Add a DNS name resolver

Package.swift

@@ -76,6 +76,10 @@ let packageDependencies: [Package.Dependency] = [
     url: "https://github.com/apple/swift-distributed-tracing.git",
     from: "1.0.0"
   ),
+  .package(
+    url: "https://github.com/apple/swift-async-dns-resolver.git",
+    branch: "main"
+  )
 ].appending(
   .package(
     url: "https://github.com/apple/swift-nio-ssl.git",
@@ -206,7 +210,8 @@ extension Target {
       .nioCore,
       .nioHTTP2,
       .cgrpcZlib,
-      .dequeModule
+      .dequeModule,
+      .product(name: "AsyncDNSResolver", package: "swift-async-dns-resolver")
     ]
   )
 

Sources/GRPCHTTP2Core/Client/Resolver/NameResolver+DNS.swift

@@ -0,0 +1,338 @@
+/*
+ * Copyright 2024, gRPC Authors All rights reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import AsyncDNSResolver
+import GRPCCore
+
+import struct Foundation.Data
+import class Foundation.JSONDecoder
+
+extension ResolvableTargets {
+  /// A resolvable target for IPv4 addresses.
+  ///
+  /// IPv4 addresses can be resolved by the ``NameResolvers/DNS``.
+  public struct DNS: ResolvableTarget {
+    /// The host to resolve via DNS.
+    public var host: String
+
+    /// The port to use with resolved addresses.
+    public var port: Int
+
+    /// Create a new DNS target.
+    /// - Parameters:
+    ///   - host: The host to resolve via DNS.
+    ///   - port: The port to use with resolved addresses.
+    public init(host: String, port: Int) {
+      self.host = host
+      self.port = port
+    }
+  }
+}
+
+extension ResolvableTarget where Self == ResolvableTargets.DNS {
+  /// Creates a new resolvable DNS target.
+  /// - Parameters:
+  ///   - host: The host address to resolve.
+  ///   - port: The port to use for each resolved address.
+  /// - Returns: A ``ResolvableTarget``.
+  public static func dns(host: String, port: Int = 443) -> Self {
+    return Self(host: host, port: port)
+  }
+}
+
+@available(macOS 13.0, iOS 16.0, watchOS 9.0, tvOS 16.0, *)
+extension NameResolvers {
+  /// A ``NameResolverFactory`` for ``ResolvableTargets/IPv4`` targets.
+  ///
+  /// The name resolver for a given target always produces the same values, with one endpoint per
+  /// address in the target. The service configuration can be specified when creating the resolver
+  /// factory and defaults to `ServiceConfiguration.default`.
+  public struct DNS: NameResolverFactory {
+    public typealias Target = ResolvableTargets.DNS
+
+    private let dnsResolver: AsyncDNSResolver
+    private let fetchServiceConfiguration: Bool
+
+    /// Create a new DNS name resolver factory.
+    ///
+    /// - Parameters:
+    ///   - resolver: The DNS resolver to use.
+    ///   - fetchServiceConfiguration: Whether service config should be fetched from DNS TXT
+    ///       records.
+    public init(resolver: AsyncDNSResolver, fetchServiceConfiguration: Bool) {
+      self.dnsResolver = resolver
+      self.fetchServiceConfiguration = fetchServiceConfiguration
+    }
+
+    /// Creates a new DNS resolver factory.
+    ///
+    /// - Parameters:
+    ///   - fetchServiceConfiguration: Whether service config should be fetched from DNS TXT
+    ///       records.
+    public init(fetchServiceConfiguration: Bool) throws {
+      self.dnsResolver = try AsyncDNSResolver()
+      self.fetchServiceConfiguration = fetchServiceConfiguration
+    }
+
+    public func resolver(for target: Target) -> NameResolver {
+      let resolver = Self.Resolver(
+        dns: self.dnsResolver,
+        fetchServiceConfiguration: self.fetchServiceConfiguration,
+        target: target
+      )
+
+      return NameResolver(names: RPCAsyncSequence(wrapping: resolver), updateMode: .pull)
+    }
+  }
+}
+
+@available(macOS 13.0, iOS 16.0, watchOS 9.0, tvOS 16.0, *)
+extension NameResolvers.DNS {
+  struct Resolver {
+    /// The underlying raw DNS resolver.
+    let dns: AsyncDNSResolver
+    /// The target to resolve.
+    let target: ResolvableTargets.DNS
+    /// Whether service configuration should be fetched from DNS TXT records.
+    let fetchServiceConfiguration: Bool
+    /// JSON decoder used for decoding service configuration.
+    let decoder: JSONDecoder
+    /// The hostname of the system the resolver is running on. May be used when a service config
+    /// from a list of config choices.
+    let hostname: String
+
+    /// Prefix for DNS TXT records containing gRPC config.
