fix: performance.timeOrigin (#26787)

`performance.timeOrigin` was being set from when JS started executing,
but `op_now` measures from an `std::time::Instant` stored in `OpState`,
which is created at a completely different time. This caused
`performance.timeOrigin` to be very incorrect. This PR corrects the
origin and also cleans up some of the timer code.

Compared to `Date.now()`, `performance`'s time origin is now
consistently within 5us (0.005ms) of system time.


![image](https://github.com/user-attachments/assets/0a7be04a-4f6d-4816-bd25-38a2e6136926)

4 files changed, 69 insertions, 39 deletions

diff --git a/ext/web/02_timers.js b/ext/web/02_timers.js
index 89acaca42..6058febd5 100644
--- a/ext/web/02_timers.js
+++ b/ext/web/02_timers.js
@@ -1,12 +1,9 @@
 // Copyright 2018-2024 the Deno authors. All rights reserved. MIT license.
 
 import { core, primordials } from "ext:core/mod.js";
-import { op_defer, op_now } from "ext:core/ops";
+import { op_defer } from "ext:core/ops";
 const {
-  Uint8Array,
-  Uint32Array,
   PromisePrototypeThen,
-  TypedArrayPrototypeGetBuffer,
   TypeError,
   indirectEval,
   ReflectApply,
@@ -18,13 +15,6 @@ const {
 
 import * as webidl from "ext:deno_webidl/00_webidl.js";
 
-const hrU8 = new Uint8Array(8);
-const hr = new Uint32Array(TypedArrayPrototypeGetBuffer(hrU8));
-function opNow() {
-  op_now(hrU8);
-  return (hr[0] * 1000 + hr[1] / 1e6);
-}
-
 // ---------------------------------------------------------------------------
 
 function checkThis(thisArg) {
@@ -151,7 +141,6 @@ export {
   clearInterval,
   clearTimeout,
   defer,
-  opNow,
   refTimer,
   setImmediate,
   setInterval,
diff --git a/ext/web/15_performance.js b/ext/web/15_performance.js
index ea5557278..9e0e310a5 100644
--- a/ext/web/15_performance.js
+++ b/ext/web/15_performance.js
@@ -1,6 +1,7 @@
 // Copyright 2018-2024 the Deno authors. All rights reserved. MIT license.
 
 import { primordials } from "ext:core/mod.js";
+import { op_now, op_time_origin } from "ext:core/ops";
 const {
   ArrayPrototypeFilter,
   ArrayPrototypePush,
@@ -10,19 +11,34 @@ const {
   Symbol,
   SymbolFor,
   TypeError,
+  TypedArrayPrototypeGetBuffer,
+  Uint8Array,
+  Uint32Array,
 } = primordials;
 
 import * as webidl from "ext:deno_webidl/00_webidl.js";
 import { structuredClone } from "./02_structured_clone.js";
 import { createFilteredInspectProxy } from "ext:deno_console/01_console.js";
 import { EventTarget } from "./02_event.js";
-import { opNow } from "./02_timers.js";
 import { DOMException } from "./01_dom_exception.js";
 
 const illegalConstructorKey = Symbol("illegalConstructorKey");
 let performanceEntries = [];
 let timeOrigin;
 
+const hrU8 = new Uint8Array(8);
+const hr = new Uint32Array(TypedArrayPrototypeGetBuffer(hrU8));
+
+function setTimeOrigin() {
+  op_time_origin(hrU8);
+  timeOrigin = hr[0] * 1000 + hr[1] / 1e6;
+}
+
+function now() {
+  op_now(hrU8);
+  return hr[0] * 1000 + hr[1] / 1e6;
+}
+
 webidl.converters["PerformanceMarkOptions"] = webidl
   .createDictionaryConverter(
     "PerformanceMarkOptions",
@@ -90,10 +106,6 @@ webidl.converters["DOMString or PerformanceMeasureOptions"] = (
   return webidl.converters.DOMString(V, prefix, context, opts);
 };
 
-function setTimeOrigin(origin) {
-  timeOrigin = origin;
-}
-
 function findMostRecent(
   name,
   type,
@@ -135,8 +147,6 @@ function filterByNameType(
   );
 }
 
