From 0efaa838d03ace898148f6b500f59bf0a3daa31d Mon Sep 17 00:00:00 2001
From: Adam Raine <6752989+adamraine@users.noreply.github.com>
Date: Fri, 5 Apr 2024 16:33:24 -0700
Subject: [PATCH] core(trace-elements): add sentry debugging for
 `impactedNodes` (#15915)

---
 core/audits/layout-shifts.js                  |  2 +-
 core/gather/gatherers/trace-elements.js       | 50 +++++++++++++++----
 .../gather/gatherers/trace-elements-test.js   |  7 +++
 3 files changed, 48 insertions(+), 11 deletions(-)

diff --git a/core/audits/layout-shifts.js b/core/audits/layout-shifts.js
index d09da2722afb..2201ed185460 100644
--- a/core/audits/layout-shifts.js
+++ b/core/audits/layout-shifts.js
@@ -78,7 +78,7 @@ class LayoutShifts extends Audit {
       .slice(0, MAX_LAYOUT_SHIFTS);
     for (const event of topLayoutShiftEvents) {
       const biggestImpactNodeId = TraceElements.getBiggestImpactNodeForShiftEvent(
-        event.args.data.impacted_nodes || [], impactByNodeId);
+        event.args.data.impacted_nodes || [], impactByNodeId, event);
       const biggestImpactElement = traceElements.find(t => t.nodeId === biggestImpactNodeId);
 
       // Turn root causes into sub-items.
diff --git a/core/gather/gatherers/trace-elements.js b/core/gather/gatherers/trace-elements.js
index 6a939d30aadb..fb221e804538 100644
--- a/core/gather/gatherers/trace-elements.js
+++ b/core/gather/gatherers/trace-elements.js
@@ -88,19 +88,49 @@ class TraceElements extends BaseGatherer {
    *
    * @param {LH.Artifacts.TraceImpactedNode[]} impactedNodes
    * @param {Map<number, number>} impactByNodeId
+   * @param {import('../../lib/trace-engine.js').SaneSyntheticLayoutShift} event Only for debugging
    * @return {number|undefined}
    */
-  static getBiggestImpactNodeForShiftEvent(impactedNodes, impactByNodeId) {
-    let biggestImpactNodeId;
-    let biggestImpactNodeScore = Number.NEGATIVE_INFINITY;
-    for (const node of impactedNodes) {
-      const impactScore = impactByNodeId.get(node.node_id);
-      if (impactScore !== undefined && impactScore > biggestImpactNodeScore) {
-        biggestImpactNodeId = node.node_id;
-        biggestImpactNodeScore = impactScore;
+  static getBiggestImpactNodeForShiftEvent(impactedNodes, impactByNodeId, event) {
+    try {
+      let biggestImpactNodeId;
+      let biggestImpactNodeScore = Number.NEGATIVE_INFINITY;
+      for (const node of impactedNodes) {
+        const impactScore = impactByNodeId.get(node.node_id);
+        if (impactScore !== undefined && impactScore > biggestImpactNodeScore) {
+          biggestImpactNodeId = node.node_id;
+          biggestImpactNodeScore = impactScore;
+        }
       }
+      return biggestImpactNodeId;
+    } catch (err) {
+      // See https://github.com/GoogleChrome/lighthouse/issues/15870
+      // `impactedNodes` should always be an array here, but it can randomly be something else for
+      // currently unknown reasons. This exception handling will help us identify what
+      // `impactedNodes` really is and also prevent the error from being fatal.
+
+      // It's possible `impactedNodes` is not JSON serializable, so let's add more supplemental
+      // fields just in case.
+      const impactedNodesType = typeof impactedNodes;
+      const impactedNodesClassName = impactedNodes?.constructor?.name;
+
+      let impactedNodesJson;
+      let eventJson;
+      try {
+        impactedNodesJson = JSON.parse(JSON.stringify(impactedNodes));
+        eventJson = JSON.parse(JSON.stringify(event));
+      } catch {}
+
+      Sentry.captureException(err, {
+        extra: {
+          impactedNodes: impactedNodesJson,
+          event: eventJson,
+          impactedNodesType,
+          impactedNodesClassName,
+        },
+      });
+      return;
     }
-    return biggestImpactNodeId;
   }
 
   /**
@@ -129,7 +159,7 @@ class TraceElements extends BaseGatherer {
         const nodeIds = [];
         const impactedNodes = event.args.data.impacted_nodes || [];
         const biggestImpactedNodeId =
-          this.getBiggestImpactNodeForShiftEvent(impactedNodes, impactByNodeId);
+          this.getBiggestImpactNodeForShiftEvent(impactedNodes, impactByNodeId, event);
         if (biggestImpactedNodeId !== undefined) {
           nodeIds.push(biggestImpactedNodeId);
         }
diff --git a/core/test/gather/gatherers/trace-elements-test.js b/core/test/gather/gatherers/trace-elements-test.js
index 67bdad011ed0..5457355df500 100644
--- a/core/test/gather/gatherers/trace-elements-test.js
+++ b/core/test/gather/gatherers/trace-elements-test.js
@@ -175,6 +175,13 @@ describe('Trace Elements gatherer - GetTopLayoutShiftElements', () => {
       {nodeId: 15}, // score: 0.1
     ]);
   });
+
+  describe('getBiggestImpactForShiftEvent', () => {
+    it('is non fatal if impactedNodes is not iterable', () => {
+      const result = TraceElementsGatherer.getBiggestImpactNodeForShiftEvent(1, new Map());
+      expect(result).toBeUndefined();
+    });
+  });
 });
 
 describe('Trace Elements gatherer - Animated Elements', () => {