Expand async tests, minor async function updates

Author: katcharov

driver-core/src/main/com/mongodb/internal/async/AsyncSupplier.java

@@ -38,6 +38,7 @@ public interface AsyncSupplier<T> extends AsyncFunction<Void, T> {
     void unsafeFinish(SingleResultCallback<T> callback);
 
     /**
+     * This is the async variant of a supplier's get method.
      * This method must only be used when this AsyncSupplier corresponds
      * to a {@link java.util.function.Supplier} (and is therefore being
      * used within an async chain method lambda).
@@ -108,13 +109,13 @@ default AsyncRunnable thenConsume(final AsyncConsumer<T> consumer) {
 
     /**
      * @param errorCheck A check, comparable to a catch-if/otherwise-rethrow
-     * @param supplier   The branch to execute if the error matches
+     * @param errorFunction   The branch to execute if the error matches
      * @return The composition of this, and the conditional branch
      */
     default AsyncSupplier<T> onErrorIf(
             final Predicate<Throwable> errorCheck,
-            final AsyncSupplier<T> supplier) {
-        return (callback) -> this.unsafeFinish((r, e) -> {
+            final AsyncFunction<Throwable, T> errorFunction) {
+        return (callback) -> this.finish((r, e) -> {
             if (e == null) {
                 callback.onResult(r, null);
                 return;
@@ -128,7 +129,7 @@ default AsyncSupplier<T> onErrorIf(
                 return;
             }
             if (errorMatched) {
-                supplier.unsafeFinish(callback);
+                errorFunction.unsafeFinish(e, callback);
             } else {
                 callback.onResult(null, e);
             }

driver-core/src/main/com/mongodb/internal/async/SingleResultCallback.java

@@ -16,6 +16,7 @@
 
 package com.mongodb.internal.async;
 
+import com.mongodb.connection.AsyncCompletionHandler;
 import com.mongodb.internal.async.function.AsyncCallbackFunction;
 import com.mongodb.lang.Nullable;
 
@@ -34,4 +35,25 @@ public interface SingleResultCallback<T> {
      * @throws Error Never, on the best effort basis.
      */
     void onResult(@Nullable T result, @Nullable Throwable t);
+
+    default AsyncCompletionHandler<T> toHandler() {
+        return new AsyncCompletionHandler<T>() {
+            @Override
+            public void completed(@Nullable final T result) {
+                onResult(result, null);
+            }
+            @Override
+            public void failed(final Throwable t) {
+                onResult(null, t);
+            }
+        };
+    }
+
+    default void complete() {
+        this.onResult(null, null);
+    }
+
+    default void complete(final T result) {
+        this.onResult(result, null);
+    }
 }