-const now = opNow;
-
 const _name = Symbol("[[name]]");
 const _entryType = Symbol("[[entryType]]");
 const _startTime = Symbol("[[startTime]]");
diff --git a/ext/web/lib.rs b/ext/web/lib.rs
index 4935af5bd..af0fc2c27 100644
--- a/ext/web/lib.rs
+++ b/ext/web/lib.rs
@@ -52,7 +52,8 @@ pub use crate::message_port::Transferable;
 
 use crate::timers::op_defer;
 use crate::timers::op_now;
-use crate::timers::StartTime;
+use crate::timers::op_time_origin;
+pub use crate::timers::StartTime;
 pub use crate::timers::TimersPermission;
 
 deno_core::extension!(deno_web,
@@ -84,6 +85,7 @@ deno_core::extension!(deno_web,
     compression::op_compression_write,
     compression::op_compression_finish,
     op_now<P>,
+    op_time_origin<P>,
     op_defer,
     stream_resource::op_readable_stream_resource_allocate,
     stream_resource::op_readable_stream_resource_allocate_sized,
@@ -123,7 +125,7 @@ deno_core::extension!(deno_web,
     if let Some(location) = options.maybe_location {
       state.put(Location(location));
     }
-    state.put(StartTime::now());
+    state.put(StartTime::default());
   }
 );
 
diff --git a/ext/web/timers.rs b/ext/web/timers.rs
index a9ab7c97e..06444ed34 100644
--- a/ext/web/timers.rs
+++ b/ext/web/timers.rs
@@ -4,7 +4,10 @@
 
 use deno_core::op2;
 use deno_core::OpState;
+use std::time::Duration;
 use std::time::Instant;
+use std::time::SystemTime;
+use std::time::UNIX_EPOCH;
 
 pub trait TimersPermission {
   fn allow_hrtime(&mut self) -> bool;
@@ -17,21 +20,28 @@ impl TimersPermission for deno_permissions::PermissionsContainer {
   }
 }
 
-pub type StartTime = Instant;
+pub struct StartTime(Instant);
 
-// Returns a milliseconds and nanoseconds subsec
-// since the start time of the deno runtime.
-// If the High precision flag is not set, the
-// nanoseconds are rounded on 2ms.
-#[op2(fast)]
-pub fn op_now<TP>(state: &mut OpState, #[buffer] buf: &mut [u8])
+impl Default for StartTime {
+  fn default() -> Self {
+    Self(Instant::now())
+  }
+}
+
+impl std::ops::Deref for StartTime {
+  type Target = Instant;
+
+  fn deref(&self) -> &Self::Target {
+    &self.0
+  }
+}
+
+fn expose_time<TP>(state: &mut OpState, duration: Duration, out: &mut [u8])
 where
   TP: TimersPermission + 'static,
 {
-  let start_time = state.borrow::<StartTime>();
-  let elapsed = start_time.elapsed();
-  let seconds = elapsed.as_secs();
-  let mut subsec_nanos = elapsed.subsec_nanos();
+  let seconds = duration.as_secs() as u32;
+  let mut subsec_nanos = duration.subsec_nanos();
 
   // If the permission is not enabled
   // Round the nano result on 2 milliseconds
@@ -40,14 +50,33 @@ where
     let reduced_time_precision = 2_000_000; // 2ms in nanoseconds
     subsec_nanos -= subsec_nanos % reduced_time_precision;
   }
-  if buf.len() < 8 {
-    return;
+
+  if out.len() >= 8 {
+    out[0..4].copy_from_slice(&seconds.to_ne_bytes());
+    out[4..8].copy_from_slice(&subsec_nanos.to_ne_bytes());
   }
-  let buf: &mut [u32] =
-    // SAFETY: buffer is at least 8 bytes long.
-    unsafe { std::slice::from_raw_parts_mut(buf.as_mut_ptr() as _, 2) };
-  buf[0] = seconds as u32;
-  buf[1] = subsec_nanos;
+}
+
+#[op2(fast)]
+pub fn op_now<TP>(state: &mut OpState, #[buffer] buf: &mut [u8])
+where
+  TP: TimersPermission + 'static,
+{
+  let start_time = state.borrow::<StartTime>();
+  let elapsed = start_time.elapsed();
+  expose_time::<TP>(state, elapsed, buf);
+}
+
+#[op2(fast)]
+pub fn op_time_origin<TP>(state: &mut OpState, #[buffer] buf: &mut [u8])
+where
+  TP: TimersPermission + 'static,
+{
+  // https://w3c.github.io/hr-time/#dfn-estimated-monotonic-time-of-the-unix-epoch
+  let wall_time = SystemTime::now();
+  let monotonic_time = state.borrow::<StartTime>().elapsed();
+  let epoch = wall_time.duration_since(UNIX_EPOCH).unwrap() - monotonic_time;
+  expose_time::<TP>(state, epoch, buf);
 }
 
 #[allow(clippy::unused_async)]