VPN: DNSResolver: Resolve multiple endpoints in parallel

Signed-off-by: Roopesh Chander <roop@roopc.net>

WireGuard/WireGuard/VPN/DNSResolver.swift

@@ -6,40 +6,51 @@ import Foundation
 
 class DNSResolver {
     let endpoints: [Endpoint?]
+    let dispatchGroup: DispatchGroup
+    var dispatchWorkItems: [DispatchWorkItem]
 
     init(endpoints: [Endpoint?]) {
         self.endpoints = endpoints
+        self.dispatchWorkItems = []
+        self.dispatchGroup = DispatchGroup()
     }
 
     func resolve(completionHandler: @escaping ([Endpoint?]?) -> Void) {
         let endpoints = self.endpoints
-        DispatchQueue.global(qos: .userInitiated).async {
-            var resolvedEndpoints: [Endpoint?] = []
-            var isError = false
-            for endpoint in endpoints {
-                if let endpoint = endpoint {
-                    if let resolvedEndpoint = DNSResolver.resolveSync(endpoint: endpoint) {
-                        resolvedEndpoints.append(resolvedEndpoint)
-                    } else {
-                        isError = true
-                        break
+        let dispatchGroup = self.dispatchGroup
+        dispatchWorkItems = []
+        var resolvedEndpoints: [Endpoint?] = Array<Endpoint?>(repeating: nil, count: endpoints.count)
+        let numberOfEndpointsToResolve = endpoints.compactMap { $0 }.count
+        for (i, endpoint) in self.endpoints.enumerated() {
+            guard let endpoint = endpoint else { return }
+            let workItem = DispatchWorkItem {
+                resolvedEndpoints[i] = DNSResolver.resolveSync(endpoint: endpoint)
             }
-                } else {
-                    resolvedEndpoints.append(nil)
+            dispatchWorkItems.append(workItem)
+            DispatchQueue.global(qos: .userInitiated).async(group: dispatchGroup, execute: workItem)
         }
-            }
-            if (isError) {
-                DispatchQueue.main.async {
+        dispatchGroup.notify(queue: .main) {
+            let numberOfResolvedEndpoints = resolvedEndpoints.compactMap { $0 }.count
+            if (numberOfResolvedEndpoints < numberOfEndpointsToResolve) {
                 completionHandler(nil)
-                }
-                return
-            }
-            DispatchQueue.main.async {
+            } else {
                 completionHandler(resolvedEndpoints)
             }
         }
     }
 
+    func cancel() {
+        for workItem in dispatchWorkItems {
+            workItem.cancel()
+        }
+    }
+
+    deinit {
+        cancel()
+    }
+}
+
+extension DNSResolver {
     // Based on DNS resolution code by Jason Donenfeld <jason@zx2c4.com>
     // in parse_endpoint() in src/tools/config.c in the WireGuard codebase
     private static func resolveSync(endpoint: Endpoint) -> Endpoint? {