Do not propagate exceptions to CoroutineExceptionHandler in 'future' builder if it has been cancelled in order to be consistent with other future implementations (#3580)

Fixes #3452

Author: qwwdfsad

kotlinx-coroutines-core/jdk8/src/future/Future.kt

@@ -58,10 +58,12 @@ private class CompletableFutureCoroutine<T>(
     }
 
     override fun onCancelled(cause: Throwable, handled: Boolean) {
-        if (!future.completeExceptionally(cause) && !handled) {
-            // prevents loss of exception that was not handled by parent & could not be set to CompletableFuture
-            handleCoroutineException(context, cause)
-        }
+        /*
+         * Here we can potentially lose the cause if the failure is racing with future's
+         * external cancellation. We are consistent with other future implementations
+         * (LF, FT, CF) and give up on such exception.
+         */
+        future.completeExceptionally(cause)
     }
 }
 

kotlinx-coroutines-core/jvm/test/jdk8/future/FutureTest.kt

@@ -392,11 +392,7 @@ class FutureTest : TestBase() {
     }
 
     @Test
-    fun testUnhandledExceptionOnExternalCompletion() = runTest(
-        unhandled = listOf(
-            { it -> it is TestException } // exception is unhandled because there is no parent
-        )
-    ) {
+    fun testUnhandledExceptionOnExternalCompletionIsNotReported() = runTest {
         expect(1)
         // No parent here (NonCancellable), so nowhere to propagate exception
         val result = future(NonCancellable + Dispatchers.Unconfined) {