Fix darwin ulock usage on macOS (#9545)

* Fix darwin ulock usage on macOS

* Fix review comments, and correctly handle timeout overflow on darwin

* Apply more review suggestions

Author: thomcc

lib/std/Thread/Futex.zig

@@ -180,32 +180,48 @@ const DarwinFutex = struct {
     const darwin = std.os.darwin;
 
     fn wait(ptr: *const Atomic(u32), expect: u32, timeout: ?u64) error{TimedOut}!void {
-        // ulock_wait() uses micro-second timeouts, where 0 = INIFITE or no-timeout
-        var timeout_us: u32 = 0;
-        if (timeout) |timeout_ns| {
-            timeout_us = @intCast(u32, timeout_ns / std.time.ns_per_us);
-        }
-
         // Darwin XNU 7195.50.7.100.1 introduced __ulock_wait2 and migrated code paths (notably pthread_cond_t) towards it:
         // https://github.com/apple/darwin-xnu/commit/d4061fb0260b3ed486147341b72468f836ed6c8f#diff-08f993cc40af475663274687b7c326cc6c3031e0db3ac8de7b24624610616be6
         //
         // This XNU version appears to correspond to 11.0.1:
         // https://kernelshaman.blogspot.com/2021/01/building-xnu-for-macos-big-sur-1101.html
+        //
+        // ulock_wait() uses 32-bit micro-second timeouts where 0 = INFINITE or no-timeout
+        // ulock_wait2() uses 64-bit nano-second timeouts (with the same convention)
+        var timeout_ns: u64 = 0;
+        if (timeout) |timeout_value| {
+            // This should be checked by the caller.
+            assert(timeout_value != 0);
+            timeout_ns = timeout_value;
+        }
         const addr = @ptrCast(*const c_void, ptr);
         const flags = darwin.UL_COMPARE_AND_WAIT | darwin.ULF_NO_ERRNO;
+        // If we're using `__ulock_wait` and `timeout` is too big to fit inside a `u32` count of
+        // micro-seconds (around 70min), we'll request a shorter timeout. This is fine (users
+        // should handle spurious wakeups), but we need to remember that we did so, so that
+        // we don't return `TimedOut` incorrectly. If that happens, we set this variable to
+        // true so that we we know to ignore the ETIMEDOUT result.
+        var timeout_overflowed = false;
         const status = blk: {
             if (target.os.version_range.semver.max.major >= 11) {
-                break :blk darwin.__ulock_wait2(flags, addr, expect, timeout_us, 0);
+                break :blk darwin.__ulock_wait2(flags, addr, expect, timeout_ns, 0);
             } else {
+                const timeout_us = std.math.cast(u32, timeout_ns / std.time.ns_per_us) catch overflow: {
+                    timeout_overflowed = true;
+                    break :overflow std.math.maxInt(u32);
+                };
                 break :blk darwin.__ulock_wait(flags, addr, expect, timeout_us);
             }
         };
 
         if (status >= 0) return;
         switch (-status) {
             darwin.EINTR => {},
-            darwin.EFAULT => unreachable,
-            darwin.ETIMEDOUT => return error.TimedOut,
+            // Address of the futex is paged out. This is unlikely, but possible in theory, and
+            // pthread/libdispatch on darwin bother to handle it. In this case we'll return
+            // without waiting, but the caller should retry anyway.
+            darwin.EFAULT => {},
+            darwin.ETIMEDOUT => if (!timeout_overflowed) return error.TimedOut,
             else => unreachable,
         }
     }
@@ -223,6 +239,7 @@ const DarwinFutex = struct {
             if (status >= 0) return;
             switch (-status) {
                 darwin.EINTR => continue, // spurious wake()
+                darwin.EFAULT => continue, // address of the lock was paged out
                 darwin.ENOENT => return, // nothing was woken up
                 darwin.EALREADY => unreachable, // only for ULF_WAKE_THREAD
                 else => unreachable,

lib/std/c/darwin.zig

@@ -246,7 +246,7 @@ pub const UL_COMPARE_AND_WAIT64 = 5;
 pub const UL_COMPARE_AND_WAIT64_SHARED = 6;
 pub const ULF_WAIT_ADAPTIVE_SPIN = 0x40000;
 
-pub extern "c" fn __ulock_wait2(op: u32, addr: ?*const c_void, val: u64, timeout_us: u32, val2: u64) c_int;
+pub extern "c" fn __ulock_wait2(op: u32, addr: ?*const c_void, val: u64, timeout_ns: u64, val2: u64) c_int;
 pub extern "c" fn __ulock_wait(op: u32, addr: ?*const c_void, val: u64, timeout_us: u32) c_int;
 pub extern "c" fn __ulock_wake(op: u32, addr: ?*const c_void, val: u64) c_int;