Allow adding ClientInterceptors to specific services and methods (grpc#2113)

## Motivation
We want to allow users to customise the RPCs a registered interceptor
should apply to on the client:
- Intercept all requests
- Intercept requests only meant for specific services
- Intercept requests only meant for specific methods

## Modifications
This PR adds a new `ClientInterceptorPipelineOperation` type that allows
users to specify what the target of the interceptor should be.
Existing APIs accepting `[any ClientInterceptor]` have been kept, but
new initialisers taking `[ClientInterceptorPipelineOperation]` instead
have been added.

## Result
Users can have more control over to which requests interceptors are
applied.

Author: gjcairo

Sources/GRPCCore/Call/Client/ClientInterceptor.swift

@@ -21,10 +21,11 @@
 /// received from the transport. They are typically used for cross-cutting concerns like injecting
 /// metadata, validating messages, logging additional data, and tracing.
 ///
-/// Interceptors are registered with a client and apply to all RPCs. If you need to modify the
-/// behavior of an interceptor on a per-RPC basis then you can use the
-/// ``ClientContext/descriptor`` to determine which RPC is being called and
-/// conditionalise behavior accordingly.
+/// Interceptors are registered with the server via ``ClientInterceptorPipelineOperation``s.
+/// You may register them for all services registered with a server, for RPCs directed to specific services, or
+/// for RPCs directed to specific methods. If you need to modify the behavior of an interceptor on a
+/// per-RPC basis in more detail, then you can use the ``ClientContext/descriptor`` to determine
+/// which RPC is being called and conditionalise behavior accordingly.
 ///
 /// - TODO: Update example and documentation to show how to register an interceptor.
 ///

Sources/GRPCCore/Call/Client/ClientInterceptorPipelineOperation.swift

@@ -0,0 +1,99 @@
+/*
+ * 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.
+ */
+
+/// A `ClientInterceptorPipelineOperation` describes to which RPCs a client interceptor should be applied.
+///
+/// You can configure a client interceptor to be applied to:
+/// - all RPCs and services;
+/// - requests directed only to specific services; or
+/// - requests directed only to specific methods (of a specific service).
+///
+/// - SeeAlso: ``ClientInterceptor`` for more information on client interceptors, and
+///  ``ServerInterceptorPipelineOperation`` for the server-side version of this type.
+public struct ClientInterceptorPipelineOperation: Sendable {
+  /// The subject of a ``ClientInterceptorPipelineOperation``.
+  /// The subject of an interceptor can either be all services and methods, only specific services, or only specific methods.
+  public struct Subject: Sendable {
+    internal enum Wrapped: Sendable {
+      case all
+      case services(Set<ServiceDescriptor>)
+      case methods(Set<MethodDescriptor>)
+    }
+
+    private let wrapped: Wrapped
+
+    /// An operation subject specifying an interceptor that applies to all RPCs across all services will be registered with this client.
+    public static var all: Self { .init(wrapped: .all) }
+
+    /// An operation subject specifying an interceptor that will be applied only to RPCs directed to the specified services.
+    /// - Parameters:
+    ///   - services: The list of service names for which this interceptor should intercept RPCs.
+    /// - Returns: A ``ClientInterceptorPipelineOperation``.
+    public static func services(_ services: Set<ServiceDescriptor>) -> Self {
+      Self(wrapped: .services(services))
+    }
+
+    /// An operation subject specifying an interceptor that will be applied only to RPCs directed to the specified service methods.
+    /// - Parameters:
+    ///   - methods: The list of method descriptors for which this interceptor should intercept RPCs.
+    /// - Returns: A ``ClientInterceptorPipelineOperation``.
+    public static func methods(_ methods: Set<MethodDescriptor>) -> Self {
+      Self(wrapped: .methods(methods))
+    }
+
+    @usableFromInline
+    internal func applies(to descriptor: MethodDescriptor) -> Bool {
+      switch self.wrapped {
+      case .all:
+        return true
+
+      case .services(let services):
+        return services.map({ $0.fullyQualifiedService }).contains(descriptor.service)
+
+      case .methods(let methods):
+        return methods.contains(descriptor)
+      }
+    }
+  }
+
+  /// The interceptor specified for this operation.
+  public let interceptor: any ClientInterceptor
+
+  @usableFromInline
+  internal let subject: Subject
+
+  private init(interceptor: any ClientInterceptor, appliesTo: Subject) {
+    self.interceptor = interceptor
+    self.subject = appliesTo
+  }
+
+  /// Create an operation, specifying which ``ClientInterceptor`` to apply and to which ``Subject``.
+  /// - Parameters:
+  ///   - interceptor: The ``ClientInterceptor`` to register with the client.
+  ///   - subject: The ``Subject`` to which the `interceptor` applies.
+  /// - Returns: A ``ClientInterceptorPipelineOperation``.
+  public static func apply(_ interceptor: any ClientInterceptor, to subject: Subject) -> Self {
+    Self(interceptor: interceptor, appliesTo: subject)
+  }
+
+  /// Returns whether this ``ClientInterceptorPipelineOperation`` applies to the given `descriptor`.
+  /// - Parameter descriptor: A ``MethodDescriptor`` for which to test whether this interceptor applies.
+  /// - Returns: `true` if this interceptor applies to the given `descriptor`, or `false` otherwise.
+  @inlinable
+  internal func applies(to descriptor: MethodDescriptor) -> Bool {
+    self.subject.applies(to: descriptor)
+  }
+}

