Fix cursor leaks when canceling AsyncTask

We discovered that AsyncTask.cancel() doesn't quite perform as expected;
In particular, if you call cancel() during a particularly slow background
worker, the result is discarded and onPostExecute() is never called.  If
the result is an open cursor, then we "leak" by not closing it.

For AccountFolderList, which has a multi-step doInBackground():
1.  Check for isCancelled() during the long doInBackground() which will
    reduce the number of discarded cursors in the first place.
2.  Check for isCancelled() at the end of the long doInBackground() and
    if true, close the result cursors and return null.
3.  In the existing isCancelled() code in onPostExecute(), close the
    cursors.

For other AsyncTasks (with simpler configurations):
1.  Check for isCancelled() at the end of doInBackground() and if true,
    close the just-opened cursor and return null.

See also Change-Id: Ie63a3197af563baa8bb0fe6f1ef9423e281cbf4c
Bug: 3088870

Change-Id: I51b11bb5b23f209f8b61962500a063040484fd24

src/com/android/email/activity/AccountFolderListFragment.java

@@ -386,19 +386,46 @@ public class AccountFolderListFragment extends ListFragment
         @Override
         protected Object[] doInBackground(Void... params) {
             // Create the summaries cursor
-            Cursor c1 = getSummaryChildCursor();
+            Cursor c1 = null;
+            Cursor c2 = null;
+            Long defaultAccount = null;
+            if (!isCancelled()) {
+                // Create the summaries cursor
+                c1 = getSummaryChildCursor();
+            }
 
-            // TODO use a custom projection and don't have to sample all of these columns
-            Cursor c2 = mActivity.getContentResolver().query(
-                    EmailContent.Account.CONTENT_URI,
-                    EmailContent.Account.CONTENT_PROJECTION, null, null, null);
-            Long defaultAccount = Account.getDefaultAccountId(mActivity);
+            if (!isCancelled()) {
+                // TODO use a custom projection and don't have to sample all of these columns
+                c2 = mActivity.getContentResolver().query(
+                        EmailContent.Account.CONTENT_URI,
+                        EmailContent.Account.CONTENT_PROJECTION, null, null, null);
+            }
+
+            if (!isCancelled()) {
+                defaultAccount = Account.getDefaultAccountId(mActivity);
+            }
+
+            if (isCancelled()) {
+                if (c1 != null) c1.close();
+                if (c2 != null) c2.close();
+                return null;
+            }
             return new Object[] { c1, c2 , defaultAccount};
         }
 
         @Override
         protected void onPostExecute(Object[] params) {
-            if (isCancelled() || params == null || ((Cursor)params[1]).isClosed()) {
+            if (isCancelled() || params == null) {
+                if (params != null) {
+                    Cursor c1 = (Cursor)params[0];
+                    if (c1 != null) {
+                        c1.close();
+                    }
+                    Cursor c2 = (Cursor)params[1];
+                    if (c2 != null) {
+                        c2.close();
+                    }
+                }
                 return;
             }
             // Before writing a new list adapter into the listview, we need to

src/com/android/email/activity/AccountShortcutPicker.java

@@ -131,7 +131,7 @@ public class AccountShortcutPicker extends ListActivity
         @Override
         protected void onPostExecute(Cursor cursor) {
             // If canceled, get out immediately
-            if (cursor == null) {
+            if (cursor == null || cursor.isClosed()) {
                 return;
             }
             // If there are no accounts, we should never have been here - exit the activity

src/com/android/email/activity/MessageOrderManager.java

@@ -266,7 +266,12 @@ public class MessageOrderManager {
     private class LoadMessageListTask extends AsyncTask<Void, Void, Cursor> {
         @Override
         protected Cursor doInBackground(Void... params) {
-            return openNewCursor();
+            Cursor c = openNewCursor();
+            if (isCancelled()) {
+                c.close();
+                c = null;
+            }
+            return c;
         }
 
         @Override