Support cancellation of requests in InProcessClient

Sources/InProcessClient/InProcessSourceKitLSPClient.swift

@@ -92,16 +92,41 @@ public final class InProcessSourceKitLSPClient: Sendable {
   ///   necessary and the response of the request is not awaited, use the version of the function that takes a
   ///   completion handler
   public func send<R: RequestType>(_ request: R) async throws -> R.Response {
-    return try await withCheckedThrowingContinuation { continuation in
-      self.send(request) {
-        continuation.resume(with: $0)
+    let requestId = ThreadSafeBox<RequestID?>(initialValue: nil)
+    return try await withTaskCancellationHandler {
+      return try await withCheckedThrowingContinuation { continuation in
+        if Task.isCancelled {
+          // Check if the task has been cancelled before we send the request to LSP to avoid any kind of work if
+          // possible.
+          return continuation.resume(throwing: CancellationError())
+        }
+        requestId.value = self.send(request) {
+          continuation.resume(with: $0)
+        }
+        if Task.isCancelled, let requestId = requestId.takeValue() {
+          // The task might have been cancelled after the above cancellation check but before `requestId` was assigned
+          // a value. To cover that case, check for cancellation here again. Note that we won't cancel twice from here
+          // and the `onCancel` handler because we take the request ID out of the `ThreadSafeBox` before sending the
+          // `CancelRequestNotification`.
+          self.send(CancelRequestNotification(id: requestId))
+        }
+      }
+    } onCancel: {
+      if let requestId = requestId.takeValue() {
+        self.send(CancelRequestNotification(id: requestId))
       }
     }
   }
 
   /// Send the request to `server` and return the request result via a completion handler.
-  public func send<R: RequestType>(_ request: R, reply: @Sendable @escaping (LSPResult<R.Response>) -> Void) {
-    server.handle(request, id: .number(Int(nextRequestID.fetchAndIncrement())), reply: reply)
+  @discardableResult
+  public func send<R: RequestType>(
+    _ request: R,
+    reply: @Sendable @escaping (LSPResult<R.Response>) -> Void
+  ) -> RequestID {
+    let requestID = RequestID.number(Int(nextRequestID.fetchAndIncrement()))
+    server.handle(request, id: requestID, reply: reply)
+    return requestID
   }
 
   /// Send the notification to `server`.