Fix SkPath::arcTo when sweepAngle is tiny and radius is big

src/core/SkPath.cpp

 /*
  * Copyright 2006 The Android Open Source Project
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include <cmath>
 #include "SkBuffer.h"
 #include "SkCubicClipper.h"
@@ skipping 1272 matching lines @@
     if (arc_is_lone_point(oval, startAngle, sweepAngle, &lonePt)) {
         forceMoveTo ? this->moveTo(lonePt) : this->lineTo(lonePt);
         return;
     }
 
     SkVector startV, stopV;
     SkRotationDirection dir;
     angles_to_unit_vectors(startAngle, sweepAngle, &startV, &stopV, &dir);
 
     SkPoint singlePt;
+
+    // At this point, we know that the arc is not a lone point, but startV == stopV
+    // indicates that the sweepAngle is too small such that angles_to_unit_vectors
+    // cannot handle it.
+    if (startV == stopV) {
+        SkScalar endAngle = SkDegreesToRadians(startAngle + sweepAngle);
+        SkScalar radiusX = oval.width() / 2;
+        SkScalar radiusY = oval.height() / 2;
+        // We cannot use SkScalarSinCos function in the next line because
+        // SkScalarSinCos has a threshold *SkScalarNearlyZero*. When sin(startAngle)
+        // is 0 and sweepAngle is very small and radius is huge, the expected
+        // behavior here is to draw a line. But calling SkScalarSinCos will 
+        // make sin(endAngle) to be 0 which will then draw a dot.
+        singlePt.set(oval.centerX() + radiusX * sk_float_cos(endAngle),
+            oval.centerY() + radiusY * sk_float_sin(endAngle));
+        forceMoveTo ? this->moveTo(singlePt) : this->lineTo(singlePt);
+        return;
+    }
+
     SkConic conics[SkConic::kMaxConicsForArc];
     int count = build_arc_conics(oval, startV, stopV, dir, conics, &singlePt);
     if (count) {
         this->incReserve(count * 2 + 1);
         const SkPoint& pt = conics[0].fPts[0];
         forceMoveTo ? this->moveTo(pt) : this->lineTo(pt);
         for (int i = 0; i < count; ++i) {
             this->conicTo(conics[i].fPts[1], conics[i].fPts[2], conics[i].fW);
         }
     } else {
@@ skipping 2073 matching lines @@
         path->arcTo(oval, startAngle, 180.f, false);
         startAngle += 180.f;
         forceMoveTo = false;
         sweepAngle -= 360.f;
     }
     path->arcTo(oval, startAngle, sweepAngle, forceMoveTo);
     if (useCenter) {
         path->close();
     }
 }