test(webapp): sample benchmark ELU as a per-window delta and build poison JSON iteratively

The ELU monitor called eventLoopUtilization() with no args, which returns the
cumulative average since thread start, so each sample smoothed out the spikes
the percentile stats are meant to catch; sample the delta between consecutive
readings instead. The benchmark producer now builds the deeply nested poison
JSON as a string iteratively (and validates the depth) so it does not depend on
runtime-specific deep JSON.stringify support.

Author: ericallam

apps/webapp/test/runsReplicationBenchmark.producer.ts

@@ -19,12 +19,13 @@ interface ProducerConfig {
   poisonDepth?: number;
 }
 
-function deeplyNestedOutput(depth: number): Record<string, unknown> {
-  let node: Record<string, unknown> = { leaf: 1 };
-  for (let i = 0; i < depth; i++) {
-    node = { [`k${i}`]: node };
+function deeplyNestedJson(depth: number): string {
+  const safeDepth = Number.isSafeInteger(depth) && depth >= 0 ? depth : 0;
+  const open: string[] = [];
+  for (let i = 0; i < safeDepth; i++) {
+    open.push(`{"k${i}":`);
   }
-  return node;
+  return open.join("") + '{"leaf":1}' + "}".repeat(safeDepth);
 }
 
 // Error templates for realistic variety
@@ -158,7 +159,7 @@ async function runProducer(config: ProducerConfig) {
         }
 
         if (isPoison) {
-          runData.output = JSON.stringify(deeplyNestedOutput(poisonDepth));
+          runData.output = deeplyNestedJson(poisonDepth);
           runData.outputType = "application/json";
           poisoned++;
         }

apps/webapp/test/runsReplicationJsonRecoveryBenchmark.test.ts

@@ -27,9 +27,11 @@ class ELUMonitor {
 
   start(intervalMs = 100) {
     this.samples = [];
-    performance.eventLoopUtilization();
+    let last = performance.eventLoopUtilization();
     this.interval = setInterval(() => {
-      this.samples.push(performance.eventLoopUtilization().utilization * 100);
+      const current = performance.eventLoopUtilization();
+      this.samples.push(performance.eventLoopUtilization(current, last).utilization * 100);
+      last = current;
     }, intervalMs);
   }