ref(core): Avoid side-effect of & streamline timestampInSeconds method (#16890)

Related to #16846

This rewrites `timestampInSeconds` to avoid side effects.

As a part of this, I also streamlined this a bit to remove special
handling for missing `timeOrigin`. If that is missing, we just fall back
to using `dateTimestampInSeconds`, instead of doing some custom handling
of `timeOrigin` - IMHO that should not be any less accurate, as
otherwise we are currently mixing `performance.now` with `Date.now()`
which is likely not necessarily accurate either.

Author: mydea

dev-packages/browser-integration-tests/suites/tracing/browserTracingIntegration/linked-traces/consistent-sampling/meta-precedence/test.ts

@@ -25,9 +25,6 @@ sentryTest.describe('When `consistentTraceSampling` is `true` and page contains
         sentryTest.skip();
       }
 
-
-
-
       const url = await getLocalTestUrl({ testDir: __dirname });
 
       const clientReportPromise = waitForClientReportRequest(page);

packages/core/src/utils/time.ts

@@ -32,14 +32,13 @@ export function dateTimestampInSeconds(): number {
  */
 function createUnixTimestampInSecondsFunc(): () => number {
   const { performance } = GLOBAL_OBJ as typeof GLOBAL_OBJ & { performance?: Performance };
-  if (!performance?.now) {
+  // Some browser and environments don't have a performance or timeOrigin, so we fallback to
+  // using Date.now() to compute the starting time.
+  if (!performance?.now || !performance.timeOrigin) {
     return dateTimestampInSeconds;
   }
 
-  // Some browser and environments don't have a timeOrigin, so we fallback to
-  // using Date.now() to compute the starting time.
-  const approxStartingTimeOrigin = Date.now() - performance.now();
-  const timeOrigin = performance.timeOrigin == undefined ? approxStartingTimeOrigin : performance.timeOrigin;
+  const timeOrigin = performance.timeOrigin;
 
   // performance.now() is a monotonic clock, which means it starts at 0 when the process begins. To get the current
   // wall clock time (actual UNIX timestamp), we need to add the starting time origin and the current time elapsed.
@@ -55,6 +54,8 @@ function createUnixTimestampInSecondsFunc(): () => number {
   };
 }
 
+let _cachedTimestampInSeconds: (() => number) | undefined;
+
 /**
  * Returns a timestamp in seconds since the UNIX epoch using either the Performance or Date APIs, depending on the
  * availability of the Performance API.
@@ -64,7 +65,11 @@ function createUnixTimestampInSecondsFunc(): () => number {
  * skew can grow to arbitrary amounts like days, weeks or months.
  * See https://github.com/getsentry/sentry-javascript/issues/2590.
  */
-export const timestampInSeconds = createUnixTimestampInSecondsFunc();
+export function timestampInSeconds(): number {
+  // We store this in a closure so that we don't have to create a new function every time this is called.
+  const func = _cachedTimestampInSeconds ?? (_cachedTimestampInSeconds = createUnixTimestampInSecondsFunc());
+  return func();
+}
 
 /**
  * Cached result of getBrowserTimeOrigin.