Add DiscardingTaskGroup and Stream polyfills for cluster support (#94)

Author: fabianfett

Sources/RediStack/Cluster/SwiftPolyfill.swift

@@ -0,0 +1,46 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the RediStack open source project
+//
+// Copyright (c) 2023 RediStack project authors
+// Licensed under Apache License v2.0
+//
+// See LICENSE.txt for license information
+// See CONTRIBUTORS.txt for the list of RediStack project authors
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+//===----------------------------------------------------------------------===//
+
+/// We use this protocol internally to abstract over TaskGroups. On Linux we can always use a `DiscardingTaskGroup`,
+/// but to support Swift 5.8 on macOS we need to fallback to the original TaskGroup.
+protocol DiscardingTaskGroupProtocol {
+    mutating func addTask(priority: TaskPriority?, operation: @escaping @Sendable () async -> Void)
+}
+
+extension DiscardingTaskGroupProtocol {
+    mutating func addTask(_ operation: @escaping @Sendable () async -> Void) {
+        self.addTask(priority: nil, operation: operation)
+    }
+}
+
+extension TaskGroup: DiscardingTaskGroupProtocol where ChildTaskResult == Void {}
+
+#if swift(>=5.9) || (swift(>=5.8) && os(Linux))
+@available(macOS 14.0, iOS 17.0, watchOS 10.0, tvOS 17.0, *)
+extension DiscardingTaskGroup: DiscardingTaskGroupProtocol {}
+#endif
+
+#if swift(<5.9)
+// This should be removed once we support Swift 5.9+ only
+extension AsyncStream {
+    static func makeStream(
+        of elementType: Element.Type = Element.self,
+        bufferingPolicy limit: Continuation.BufferingPolicy = .unbounded
+    ) -> (stream: AsyncStream<Element>, continuation: AsyncStream<Element>.Continuation) {
+        var continuation: AsyncStream<Element>.Continuation!
+        let stream = AsyncStream<Element>(bufferingPolicy: limit) { continuation = $0 }
+        return (stream: stream, continuation: continuation!)
+    }
+}
+#endif