Add cleanUp function to Future.wait.

sdk/lib/async/future.dart

Index: sdk/lib/async/future.dart
diff --git a/sdk/lib/async/future.dart b/sdk/lib/async/future.dart
index 10aa237f9f08d0a8cca20e86d44fcf227ea69874..4bcc36f1bd88253d33487c1e411c110744f5f93e 100644
--- a/sdk/lib/async/future.dart
+++ b/sdk/lib/async/future.dart
@@ -239,14 +239,22 @@ abstract class Future<T> {
    * Returns a future which will complete once all the futures in a list are
    * complete. If any of the futures in the list completes with an error,
    * the resulting future also completes with an error. Otherwise the value
-   * of the returned future will be a list of all the values that were produced.
+   * of the returned future will be a list of all the values that were
+   * produced.
    *
    * If `eagerError` is true, the future completes with an error immediately on
    * the first error from one of the futures. Otherwise all futures must
    * complete before the returned future is completed (still with the first
    * error to occur, the remaining errors are silently dropped).
+   *
+   * If [cleanUp] is provided, in the case of an error, any non-null result of
+   * a successful future is passed to `cleanUp`, which can then release any
+   * resources that the successful operation allocated.
+   * The call to `cleanUp` must not throw.

[Søren Gjesse, 2015/01/09 10:04:11]

Please explain what happens if cleanUp actually throws.

[Lasse Reichstein Nielsen, 2015/01/09 10:52:09]

Done.

    */
-  static Future<List> wait(Iterable<Future> futures, {bool eagerError: false}) {
+  static Future<List> wait(Iterable<Future> futures,
+                           {bool eagerError: false,
+                            void cleanUp(successValue)}) {
     final _Future<List> result = new _Future<List>();
     List values;  // Collects the values. Set to null on error.
     int remaining = 0;  // How many futures are we waiting for.
@@ -254,11 +262,18 @@ abstract class Future<T> {
     StackTrace stackTrace;  // The stackTrace that came with the error.
 
     // Handle an error from any of the futures.
-    handleError(theError, theStackTrace) {
-      final bool isFirstError = (values != null);
-      values = null;
+    void handleError(theError, theStackTrace) {
       remaining--;
-      if (isFirstError) {
+      if (values != null) {
+        if (cleanUp != null) {
+          for (var value in values) {
+            if (value != null) {
+              // Ensure errors from cleanUp are uncaught.
+              new Future.sync(() { cleanUp(value); });
+            }
+          }
+        }
+        values = null;
         if (remaining == 0 || eagerError) {
           result._completeError(theError, theStackTrace);
         } else {
@@ -281,8 +296,14 @@ abstract class Future<T> {
           if (remaining == 0) {
             result._completeWithValue(values);
           }
-        } else if (remaining == 0 && !eagerError) {
-          result._completeError(error, stackTrace);
+        } else {
+          if (cleanUp != null && value != null) {
+            // Ensure errors from cleanUp are uncaught.
+            new Future.sync(() { cleanUp(value); });
+          }
+          if (remaining == 0 && !eagerError) {
+            result._completeError(error, stackTrace);
+          }
         }
       }, onError: handleError);
     }