Add TLS support for NIOPosix H2 client (grpc#2036)

## Motivation
We currently have a `NIOPosix` client transport implementation in gRPC
v2, but it doesn't support TLS.

## Modifications
This PR adds support for TLS in the NIOPosix-backed HTTP/2
implementation of the client transport for gRPC v2.

## Result
We now support TLS when using the NIOPosix client transport in gRPC V2.

Author: gjcairo

Examples/v2/echo/Subcommands/Collect.swift

@@ -32,7 +32,7 @@ struct Collect: AsyncParsableCommand {
     let client = GRPCClient(
       transport: try .http2NIOPosix(
         target: self.arguments.target,
-        config: .defaults()
+        config: .defaults(transportSecurity: .plaintext)
       )
     )
 

Examples/v2/echo/Subcommands/Expand.swift

@@ -32,7 +32,7 @@ struct Expand: AsyncParsableCommand {
     let client = GRPCClient(
       transport: try .http2NIOPosix(
         target: self.arguments.target,
-        config: .defaults()
+        config: .defaults(transportSecurity: .plaintext)
       )
     )
 

Examples/v2/echo/Subcommands/Get.swift

@@ -30,7 +30,7 @@ struct Get: AsyncParsableCommand {
     let client = GRPCClient(
       transport: try .http2NIOPosix(
         target: self.arguments.target,
-        config: .defaults()
+        config: .defaults(transportSecurity: .plaintext)
       )
     )
 

Examples/v2/echo/Subcommands/Update.swift

@@ -32,7 +32,7 @@ struct Update: AsyncParsableCommand {
     let client = GRPCClient(
       transport: try .http2NIOPosix(
         target: self.arguments.target,
-        config: .defaults()
+        config: .defaults(transportSecurity: .plaintext)
       )
     )
 

Examples/v2/hello-world/Subcommands/Greet.swift

@@ -33,7 +33,7 @@ struct Greet: AsyncParsableCommand {
       let client = GRPCClient(
         transport: try .http2NIOPosix(
           target: .ipv4(host: "127.0.0.1", port: self.port),
-          config: .defaults()
+          config: .defaults(transportSecurity: .plaintext)
         )
       )
 

Examples/v2/route-guide/Subcommands/GetFeature.swift

@@ -39,7 +39,7 @@ struct GetFeature: AsyncParsableCommand {
   func run() async throws {
     let transport = try HTTP2ClientTransport.Posix(
       target: .ipv4(host: "127.0.0.1", port: self.port),
-      config: .defaults()
+      config: .defaults(transportSecurity: .plaintext)
     )
     let client = GRPCClient(transport: transport)
 

Examples/v2/route-guide/Subcommands/ListFeatures.swift

@@ -53,7 +53,7 @@ struct ListFeatures: AsyncParsableCommand {
   func run() async throws {
     let transport = try HTTP2ClientTransport.Posix(
       target: .ipv4(host: "127.0.0.1", port: self.port),
-      config: .defaults()
+      config: .defaults(transportSecurity: .plaintext)
     )
     let client = GRPCClient(transport: transport)
 

Examples/v2/route-guide/Subcommands/RecordRoute.swift

@@ -32,7 +32,7 @@ struct RecordRoute: AsyncParsableCommand {
   func run() async throws {
     let transport = try HTTP2ClientTransport.Posix(
       target: .ipv4(host: "127.0.0.1", port: self.port),
-      config: .defaults()
+      config: .defaults(transportSecurity: .plaintext)
     )
     let client = GRPCClient(transport: transport)
 

Examples/v2/route-guide/Subcommands/RouteChat.swift

@@ -32,7 +32,7 @@ struct RouteChat: AsyncParsableCommand {
   func run() async throws {
     let transport = try HTTP2ClientTransport.Posix(
       target: .ipv4(host: "127.0.0.1", port: self.port),
-      config: .defaults()
+      config: .defaults(transportSecurity: .plaintext)
     )
     let client = GRPCClient(transport: transport)
 

Sources/GRPCHTTP2TransportNIOPosix/HTTP2ClientTransport+Posix.swift

@@ -19,31 +19,38 @@ public import GRPCHTTP2Core  // should be @usableFromInline
 public import NIOCore
 public import NIOPosix  // has to be public because of default argument value in init
 
