1 files changed, 12 insertions, 2 deletions
diff --git a/src/mongo/s/d_logic.cpp b/src/mongo/s/d_logic.cpp
index 4d04eea2be9..dcf66033428 100644
--- a/src/mongo/s/d_logic.cpp
+++ b/src/mongo/s/d_logic.cpp
@@ -100,7 +100,7 @@ namespace mongo {
             dbresponse->responseTo = m.header()->id;
             return true;
         }
-        
+
         uassert( 9517 , "writeback" , ( d.reservedField() & Reserved_FromWriteback ) == 0 );
 
         OID writebackID;
@@ -109,6 +109,13 @@ namespace mongo {
         const OID& clientID = ShardedConnectionInfo::get(false)->getID();
         massert( 10422 ,  "write with bad shard config and no server id!" , clientID.isSet() );
 
+        // We need to check this here, since otherwise we'll get errors wrapping the writeback -
+        // not just here, but also when returning as a command result.
+        // We choose 1/2 the overhead of the internal maximum so that we can still handle ops of
+        // 16MB exactly.
+        massert( 16437, "data size of operation is too large to queue for writeback",
+                 m.dataSize() < BSONObjMaxInternalSize - (8 * 1024));
+
         LOG(1) << "writeback queued for " << m.toString() << endl;
 
         BSONObjBuilder b;
@@ -123,11 +130,14 @@ namespace mongo {
         b.appendBinData( "msg" , m.header()->len , bdtCustom , (char*)(m.singleData()) );
         LOG(2) << "writing back msg with len: " << m.header()->len << " op: " << m.operation() << endl;
 
+        // Convert to new BSONObj here just to be safe
+        BSONObj wbObj = b.obj();
+
         // Don't register the writeback until immediately before we queue it -
         // after this line, mongos will wait for an hour if we don't queue correctly
         lastError.getSafe()->writeback( writebackID );
 
-        writeBackManager.queueWriteBack( clientID.str() , b.obj() );
+        writeBackManager.queueWriteBack( clientID.str() , wbObj );
 
         return true;
     }