Enable strict concurrency (#54)

### Motivation:

Catch potential data races at build time.

### Modifications:

- Enabled strict concurrency checking in the Package.swift.
- Made a few types Sendable: `AresChannel`, `CAresDNSResolver`, `Ares`,
`QueryProcessor`, `DNSSDDNSResolver`.

### Result:

Fewer potential data races can sneak in.

### Test Plan

Ran tests locally, did not see any concurrency warnings or errors, all
tests passed.

Author: czechboy0

Package.swift

@@ -50,3 +50,9 @@ let package = Package(
     ],
     cLanguageStandard: .gnu11
 )
+
+for target in package.targets {
+    var settings = target.swiftSettings ?? []
+    settings.append(.enableExperimentalFeature("StrictConcurrency=complete"))
+    target.swiftSettings = settings
+}

Sources/AsyncDNSResolver/c-ares/AresChannel.swift

@@ -18,17 +18,20 @@ import Foundation
 // MARK: - ares_channel
 
 @available(macOS 10.15, iOS 13, tvOS 13, watchOS 6, *)
-class AresChannel {
-    let pointer: UnsafeMutablePointer<ares_channel?>
-    let lock = NSLock()
+final class AresChannel: @unchecked Sendable {
+    private let locked_pointer: UnsafeMutablePointer<ares_channel?>
+    private let lock = NSLock()
 
-    private var underlying: ares_channel? {
-        self.pointer.pointee
+    // For testing only.
+    var underlying: ares_channel? {
+        self.locked_pointer.pointee
     }
 
     deinit {
-        ares_destroy(pointer.pointee)
-        pointer.deallocate()
+        // Safe to perform without the lock, as in deinit we know that no more
+        // strong references to self exist, so nobody can be holding the lock.
+        ares_destroy(locked_pointer.pointee)
+        locked_pointer.deallocate()
         ares_library_cleanup()
     }
 
@@ -49,7 +52,7 @@ class AresChannel {
             try checkAresResult { ares_set_sortlist(pointer.pointee, sortlist) }
         }
 
-        self.pointer = pointer
+        self.locked_pointer = pointer
     }
 
     func withChannel(_ body: (ares_channel) -> Void) {

Sources/AsyncDNSResolver/c-ares/AresOptions.swift

@@ -19,7 +19,7 @@ import CAsyncDNSResolver
 @available(macOS 10.15, iOS 13, tvOS 13, watchOS 6, *)
 extension CAresDNSResolver {
     /// Options for ``CAresDNSResolver``.
-    public struct Options {
+    public struct Options: Sendable {
         public static var `default`: Options {
             .init()
         }
@@ -91,7 +91,7 @@ extension CAresDNSResolver {
 
 @available(macOS 10.15, iOS 13, tvOS 13, watchOS 6, *)
 extension CAresDNSResolver.Options {
-    public struct Flags: OptionSet {
+    public struct Flags: OptionSet, Sendable {
         public let rawValue: Int32
 
         public init(rawValue: Int32) {

Sources/AsyncDNSResolver/c-ares/DNSResolver_c-ares.swift

@@ -13,10 +13,11 @@
 //===----------------------------------------------------------------------===//
 
 import CAsyncDNSResolver
+import Foundation
 
 /// ``DNSResolver`` implementation backed by c-ares C library.
 @available(macOS 10.15, iOS 13, tvOS 13, watchOS 6, *)
-public class CAresDNSResolver: DNSResolver {
+public final class CAresDNSResolver: DNSResolver, Sendable {
     let options: Options
     let ares: Ares
 
@@ -121,18 +122,15 @@ extension QueryType {
 // MARK: - c-ares query wrapper
 
 @available(macOS 10.15, iOS 13, tvOS 13, watchOS 6, *)
-class Ares {
+final class Ares: Sendable {
     typealias QueryCallback = @convention(c) (
         UnsafeMutableRawPointer?, CInt, CInt, UnsafeMutablePointer<CUnsignedChar>?, CInt
     ) -> Void
 
-    let options: AresOptions
-    let channel: AresChannel
-
+    private let channel: AresChannel
     private let queryProcessor: QueryProcessor
 
     init(options: AresOptions) throws {
-        self.options = options
         self.channel = try AresChannel(options: options)
 