Sources/GRPCCore/Call/Server/Internal/ServerRPCExecutor.swift

@@ -23,7 +23,8 @@ struct ServerRPCExecutor {
   ///   - stream: The accepted stream to execute the RPC on.
   ///   - deserializer: A deserializer for messages received from the client.
   ///   - serializer: A serializer for messages to send to the client.
-  ///   - interceptors: Server interceptors to apply to this RPC.
+  ///   - interceptors: Server interceptors to apply to this RPC.  The
+  ///       interceptors will be called in the order of the array.
   ///   - handler: A handler which turns the request into a response.
   @inlinable
   static func execute<Input, Output>(

Sources/GRPCCore/Call/Server/ServerInterceptorPipelineOperation.swift

@@ -21,7 +21,8 @@
 /// - requests directed only to specific services registered with your server; or
 /// - requests directed only to specific methods (of a specific service).
 ///
-/// - SeeAlso: ``ServerInterceptor`` for more information on server interceptors.
+/// - SeeAlso: ``ServerInterceptor`` for more information on server interceptors, and
+///  ``ClientInterceptorPipelineOperation`` for the client-side version of this type.
 public struct ServerInterceptorPipelineOperation: Sendable {
   /// The subject of a ``ServerInterceptorPipelineOperation``.
   /// The subject of an interceptor can either be all services and methods, only specific services, or only specific methods.

Sources/GRPCCore/GRPCClient.swift

@@ -112,19 +112,12 @@ public final class GRPCClient: Sendable {
   /// The transport which provides a bidirectional communication channel with the server.
   private let transport: any ClientTransport
 
-  /// A collection of interceptors providing cross-cutting functionality to each accepted RPC.
-  ///
-  /// The order in which interceptors are added reflects the order in which they are called. The
-  /// first interceptor added will be the first interceptor to intercept each request. The last
-  /// interceptor added will be the final interceptor to intercept each request before calling
-  /// the appropriate handler.
-  private let interceptors: [any ClientInterceptor]
-
   /// The current state of the client.
-  private let state: Mutex<State>
+  private let stateMachine: Mutex<StateMachine>
 
   /// The state of the client.
   private enum State: Sendable {
+
     /// The client hasn't been started yet. Can transition to `running` or `stopped`.
     case notStarted
     /// The client is running and can send RPCs. Can transition to `stopping`.
@@ -187,22 +180,79 @@ public final class GRPCClient: Sendable {
     }
   }
 
+  private struct StateMachine {
+    var state: State
+
+    private let interceptorPipeline: [ClientInterceptorPipelineOperation]
+
+    /// A collection of interceptors providing cross-cutting functionality to each accepted RPC, keyed by the method to which they apply.
+    ///
+    /// The list of interceptors for each method is computed from `interceptorsPipeline` when calling a method for the first time.
+    /// This caching is done to avoid having to compute the applicable interceptors for each request made.
+    ///
+    /// The order in which interceptors are added reflects the order in which they are called. The
+    /// first interceptor added will be the first interceptor to intercept each request. The last
+    /// interceptor added will be the final interceptor to intercept each request before calling
+    /// the appropriate handler.
+    var interceptorsPerMethod: [MethodDescriptor: [any ClientInterceptor]]
+
+    init(interceptorPipeline: [ClientInterceptorPipelineOperation]) {
+      self.state = .notStarted
+      self.interceptorPipeline = interceptorPipeline
+      self.interceptorsPerMethod = [:]
+    }
+
+    mutating func checkExecutableAndGetApplicableInterceptors(
+      for method: MethodDescriptor
+    ) throws -> [any ClientInterceptor] {
+      try self.state.checkExecutable()
+
+      guard let applicableInterceptors = self.interceptorsPerMethod[method] else {
+        let applicableInterceptors = self.interceptorPipeline
+          .filter { $0.applies(to: method) }
+          .map { $0.interceptor }
+        self.interceptorsPerMethod[method] = applicableInterceptors
+        return applicableInterceptors
+      }
+
+      return applicableInterceptors
+    }
+  }
+
   /// Creates a new client with the given transport, interceptors and configuration.
   ///
   /// - Parameters:
   ///   - transport: The transport used to establish a communication channel with a server.
-  ///   - interceptors: A collection of interceptors providing cross-cutting functionality to each
+  ///   - interceptors: A collection of ``ClientInterceptor``s providing cross-cutting functionality to each
   ///       accepted RPC. The order in which interceptors are added reflects the order in which they
   ///       are called. The first interceptor added will be the first interceptor to intercept each
   ///       request. The last interceptor added will be the final interceptor to intercept each
   ///       request before calling the appropriate handler.
-  public init(
+  convenience public init(
     transport: some ClientTransport,
     interceptors: [any ClientInterceptor] = []
+  ) {
+    self.init(
+      transport: transport,
+      interceptorPipeline: interceptors.map { .apply($0, to: .all) }
+    )
+  }
+
+  /// Creates a new client with the given transport, interceptors and configuration.
+  ///
+  /// - Parameters:
+  ///   - transport: The transport used to establish a communication channel with a server.
+  ///   - interceptorPipeline: A collection of ``ClientInterceptorPipelineOperation`` providing cross-cutting
+  ///       functionality to each accepted RPC. Only applicable interceptors from the pipeline will be applied to each RPC.
+  ///       The order in which interceptors are added reflects the order in which they are called.
+  ///       The first interceptor added will be the first interceptor to intercept each request.
+  ///       The last interceptor added will be the final interceptor to intercept each request before calling the appropriate handler.
+  public init(
+    transport: some ClientTransport,
+    interceptorPipeline: [ClientInterceptorPipelineOperation]
   ) {
     self.transport = transport
-    self.interceptors = interceptors
-    self.state = Mutex(.notStarted)
+    self.stateMachine = Mutex(StateMachine(interceptorPipeline: interceptorPipeline))
   }
 
