Optimized asyncUnaryCall implementation with cancellation support

core/src/main/scala/org/ivovk/connect_rpc_scala/ConnectHandler.scala

@@ -9,13 +9,12 @@ import io.grpc.stub.MetadataUtils
 import org.http4s.dsl.Http4sDsl
 import org.http4s.{Header, MediaType, MessageFailure, Method, Response}
 import org.ivovk.connect_rpc_scala.Mappings.*
-import org.ivovk.connect_rpc_scala.grpc.{MethodName, MethodRegistry}
+import org.ivovk.connect_rpc_scala.grpc.{GrpcClientCalls, MethodName, MethodRegistry}
 import org.ivovk.connect_rpc_scala.http.Headers.`X-Test-Case-Name`
 import org.ivovk.connect_rpc_scala.http.codec.MessageCodec.given
 import org.ivovk.connect_rpc_scala.http.codec.{MessageCodec, MessageCodecRegistry}
 import org.ivovk.connect_rpc_scala.http.{MediaTypes, RequestEntity}
 import org.slf4j.{Logger, LoggerFactory}
-import scalapb.grpc.ClientCalls
 import scalapb.{GeneratedMessage, GeneratedMessageCompanion, TextFormat}
 
 import java.util.concurrent.atomic.AtomicReference
@@ -103,32 +102,35 @@ class ConnectHandler[F[_] : Async](
           logger.trace(s">>> Method: ${method.descriptor.getFullMethodName}, Entity: $message")
         }
 
-        Async[F].fromFuture(Async[F].delay {
-          ClientCalls.asyncUnaryCall[GeneratedMessage, GeneratedMessage](
+        val callOptions = CallOptions.DEFAULT
+          .pipe(
+            req.timeout match {
+              case Some(timeout) => _.withDeadlineAfter(timeout, MILLISECONDS)
+              case None => identity
+            }
+          )
+
+        GrpcClientCalls
+          .asyncUnaryCall2[F, GeneratedMessage, GeneratedMessage](
             ClientInterceptors.intercept(
               channel,
               MetadataUtils.newAttachHeadersInterceptor(req.headers.toMetadata),
               MetadataUtils.newCaptureMetadataInterceptor(responseHeaderMetadata, responseTrailerMetadata),
             ),
             method.descriptor,
-            CallOptions.DEFAULT.pipe(
-              req.timeout match {
-                case Some(timeout) => _.withDeadlineAfter(timeout, MILLISECONDS)
-                case None => identity
-              }
-            ),
+            callOptions,
             message
           )
-        }).map { response =>
-          val headers = responseHeaderMetadata.get.toHeaders() ++
-            responseTrailerMetadata.get.toHeaders(trailing = !treatTrailersAsHeaders)
+          .map { response =>
+            val headers = responseHeaderMetadata.get.toHeaders() ++
+              responseTrailerMetadata.get.toHeaders(trailing = !treatTrailersAsHeaders)
 
-          if (logger.isTraceEnabled) {
-            logger.trace(s"<<< Headers: ${headers.redactSensitive()}")
-          }
+            if (logger.isTraceEnabled) {
+              logger.trace(s"<<< Headers: ${headers.redactSensitive()}")
+            }
 
-          Response(Ok, headers = headers).withEntity(response)
-        }
+            Response(Ok, headers = headers).withEntity(response)
+          }
       }
       .recover { case e =>
         val grpcStatus = e match {

core/src/main/scala/org/ivovk/connect_rpc_scala/grpc/GrpcClientCalls.scala

@@ -0,0 +1,87 @@
+package org.ivovk.connect_rpc_scala.grpc
+
+import cats.effect.Async
+import com.google.common.util.concurrent.{FutureCallback, Futures, MoreExecutors}
+import io.grpc.stub.{ClientCalls, StreamObserver}
+import io.grpc.{CallOptions, Channel, MethodDescriptor}
+
+object GrpcClientCalls {
+
+  /**
+   * Asynchronous unary call.
+   *
+   * Optimized version of the `scalapb.grpc.ClientCalls.asyncUnaryCall` that skips Scala's Future instantiation
+   * and supports cancellation.
+   */
+  def asyncUnaryCall[F[_] : Async, Req, Resp](
+    channel: Channel,
+    method: MethodDescriptor[Req, Resp],
+    options: CallOptions,
+    request: Req,
+  ): F[Resp] = {
+    Async[F].async[Resp] { cb =>
+      Async[F].delay {
+        val future = ClientCalls.futureUnaryCall(channel.newCall(method, options), request)
+
+        Futures.addCallback(
+          future,
+          new FutureCallback[Resp] {
+            def onSuccess(result: Resp): Unit = cb(Right(result))
+
+            def onFailure(t: Throwable): Unit = cb(Left(t))
+          },
+          MoreExecutors.directExecutor(),
+        )
+
+        Some(Async[F].delay(future.cancel(true)))
+      }
+    }
+  }
+
+  /**
+   * Implementation that should be faster than the [[asyncUnaryCall]].
+   */
+  def asyncUnaryCall2[F[_] : Async, Req, Resp](
+    channel: Channel,
+    method: MethodDescriptor[Req, Resp],
+    options: CallOptions,
+    request: Req,
+  ): F[Resp] = {
+    Async[F].async[Resp] { cb =>
+      Async[F].delay {
+        val call = channel.newCall(method, options)
+
+        ClientCalls.asyncUnaryCall(call, request, new CallbackObserver(cb))
+
+        Some(Async[F].delay(call.cancel("Cancelled", null)))
+      }
+    }
+  }
+
+  /**
+   * [[CallbackObserver]] either executes [[onNext]] -> [[onCompleted]] during the happy path or just [[onError]] in case of
+   * an error.
+   */
+  private class CallbackObserver[F[_] : Async, Resp](cb: Either[Throwable, Resp] => Unit) extends StreamObserver[Resp] {
+    private var value: Option[Either[Throwable, Resp]] = None
+
+    override def onNext(value: Resp): Unit = {
+      if this.value.isDefined then
+        throw new IllegalStateException("Value already received")
+
+      this.value = Some(Right(value))
+    }
+
+    override def onError(t: Throwable): Unit = {
+      cb(Left(t))
+    }
+
+    override def onCompleted(): Unit = {
+      this.value match
+        case Some(v) => cb(v)
+        case None => cb(Left(new IllegalStateException("No value received or call to onCompleted after onError")))
+    }
+
+  }
+
+}