+    private static let txtRecordPrefix = "grpc_config="
+    /// Name prefix for DNS TXT records containing gRPC config. E.g. if the name to resolve is
+    /// `grpc.io` then the name of the TXT records to resolve is `_grpc_config.grpc.io`.
+    private static let txtRecordNamePrefix = "_grpc_config."
+
+    init(
+      dns: AsyncDNSResolver,
+      fetchServiceConfiguration: Bool,
+      target: ResolvableTargets.DNS
+    ) {
+      self.dns = dns
+      self.target = target
+      self.fetchServiceConfiguration = fetchServiceConfiguration
+      self.decoder = JSONDecoder()
+      self.hostname = System.hostname()
+    }
+
+    func resolve() async throws -> NameResolutionResult {
+      // Kick off address resolution
+      async let _endpoints = self.resolveEndpoints()
+
+      // Fetch the service configuration and pick an appropriate choice.
+      let serviceConfig: Result<ServiceConfiguration, RPCError>?
+      if self.fetchServiceConfiguration {
+        switch await self.resolveServiceConfigChoices() {
+        case .success(let choices):
+          serviceConfig = self.selectServiceConfig(choices: choices).map { .success($0) }
+        case .failure(let error):
+          serviceConfig = .failure(error)
+        }
+      } else {
+        serviceConfig = nil
+      }
+
+      let endpoints = try await _endpoints.get()
+      return NameResolutionResult(endpoints: endpoints, serviceConfiguration: serviceConfig)
+    }
+
+    private func resolveEndpoints() async -> Result<[Endpoint], RPCError> {
+      return await withTaskGroup(of: Result<[Endpoint], RPCError>.self) { group in
+        group.addTask {
+          do {
+            let records = try await self.dns.queryA(name: self.target.host)
+            let endpoints = records.map { record in
+              let address = SocketAddress.ipv4(host: record.address.address, port: self.target.port)
+              return Endpoint(addresses: [address])
+            }
+            return .success(endpoints)
+          } catch {
+            let error = RPCError(
+              code: .internalError,
+              message: "DNS lookup for A records associated with '\(self.target.host)' failed",
+              cause: error
+            )
+            return .failure(error)
+          }
+        }
+
+        group.addTask {
+          do {
+            let records = try await self.dns.queryAAAA(name: self.target.host)
+            let endpoints = records.map { record in
+              let address = SocketAddress.ipv6(host: record.address.address, port: self.target.port)
+              return Endpoint(addresses: [address])
+            }
+            return .success(endpoints)
+          } catch {
+            let error = RPCError(
+              code: .internalError,
+              message: "DNS lookup for AAAA records associated with '\(self.target.host)' failed",
+              cause: error
+            )
+            return .failure(error)
+          }
+        }
+
+        var all = [Endpoint]()
+        for await result in group {
+          switch result {
+          case .success(let endpoints):
+            all.append(contentsOf: endpoints)
+          case .failure(let error):
+            return .failure(error)
+          }
+        }
+
+        return .success(all)
+      }
+    }
+
+    private func resolveServiceConfigChoices() async -> Result<[ServiceConfigChoice], RPCError> {
+      let name = Self.txtRecordNamePrefix + self.target.host
+
+      do {
+        let records = try await self.dns.queryTXT(name: name)
+        return self.parseTXTRecords(records)
+      } catch {
+        let error = RPCError(
+          code: .internalError,
+          message: "DNS lookup for TXT records associated with '\(name)' failed",
+          cause: error
+        )
+        return .failure(error)
+      }
+    }
+
+    private func parseTXTRecords(
+      _ records: [TXTRecord]
+    ) -> Result<[ServiceConfigChoice], RPCError> {
+      var allChoices = [ServiceConfigChoice]()
+
+      for record in records {
+        // The records are prefixed with "grpc_config=". The suffix is an array of service config
+        // choice objects encoded as a JSON array.
+        guard record.txt.hasPrefix(Self.txtRecordPrefix) else { continue }
+
+        // Drop the prefix, the rest of the content should be a JSON array.
