Rewrite CompletableFutureExtension.allSettled to not use CompletableFuture.supplyAsync (#1872)

We noticed a big drop in performance after switching to the new
`CompletableFuture` based `FederatedTypeResolver`. Our investigations
led us to the implementation of `CompletableFutureExtension.allSettled`
which has the following issues:

- Bug: It uses `CompletableFuture.supplyAsync` which looses all thread
local data (as we switch to another thread pool we have no control
over), which breaks most tracing frameworks and all other thread local
usages
- Performance: 100 futures get wrapped into 100 separately scheduled
futures, this is unnecessary as `thenApply` and `exceptionally` work
without nesting of futures. This avoids non-neglegible overhead

I changed the accompanying test to expect another exception type as only
`get()` throw a `ExecutionException`. This should not affect your public
API as its an internal method and I could not find any other references
to `ExecutionException` in `graphql-kotlin`s codebase.

Co-authored-by: Heller, Nico [CBC] <[email protected]>

Author: roookeee

generator/graphql-kotlin-federation/src/main/kotlin/com/expediagroup/graphql/generator/federation/extensions/CompletableFutureExtensions.kt

@@ -42,18 +42,9 @@ internal fun <T : Any?> List<CompletableFuture<out T>>.joinAll(): CompletableFut
 @Suppress("TooGenericExceptionCaught")
 internal fun <T : Any?> List<CompletableFuture<out T>>.allSettled(): CompletableFuture<List<Result<T>>> {
     val resultFutures = map { future ->
-        CompletableFuture.supplyAsync {
-            try {
-                Result.success(future.get())
-            } catch (e: Exception) {
-                Result.failure<T>(e)
-            }
-        }
+        future.thenApply { result -> Result.success(result) }
+            .exceptionally { exception -> Result.failure<T>(exception) }
     }
 
-    return CompletableFuture.allOf(
-        *resultFutures.toTypedArray()
-    ).thenApply {
-        resultFutures.map(CompletableFuture<Result<T>>::join)
-    }
+    return resultFutures.joinAll()
 }

generator/graphql-kotlin-federation/src/test/kotlin/com/expediagroup/graphql/generator/federation/extensions/CompletableFutureExtensionsKtTest.kt

@@ -27,7 +27,6 @@ import reactor.kotlin.core.publisher.toMono
 import java.time.Duration
 import java.util.concurrent.CompletableFuture
 import java.util.concurrent.CompletionException
-import java.util.concurrent.ExecutionException
 import kotlin.test.assertEquals
 import kotlin.test.assertIs
 import kotlin.test.assertTrue
@@ -101,7 +100,7 @@ class CompletableFutureExtensionsKtTest {
         assertEquals("first promise", result[0].getOrNull())
         assertEquals("second promise", result[1].getOrNull())
         assertTrue(result[2].isFailure)
-        assertIs<ExecutionException>(result[2].exceptionOrNull())
+        assertIs<CompletionException>(result[2].exceptionOrNull())
         assertEquals("async exception", result[2].exceptionOrNull()?.cause?.message)
     }
 }