Fix a race in some tests for JavaRX integration

An extremely rare race could happen in any of the tests in
`LeakedExceptionTest` in the following case:
* `withExceptionHandler` runs the block passed to it;
* In one of the last iterations of `repeat`, `select` in `combine`
  happens on both flows at the same time, that is, the block that
  was passed to `rx[Something]` runs in two threads
  simultaneously;
* One of these two threads (thread A) runs anomalously slow;
* The other thread successfully throws an exception;
* This exception is propagated to `catch`, so `collect` is
  finished;
* `repeat` is exited, the block passed to `withExceptionHandler` is
  done executing;
* `withExceptionHandler` sets back the usual exception handler,
  which fails when an exception in JavaRX happens (see
  https://github.com/ReactiveX/RxJava/wiki/What's-different-in-2.0#error-handling);
* Thread A wakes up and throws an exception. This time, it is
  passed not to `handler`, which is made specifically to deal with
  this, but to the default handler.

The fix is simple: now a special coroutine context passed to
`rx[Something]` ensures that the spawned executions are finished
before the block passed to `withExceptionHandler` is exited.

Author: dkhalanskyjb

reactive/kotlinx-coroutines-rx2/test/LeakedExceptionTest.kt

@@ -6,12 +6,10 @@ package kotlinx.coroutines.rx2
 
 import io.reactivex.*
 import io.reactivex.exceptions.*
-import io.reactivex.plugins.*
 import kotlinx.coroutines.*
 import kotlinx.coroutines.flow.*
 import kotlinx.coroutines.reactive.*
 import org.junit.Test
-import java.io.*
 import kotlin.test.*
 
 // Check that exception is not leaked to the global exception handler
@@ -22,8 +20,9 @@ class LeakedExceptionTest : TestBase() {
 
     @Test
     fun testSingle() = withExceptionHandler(handler) {
-        val flow = rxSingle<Unit> { throw TestException() }.toFlowable().asFlow()
         runBlocking {
+            val dispatcher = wrapperDispatcher()
+            val flow = rxSingle<Unit>(dispatcher) { throw TestException() }.toFlowable().asFlow()
             repeat(10000) {
                 combine(flow, flow) { _, _ -> Unit }
                     .catch {}
@@ -34,8 +33,11 @@ class LeakedExceptionTest : TestBase() {
 
     @Test
     fun testObservable() = withExceptionHandler(handler) {
-        val flow = rxObservable<Unit> { throw TestException() }.toFlowable(BackpressureStrategy.BUFFER).asFlow()
         runBlocking {
+            val dispatcher = wrapperDispatcher()
+            val flow = rxObservable<Unit>(dispatcher) { throw TestException() }
+                .toFlowable(BackpressureStrategy.BUFFER)
+                .asFlow()
             repeat(10000) {
                 combine(flow, flow) { _, _ -> Unit }
                     .catch {}
@@ -46,13 +48,41 @@ class LeakedExceptionTest : TestBase() {
 
     @Test
     fun testFlowable() = withExceptionHandler(handler) {
-        val flow = rxFlowable<Unit> { throw TestException() }.asFlow()
         runBlocking {
+            val dispatcher = wrapperDispatcher()
+            val flow = rxFlowable<Unit>(dispatcher) { throw TestException() }.asFlow()
             repeat(10000) {
                 combine(flow, flow) { _, _ -> Unit }
                     .catch {}
                     .collect { }
             }
         }
     }
+
+    /**
+     * This test doesn't test much and was added to display a problem with straighforward use of
+     * [withExceptionHandler].
+     *
+     * If one was to remove `dispatcher` and launch `rxFlowable` with an empty coroutine context,
+     * this test would fail fairly often, while other tests were also vulnerable, but the problem is
+     * much more difficult to reproduce. Thus, this test is a justification for adding `dispatcher`
+     * to other tests.
+     *
+     * See the commit that introduced this test for a better explanation.
+     */
+    @Test
+    fun testResettingExceptionHandler() = withExceptionHandler(handler) {
+        runBlocking {
+            val dispatcher = wrapperDispatcher()
+            val flow = rxFlowable<Unit>(dispatcher) {
+                if ((0..1).random() == 0) {
+                    Thread.sleep(100)
+                }
+                throw TestException()
+            }.asFlow()
+            combine(flow, flow) { _, _ -> Unit }
+                .catch {}
+                .collect { }
+        }
+    }
 }