Original summary:
MapReduce with inline enabled causes sharding to create collection with empty name

Original report:

This was caught through the mapReduce*.js suite of tests, in the case of sharded collection output, when there is an assertion about the collection name not being empty in query-related commands.

This is the assertion:

 m30999| 2014-10-03T12:06:23.586-0400 D SHARDING [Balancer] shard0001 has more chunks me:1 best: shard0000:0
 m30999| 2014-10-03T12:06:23.586-0400 D SHARDING [Balancer] collection : mrShard.mrReduceInShardedOutSharded
 m30999| 2014-10-03T12:06:23.586-0400 D SHARDING [Balancer] donor      : shard0001 chunks on 1
 m30999| 2014-10-03T12:06:23.586-0400 D SHARDING [Balancer] receiver   : shard0000 chunks on 0
 m30999| 2014-10-03T12:06:23.586-0400 D SHARDING [Balancer] threshold  : 2
 m30999| 2014-10-03T12:06:23.586-0400 D SHARDING [Balancer] no need to move any chunk
 m30999| 2014-10-03T12:06:23.586-0400 D SHARDING [Balancer] about to log balancer result: { server: "kaloianmdesktop", time: new Date(1412352383586), what: "balancer.round", details: { executionTimeMillis: 225, errorOccured: false, candidateChunks: 0, chunksMoved: 0 } }
 m30999| 2014-10-03T12:06:23.611-0400 D SHARDING [Balancer] *** end of balancing round
 m30999| 2014-10-03T12:06:23.629-0400 I SHARDING [Balancer] distributed lock 'balancer/kaloianmdesktop:30999:1412352205:41' unlocked.
 m30999| 2014-10-03T12:06:26.184-0400 I SHARDING [LockPinger] cluster localhost:30000 pinged successfully at Fri Oct 03 12:06:26 2014 by distribute

...

 m30001| 2014-10-03T12:06:26.601-0400 I -        [conn5] Invariant failure !_nss.coll().empty() src\mongo\db\client.cpp 221
 m30001| 2014-10-03T12:06:27.351-0400 I -        [conn5] mongod.exe     ...\src\mongo\util\stacktrace_win.cpp(173)                       mongo::printStackTrace+0x5b
 m30001| 2014-10-03T12:06:27.352-0400 I -        [conn5] mongod.exe     ...\src\mongo\util\log.cpp(134)                                  mongo::logContext+0xd8
 m30001| 2014-10-03T12:06:27.352-0400 I -        [conn5] mongod.exe     ...\src\mongo\util\assert_util.cpp(143)                          mongo::invariantFailed+0xe3
 m30001| 2014-10-03T12:06:27.352-0400 I -        [conn5] mongod.exe     ...\src\mongo\db\client.cpp(221)                                 mongo::AutoGetCollectionForRead::_init+0xa1
 m30001| 2014-10-03T12:06:27.352-0400 I -        [conn5] mongod.exe     ...\src\mongo\db\client.cpp(206)                                 mongo::AutoGetCollectionForRead::AutoGetCollectionForRead+0xff
 m30001| 2014-10-03T12:06:27.353-0400 I -        [conn5] mongod.exe     ...\src\mongo\db\commands\count.cpp(108)                         mongo::CmdCount::run+0xfb
 m30001| 2014-10-03T12:06:27.353-0400 I -        [conn5] mongod.exe     ...\src\mongo\db\dbcommands.cpp(1150)                            mongo::_execCommand+0xb0
 m30001| 2014-10-03T12:06:27.353-0400 I -        [conn5] mongod.exe     ...\src\mongo\db\dbcommands.cpp(1364)                            mongo::Command::execCommand+0xde3
 m30001| 2014-10-03T12:06:27.353-0400 I -        [conn5] mongod.exe     ...\src\mongo\db\dbcommands.cpp(1442)                            mongo::_runCommands+0x570
 m30001| 2014-10-03T12:06:27.354-0400 I -        [conn5] mongod.exe     ...\src\mongo\db\query\new_find.cpp(125)                         mongo::runCommands+0x88
 m30001| 2014-10-03T12:06:27.354-0400 I -        [conn5] mongod.exe     ...\src\mongo\db\query\new_find.cpp(510)                         mongo::newRunQuery+0x490
 m30001| 2014-10-03T12:06:27.354-0400 I -        [conn5] mongod.exe     ...\src\mongo\db\instance.cpp(271)                               mongo::receivedQuery+0x2ae
 m30001| 2014-10-03T12:06:27.354-0400 I -        [conn5] mongod.exe     ...\src\mongo\db\instance.cpp(444)                               mongo::assembleResponse+0x4e1
 m30001| 2014-10-03T12:06:27.354-0400 I -        [conn5] mongod.exe     ...\src\mongo\db\db.cpp(186)                                     mongo::MyMessageHandler::process+0x1af
 m30001| 2014-10-03T12:06:27.355-0400 I -        [conn5] mongod.exe     ...\src\mongo\util\net\message_server_port.cpp(228)              mongo::PortMessageServer::handleIncomingMsg+0x5e4

