fix linear gradient assert, by explicitly clamping

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1556993003

Review URL: https://codereview.chromium.org/1556993003

2 files changed, 29 insertions, 4 deletions

diff --git a/src/effects/gradients/SkLinearGradient.cpp b/src/effects/gradients/SkLinearGradient.cpp
index 298466c8a6..4f90c27c8c 100644
--- a/src/effects/gradients/SkLinearGradient.cpp
+++ b/src/effects/gradients/SkLinearGradient.cpp
@@ -668,12 +668,12 @@ void SkLinearGradient::LinearGradientContext::shade4_dx_clamp(SkPMColor dstC[],
         }
 
         fx += n * dx;
-        count -= n;
-        SkASSERT(count >= 0);
+        // fx should now outside of the p0..p1 interval. However, due to float precision loss,
+        // its possible that fx is slightly too small/large, so we clamp it.
         if (dx_is_pos) {
-            SkASSERT(0 == count || fx >= p1);
+            fx = SkTMax(fx, p1);
         } else {
-            SkASSERT(0 == count || fx <= p0);
+            fx = SkTMin(fx, p0);
         }
 
         ramp<apply_alpha>(dstC, n, c, dc, dither0, dither1);
@@ -683,6 +683,9 @@ void SkLinearGradient::LinearGradientContext::shade4_dx_clamp(SkPMColor dstC[],
         if (n & 1) {
             SkTSwap(dither0, dither1);
         }
+
+        count -= n;
+        SkASSERT(count >= 0);
     }
 }
 
diff --git a/tests/GradientTest.cpp b/tests/GradientTest.cpp
index 8d73569f64..6a277d520e 100644
--- a/tests/GradientTest.cpp
+++ b/tests/GradientTest.cpp
@@ -212,9 +212,31 @@ static void test_nearly_vertical(skiatest::Reporter* reporter) {
     surface->getCanvas()->drawPaint(paint);
 }
 
+// A linear gradient interval can, due to numerical imprecision (likely in the divide)
+// finish an interval with the final fx not landing outside of [p0...p1].
+// The old code had an assert which this test triggered.
+// We now explicitly clamp the resulting fx value.
+static void test_linear_fuzz(skiatest::Reporter* reporter) {
+    SkAutoTUnref<SkSurface> surface(SkSurface::NewRasterN32Premul(1300, 630));
+
+    const SkPoint pts[] = {{ 179.5f, -179.5f }, { 1074.5f, 715.5f }};
+    const SkColor colors[] = { SK_ColorBLACK, SK_ColorWHITE, SK_ColorBLACK, SK_ColorWHITE };
+    const SkScalar pos[] = {0, 0.200000003f, 0.800000012f, 1 };
+
+
+    SkAutoTUnref<SkShader> gradient(
+                   SkGradientShader::CreateLinear(pts, colors, pos, 4, SkShader::kClamp_TileMode));
+
+    SkPaint paint;
+    paint.setShader(gradient);
+    SkRect r = {0, 83, 1254, 620};
+    surface->getCanvas()->drawRect(r, paint);
+}
+
 DEF_TEST(Gradient, reporter) {
     TestGradientShaders(reporter);
     TestConstantGradient(reporter);
     test_big_grad(reporter);
     test_nearly_vertical(reporter);
+    test_linear_fuzz(reporter);
 }