         // Need to call `ares_process` or `ares_process_fd` for query callbacks to happen
@@ -145,7 +143,8 @@ class Ares {
         name: String,
         replyParser: ReplyParser
     ) async throws -> ReplyParser.Reply {
-        try await withTaskCancellationHandler(
+        let channel = self.channel
+        return try await withTaskCancellationHandler(
             operation: {
                 try await withCheckedThrowingContinuation { continuation in
                     let handler = QueryReplyHandler(parser: replyParser, continuation)
@@ -178,7 +177,7 @@ class Ares {
                 }
             },
             onCancel: {
-                self.channel.withChannel { channel in
+                channel.withChannel { channel in
                     ares_cancel(channel)
                 }
             }
@@ -198,16 +197,18 @@ extension Ares {
     // https://github.com/dimbleby/c-ares-resolver/blob/master/src/unix/eventloop.rs  // ignore-unacceptable-language
     // https://github.com/dimbleby/rust-c-ares/blob/master/src/channel.rs  // ignore-unacceptable-language
     // https://github.com/dimbleby/rust-c-ares/blob/master/examples/event-loop.rs  // ignore-unacceptable-language
-    class QueryProcessor {
+    final class QueryProcessor: @unchecked Sendable {
         static let defaultPollInterval: UInt64 = 10 * 1_000_000  // 10ms
 
         private let channel: AresChannel
         private let pollIntervalNanos: UInt64
 
-        private var pollingTask: Task<Void, Error>?
+        private let lock = NSLock()
+        private var locked_pollingTask: Task<Void, Error>?
 
         deinit {
-            self.pollingTask?.cancel()
+            // No need to lock here as there can exist no more strong references to self.
+            self.locked_pollingTask?.cancel()
         }
 
         init(channel: AresChannel, pollIntervalNanos: UInt64 = QueryProcessor.defaultPollInterval) {
@@ -218,7 +219,7 @@ extension Ares {
         /// Asks c-ares for the set of socket descriptors we are waiting on for the `ares_channel`'s pending queries
         /// then call `ares_process_fd` if any is ready for read and/or write.
         /// c-ares returns up to `ARES_GETSOCK_MAXNUM` socket descriptors only. If more are in use (unlikely) they are not reported back.
-        func poll() async {
+        func poll() {
             var socks = [ares_socket_t](repeating: ares_socket_t(), count: Int(ARES_GETSOCK_MAXNUM))
 
             self.channel.withChannel { channel in
@@ -249,12 +250,14 @@ extension Ares {
         }
 
         private func schedule() {
-            self.pollingTask = Task { [weak self] in
+            self.lock.lock()
+            defer { self.lock.unlock() }
+            self.locked_pollingTask = Task { [weak self] in
                 guard let s = self else {
                     return
                 }
                 try await Task.sleep(nanoseconds: s.pollIntervalNanos)
-                await s.poll()
+                s.poll()
             }
         }
     }
@@ -291,7 +294,7 @@ extension Ares {
 // MARK: - c-ares query reply parsers
 
 protocol AresQueryReplyParser {
-    associatedtype Reply
+    associatedtype Reply: Sendable
 
     func parse(buffer: UnsafeMutablePointer<CUnsignedChar>?, length: CInt) throws -> Reply
 }

Sources/AsyncDNSResolver/dnssd/DNSResolver_dnssd.swift

@@ -17,7 +17,7 @@ import dnssd
 
 /// ``DNSResolver`` implementation backed by dnssd framework.
 @available(macOS 10.15, iOS 13, tvOS 13, watchOS 6, *)
-public struct DNSSDDNSResolver: DNSResolver {
+public struct DNSSDDNSResolver: DNSResolver, Sendable {
     let dnssd: DNSSD
 
     init() {
@@ -100,7 +100,7 @@ extension QueryType {
 // MARK: - dnssd query wrapper
 
 @available(macOS 10.15, iOS 13, tvOS 13, watchOS 6, *)
-struct DNSSD {
+struct DNSSD: Sendable {
     // Reference: https://gist.github.com/fikeminkel/a9c4bc4d0348527e8df3690e242038d3
     func query<ReplyHandler: DNSSDQueryReplyHandler>(
         type: QueryType,
@@ -225,7 +225,7 @@ extension DNSSD {
 // MARK: - dnssd query reply handlers
 
 protocol DNSSDQueryReplyHandler {
-    associatedtype Record
+    associatedtype Record: Sendable
     associatedtype Reply
 
     func parseRecord(data: UnsafeRawPointer?, length: UInt16) throws -> Record?