This is the command, which gets executed on MongoS and the call stack:

 m30999| 2014-10-03T15:30:00.360-0400 D COMMANDS [conn1] COMMAND OBJECT: { mapreduce: "srcSharded", map: function map() { emit(this.i, 1); }, reduce: function reduce(key, values) { return Array.sum(values) }, out: { inline: 1.0, sharded: true } }

Call stack:

 m30999| 2014-10-03T12:06:29.351-0400 I -        [conn1] mongos.exe     ...\src\mongo\util\stacktrace_win.cpp(173)                       mongo::printStackTrace+0x5b
 m30999| 2014-10-03T12:06:29.352-0400 I -        [conn1] mongos.exe     ...\src\mongo\util\assert_util.cpp(80)                           mongo::DBException::traceIfNeeded+0x13c
 m30999| 2014-10-03T12:06:29.352-0400 I -        [conn1] mongos.exe     ...\src\mongo\util\assert_util.h(116)                            mongo::DBException::DBException+0x84
 m30999| 2014-10-03T12:06:29.352-0400 I -        [conn1] mongos.exe     ...\src\mongo\util\net\sock.h(166)                               mongo::SocketException::SocketException+0x15a
 m30999| 2014-10-03T12:06:29.352-0400 I -        [conn1] mongos.exe     ...\src\mongo\util\net\sock.cpp(806)                             mongo::Socket::handleRecvError+0x65c
 m30999| 2014-10-03T12:06:29.352-0400 I -        [conn1] mongos.exe     ...\src\mongo\util\net\sock.cpp(749)                             mongo::Socket::_recv+0xd3
 m30999| 2014-10-03T12:06:29.352-0400 I -        [conn1] mongos.exe     ...\src\mongo\util\net\sock.cpp(734)                             mongo::Socket::unsafe_recv+0x3c
 m30999| 2014-10-03T12:06:29.352-0400 I -        [conn1] mongos.exe     ...\src\mongo\util\net\sock.cpp(724)                             mongo::Socket::recv+0x8b
 m30999| 2014-10-03T12:06:29.352-0400 I -        [conn1] mongos.exe     ...\src\mongo\util\net\message_port.cpp(180)                     mongo::MessagingPort::recv+0x8c
 m30999| 2014-10-03T12:06:29.352-0400 I -        [conn1] mongos.exe     ...\src\mongo\util\net\message_port.cpp(272)                     mongo::MessagingPort::recv+0x6c
 m30999| 2014-10-03T12:06:29.352-0400 I -        [conn1] mongos.exe     ...\src\mongo\util\net\message_port.cpp(268)                     mongo::MessagingPort::call+0x4e
 m30999| 2014-10-03T12:06:29.352-0400 I -        [conn1] mongos.exe     ...\src\mongo\client\dbclient.cpp(1525)                          mongo::DBClientConnection::call+0xa3
 m30999| 2014-10-03T12:06:29.352-0400 I -        [conn1] mongos.exe     ...\src\mongo\client\dbclientcursor.cpp(80)                      mongo::DBClientCursor::init+0x131
 m30999| 2014-10-03T12:06:29.352-0400 I -        [conn1] mongos.exe     ...\src\mongo\client\dbclient.cpp(1118)                          mongo::DBClientBase::query+0x132
 m30999| 2014-10-03T12:06:29.352-0400 I -        [conn1] mongos.exe     ...\src\mongo\client\dbclientinterface.h(1272)                   mongo::DBClientConnection::query+0xd3
