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..d2c120179f333c2fa4e332726e03b75a44a25631 100644
--- a/sdk/lib/async/future.dart
+++ b/sdk/lib/async/future.dart
@@ -233,20 +233,36 @@ abstract class Future<T> {
     return result;
   }
 
+  /** Helper function that moves try/catch out of the main wait function. */
+  static void _safeCleanUp(var value, void cleanUp(value)) {
+    try {
+      cleanUp(value);
+    } on Object {
+      // Swallow any error. We are already reporting an earlier error.
+    }
+  }
+
   /**
    * Wait for all the given futures to complete and collect their values.
    *
    * 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

[kevmoo, 2014/12/18 23:17:50]

This could be a bit more clear.

...in case of an error, all non-null results generated before the error are
passed to `cleanUp`, which can release any...

[Lasse Reichstein Nielsen, 2014/12/19 06:54:11]

Any futures completing successfully after the first error will also be passed to
cleanUp. It could do with a rewrite for clarity.

We also swallow any error after the first - we consider the n futures to be one
operation, and it fails or completes a one.

I'll see if I can make the clean-up errors become uncaught errors instead of
being swallowed. There shouldn't be any errors, so having them uncaught is
reasonable too.

+   * a successful future is passed to `cleanUp`, which can then release any
+   * resources that the successful operation allocated.
    */
-  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 +270,15 @@ 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) _safeCleanUp(value, cleanUp);
+          }
+        }
+        values = null;
         if (remaining == 0 || eagerError) {
           result._completeError(theError, theStackTrace);
         } else {
@@ -281,8 +301,13 @@ 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) {
+            _safeCleanUp(value, cleanUp);
+          }
+          if (remaining == 0 && !eagerError) {
+            result._completeError(error, stackTrace);
+          }
         }
       }, onError: handleError);
     }