std: Force Instant::now() to be monotonic

This commit is an attempt to force `Instant::now` to be monotonic
through any means possible. We tried relying on OS/hardware/clock
implementations, but those seem buggy enough that we can't rely on them
in practice. This commit implements the same hammer Firefox recently
implemented (noted in rust-lang#56612) which is to just keep whatever the lastest
`Instant::now()` return value was in memory, returning that instead of
the OS looks like it's moving backwards.

Closes rust-lang#48514
Closes rust-lang#49281
cc rust-lang#51648
cc rust-lang#56560
Closes rust-lang#56612
Closes rust-lang#56940

src/librustc/util/profiling.rs

@@ -2,7 +2,7 @@ use session::config::Options;
 
 use std::fs;
 use std::io::{self, StderrLock, Write};
-use std::time::{Duration, Instant};
+use std::time::Instant;
 
 macro_rules! define_categories {
     ($($name:ident,)*) => {
@@ -203,20 +203,7 @@ impl SelfProfiler {
     }
 
     fn stop_timer(&mut self) -> u64 {
-        let elapsed = if cfg!(windows) {
-            // On Windows, timers don't always appear to be monotonic (see #51648)
-            // which can lead to panics when calculating elapsed time.
-            // Work around this by testing to see if the current time is less than
-            // our recorded time, and if it is, just returning 0.
-            let now = Instant::now();
-            if self.current_timer >= now {
-                Duration::new(0, 0)
-            } else {
-                self.current_timer.elapsed()
-            }
-        } else {
-            self.current_timer.elapsed()
-        };
+        let elapsed = self.current_timer.elapsed();
 
         self.current_timer = Instant::now();
 

src/libstd/sys/cloudabi/time.rs

@@ -25,6 +25,14 @@ impl Instant {
         }
     }
 
+    pub fn actually_monotonic() -> bool {
+        true
+    }
+
+    pub const fn zero() -> Instant {
+        Instant { t: 0 }
+    }
+
     pub fn sub_instant(&self, other: &Instant) -> Duration {
         let diff = self.t
             .checked_sub(other.t)

src/libstd/sys/redox/time.rs

@@ -128,6 +128,14 @@ impl Instant {
         Instant { t: now(syscall::CLOCK_MONOTONIC) }
     }
 
+    pub const fn zero() -> Instant {
+        Instant { t: Timespec { t: syscall::TimeSpec { tv_sec: 0, tv_nsec: 0 } } }
+    }
+
+    pub fn actually_monotonic() -> bool {
+        false
+    }
+
     pub fn sub_instant(&self, other: &Instant) -> Duration {
         self.t.sub_timespec(&other.t).unwrap_or_else(|_| {
             panic!("specified instant was later than self")

src/libstd/sys/unix/time.rs

@@ -14,6 +14,12 @@ struct Timespec {
 }
 
 impl Timespec {
+    const fn zero() -> Timespec {
+        Timespec {
+            t: libc::timespec { tv_sec: 0, tv_nsec: 0 },
+        }
+    }
+
     fn sub_timespec(&self, other: &Timespec) -> Result<Duration, Duration> {
         if self >= other {
             Ok(if self.t.tv_nsec >= other.t.tv_nsec {
@@ -128,19 +134,22 @@ mod inner {
     }
 
     pub const UNIX_EPOCH: SystemTime = SystemTime {
-        t: Timespec {
-            t: libc::timespec {
-                tv_sec: 0,
-                tv_nsec: 0,
-            },
-        },
+        t: Timespec::zero(),
     };
 
     impl Instant {
         pub fn now() -> Instant {
             Instant { t: unsafe { libc::mach_absolute_time() } }
         }
 
+        pub const fn zero() -> Instant {
+            Instant { t: 0 }
+        }
+
+        pub fn actually_monotonic() -> bool {
+            true
+        }
+
         pub fn sub_instant(&self, other: &Instant) -> Duration {
             let info = info();
             let diff = self.t.checked_sub(other.t)
@@ -258,19 +267,26 @@ mod inner {
     }
 
     pub const UNIX_EPOCH: SystemTime = SystemTime {
-        t: Timespec {
-            t: libc::timespec {
-                tv_sec: 0,
-                tv_nsec: 0,
-            },
-        },
+        t: Timespec::zero(),
     };
 
     impl Instant {
         pub fn now() -> Instant {
             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")) ||
+            false // last clause, used so `||` is always trailing above
+        }
+
         pub fn sub_instant(&self, other: &Instant) -> Duration {
             self.t.sub_timespec(&other.t).unwrap_or_else(|_| {
                 panic!("specified instant was later than self")

src/libstd/sys/wasm/time.rs

@@ -14,6 +14,14 @@ impl Instant {
         Instant(TimeSysCall::perform(TimeClock::Monotonic))
     }
 
+    pub const fn zero() -> Instant {
+        Instant(Duration::from_secs(0))
+    }
+
+    pub fn actually_monotonic() -> bool {
+        false
+    }
+
     pub fn sub_instant(&self, other: &Instant) -> Duration {
         self.0 - other.0
     }

src/libstd/sys/windows/time.rs

@@ -40,6 +40,14 @@ impl Instant {
         t
     }
 
+    pub fn actually_monotonic() -> bool {
+        false
+    }
+
+    pub const fn zero() -> Instant {
+        Instant { t: 0 }
+    }
+
     pub fn sub_instant(&self, other: &Instant) -> Duration {
         // Values which are +- 1 need to be considered as basically the same
         // units in time due to various measurement oddities, according to

src/libstd/time.rs

@@ -12,11 +12,13 @@
 
 #![stable(feature = "time", since = "1.3.0")]
 
+use cmp;
 use error::Error;
 use fmt;
 use ops::{Add, Sub, AddAssign, SubAssign};
 use sys::time;
 use sys_common::FromInner;
+use sys_common::mutex::Mutex;
 
 #[stable(feature = "time", since = "1.3.0")]
 pub use core::time::Duration;
@@ -150,7 +152,45 @@ impl Instant {
     /// ```
     #[stable(feature = "time2", since = "1.8.0")]
     pub fn now() -> Instant {
-        Instant(time::Instant::now())
+        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 simply seems that this it just happens so that 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 though a few platforms are
+        // whitelisted as "these at least haven't gone backwards yet".
+        if time::Instant::actually_monotonic() {
+            return Instant(os_now)
+        }
+
+        static LOCK: Mutex = Mutex::new();
+        static mut LAST_NOW: time::Instant = time::Instant::zero();
+        unsafe {
+            let _lock = LOCK.lock();
+            let now = cmp::max(LAST_NOW, os_now);
+            LAST_NOW = now;
+            Instant(now)
+        }
     }
 
     /// Returns the amount of time elapsed from another instant to this one.