 m30999| 2014-10-03T12:06:29.353-0400 I -        [conn1] mongos.exe     ...\src\mongo\client\dbclient.cpp(959)                           mongo::DBClientInterface::findN+0x104
 m30999| 2014-10-03T12:06:29.353-0400 I -        [conn1] mongos.exe     ...\src\mongo\client\dbclient.cpp(979)                           mongo::DBClientInterface::findOne+0xc9
 m30999| 2014-10-03T12:06:29.353-0400 I -        [conn1] mongos.exe     ...\src\mongo\client\dbclient.cpp(447)                           mongo::DBClientWithCommands::runCommand+0x18b
 m30999| 2014-10-03T12:06:29.353-0400 I -        [conn1] mongos.exe     ...\src\mongo\client\dbclient.cpp(1049)                          mongo::DBClientConnection::runCommand+0x51
 m30999| 2014-10-03T12:06:29.353-0400 I -        [conn1] mongos.exe     ...\src\mongo\client\dbclient.cpp(473)                           mongo::DBClientWithCommands::count+0x115
 m30999| 2014-10-03T12:06:29.353-0400 I -        [conn1] mongos.exe     ...\src\mongo\s\chunk.cpp(1101)                                  mongo::ChunkManager::calcInitSplitsAndShards+0x2b6
 m30999| 2014-10-03T12:06:29.353-0400 I -        [conn1] mongos.exe     ...\src\mongo\s\chunk.cpp(1147)                                  mongo::ChunkManager::createFirstChunks+0xb1
 m30999| 2014-10-03T12:06:29.353-0400 I -        [conn1] mongos.exe     ...\src\mongo\s\config.cpp(229)                                  mongo::DBConfig::shardCollection+0x733
 m30999| 2014-10-03T12:06:29.353-0400 I -        [conn1] mongos.exe     ...\src\mongo\s\commands_public.cpp(2029)                        mongo::dbgrid_pub_cmds::MRCmd::run+0x2063
 m30999| 2014-10-03T12:06:29.353-0400 I -        [conn1] mongos.exe     ...\src\mongo\s\commands_public.cpp(1785)                        mongo::dbgrid_pub_cmds::MRCmd::run+0x65
 m30999| 2014-10-03T12:06:29.353-0400 I -        [conn1] mongos.exe     ...\src\mongo\s\s_only.cpp(145)                                  mongo::Command::execCommandClientBasic+0x386
 m30999| 2014-10-03T12:06:29.353-0400 I -        [conn1] mongos.exe     ...\src\mongo\s\commands_public.cpp(2683)                        mongo::Command::runAgainstRegistered+0x292
 m30999| 2014-10-03T12:06:29.353-0400 I -        [conn1] mongos.exe     ...\src\mongo\s\strategy.cpp(293)                                mongo::Strategy::clientCommandOp+0x75c
 m30999| 2014-10-03T12:06:29.353-0400 I -        [conn1] mongos.exe     ...\src\mongo\s\request.cpp(124)                                 mongo::Request::process+0x48a
 m30999| 2014-10-03T12:06:29.354-0400 I -        [conn1] mongos.exe     ...\src\mongo\s\server.cpp(148)                                  mongo::ShardedMessageHandler::process+0x129
 m30999| 2014-10-03T12:06:29.354-0400 I -        [conn1] mongos.exe     ...\src\mongo\util\net\message_server_port.cpp(228)              mongo::PortMessageServer::handleIncomingMsg+0x5e4