fix path ops fuzz bug

If one path is empty and the other has extreme values, the
intermediate coincident paths cannot be resolved, but triggers
an assert that a data structure unexpectedly has zero-length.

Tunnel this failure back up to the top and return that the
entire path op fails.

A future optimization could detect the empty path and avoid
this, allowing the op to succeed -- not sure that it's worth
the additional logic though.

TBR=reed@google.com
BUG=535151
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1730293002

Review URL: https://codereview.chromium.org/1730293002

1 files changed, 8 insertions, 3 deletions

diff --git a/src/pathops/SkPathOpsCommon.cpp b/src/pathops/SkPathOpsCommon.cpp
index 0060db2f30..829f8a50d5 100644
--- a/src/pathops/SkPathOpsCommon.cpp
+++ b/src/pathops/SkPathOpsCommon.cpp
@@ -425,11 +425,14 @@ static bool missingCoincidence(SkOpContourHead* contourList,
     return result;
 }
 
-static void moveMultiples(SkOpContourHead* contourList) {
+static bool moveMultiples(SkOpContourHead* contourList) {
     SkOpContour* contour = contourList;
     do {
-        contour->moveMultiples();
+        if (!contour->moveMultiples()) {
+            return false;
+        }
     } while ((contour = contour->next()));
+    return true;
 }
 
 static void moveNearby(SkOpContourHead* contourList) {
@@ -451,7 +454,9 @@ bool HandleCoincidence(SkOpContourHead* contourList, SkOpCoincidence* coincidenc
     SkOpGlobalState* globalState = contourList->globalState();
     // combine t values when multiple intersections occur on some segments but not others
     DEBUG_COINCIDENCE_HEALTH(contourList, "start");
-    moveMultiples(contourList);
+    if (!moveMultiples(contourList)) {
+        return false;
+    }
     DEBUG_COINCIDENCE_HEALTH(contourList, "moveMultiples");
     findCollapsed(contourList);
     DEBUG_COINCIDENCE_HEALTH(contourList, "findCollapsed");