shim Apple's futex primitives

This is necessary to unblock rust-lang/rust#122408. The documentation for these is available [here](https://developer.apple.com/documentation/os/os_sync_wait_on_address?language=objc).

Because the futex wait operations (`os_sync_wait_on_address` et al.) return the number of remaining waiters after returning, this required some changes to the common futex infrastructure, which I've changed to take a callback instead of precalculating the return values.

Author: joboet

src/concurrency/sync.rs

@@ -140,6 +140,12 @@ struct Futex {
 #[derive(Default, Clone)]
 pub struct FutexRef(Rc<RefCell<Futex>>);
 
+impl FutexRef {
+    pub fn waiters(&self) -> usize {
+        self.0.borrow().waiters.len()
+    }
+}
+
 impl VisitProvenance for FutexRef {
     fn visit_provenance(&self, _visit: &mut VisitWith<'_>) {
         // No provenance in `Futex`.
@@ -729,17 +735,16 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
     }
 
     /// Wait for the futex to be signaled, or a timeout.
-    /// On a signal, `retval_succ` is written to `dest`.
-    /// On a timeout, `retval_timeout` is written to `dest` and `errno_timeout` is set as the last error.
+    /// * On a signal, `retval_succ` is called with the number of waiters on the
+    /// futex and its result is written to `dest`.
+    /// * On a timeout, `retval_timeout` is written to `dest` and `errno_timeout`
+    /// is set as the last error.
     fn futex_wait(
         &mut self,
         futex_ref: FutexRef,
         bitset: u32,
         timeout: Option<(TimeoutClock, TimeoutAnchor, Duration)>,
-        retval_succ: Scalar,
-        retval_timeout: Scalar,
-        dest: MPlaceTy<'tcx>,
-        errno_timeout: IoError,
+        callback: DynUnblockCallback<'tcx>,
     ) {
         let this = self.eval_context_mut();
         let thread = this.active_thread();
@@ -755,10 +760,7 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
             callback!(
                 @capture<'tcx> {
                     futex_ref: FutexRef,
-                    retval_succ: Scalar,
-                    retval_timeout: Scalar,
-                    dest: MPlaceTy<'tcx>,
-                    errno_timeout: IoError,
+                    callback: DynUnblockCallback<'tcx>,
                 }
                 |this, unblock: UnblockKind| {
                     match unblock {
@@ -768,21 +770,16 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
                             if let Some(data_race) = &this.machine.data_race {
                                 data_race.acquire_clock(&futex.clock, &this.machine.threads);
                             }
-                            // Write the return value.
-                            this.write_scalar(retval_succ, &dest)?;
-                            interp_ok(())
                         },
                         UnblockKind::TimedOut => {
                             // Remove the waiter from the futex.
                             let thread = this.active_thread();
                             let mut futex = futex_ref.0.borrow_mut();
                             futex.waiters.retain(|waiter| waiter.thread != thread);
-                            // Set errno and write return value.
-                            this.set_last_error(errno_timeout)?;
-                            this.write_scalar(retval_timeout, &dest)?;
-                            interp_ok(())
                         },
                     }
+
+                    callback.call(this, unblock)
                 }
             ),
         );

src/shims/unix/linux_like/sync.rs

@@ -156,14 +156,24 @@ pub fn futex<'tcx>(
                     .futex
                     .clone();
 
+                let dest = dest.clone();
                 ecx.futex_wait(
                     futex_ref,
                     bitset,
                     timeout,
-                    Scalar::from_target_isize(0, ecx), // retval_succ
-                    Scalar::from_target_isize(-1, ecx), // retval_timeout
-                    dest.clone(),
-                    LibcError("ETIMEDOUT"), // errno_timeout
+                    callback!(
+                        @capture<'tcx> {
+                            dest: MPlaceTy<'tcx>,
+                        }
+                        |ecx, unblock: UnblockKind| match unblock {
+                            UnblockKind::Ready => {
+                                ecx.write_scalar(Scalar::from_target_isize(0, ecx), &dest)
+                            }
+                            UnblockKind::TimedOut => {
+                                ecx.set_last_error_and_return(LibcError("ETIMEDOUT"), &dest)
+                            }
+                        }
+                    ),
                 );
             } else {
                 // The futex value doesn't match the expected value, so we return failure

src/shims/unix/macos/foreign_items.rs

@@ -2,13 +2,21 @@ use rustc_middle::ty::Ty;
 use rustc_span::Symbol;
 use rustc_target::callconv::{Conv, FnAbi};
 
-use super::sync::EvalContextExt as _;
+use super::sync::{EvalContextExt as _, MacOsFutexTimeout};
 use crate::helpers::check_min_arg_count;
 use crate::shims::unix::*;
 use crate::*;
 
-pub fn is_dyn_sym(_name: &str) -> bool {
-    false
+pub fn is_dyn_sym(name: &str) -> bool {
+    match name {
+        // These only became available with macOS 11.0, so std looks them up dynamically.
+        "os_sync_wait_on_address"
+        | "os_sync_wait_on_address_with_deadline"
+        | "os_sync_wait_on_address_with_timeout"
+        | "os_sync_wake_by_address_any"
+        | "os_sync_wake_by_address_all" => true,
+        _ => false,
+    }
 }
 
 impl<'tcx> EvalContextExt<'tcx> for crate::MiriInterpCx<'tcx> {}
@@ -216,6 +224,53 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
                 this.write_scalar(res, dest)?;
             }
 
+            "os_sync_wait_on_address" => {
+                let [addr_op, value_op, size_op, flags_op] =
+                    this.check_shim(abi, Conv::C, link_name, args)?;
+                this.os_sync_wait_on_address(
+                    addr_op,
+                    value_op,
+                    size_op,
+                    flags_op,
+                    MacOsFutexTimeout::None,
+                    dest,
+                )?;
+            }
+            "os_sync_wait_on_address_with_deadline" => {
+                let [addr_op, value_op, size_op, flags_op, clock_op, timeout_op] =
+                    this.check_shim(abi, Conv::C, link_name, args)?;
+                this.os_sync_wait_on_address(
+                    addr_op,
+                    value_op,
+                    size_op,
+                    flags_op,
+                    MacOsFutexTimeout::Absolute { clock_op, timeout_op },
+                    dest,
+                )?;
+            }
+            "os_sync_wait_on_address_with_timeout" => {
+                let [addr_op, value_op, size_op, flags_op, clock_op, timeout_op] =
+                    this.check_shim(abi, Conv::C, link_name, args)?;
+                this.os_sync_wait_on_address(
+                    addr_op,
+                    value_op,
+                    size_op,
+                    flags_op,
+                    MacOsFutexTimeout::Relative { clock_op, timeout_op },
+                    dest,
+                )?;
+            }
+            "os_sync_wake_by_address_any" => {
+                let [addr_op, size_op, flags_op] =
+                    this.check_shim(abi, Conv::C, link_name, args)?;
+                this.os_sync_wake_by_address(addr_op, size_op, flags_op, false, dest)?;
+            }
+            "os_sync_wake_by_address_all" => {
+                let [addr_op, size_op, flags_op] =
+                    this.check_shim(abi, Conv::C, link_name, args)?;
+                this.os_sync_wake_by_address(addr_op, size_op, flags_op, true, dest)?;
+            }
+
             "os_unfair_lock_lock" => {
                 let [lock_op] = this.check_shim(abi, Conv::C, link_name, args)?;
                 this.os_unfair_lock_lock(lock_op)?;