Tests/AsyncDNSResolverTests/c-ares/AresChannelTests.swift

@@ -29,7 +29,7 @@ final class AresChannelTests: XCTestCase {
         guard let channel = try? AresChannel(options: options) else {
             return XCTFail("Channel not initialized")
         }
-        guard let _ = channel.pointer.pointee else {
+        guard let _ = channel.underlying else {
             return XCTFail("Underlying ares_channel is nil")
         }
     }

Tests/AsyncDNSResolverTests/c-ares/CAresDNSResolverTests.swift

@@ -124,7 +124,7 @@ final class CAresDNSResolverTests: XCTestCase {
     func test_concurrency() async throws {
         func run(
             times: Int = 100,
-            _ query: @escaping (_ index: Int) async throws -> Void
+            _ query: @Sendable @escaping (_ index: Int) async throws -> Void
         ) async throws {
             try await withThrowingTaskGroup(of: Void.self) { group in
                 for i in 1...times {
@@ -136,65 +136,67 @@ final class CAresDNSResolverTests: XCTestCase {
             }
         }
 
+        let resolver = self.resolver!
+        let verbose = self.verbose
         try await run { i in
-            let reply = try await self.resolver.queryA(name: "apple.com")
-            if self.verbose {
+            let reply = try await resolver.queryA(name: "apple.com")
+            if verbose {
                 print("[A] run #\(i) result: \(reply)")
             }
         }
 
         try await run { i in
-            let reply = try await self.resolver.queryAAAA(name: "apple.com")
-            if self.verbose {
+            let reply = try await resolver.queryAAAA(name: "apple.com")
+            if verbose {
                 print("[AAAA] run #\(i) result: \(reply)")
             }
         }
 
         try await run { i in
-            let reply = try await self.resolver.queryNS(name: "apple.com")
-            if self.verbose {
+            let reply = try await resolver.queryNS(name: "apple.com")
+            if verbose {
                 print("[NS] run #\(i) result: \(reply)")
             }
         }
 
         try await run { i in
-            let reply = try await self.resolver.queryCNAME(name: "www.apple.com")
-            if self.verbose {
+            let reply = try await resolver.queryCNAME(name: "www.apple.com")
+            if verbose {
                 print("[CNAME] run #\(i) result: \(String(describing: reply))")
             }
         }
 
         try await run { i in
-            let reply = try await self.resolver.querySOA(name: "apple.com")
-            if self.verbose {
+            let reply = try await resolver.querySOA(name: "apple.com")
+            if verbose {
                 print("[SOA] run #\(i) result: \(String(describing: reply))")
             }
         }
 
         try await run { i in
-            let reply = try await self.resolver.queryPTR(name: "47.224.172.17.in-addr.arpa")
-            if self.verbose {
+            let reply = try await resolver.queryPTR(name: "47.224.172.17.in-addr.arpa")
+            if verbose {
                 print("[PTR] run #\(i) result: \(reply)")
             }
         }
 
         try await run { i in
-            let reply = try await self.resolver.queryMX(name: "apple.com")
-            if self.verbose {
+            let reply = try await resolver.queryMX(name: "apple.com")
+            if verbose {
                 print("[MX] run #\(i) result: \(reply)")
             }
         }
 
         try await run { i in
-            let reply = try await self.resolver.queryTXT(name: "apple.com")
-            if self.verbose {
+            let reply = try await resolver.queryTXT(name: "apple.com")
+            if verbose {
                 print("[TXT] run #\(i) result: \(reply)")
             }
         }
 
         try await run { i in
-            let reply = try await self.resolver.querySRV(name: "_caldavs._tcp.google.com")
-            if self.verbose {
+            let reply = try await resolver.querySRV(name: "_caldavs._tcp.google.com")
+            if verbose {
                 print("[SRV] run #\(i) result: \(reply)")
             }
         }