From 73fbd61bd016eebbf2776dc94c15a26bf39668d6 Mon Sep 17 00:00:00 2001
From: snek <snek@deno.com>
Date: Fri, 8 Nov 2024 23:20:24 +0100
Subject: 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)
---
 ext/web/timers.rs | 65 ++++++++++++++++++++++++++++++++++++++++---------------
 1 file changed, 47 insertions(+), 18 deletions(-)

(limited to 'ext/web/timers.rs')

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)]
-- 
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