SERVER-17672 Helpers::getSingleton()/getLast() return owned object

(cherry picked from commit 8f931d4443e07e3bfdfab97377b98bb1fdd53548)

2 files changed, 31 insertions, 20 deletions

diff --git a/src/mongo/db/dbhelpers.cpp b/src/mongo/db/dbhelpers.cpp
index 32fa31f3725..16d1b8fe809 100644
--- a/src/mongo/db/dbhelpers.cpp
+++ b/src/mongo/db/dbhelpers.cpp
@@ -205,19 +205,18 @@ namespace mongo {
         return accessMethod->findSingle( txn, idquery["_id"].wrap() );
     }
 
-    /* Get the first object from a collection.  Generally only useful if the collection
-       only ever has a single object -- which is a "singleton collection". Note that the
-       BSONObj returned is *not* owned and will become invalid if the database is closed.
-
-       Returns: true if object exists.
-    */
     bool Helpers::getSingleton(OperationContext* txn, const char *ns, BSONObj& result) {
         AutoGetCollectionForRead ctx(txn, ns);
         auto_ptr<PlanExecutor> exec(InternalPlanner::collectionScan(txn, ns, ctx.getCollection()));
-
         PlanExecutor::ExecState state = exec->getNext(&result, NULL);
+
         txn->getCurOp()->done();
-        return PlanExecutor::ADVANCED == state;
+
+        if (PlanExecutor::ADVANCED == state) {
+            result = result.getOwned();
+            return true;
+        }
+        return false;
     }
 
     bool Helpers::getLast(OperationContext* txn, const char *ns, BSONObj& result) {
@@ -226,9 +225,13 @@ namespace mongo {
                                                                     ns,
                                                                     autoColl.getCollection(),
                                                                     InternalPlanner::BACKWARD));
-
         PlanExecutor::ExecState state = exec->getNext(&result, NULL);
-        return PlanExecutor::ADVANCED == state;
+
+        if (PlanExecutor::ADVANCED == state) {
+            result = result.getOwned();
+            return true;
+        }
+        return false;
     }
 
     void Helpers::upsert( OperationContext* txn,
diff --git a/src/mongo/db/dbhelpers.h b/src/mongo/db/dbhelpers.h
index f9ead76bc20..29236a0ef75 100644
--- a/src/mongo/db/dbhelpers.h
+++ b/src/mongo/db/dbhelpers.h
@@ -107,23 +107,31 @@ namespace mongo {
         static RecordId findById(OperationContext* txn,
                                 Collection* collection, const BSONObj& query);
 
-        /** Get/put the first (or last) object from a collection.  Generally only useful if the collection
-            only ever has a single object -- which is a "singleton collection".
-
-            You do not need to set the database (Context) before calling.
-
-            @return true if object exists.
-        */
+        /**
+         * Get the first object generated from a forward natural-order scan on "ns".  Callers do not
+         * have to lock "ns".
+         *
+         * Returns true if there is such an object.  An owned copy of the object is placed into the
+         * out-argument "result".
+         *
+         * Returns false if there is no such object.
+         */
         static bool getSingleton(OperationContext* txn, const char *ns, BSONObj& result);
-        static void putSingleton(OperationContext* txn, const char *ns, BSONObj obj);
-        static void putSingletonGod(OperationContext* txn, const char *ns, BSONObj obj, bool logTheOp);
 
         /**
-         * get last object int he collection; e.g. {$natural : -1}
+         * Same as getSingleton, but with a reverse natural-order scan on "ns".
          */
         static bool getLast(OperationContext* txn, const char *ns, BSONObj& result);
 
         /**
+         * Performs an upsert of "obj" into the collection "ns", with an empty update predicate.
+         * Callers must have "ns" locked.
+         */
+        static void putSingleton(OperationContext* txn, const char *ns, BSONObj obj);
+
+        static void putSingletonGod(OperationContext* txn, const char *ns, BSONObj obj, bool logTheOp);
+
+        /**
          * you have to lock
          * you do not have to have Context set
          * o has to have an _id field or will assert