   /// Start the client.
@@ -213,11 +263,11 @@ public final class GRPCClient: Sendable {
   /// The client, and by extension this function, can only be run once. If the client is already
   /// running or has already been closed then a ``RuntimeError`` is thrown.
   public func run() async throws {
-    try self.state.withLock { try $0.run() }
+    try self.stateMachine.withLock { try $0.state.run() }
 
     // When this function exits the client must have stopped.
     defer {
-      self.state.withLock { $0.stopped() }
+      self.stateMachine.withLock { $0.state.stopped() }
     }
 
     do {
@@ -237,7 +287,7 @@ public final class GRPCClient: Sendable {
   /// in-flight RPCs to finish executing, but no new RPCs will be accepted. You can cancel the task
   /// executing ``run()`` if you want to abruptly stop in-flight RPCs.
   public func beginGracefulShutdown() {
-    let wasRunning = self.state.withLock { $0.beginGracefulShutdown() }
+    let wasRunning = self.stateMachine.withLock { $0.state.beginGracefulShutdown() }
     if wasRunning {
       self.transport.beginGracefulShutdown()
     }
@@ -356,7 +406,9 @@ public final class GRPCClient: Sendable {
     options: CallOptions,
     handler: @Sendable @escaping (StreamingClientResponse<Response>) async throws -> ReturnValue
   ) async throws -> ReturnValue {
-    try self.state.withLock { try $0.checkExecutable() }
+    let applicableInterceptors = try self.stateMachine.withLock {
+      try $0.checkExecutableAndGetApplicableInterceptors(for: descriptor)
+    }
     let methodConfig = self.transport.config(forMethod: descriptor)
     var options = options
     options.formUnion(with: methodConfig)
@@ -368,7 +420,7 @@ public final class GRPCClient: Sendable {
       serializer: serializer,
       deserializer: deserializer,
       transport: self.transport,
-      interceptors: self.interceptors,
+      interceptors: applicableInterceptors,
       handler: handler
     )
   }

Tests/GRPCCoreTests/Call/Client/ClientInterceptorPipelineOperationTests.swift

@@ -0,0 +1,68 @@
+/*
+ * 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 Testing
+
+@testable import GRPCCore
+
+@Suite("ClientInterceptorPipelineOperation")
+struct ClientInterceptorPipelineOperationTests {
+  @Test(
+    "Applies to",
+    arguments: [
+      (
+        .all,
+        [.fooBar, .fooBaz, .barFoo, .barBaz],
+        []
+      ),
+      (
+        .services([ServiceDescriptor(package: "pkg", service: "foo")]),
+        [.fooBar, .fooBaz],
+        [.barFoo, .barBaz]
+      ),
+      (
+        .methods([.barFoo]),
+        [.barFoo],
+        [.fooBar, .fooBaz, .barBaz]
+      ),
+    ] as [(ClientInterceptorPipelineOperation.Subject, [MethodDescriptor], [MethodDescriptor])]
+  )
+  func appliesTo(
+    operationSubject: ClientInterceptorPipelineOperation.Subject,
+    applicableMethods: [MethodDescriptor],
+    notApplicableMethods: [MethodDescriptor]
+  ) {
+    let operation = ClientInterceptorPipelineOperation.apply(
+      .requestCounter(.init()),
+      to: operationSubject
+    )
+
+    for applicableMethod in applicableMethods {
+      #expect(operation.applies(to: applicableMethod))
+    }
+
+    for notApplicableMethod in notApplicableMethods {
+      #expect(!operation.applies(to: notApplicableMethod))
+    }
+  }
+}
+
+extension MethodDescriptor {
+  fileprivate static let fooBar = Self(service: "pkg.foo", method: "bar")
+  fileprivate static let fooBaz = Self(service: "pkg.foo", method: "baz")
+  fileprivate static let barFoo = Self(service: "pkg.bar", method: "foo")
+  fileprivate static let barBaz = Self(service: "pkg.bar", method: "Baz")
+}