Add support for Coroutines transactions

This commit adds Coroutines extensions for
TransactionalOperator.transactional that accept suspending lambda or
Kotlin Flow parameters.

@Transactional on suspending functions is not supported yet, gh-23575
has been created for that purpose.

Closes gh-22915

spring-tx/src/test/kotlin/org/springframework/transaction/reactive/TransactionalOperatorExtensionsTests.kt

@@ -0,0 +1,100 @@
+/*
+ * Copyright 2002-2019 the original author or 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
+ *
+ *      https://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 org.springframework.transaction.reactive
+
+import kotlinx.coroutines.ExperimentalCoroutinesApi
+import kotlinx.coroutines.delay
+import kotlinx.coroutines.flow.flow
+import kotlinx.coroutines.flow.toList
+import kotlinx.coroutines.runBlocking
+import org.assertj.core.api.Assertions.assertThat
+import org.junit.jupiter.api.Test
+import org.junit.jupiter.api.fail
+import org.springframework.transaction.support.DefaultTransactionDefinition
+
+class TransactionalOperatorExtensionsTests {
+
+	private val tm = ReactiveTestTransactionManager(false, true)
+
+	@Test
+	fun commitWithSuspendingFunction() {
+		val operator = TransactionalOperator.create(tm, DefaultTransactionDefinition())
+		runBlocking {
+			operator.transactional {
+				delay(1)
+				true
+			}
+		}
+		assertThat(tm.commit).isTrue()
+		assertThat(tm.rollback).isFalse()
+	}
+
+	@Test
+	fun rollbackWithSuspendingFunction() {
+		val operator = TransactionalOperator.create(tm, DefaultTransactionDefinition())
+		runBlocking {
+			try {
+				operator.transactional {
+					delay(1)
+					throw IllegalStateException()
+				}
+			} catch (ex: IllegalStateException) {
+				assertThat(tm.commit).isFalse()
+				assertThat(tm.rollback).isTrue()
+				return@runBlocking
+			}
+			fail("")
+		}
+	}
+
+	@Test
+	@ExperimentalCoroutinesApi
+	fun commitWithFlow() {
+		val operator = TransactionalOperator.create(tm, DefaultTransactionDefinition())
+		val flow = flow {
+			emit(1)
+			emit(2)
+			emit(3)
+			emit(4)
+		}
+		runBlocking {
+			val list = operator.transactional(flow).toList()
+			assertThat(list).hasSize(4)
+		}
+		assertThat(tm.commit).isTrue()
+		assertThat(tm.rollback).isFalse()
+	}
+
+	@Test
+	@ExperimentalCoroutinesApi
+	fun rollbackWithFlow() {
+		val operator = TransactionalOperator.create(tm, DefaultTransactionDefinition())
+		val flow = flow<Int> {
+			delay(1)
+			throw IllegalStateException()
+		}
+		runBlocking {
+			try {
+				operator.transactional(flow).toList()
+			} catch (ex: IllegalStateException) {
+				assertThat(tm.commit).isFalse()
+				assertThat(tm.rollback).isTrue()
+				return@runBlocking
+			}
+		}
+	}
+}