Replace NSRecursiveLock with pthread mutex in PropertyBox.

Sources/Property.swift

@@ -1,4 +1,8 @@
-import Foundation
+#if os(iOS) || os(macOS) || os(tvOS) || os(watchOS)
+import Darwin.POSIX.pthread
+#else
+import Glibc
+#endif
 import enum Result.NoError
 
 /// Represents a property that allows observation of its changes.
@@ -698,16 +702,51 @@ public final class MutableProperty<Value>: ComposableMutablePropertyProtocol {
 /// The requirement of a `Value?` storage from composed properties prevents further
 /// implementation sharing with `MutableProperty`.
 private final class PropertyBox<Value> {
-	private let lock = NSRecursiveLock()
+	private let mutex: UnsafeMutablePointer<pthread_mutex_t>
 	private var _value: Value
 	var value: Value { return modify { $0 } }
 
 	init(_ value: Value) {
-		self._value = value
+		_value = value
+		mutex = .allocate(capacity: 1)
+		mutex.initialize(to: pthread_mutex_t())
+
+		let attr = UnsafeMutablePointer<pthread_mutexattr_t>.allocate(capacity: 1)
+		attr.initialize(to: pthread_mutexattr_t())
+
+		defer {
+			attr.deinitialize()
+			attr.deallocate(capacity: 1)
+		}
+
+		pthread_mutexattr_init(attr)
+		pthread_mutexattr_settype(attr, Int32(PTHREAD_MUTEX_RECURSIVE))
+		#if os(iOS) || os(macOS) || os(tvOS) || os(watchOS)
+		// The pthread mutex implementation of Darwin guarantees fairness by
+		// default. Since fairness is not a concern of properties anyway, it can
+		// be disabled.
+		pthread_mutexattr_setpolicy_np(attr, _PTHREAD_MUTEX_POLICY_FIRSTFIT)
+		#endif
+
+		pthread_mutex_init(mutex, attr)
+		pthread_mutexattr_destroy(attr)
+	}
+
+	deinit {
+		pthread_mutex_destroy(mutex)
+		mutex.deinitialize()
+		mutex.deallocate(capacity: 1)
 	}
 
 	func modify<Result>(didSet: (Value) -> Void = { _ in }, _ action: (inout Value) throws -> Result) rethrows -> Result {
-		lock.lock(); defer { didSet(_value); lock.unlock() }
+		let status = pthread_mutex_lock(mutex)
+		assert(status == 0, "Failed to acquire the property box recursive lock. Status code: \(status)")
+
+		defer {
+			didSet(_value)
+			pthread_mutex_unlock(mutex)
+		}
+
 		return try action(&_value)
 	}
 }