+#if canImport(NIOSSL)
+import NIOSSL
+#endif
+
 @available(macOS 15.0, iOS 18.0, watchOS 11.0, tvOS 18.0, visionOS 2.0, *)
 extension HTTP2ClientTransport {
-  /// A `ClientTransport` using HTTP/2 built on top of `NIOPosix`.
+  /// A ``GRPCCore/ClientTransport`` using HTTP/2 built on top of `NIOPosix`.
   ///
   /// This transport builds on top of SwiftNIO's Posix networking layer and is suitable for use
   /// on Linux and Darwin based platform (macOS, iOS, etc.) However, it's *strongly* recommended
   /// that if you are targeting Darwin platforms then you should use the `NIOTS` variant of
-  /// the `HTTP2ClientTransport`.
+  /// the ``GRPCHTTP2Core/HTTP2ClientTransport``.
   ///
   /// To use this transport you need to provide a 'target' to connect to which will be resolved
   /// by an appropriate resolver from the resolver registry. By default the resolver registry can
   /// resolve DNS targets, IPv4 and IPv6 targets, Unix domain socket targets, and Virtual Socket
   /// targets. If you use a custom target you must also provide an appropriately configured
   /// registry.
   ///
-  /// You can control various aspects of connection creation, management and RPC behavior via the
-  /// ``Config``. Load balancing policies and other RPC specific behavior can be configured via
+  /// You can control various aspects of connection creation, management, security and RPC behavior via
+  /// the ``Config``. Load balancing policies and other RPC specific behavior can be configured via
   /// the ``ServiceConfig`` (if it isn't provided by a resolver).
   ///
   /// Beyond creating the transport you don't need to interact with it directly, instead, pass it
   /// to a `GRPCClient`:
   ///
   /// ```swift
-  /// try await withThrowingDiscardingTaskGroup {
-  ///   let transport = try HTTP2ClientTransport.Posix(target: .dns(host: "example.com"))
+  /// try await withThrowingDiscardingTaskGroup { group in
+  ///   let transport = try HTTP2ClientTransport.Posix(
+  ///     target: .ipv4(host: "example.com"),
+  ///     config: .defaults(transportSecurity: .plaintext)
+  ///   )
   ///   let client = GRPCClient(transport: transport)
   ///   group.addTask {
   ///     try await client.run()
@@ -87,7 +94,7 @@ extension HTTP2ClientTransport {
       // Configure a connector.
       self.channel = GRPCChannel(
         resolver: resolver,
-        connector: Connector(eventLoopGroup: eventLoopGroup, config: config),
+        connector: try Connector(eventLoopGroup: eventLoopGroup, config: config),
         config: GRPCChannel.Config(posix: config),
         defaultServiceConfig: serviceConfig
       )
@@ -125,9 +132,33 @@ extension HTTP2ClientTransport.Posix {
     private let config: HTTP2ClientTransport.Posix.Config
     private let eventLoopGroup: any EventLoopGroup
 
-    init(eventLoopGroup: any EventLoopGroup, config: HTTP2ClientTransport.Posix.Config) {
+    #if canImport(NIOSSL)
+    private let nioSSLContext: NIOSSLContext?
+    private let serverHostname: String?
+    #endif
+
+    init(eventLoopGroup: any EventLoopGroup, config: HTTP2ClientTransport.Posix.Config) throws {
       self.eventLoopGroup = eventLoopGroup
       self.config = config
+
+      #if canImport(NIOSSL)
+      switch self.config.transportSecurity.wrapped {
+      case .plaintext:
+        self.nioSSLContext = nil
+        self.serverHostname = nil
+      case .tls(let tlsConfig):
+        do {
+          self.nioSSLContext = try NIOSSLContext(configuration: TLSConfiguration(tlsConfig))
+          self.serverHostname = tlsConfig.serverHostname
+        } catch {
+          throw RuntimeError(
+            code: .transportError,
+            message: "Couldn't create SSL context, check your TLS configuration.",
+            cause: error
+          )
+        }
+      }
+      #endif
     }
 
     func establishConnection(
@@ -137,7 +168,18 @@ extension HTTP2ClientTransport.Posix {
         group: self.eventLoopGroup
       ).connect(to: address) { channel in
         channel.eventLoop.makeCompletedFuture {
-          try channel.pipeline.syncOperations.configureGRPCClientPipeline(
+          #if canImport(NIOSSL)
+          if let nioSSLContext = self.nioSSLContext {
+            try channel.pipeline.syncOperations.addHandler(
+              NIOSSLClientHandler(
+                context: nioSSLContext,
+                serverHostname: self.serverHostname
+              )
+            )
+          }
+          #endif
+
+          return try channel.pipeline.syncOperations.configureGRPCClientPipeline(
             channel: channel,
             config: GRPCChannel.Config(posix: self.config)
           )
@@ -164,31 +206,48 @@ extension HTTP2ClientTransport.Posix {
     /// Compression configuration.
     public var compression: HTTP2ClientTransport.Config.Compression
 
+    /// The transport's security.
+    public var transportSecurity: TransportSecurity
+
     /// Creates a new connection configuration.
     ///
-    /// See also ``defaults``.
+    /// - Parameters:
+    ///   - http2: HTTP2 configuration.
+    ///   - backoff: Backoff configuration.
+    ///   - connection: Connection configuration.
+    ///   - compression: Compression configuration.
+    ///   - transportSecurity: The transport's security configuration.
+    ///
+    /// - SeeAlso: ``defaults(_:)``.
     public init(
       http2: HTTP2ClientTransport.Config.HTTP2,
       backoff: HTTP2ClientTransport.Config.Backoff,
       connection: HTTP2ClientTransport.Config.Connection,
-      compression: HTTP2ClientTransport.Config.Compression
+      compression: HTTP2ClientTransport.Config.Compression,
+      transportSecurity: TransportSecurity
     ) {
       self.http2 = http2
       self.connection = connection
       self.backoff = backoff
       self.compression = compression
+      self.transportSecurity = transportSecurity
     }
 
     /// Default values.
     ///
     /// - Parameters:
+    ///   - transportSecurity: The security settings applied to the transport.
     ///   - configure: A closure which allows you to modify the defaults before returning them.
-    public static func defaults(_ configure: (_ config: inout Self) -> Void = { _ in }) -> Self {
+    public static func defaults(
+      transportSecurity: TransportSecurity,
+      configure: (_ config: inout Self) -> Void = { _ in }
+    ) -> Self {
       var config = Self(
         http2: .defaults,
         backoff: .defaults,
         connection: .defaults,
-        compression: .defaults
+        compression: .defaults,
+        transportSecurity: transportSecurity
       )
       configure(&config)
       return config