fix zero-length tangent

gm/strokes.cpp

 /*
  * Copyright 2011 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "gm.h"
 #include "SkPath.h"
 #include "SkRandom.h"
@@ skipping 248 matching lines @@
         paint.setStrokeWidth(0.055f);
     
         canvas->scale(1000, 1000);
         canvas->drawCircle(0, 2, 1.97f, paint);
     }
 
 private:
     typedef skiagm::GM INHERITED;
 };
 
+// Test stroking for curves that produce degenerate tangents when t is 0 or 1 (see bug 4191)
+class Strokes5GM : public skiagm::GM {
+public:
+    Strokes5GM() {}
+
+protected:
+
+    SkString onShortName() override {
+        return SkString("zero_control_stroke");
+    }
+
+    SkISize onISize() override {
+        return SkISize::Make(W, H*2);
+    }
+
+    void onDraw(SkCanvas* canvas) override {
+        SkPaint p;
+        p.setColor(SK_ColorRED);
+        p.setAntiAlias(true);
+        p.setStyle(SkPaint::kStroke_Style);
+        p.setStrokeWidth(40);
+        p.setStrokeCap(SkPaint::kButt_Cap);
+
+        SkPath path;
+        path.moveTo(157.474f,111.753f);
+        path.cubicTo(128.5f,111.5f,35.5f,29.5f,35.5f,29.5f);
+        canvas->drawPath(path, p);
+        path.reset();
+        path.moveTo(250, 50);
+        path.quadTo(280, 80, 280, 80);
+        canvas->drawPath(path, p);
+        path.reset();
+        path.moveTo(150, 50);
+        path.conicTo(180, 80, 180, 80, 0.707f);
+        canvas->drawPath(path, p);
+
+        path.reset();
+        path.moveTo(157.474f,311.753f);
+        path.cubicTo(157.474f,311.753f,85.5f,229.5f,35.5f,229.5f);
+        canvas->drawPath(path, p);
+        path.reset();
+        path.moveTo(280, 250);
+        path.quadTo(280, 250, 310, 280);
+        canvas->drawPath(path, p);
+        path.reset();
+        path.moveTo(180, 250);
+        path.conicTo(180, 250, 210, 280, 0.707f);
+        canvas->drawPath(path, p);
+    }
+
+private:
+    typedef skiagm::GM INHERITED;
+};
+
 
 //////////////////////////////////////////////////////////////////////////////
 
 static skiagm::GM* F0(void*) { return new StrokesGM; }
 static skiagm::GM* F1(void*) { return new Strokes2GM; }
 static skiagm::GM* F2(void*) { return new Strokes3GM; }
 static skiagm::GM* F3(void*) { return new Strokes4GM; }
+static skiagm::GM* F4(void*) { return new Strokes5GM; }
 
 static skiagm::GMRegistry R0(F0);
 static skiagm::GMRegistry R1(F1);
 static skiagm::GMRegistry R2(F2);
 static skiagm::GMRegistry R3(F3);
+static skiagm::GMRegistry R4(F4);