don't fix winding for empty paths

A path can be non-empty but become empty when it is simplified.
For instance, a path with the same rectangle, twice, with opposite
windings.

No contours are created for empty paths, so don't try to
fix their winding direction.

Additionally, check for a NULL coincidence since the
OpBuilder assumes that no concidence edges can be present
after the paths are simplified. This code should not get
called, but it's worth the future-proofing to check.

R=fmalita@chromium.org
BUG=502792

Review URL: https://codereview.chromium.org/1218863005

3 files changed, 24 insertions, 4 deletions

diff --git a/src/pathops/SkOpBuilder.cpp b/src/pathops/SkOpBuilder.cpp
index 2fbd4f0460..0d229a8c40 100644
--- a/src/pathops/SkOpBuilder.cpp
+++ b/src/pathops/SkOpBuilder.cpp
@@ -158,9 +158,11 @@ bool SkOpBuilder::resolve(SkPath* result) {
             *result = original;
             return false;
         }
-        // convert the even odd result back to winding form before accumulating it
-        FixWinding(&fPathRefs[index]);
-        sum.addPath(fPathRefs[index]);
+        if (!fPathRefs[index].isEmpty()) {
+            // convert the even odd result back to winding form before accumulating it
+            FixWinding(&fPathRefs[index]);
+            sum.addPath(fPathRefs[index]);
+        }
     }
     reset();
     bool success = Simplify(sum, result);
diff --git a/src/pathops/SkOpSpan.cpp b/src/pathops/SkOpSpan.cpp
index e89ec3e204..ae4771cffc 100755
--- a/src/pathops/SkOpSpan.cpp
+++ b/src/pathops/SkOpSpan.cpp
@@ -279,7 +279,10 @@ void SkOpSpan::detach(SkOpPtT* kept) {
     prev->setNext(next);
     next->setPrev(prev);
     this->segment()->detach(this);
-    this->globalState()->coincidence()->fixUp(this->ptT(), kept);
+    SkOpCoincidence* coincidence = this->globalState()->coincidence();
+    if (coincidence) {
+        coincidence->fixUp(this->ptT(), kept);
+    }
     this->ptT()->setDeleted();
 }
 
diff --git a/tests/PathOpsBuilderTest.cpp b/tests/PathOpsBuilderTest.cpp
index d36072f077..df77100b01 100644
--- a/tests/PathOpsBuilderTest.cpp
+++ b/tests/PathOpsBuilderTest.cpp
@@ -135,3 +135,18 @@ DEF_TEST(BuilderIssue3838_3, reporter) {
     int pixelDiff = comparePaths(reporter, __FUNCTION__, path, result);
     REPORTER_ASSERT(reporter, pixelDiff == 0);
 }
+
+DEF_TEST(BuilderIssue502792_2, reporter) {
+    SkPath path, pathB;
+    path.setFillType(SkPath::kWinding_FillType);
+    path.addRect(0, 0, 1, 1, SkPath::kCW_Direction);
+    path.addRect(2, 2, 3, 3, SkPath::kCW_Direction);
+    pathB.setFillType(SkPath::kEvenOdd_FillType);
+    pathB.addRect(3, 3, 4, 4, SkPath::kCW_Direction);
+    pathB.addRect(3, 3, 4, 4, SkPath::kCW_Direction);
+    SkOpBuilder builder;
+    builder.add(path, kUnion_SkPathOp);
+    builder.add(pathB, kDifference_SkPathOp);
+    SkPath result;
+    builder.resolve(&result);
+}