Simplify UnaryBlockingCall

library/src/main/kotlin/com/connectrpc/UnaryBlockingCall.kt

@@ -14,57 +14,18 @@
 
 package com.connectrpc
 
-import java.util.concurrent.CountDownLatch
-import java.util.concurrent.atomic.AtomicReference
-
 /**
  * A [UnaryBlockingCall] contains the way to make a blocking RPC call and cancelling the RPC.
  */
-class UnaryBlockingCall<Output> {
-    private var executable: ((ResponseMessage<Output>) -> Unit) -> Unit = { }
-    private var cancelFn: () -> Unit = { }
-
+interface UnaryBlockingCall<Output> {
     /**
-     * Execute the underlying request.
-     * Subsequent calls will create a new request.
+     * Execute the underlying request. Can only be called once.
+     * Subsequent calls will throw IllegalStateException.
      */
-    fun execute(): ResponseMessage<Output> {
-        val countDownLatch = CountDownLatch(1)
-        val reference = AtomicReference<ResponseMessage<Output>>()
-        executable { responseMessage ->
-            reference.set(responseMessage)
-            countDownLatch.countDown()
-        }
-        countDownLatch.await()
-        return reference.get()
-    }
+    fun execute(): ResponseMessage<Output>
 
     /**
      * Cancel the underlying request.
      */
-    fun cancel() {
-        cancelFn()
-    }
-
-    /**
-     * Gives the blocking call a cancellation function to cancel the
-     * underlying request.
-     *
-     * @param cancel The function to call in order to cancel the
-     * underlying request.
-     */
-    internal fun setCancel(cancel: () -> Unit) {
-        this.cancelFn = cancel
-    }
-
-    /**
-     * Gives the blocking call the execution function to initiate
-     * the underlying request.
-     *
-     * @param executable The function to call in order to initiate
-     * a request.
-     */
-    internal fun setExecute(executable: ((ResponseMessage<Output>) -> Unit) -> Unit) {
-        this.executable = executable
-    }
+    fun cancel()
 }

library/src/main/kotlin/com/connectrpc/impl/ProtocolClient.kt

@@ -41,7 +41,6 @@ import kotlinx.coroutines.suspendCancellableCoroutine
 import kotlinx.coroutines.withContext
 import okio.Buffer
 import java.net.URI
-import java.util.concurrent.CountDownLatch
 import kotlin.coroutines.resume
 
 /**
@@ -167,18 +166,9 @@ class ProtocolClient(
         headers: Headers,
         methodSpec: MethodSpec<Input, Output>,
     ): UnaryBlockingCall<Output> {
-        val countDownLatch = CountDownLatch(1)
-        val call = UnaryBlockingCall<Output>()
-        // Set the unary synchronous executable.
-        call.setExecute { callback: (ResponseMessage<Output>) -> Unit ->
-            val cancellationFn = unary(request, headers, methodSpec) { responseMessage ->
-                callback(responseMessage)
-                countDownLatch.countDown()
-            }
-            // Set the cancellation function .
-            call.setCancel(cancellationFn)
+        return UnaryCall { callback ->
+            unary(request, headers, methodSpec, callback)
         }
-        return call
     }
 
