0;
-    if (length < 4) {
+    if (!drawWay || drawWay.isDegenerate()) {
       if (DEBUG) {
         console.log('DrawAreaMode: draw way invalid, rolling back');  // eslint-disable-line no-console
       }

modules/modes/DrawLineMode.js

@@ -186,8 +186,7 @@ export class DrawLineMode extends AbstractMode {
     // If any issues, revert back to how things were before we started.
     const graph = editor.stable.graph;
     const drawWay = this.drawWayID && graph.hasEntity(this.drawWayID);
-    const length = drawWay?.nodes?.length || 0;
-    if (length < 2 || this.firstNodeID === this.lastNodeID) {
+    if (!drawWay || drawWay.isDegenerate()) {
       if (DEBUG) {
         console.log('DrawLineMode: draw way invalid, rolling back');  // eslint-disable-line no-console
       }

modules/osm/way.js

@@ -236,7 +236,7 @@ Object.assign(osmWay.prototype, {
 
 
     isDegenerate: function() {
-        return (new Set(this.nodes).size < (this.isArea() ? 3 : 2));
+        return (new Set(this.nodes).size < (this.isClosed() ? 3 : 2));
     },
 
 

test/spec/actions/circularize.js

@@ -278,20 +278,6 @@ describe('actionCircularize', function () {
         expect(graph.entity('-').nodes).to.have.length(20);
     });
 
-    it('circularizes a closed single line way', function () {
-        var graph = new Rapid.Graph([
-                Rapid.osmNode({id: 'a', loc: [0, 0]}),
-                Rapid.osmNode({id: 'b', loc: [0, 2]}),
-                Rapid.osmWay({id: '-', nodes: ['a', 'b', 'a']})
-            ]);
-
-        expect(area('-', graph)).to.eql(0);
-
-        graph = Rapid.actionCircularize('-', projection)(graph);
-
-        expect(isCircular('-', graph)).to.be.ok;
-    });
-
     it('not disable circularize when its not circular', function(){
         var graph = new Rapid.Graph([
             Rapid.osmNode({id: 'a', loc: [0, 0]}),

test/spec/osm/way.js

@@ -466,6 +466,10 @@ describe('osmWay', function () {
             expect(Rapid.osmWay({nodes: ['a', 'b']}).isDegenerate()).to.equal(false);
         });
 
+        it('returns true for a linear way that doubles back on itself', function () {
+            expect(iD.osmWay({nodes: ['a', 'b', 'a']}).isDegenerate()).to.equal(true);
+        });
+
         it('returns true for an area with zero, one, or two unique nodes', function () {
             expect(Rapid.osmWay({tags: {area: 'yes'}, nodes: []}).isDegenerate()).to.equal(true);
             expect(Rapid.osmWay({tags: {area: 'yes'}, nodes: ['a', 'a']}).isDegenerate()).to.equal(true);