Rollup merge of #89926 - the8472:saturate-instant, r=Mark-Simulacrum

make `Instant::{duration_since, elapsed, sub}` saturating and remove workarounds

This removes all mutex/atomic-based workarounds for non-monotonic clocks and makes the previously panicking methods saturating instead. Additionally `saturating_duration_since` becomes deprecated since `duration_since` now fills that role.

Effectively this moves the fixup from `Instant` construction to the comparisons.

This has some observable effects, especially on platforms without monotonic clocks:

* Incorrectly ordered Instant comparisons no longer panic in release mode. This could hide some programming errors, but since debug mode still panics tests can still catch them.
* `checked_duration_since` will now return `None` in more cases. Previously it only happened when one compared instants obtained in the wrong order or manually created ones. Now it also does on backslides.
* non-monotonic intervals will not be transitive, i.e. `b.duration_since(a) + c.duration_since(b) != c.duration_since(a)`

The upsides are reduced complexity and lower overhead of `Instant::now`.

## Motivation

Currently we must choose between two poisons. One is high worst-case latency and jitter of `Instant::now()` due to explicit synchronization; see #83093 for benchmarks, the worst-case overhead is > 100x. The other is sporadic panics on specific, rare combinations of CPU/hypervisor/operating system due to platform bugs.

Use-cases where low-overhead, fine-grained timestamps are needed - such as syscall tracing, performance profiles or sensor data acquisition (drone flight controllers were mentioned in a libs meeting) in multi-threaded programs - are negatively impacted by the synchronization.

The panics are user-visible (program crashes), hard to reproduce and can be triggered by any dependency that might be using Instants for any reason.

A solution that is fast _and_ doesn't panic is desirable.

----

closes #84448
closes #86470

library/std/src/sys/hermit/time.rs

@@ -115,14 +115,6 @@ impl Instant {
         Instant { t: time }
     }
 
-    pub const fn zero() -> Instant {
-        Instant { t: Timespec::zero() }
-    }
-
-    pub fn actually_monotonic() -> bool {
-        true
-    }
-
     pub fn checked_sub_instant(&self, other: &Instant) -> Option<Duration> {
         self.t.sub_timespec(&other.t).ok()
     }

library/std/src/sys/itron/time.rs

@@ -14,15 +14,6 @@ impl Instant {
         }
     }
 
-    pub const fn zero() -> Instant {
-        Instant(0)
-    }
-
-    pub fn actually_monotonic() -> bool {
-        // There are ways to change the system time
-        false
-    }
-
     pub fn checked_sub_instant(&self, other: &Instant) -> Option<Duration> {
         self.0.checked_sub(other.0).map(|ticks| {
             // `SYSTIM` is measured in microseconds

library/std/src/sys/sgx/time.rs

@@ -25,14 +25,6 @@ impl Instant {
     pub fn checked_sub_duration(&self, other: &Duration) -> Option<Instant> {
         Some(Instant(self.0.checked_sub(*other)?))
     }
-
-    pub fn actually_monotonic() -> bool {
-        false
-    }
-
-    pub const fn zero() -> Instant {
-        Instant(Duration::from_secs(0))
-    }
 }
 
 impl SystemTime {

library/std/src/sys/unix/time.rs

@@ -154,14 +154,6 @@ mod inner {
             Instant { t: unsafe { mach_absolute_time() } }
         }
 
-        pub const fn zero() -> Instant {
-            Instant { t: 0 }
-        }
-
-        pub fn actually_monotonic() -> bool {
-            true
-        }
-
         pub fn checked_sub_instant(&self, other: &Instant) -> Option<Duration> {
             let diff = self.t.checked_sub(other.t)?;
             let info = info();
@@ -296,17 +288,6 @@ mod inner {
             Instant { t: now(libc::CLOCK_MONOTONIC) }
         }
 
-        pub const fn zero() -> Instant {
-            Instant { t: Timespec::zero() }
-        }
-
-        pub fn actually_monotonic() -> bool {
-            (cfg!(target_os = "linux") && cfg!(target_arch = "x86_64"))
-                || (cfg!(target_os = "linux") && cfg!(target_arch = "x86"))
-                || (cfg!(target_os = "linux") && cfg!(target_arch = "aarch64"))
-                || cfg!(target_os = "fuchsia")
-        }
-
         pub fn checked_sub_instant(&self, other: &Instant) -> Option<Duration> {
             self.t.sub_timespec(&other.t).ok()
         }

library/std/src/sys/unsupported/time.rs

@@ -13,14 +13,6 @@ impl Instant {
         panic!("time not implemented on this platform")
     }
 
