fix(webapp): convert epoch ms to BigInt before nanosecond multiplication

Multiplying epoch milliseconds by 1e6 as a Number exceeds
Number.MAX_SAFE_INTEGER, so the product is rounded to a multiple of 256
before it is converted to BigInt. Span start times and durations could
therefore be off by up to a few hundred nanoseconds.

Fixes #3292

Author: oziass

apps/webapp/app/v3/eventRepository/common.server.ts

@@ -26,7 +26,7 @@ export function extractContextFromCarrier(carrier: Record<string, unknown>) {
 }
 
 export function getNowInNanoseconds(): bigint {
-  return BigInt(new Date().getTime() * 1_000_000);
+  return convertDateToNanoseconds(new Date());
 }
 
 export function getDateFromNanoseconds(nanoseconds: bigint): Date {
@@ -40,7 +40,7 @@ export function calculateDurationFromStart(
 ) {
   const $endtime = typeof endTime === "string" ? new Date(endTime) : endTime;
 
-  const duration = Number(BigInt($endtime.getTime() * 1_000_000) - startTime);
+  const duration = Number(convertDateToNanoseconds($endtime) - startTime);
 
   if (minimumDuration && duration < minimumDuration) {
     return minimumDuration;

apps/webapp/app/v3/eventRepository/index.server.ts

@@ -208,7 +208,7 @@ async function recordRunEvent(
         runId: foundRun.friendlyId,
         ...attributes,
       },
-      startTime: BigInt((startTime?.getTime() ?? Date.now()) * 1_000_000),
+      startTime: BigInt(startTime?.getTime() ?? Date.now()) * BigInt(1_000_000),
       ...optionsRest,
     });
 

apps/webapp/app/v3/runEngineHandlers.server.ts

@@ -557,7 +557,7 @@ export function registerRunEngineEventBusHandlers() {
         );
 
         await eventRepository.recordEvent(retryMessage, {
-          startTime: BigInt(time.getTime() * 1000000),
+          startTime: BigInt(time.getTime()) * BigInt(1_000_000),
           taskSlug: run.taskIdentifier,
           environment,
           attributes: {

apps/webapp/test/eventRepositoryCommon.test.ts

@@ -0,0 +1,41 @@
+import { afterEach, describe, expect, it, vi } from "vitest";
+import {
+  calculateDurationFromStart,
+  convertDateToNanoseconds,
+  getNowInNanoseconds,
+} from "~/v3/eventRepository/common.server";
+
+// 1700000000001 * 1e6 is not representable as a double (spacing is 256 at ~1.7e18),
+// so Number multiplication before BigInt conversion rounds it to ...000999936.
+const unsafeMs = 1_700_000_000_001;
+const unsafeNs = 1_700_000_000_001_000_000n;
+
+describe("eventRepository nanosecond conversions", () => {
+  afterEach(() => {
+    vi.useRealTimers();
+  });
+
+  it("convertDateToNanoseconds keeps exact precision beyond MAX_SAFE_INTEGER", () => {
+    expect(convertDateToNanoseconds(new Date(unsafeMs))).toBe(unsafeNs);
+  });
+
+  it("getNowInNanoseconds keeps exact precision beyond MAX_SAFE_INTEGER", () => {
+    vi.useFakeTimers();
+    vi.setSystemTime(unsafeMs);
+
+    expect(getNowInNanoseconds()).toBe(unsafeNs);
+    expect(getNowInNanoseconds() % 1_000_000n).toBe(0n);
+  });
+
+  it("calculateDurationFromStart is exact for sub-millisecond-sensitive end times", () => {
+    const startTime = convertDateToNanoseconds(new Date(unsafeMs - 1));
+
+    expect(calculateDurationFromStart(startTime, new Date(unsafeMs))).toBe(1_000_000);
+  });
+
+  it("calculateDurationFromStart applies the minimum duration", () => {
+    const startTime = convertDateToNanoseconds(new Date(unsafeMs - 1));
+
+    expect(calculateDurationFromStart(startTime, new Date(unsafeMs), 5_000_000)).toBe(5_000_000);
+  });
+});