     override suspend fun <Input : Any, Output : Any> serverStream(

library/src/main/kotlin/com/connectrpc/impl/UnaryCall.kt

@@ -0,0 +1,85 @@
+// Copyright 2022-2023 The Connect Authors
+//
+// 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.
+
+package com.connectrpc.impl
+
+import com.connectrpc.ResponseMessage
+import com.connectrpc.UnaryBlockingCall
+import com.connectrpc.http.Cancelable
+import java.util.concurrent.CountDownLatch
+import java.util.concurrent.atomic.AtomicBoolean
+import java.util.concurrent.atomic.AtomicReference
+
+/**
+ * Callback that handles asynchronous response.
+ */
+internal typealias ResponseCallback<T> = (ResponseMessage<T>) -> Unit
+
+/**
+ * Represents a cancelable asynchronous operation. When the function
+ * is invoked, the operation is initiated. When that operation completes
+ * it MUST invoke the callback, even when canceled. The value returned
+ * from the function can be called to abort the operation and have it
+ * return early.
+ */
+internal typealias AsyncOperation<T> = (callback: ResponseCallback<T>) -> Cancelable
+
+/**
+ * Concrete implementation of [UnaryBlockingCall] which transforms
+ * the given async operation into a synchronous/blocking one.
+ */
+internal class UnaryCall<Output>(
+    private val block: AsyncOperation<Output>,
+) : UnaryBlockingCall<Output> {
+    private val executed = AtomicBoolean()
+
+    /**
+     * initialized to null and then replaced with non-null
+     * function when [execute] or [cancel] is called.
+     */
+    private var cancelFunc = AtomicReference<Cancelable>()
+
+    /**
+     * Execute the underlying operation and block until it completes.
+     */
+    override fun execute(): ResponseMessage<Output> {
+        check(executed.compareAndSet(false, true)) { "already executed" }
+
+        val resultReady = CountDownLatch(1)
+        val result = AtomicReference<ResponseMessage<Output>>()
+        val cancelFn = block { responseMessage ->
+            result.set(responseMessage)
+            resultReady.countDown()
+        }
+
+        if (!cancelFunc.compareAndSet(null, cancelFn)) {
+            // concurrently cancelled before we could set the
+            // cancel function, so we need to cancel what we
+            // just started
+            cancelFn()
+        }
+        resultReady.await()
+        return result.get()
+    }
+
+    /**
+     * Cancel the underlying request.
+     */
+    override fun cancel() {
+        val cancelFn = cancelFunc.getAndSet {} // set to (non-null) no-op
+        if (cancelFn != null) {
+            cancelFn()
+        }
+    }
+}

library/src/test/kotlin/com/connectrpc/impl/UnaryCallTest.kt

@@ -0,0 +1,106 @@
+// Copyright 2022-2023 The Connect Authors
+//
+// 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.
+
+package com.connectrpc.impl
+
+import com.connectrpc.Code
+import com.connectrpc.ConnectException
+import com.connectrpc.ResponseMessage
+import org.assertj.core.api.Assertions.assertThat
+import org.junit.Test
+import java.util.concurrent.CountDownLatch
+import java.util.concurrent.Executors
+
+class UnaryCallTest {
+    @Test
+    fun testExecute() {
+        val executor = Executors.newSingleThreadExecutor()
+        try {
+            val result = Object()
+            val call = UnaryCall<Any> { callback ->
+                executor.execute {
+                    callback.invoke(
+                        ResponseMessage.Success(
+                            result,
+                            headers = emptyMap(),
+                            trailers = emptyMap(),
+                        ),
+                    )
+                }
+                return@UnaryCall { }
+            }
+            val resp = call.execute()
+            assertThat(resp).isInstanceOf(ResponseMessage.Success::class.java)
+            val msg = resp.success { it.message }!!
+            assertThat(msg).isEqualTo(result)
+        } finally {
+            assertThat(executor.shutdownNow()).isEmpty()
+        }
+    }
+
+    @Test
+    fun testCancelAfterExecute() {
+        testCancel(false)
+    }
+
+    @Test
+    fun testCancelBeforeExecute() {
+        testCancel(true)
+    }
+
+    private fun testCancel(cancelFirst: Boolean) {
+        val executor = Executors.newFixedThreadPool(2)
+        try {
+            // Indicates when the async task has begun.
+            val taskRunning = CountDownLatch(1)
+            // Indicates when the async task has been canceled.
+            val taskCanceled = CountDownLatch(1)
+
+            val call = UnaryCall<Any> { callback ->
+                executor.execute {
+                    taskRunning.countDown()
+                    taskCanceled.await()
+                    callback.invoke(
+                        ResponseMessage.Failure(
+                            headers = emptyMap(),
+                            trailers = emptyMap(),
+                            cause = ConnectException(code = Code.CANCELED),
+                        ),
+                    )
+                }
+                return@UnaryCall {
+                    taskCanceled.countDown()
+                }
+            }
+            if (cancelFirst) {
+                // When we execute the task below, the call will observe
+                // that it has already been canceled and immediately
+                // cancel the just-started task.
+                call.cancel()
+            } else {
+                // This will cancel the task right after it has started running.
+                executor.execute {
+                    taskRunning.await()
+                    call.cancel()
+                }
+            }
+            val resp = call.execute()
+            assertThat(resp).isInstanceOf(ResponseMessage.Failure::class.java)
+            val connEx = resp.failure { it.cause }!!
+            assertThat(connEx.code).isEqualTo(Code.CANCELED)
+        } finally {
+            assertThat(executor.shutdownNow()).isEmpty()
+        }
+    }
+}