-    pub const fn zero() -> Instant {
-        Instant(Duration::from_secs(0))
-    }
-
-    pub fn actually_monotonic() -> bool {
-        false
-    }
-
     pub fn checked_sub_instant(&self, other: &Instant) -> Option<Duration> {
         self.0.checked_sub(other.0)
     }

library/std/src/sys/wasi/time.rs

@@ -25,14 +25,6 @@ impl Instant {
         Instant(current_time(wasi::CLOCKID_MONOTONIC))
     }
 
-    pub const fn zero() -> Instant {
-        Instant(Duration::from_secs(0))
-    }
-
-    pub fn actually_monotonic() -> bool {
-        true
-    }
-
     pub fn checked_sub_instant(&self, other: &Instant) -> Option<Duration> {
         self.0.checked_sub(other.0)
     }

library/std/src/sys/windows/time.rs

@@ -41,14 +41,6 @@ impl Instant {
         perf_counter::PerformanceCounterInstant::now().into()
     }
 
-    pub fn actually_monotonic() -> bool {
-        false
-    }
-
-    pub const fn zero() -> Instant {
-        Instant { t: Duration::from_secs(0) }
-    }
-
     pub fn checked_sub_instant(&self, other: &Instant) -> Option<Duration> {
         // On windows there's a threshold below which we consider two timestamps
         // equivalent due to measurement error. For more details + doc link,

library/std/src/time.rs

@@ -31,7 +31,6 @@
 
 #![stable(feature = "time", since = "1.3.0")]
 
-mod monotonic;
 #[cfg(test)]
 mod tests;
 
@@ -50,8 +49,8 @@ pub use core::time::FromFloatSecsError;
 /// A measurement of a monotonically nondecreasing clock.
 /// Opaque and useful only with [`Duration`].
 ///
-/// Instants are always guaranteed to be no less than any previously measured
-/// instant when created, and are often useful for tasks such as measuring
+/// Instants are always guaranteed, barring [platform bugs], to be no less than any previously
+/// measured instant when created, and are often useful for tasks such as measuring
 /// benchmarks or timing how long an operation takes.
 ///
 /// Note, however, that instants are **not** guaranteed to be **steady**. In other
@@ -84,6 +83,8 @@ pub use core::time::FromFloatSecsError;
 /// }
 /// ```
 ///
+/// [platform bugs]: Instant#monotonicity
+///
 /// # OS-specific behaviors
 ///
 /// An `Instant` is a wrapper around system-specific types and it may behave
@@ -125,6 +126,26 @@ pub use core::time::FromFloatSecsError;
 /// > structure cannot represent the new point in time.
 ///
 /// [`add`]: Instant::add
+///
+/// ## Monotonicity
+///
+/// On all platforms `Instant` will try to use an OS API that guarantees monotonic behavior
+/// if available, which is the case for all [tier 1] platforms.
+/// In practice such guarantees are – under rare circumstances – broken by hardware, virtualization
+/// or operating system bugs. To work around these bugs and platforms not offering monotonic clocks
+/// [`duration_since`], [`elapsed`] and [`sub`] saturate to zero. In older Rust versions this
+/// lead to a panic instead. [`checked_duration_since`] can be used to detect and handle situations
+/// where monotonicity is violated, or `Instant`s are subtracted in the wrong order.
+///
+/// This workaround obscures programming errors where earlier and later instants are accidentally
+/// swapped. For this reason future rust versions may reintroduce panics.
+///
+/// [tier 1]: https://doc.rust-lang.org/rustc/platform-support.html
+/// [`duration_since`]: Instant::duration_since
+/// [`elapsed`]: Instant::elapsed
+/// [`sub`]: Instant::sub
+/// [`checked_duration_since`]: Instant::checked_duration_since
+///
 #[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
 #[stable(feature = "time2", since = "1.8.0")]
 pub struct Instant(time::Instant);
@@ -247,59 +268,19 @@ impl Instant {
     #[must_use]
     #[stable(feature = "time2", since = "1.8.0")]
     pub fn now() -> Instant {
-        let os_now = time::Instant::now();
-
-        // And here we come upon a sad state of affairs. The whole point of
-        // `Instant` is that it's monotonically increasing. We've found in the
-        // wild, however, that it's not actually monotonically increasing for
-        // one reason or another. These appear to be OS and hardware level bugs,
-        // and there's not really a whole lot we can do about them. Here's a
-        // taste of what we've found:
-        //
-        // * #48514 - OpenBSD, x86_64
-        // * #49281 - linux arm64 and s390x
-        // * #51648 - windows, x86
-        // * #56560 - windows, x86_64, AWS
-        // * #56612 - windows, x86, vm (?)
-        // * #56940 - linux, arm64
-        // * https://bugzilla.mozilla.org/show_bug.cgi?id=1487778 - a similar
-        //   Firefox bug
-        //
-        // It seems that this just happens a lot in the wild.
-        // We're seeing panics across various platforms where consecutive calls
-        // to `Instant::now`, such as via the `elapsed` function, are panicking
-        // as they're going backwards. Placed here is a last-ditch effort to try
-        // to fix things up. We keep a global "latest now" instance which is
-        // returned instead of what the OS says if the OS goes backwards.
-        //
-        // To hopefully mitigate the impact of this, a few platforms are
-        // excluded as "these at least haven't gone backwards yet".
-        //
-        // While issues have been seen on arm64 platforms the Arm architecture
-        // requires that the counter monotonically increases and that it must
-        // provide a uniform view of system time (e.g. it must not be possible
-        // for a core to receive a message from another core with a time stamp
-        // and observe time going backwards (ARM DDI 0487G.b D11.1.2). While
-        // there have been a few 64bit SoCs that have bugs which cause time to
-        // not monoticially increase, these have been fixed in the Linux kernel
-        // and we shouldn't penalize all Arm SoCs for those who refuse to
-        // update their kernels:
-        // SUN50I_ERRATUM_UNKNOWN1 - Allwinner A64 / Pine A64 - fixed in 5.1
-        // FSL_ERRATUM_A008585 - Freescale LS2080A/LS1043A - fixed in 4.10
-        // HISILICON_ERRATUM_161010101 - Hisilicon 1610 - fixed in 4.11
-        // ARM64_ERRATUM_858921 - Cortex A73 - fixed in 4.12
-        if time::Instant::actually_monotonic() {
-            return Instant(os_now);
-        }
-
-        Instant(monotonic::monotonize(os_now))
+        Instant(time::Instant::now())
     }
 
