mongos can cause the in-memory sort limit to be hit on shards by requesting more results than needed

// Use mtools to setup the cluster and populate it with data.

$ mlaunch --sharded 2 --single -v

$ mgenerate '{"foo": {"$string": {"length": 10000}}, "bar": "$number", "baz": 0}' -n 20000

// Connect to mongos and shard the collection. Disable the balancer so that the data remains imbalanced,

// as there needs to be enough data on one of the shards to exceed the in-memory sort limit.

$ mongo

MongoDB shell version: 2.8.0-rc2

connecting to: test

mongos> db.version()

2.8.0-rc2

mongos> db.adminCommand({enableSharding: "test"})

{ "ok" : 1 }

mongos> db.adminCommand({shardCollection: "test.mgendata", key: {_id: 1}})

{ "collectionsharded" : "test.mgendata", "ok" : 1 }

mongos> sh.disableBalancing("test.mgendata")

// Connect to the shard with most of the data. The query succeeds on the shard until the limit

// becomes 3337 or higher. Then it fails due to hitting a memory limit. This is expected behavior.

$ mongo --port 27018

MongoDB shell version: 2.8.0-rc2

connecting to: 127.0.0.1:27018/test

> db.mgendata.count()

17950

> db.mgendata.find().sort({bar: 1}).limit(2000).itcount()

2000

> db.mgendata.find().sort({bar: 1}).limit(3336).itcount()

3336

> db.mgendata.find().sort({bar: 1}).limit(3337).itcount()

2014-12-10T20:39:17.781-0500 I QUERY    Error: error: {

        "$err" : "Executor error: Overflow sort stage buffered data usage of 33563546 bytes exceeds internal limit of 33554432 bytes",

        "code" : 17144

}

    at Error (<anonymous>)

    at DBQuery.next (src/mongo/shell/query.js:259:15)

    at DBQuery.itcount (src/mongo/shell/query.js:352:14)

    at (shell):1:47 at src/mongo/shell/query.js:259

// Connect to mongos and do the same thing. This time we hit the memory limit even

// if the limit is small, which is the bug!

mongos> db.mgendata.find().sort({bar: 1}).limit(500).itcount()

2014-12-10T20:41:55.832-0500 I QUERY    Error: error: {

        "$err" : "getMore executor error: Overflow sort stage buffered data usage of 33563546 bytes exceeds internal limit of 33554432 bytes",

        "code" : 17406

}

    at Error (<anonymous>)

    at DBQuery.next (src/mongo/shell/query.js:259:15)

    at DBQuery.itcount (src/mongo/shell/query.js:352:14)

    at (shell):1:46 at src/mongo/shell/query.js:259

st = new ShardingTest({ shards: 2, chunkSize: 1, other: { separateConfig: true, nopreallocj: 1 }});

var db = st.s.getDB('test');                                                                         

var mongosCol = db.getCollection('skip');                                                                  

db.adminCommand({ enableSharding: 'test' });                                                         

db.adminCommand({ shardCollection: 'test.skip', key: { x: 1 }});                                     

var i = 0;                                                                                           

var filler = new Array(1024).toString();                                                             

// create enough data to exceed 32MB limit

while (i < 32*1024 ) {                                                                               

    var bulk = []; 

    for (j = 0; j < 1024; j++) {                                                                     

        bulk.push({x:i+j, y:i+j, z:filler});                                                           

    }                                                                                                

    mongosCol.insert(bulk);                                                                     

    i += j;                                                                                          

}                                                                                                    

jsTest.log(mongosCol.count() + " documents in " + mongosCol);                                              

// test that direct connect to shard returns a document just below in-mem sort limit

var shardCol = st.shard0.getDB('test').getCollection('skip');

jsTest.log('test query succeeds directly on shard with skip 30000');

assert.eq(1, shardCol.find().sort({y:1}).skip(30000).limit(1).itcount());

// test that exceeding the in-memory sort limit errors on mongod

jsTest.log('test that there's an error on mongos with skip 32000');

assert.throws( function(){ mongosCol.find().sort({y:1}).skip(32000).limit(1).itcount() });

// test that exceeding the in-memory sort limit errors on mongos

jsTest.log('test that there's an error on mongos with skip 32000');

assert.throws( function(){ mongosCol.find().sort({y:1}).skip(32000).limit(1).itcount() });

// test that below limit should succeed on mongos 

jsTest.log('test query succeeds on mongos with skip 30000');

assert.eq(1, mongosCol.find().sort({y:1}).skip(30000).limit(1).itcount());