break out of cubic stroker loop on degenerate case

The looper can generate more than one quad, but if any one is degenerate,
give up, but not before generating the state for the line join to
produce the correct end.

Before, the early return allowed the inside path to contain multiple
movetos that caused reversePath to assert.

R=reed@google.com

Review URL: https://codereview.chromium.org/948043002

2 files changed, 35 insertions, 21 deletions

diff --git a/gm/dashcubics.cpp b/gm/dashcubics.cpp
index 9a7d6d7482..246d0548ca 100644
--- a/gm/dashcubics.cpp
+++ b/gm/dashcubics.cpp
@@ -26,30 +26,16 @@ protected:
     }
 
     virtual SkISize onISize() {
-        return SkISize::Make(640, 480);
+        return SkISize::Make(860, 700);
     }
 
-    virtual void onDraw(SkCanvas* canvas) {
-        SkPath path;
-        const char* d = "M 337,98 C 250,141 250,212 250,212 C 250,212 250,212 250,212"
-        "C 250,212 250,212 250,212 C 250,212 250,141 163,98 C 156,195 217,231 217,231"
-        "C 217,231 217,231 217,231 C 217,231 217,231 217,231 C 217,231 156,195 75,250"
-        "C 156,305 217,269 217,269 C 217,269 217,269 217,269 C 217,269 217,269 217,269"
-        "C 217,269 156,305 163,402 C 250,359 250,288 250,288 C 250,288 250,288 250,288"
-        "C 250,288 250,288 250,288 C 250,288 250,359 338,402 C 345,305 283,269 283,269"
-        "C 283,269 283,269 283,269 C 283,269 283,269 283,269 C 283,269 345,305 425,250"
-        "C 344,195 283,231 283,231 C 283,231 283,231 283,231 C 283,231 283,231 283,231"
-        "C 283,231 344,195 338,98";
-
-        SkParsePath::FromSVGString(d, &path);
-
-        SkScalar intervals[] = { 5, 10 };
+    void flower(SkCanvas* canvas, const SkPath& path, SkScalar intervals[2], SkPaint::Join join) {
         SkPathEffect* pe = SkDashPathEffect::Create(intervals, 2, 0);
 
         SkPaint paint;
         paint.setAntiAlias(true);
         paint.setStyle(SkPaint::kStroke_Style);
-
+        paint.setStrokeJoin(join);
         paint.setStrokeWidth(42);
         canvas->drawPath(path, paint);
 
@@ -64,6 +50,31 @@ protected:
         canvas->drawPath(path, paint);
     }
 
+    virtual void onDraw(SkCanvas* canvas) {
+        SkPath path;
+        const char* d = "M 337,98 C 250,141 250,212 250,212 C 250,212 250,212 250,212"
+        "C 250,212 250,212 250,212 C 250,212 250,141 163,98 C 156,195 217,231 217,231"
+        "C 217,231 217,231 217,231 C 217,231 217,231 217,231 C 217,231 156,195 75,250"
+        "C 156,305 217,269 217,269 C 217,269 217,269 217,269 C 217,269 217,269 217,269"
+        "C 217,269 156,305 163,402 C 250,359 250,288 250,288 C 250,288 250,288 250,288"
+        "C 250,288 250,288 250,288 C 250,288 250,359 338,402 C 345,305 283,269 283,269"
+        "C 283,269 283,269 283,269 C 283,269 283,269 283,269 C 283,269 345,305 425,250"
+        "C 344,195 283,231 283,231 C 283,231 283,231 283,231 C 283,231 283,231 283,231"
+        "C 283,231 344,195 338,98";
+
+        SkParsePath::FromSVGString(d, &path);
+            canvas->translate(-35.f, -55.f);
+        for (int x = 0; x < 2; ++x) {
+            for (int y = 0; y < 2; ++y) {
+                canvas->save();
+                canvas->translate(x * 430.f, y * 355.f);
+                SkScalar intervals[] = { 5 + (x ? 0 : 0.0001f + 0.0001f), 10 };
+                flower(canvas, path, intervals, y ? SkPaint::kDefault_Join : SkPaint::kRound_Join);
+                canvas->restore();
+            }
+        }
+    }
+
 private:
     typedef GM INHERITED;
 };
diff --git a/src/core/SkStroke.cpp b/src/core/SkStroke.cpp
index 1a44a3a19f..5c65e6eb01 100644
--- a/src/core/SkStroke.cpp
+++ b/src/core/SkStroke.cpp
@@ -905,9 +905,10 @@ void SkPathStroker::quadTo(const SkPoint& pt1, const SkPoint& pt2) {
 // compute the perpendicular point and its tangent.
 void SkPathStroker::setRayPts(const SkPoint& tPt, SkVector* dxy, SkPoint* onPt,
         SkPoint* tangent) const {
+    SkPoint oldDxy = *dxy;
     if (!dxy->setLength(fRadius)) {  // consider moving double logic into SkPoint::setLength
-        double xx = dxy->fX;
-        double yy = dxy->fY;
+        double xx = oldDxy.fX;
+        double yy = oldDxy.fY;
         double dscale = fRadius / sqrt(xx * xx + yy * yy);
         dxy->fX = SkDoubleToScalar(xx * dscale);
         dxy->fY = SkDoubleToScalar(yy * dscale);
@@ -1412,14 +1413,16 @@ void SkPathStroker::cubicTo(const SkPoint& pt1, const SkPoint& pt2,
         SkQuadConstruct quadPts;
         this->init(kOuter_StrokeType, &quadPts, lastT, nextT);
         if (!this->cubicStroke(cubic, &quadPts)) {
-            return;
+            break;
         }
         this->init(kInner_StrokeType, &quadPts, lastT, nextT);
         if (!this->cubicStroke(cubic, &quadPts)) {
-            return;
+            break;
         }
         lastT = nextT;
     }
+    // emit the join even if one stroke succeeded but the last one failed
+    // this avoids reversing an inner stroke with a partial path followed by another moveto
     this->setCubicEndNormal(cubic, normalAB, unitAB, &normalCD, &unitCD);
 #else
     bool    degenerateAB = SkPath::IsLineDegenerate(fPrevPt, pt1);