Guard SkMatrix::get*Scale[s]() against negative nearly-zero values.

src/core/SkMatrix.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 "SkFloatBits.h"
 #include "SkMatrix.h"
 #include "SkNx.h"
@@ skipping 1550 matching lines @@
         SkScalar x = SkScalarHalf(SkScalarSqrt(aminusc * aminusc + 4 * bSqd));
         if (kMin_MinMaxOrBoth == MIN_MAX_OR_BOTH) {
             results[0] = apluscdiv2 - x;
         } else if (kMax_MinMaxOrBoth == MIN_MAX_OR_BOTH) {
             results[0] = apluscdiv2 + x;
         } else {
             results[0] = apluscdiv2 - x;
             results[1] = apluscdiv2 + x;
         }
     }
-    if (SkScalarIsNaN(results[0])) {
+    if (!SkScalarIsFinite(results[0])) {
         return false;
     }
+    if (results[0] < 0 && results[0] > -SK_ScalarNearlyZero) {

[jvanverth1, 2016/07/15 13:49:05]

I missed something here -- why was this changed to only clamp negative nearly
zero values? This seems asymmetric.

[reed1, 2016/07/17 00:15:17]

Agreed. This reads oddly -- we clamp nearly-zero-negative values, and then
assert that we're never negative. Why not add whatever assert you want ahead of
time, and then clamp all negative values?

+        results[0] = 0;
+    }
     SkASSERT(results[0] >= 0);
     results[0] = SkScalarSqrt(results[0]);
     if (kBoth_MinMaxOrBoth == MIN_MAX_OR_BOTH) {
-        if (SkScalarIsNaN(results[1])) {
+        if (!SkScalarIsFinite(results[1])) {
             return false;
         }
+        if (results[1] < 0 && results[1] > -SK_ScalarNearlyZero) {
+            results[1] = 0;
+        }
         SkASSERT(results[1] >= 0);
         results[1] = SkScalarSqrt(results[1]);
     }
     return true;
 }
 
 SkScalar SkMatrix::getMinScale() const {
     SkScalar factor;
     if (get_scale_factor<kMin_MinMaxOrBoth>(this->getType(), fMat, &factor)) {
         return factor;
@@ skipping 287 matching lines @@
     const SkScalar m10 = -m01;
     const SkScalar m11 = m00;
     const SkScalar m12 = fTy;
 
     quad[0].set(m02, m12);
     quad[1].set(m00 * width + m02, m10 * width + m12);
     quad[2].set(m00 * width + m01 * height + m02, m10 * width + m11 * height + m12);
     quad[3].set(m01 * height + m02, m11 * height + m12);
 #endif
 }