+        let json = Data(record.txt.utf8.dropFirst(Self.txtRecordPrefix.utf8.count))
+
+        do {
+          let choices = try self.decoder.decode([ServiceConfigChoice].self, from: json)
+          allChoices.append(contentsOf: choices)
+        } catch {
+          let error = RPCError(
+            code: .internalError,
+            message: "Can't decode service_config choice.",
+            cause: error
+          )
+          return .failure(error)
+        }
+      }
+
+      return .success(allChoices)
+    }
+
+    private func selectServiceConfig(choices: [ServiceConfigChoice]) -> ServiceConfiguration? {
+      let choice = choices.first { $0.select(hostname: self.hostname) }
+      return choice?.configuration
+    }
+  }
+}
+
+@available(macOS 13.0, iOS 16.0, watchOS 9.0, tvOS 16.0, *)
+extension NameResolvers.DNS.Resolver: AsyncSequence, AsyncIteratorProtocol {
+  typealias Element = NameResolutionResult
+
+  func makeAsyncIterator() -> NameResolvers.DNS.Resolver {
+    return self
+  }
+
+  func next() async throws -> NameResolutionResult? {
+    return try await self.resolve()
+  }
+}
+
+// Service configuration provided by DNS is contained in a JSON object with this shape.
+@available(macOS 13.0, iOS 16.0, watchOS 9.0, tvOS 16.0, *)
+struct ServiceConfigChoice: Codable {
+  var language: [String]
+  var percentage: Int?
+  var hostname: [String]
+  var configuration: ServiceConfiguration
+
+  enum CodingKeys: CodingKey {
+    case clientLanguage
+    case percentage
+    case clientHostname
+    case serviceConfig
+  }
+
+  init(
+    language: [String] = [],
+    percentage: Int? = nil,
+    hostname: [String] = [],
+    configuration: ServiceConfiguration
+  ) {
+    self.language = language
+    self.percentage = percentage
+    self.hostname = hostname
+    self.configuration = configuration
+  }
+
+  init(from decoder: any Decoder) throws {
+    let container = try decoder.container(keyedBy: CodingKeys.self)
+    self.language = try container.decodeIfPresent([String].self, forKey: .clientLanguage) ?? []
+    self.percentage = try container.decodeIfPresent(Int.self, forKey: .percentage)
+    self.hostname = try container.decodeIfPresent([String].self, forKey: .clientHostname) ?? []
+    self.configuration = try container.decode(ServiceConfiguration.self, forKey: .serviceConfig)
+
+    if let percentage = self.percentage, percentage < 0 || percentage > 100 {
+      throw RPCError(
+        code: .internalError,
+        message: "Invalid service config choice percentage '\(percentage)' (must be 0...100)"
+      )
+    }
+  }
+
+  func encode(to encoder: any Encoder) throws {
+    var container = encoder.container(keyedBy: CodingKeys.self)
+    if !self.language.isEmpty {
+      try container.encode(self.language, forKey: .clientLanguage)
+    }
+    if !self.hostname.isEmpty {
+      try container.encode(self.hostname, forKey: .clientHostname)
+    }
+    try container.encodeIfPresent(self.percentage, forKey: .percentage)
+    try container.encode(self.configuration, forKey: .serviceConfig)
+  }
+
+  func select(hostname: String) -> Bool {
+    // All three conditions must pass. Empty arrays count as matching all values.
+    let containsSwift = self.language.contains { $0.lowercased() == "swift" }
+    guard containsSwift || self.language.isEmpty else { return false }
+
+    let canarySelected = Int.random(in: 1 ... 100) <= (self.percentage ?? 100)
+    guard canarySelected else { return false }
+
+    let containsHostname = self.hostname.contains(hostname)
+    return containsHostname || self.hostname.isEmpty
+  }
+}

Sources/GRPCHTTP2Core/Client/Resolver/NameResolverRegistry.swift

@@ -116,12 +116,14 @@ public struct NameResolverRegistry {
   /// Returns a new name resolver registry with the default factories registered.
   ///
   /// The default resolvers include:
+  /// - ``NameResolvers/DNS``,
   /// - ``NameResolvers/IPv4``,
   /// - ``NameResolvers/IPv6``,
   /// - ``NameResolvers/UnixDomainSocket``,
   /// - ``NameResolvers/VirtualSocket``.
   public static var defaults: Self {
     var resolvers = NameResolverRegistry()
+    resolvers.registerFactory(try! NameResolvers.DNS(fetchServiceConfiguration: false))
     resolvers.registerFactory(NameResolvers.IPv4())
     resolvers.registerFactory(NameResolvers.IPv6())
     resolvers.registerFactory(NameResolvers.UnixDomainSocket())