Add thread safety for fonts cache via read-write lock in MTFontManager

Sources/SwiftMath/MathRender/RWLock.swift

@@ -0,0 +1,58 @@
+import Foundation
+
+final class RWLock {
+    init() {
+        pthread_rwlock_init(&lock, nil)
+    }
+
+    deinit {
+        pthread_rwlock_destroy(&lock)
+    }
+
+    func read<T>(_ block: () -> T) -> T {
+        pthread_rwlock_rdlock(&lock)
+        defer { pthread_rwlock_unlock(&lock) }
+        return block()
+    }
+
+    func readWrite<T>(_ block: () -> T) -> T {
+        pthread_rwlock_wrlock(&lock)
+        defer { pthread_rwlock_unlock(&lock) }
+        return block()
+    }
+
+    private var lock = pthread_rwlock_t()
+}
+
+@propertyWrapper
+struct RWLocked<T> {
+    init(wrappedValue: T) {
+        value = wrappedValue
+    }
+
+    var wrappedValue: T {
+        get {
+            lock.read {
+                value
+            }
+        }
+        set {
+            lock.readWrite {
+                value = newValue
+            }
+        }
+    }
+
+    @discardableResult
+    mutating func readWrite(_ block: (inout T) -> Void) -> (oldValue: T, newValue: T) {
+        lock.readWrite {
+            let old = value
+            block(&value)
+            return (old, value)
+        }
+    }
+
+    private var value: T
+    private let lock = RWLock()
+}
+