-    /// Returns the amount of time elapsed from another instant to this one.
+    /// Returns the amount of time elapsed from another instant to this one,
+    /// or zero duration if that instant is later than this one.
     ///
     /// # Panics
     ///
-    /// This function will panic if `earlier` is later than `self`.
+    /// Previous rust versions panicked when `earlier` was later than `self`. Currently this
+    /// method saturates. Future versions may reintroduce the panic in some circumstances.
+    /// See [Monotonicity].
+    ///
+    /// [Monotonicity]: Instant#monotonicity
     ///
     /// # Examples
     ///
@@ -311,16 +292,22 @@ impl Instant {
     /// sleep(Duration::new(1, 0));
     /// let new_now = Instant::now();
     /// println!("{:?}", new_now.duration_since(now));
+    /// println!("{:?}", now.duration_since(new_now)); // 0ns
     /// ```
     #[must_use]
     #[stable(feature = "time2", since = "1.8.0")]
     pub fn duration_since(&self, earlier: Instant) -> Duration {
-        self.0.checked_sub_instant(&earlier.0).expect("supplied instant is later than self")
+        self.checked_duration_since(earlier).unwrap_or_default()
     }
 
     /// Returns the amount of time elapsed from another instant to this one,
     /// or None if that instant is later than this one.
     ///
+    /// Due to [monotonicity bugs], even under correct logical ordering of the passed `Instant`s,
+    /// this method can return `None`.
+    ///
+    /// [monotonicity bugs]: Instant#monotonicity
+    ///
     /// # Examples
     ///
     /// ```no_run
@@ -364,9 +351,11 @@ impl Instant {
     ///
     /// # Panics
     ///
-    /// This function may panic if the current time is earlier than this
-    /// instant, which is something that can happen if an `Instant` is
-    /// produced synthetically.
+    /// Previous rust versions panicked when self was earlier than the current time. Currently this
+    /// method returns a Duration of zero in that case. Future versions may reintroduce the panic.
+    /// See [Monotonicity].
+    ///
+    /// [Monotonicity]: Instant#monotonicity
     ///
     /// # Examples
     ///
@@ -442,6 +431,16 @@ impl SubAssign<Duration> for Instant {
 impl Sub<Instant> for Instant {
     type Output = Duration;
 
+    /// Returns the amount of time elapsed from another instant to this one,
+    /// or zero duration if that instant is later than this one.
+    ///
+    /// # Panics
+    ///
+    /// Previous rust versions panicked when `other` was later than `self`. Currently this
+    /// method saturates. Future versions may reintroduce the panic in some circumstances.
+    /// See [Monotonicity].
+    ///
+    /// [Monotonicity]: Instant#monotonicity
     fn sub(self, other: Instant) -> Duration {
         self.duration_since(other)
     }