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  1. Core Server
  2. SERVER-27362

MapReduce can attempt to deregister a cursor without holding a collection lock

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    Details

    • Type: Bug
    • Status: Closed
    • Priority: Major - P3
    • Resolution: Fixed
    • Affects Version/s: 3.4.0
    • Fix Version/s: 3.4.3, 3.5.2
    • Component/s: MapReduce
    • Labels:
      None
    • Backwards Compatibility:
      Fully Compatible
    • Operating System:
      ALL
    • Backport Requested:
      v3.4
    • Sprint:
      Query 2017-01-23

      Description

      There is code that attempts to prevent this from happening, but it's subtly broken. During the Map/Reduce command we construct a RangePreserver to prevent migrated chunks from being deleted out from under us. The RangePreserver will create a ClientCursor on the collection, preventing the RangeDeleter from deleting migrated ranges. In order to destroy a RangeDeleter you need to hold the collection lock in at least MODE_IS. Destroying a RangePreserver will attempt to delete and de-register its ClientCursor, and thus will need to access the CursorManager. The lock must be held to prevent the CursorManager from being destroyed while its being accessed (the CursorManager is deleted when a Collection is deleted, e.g. by a drop). We attempt to ensure this is the case by using the ON_BLOCK_EXIT macro to acquire a lock and clean up the ClientCursorPin:

              // Prevent sharding state from changing during the MR.
              unique_ptr<RangePreserver> rangePreserver;
              ScopedCollectionMetadata collMetadata;
              {
                  AutoGetCollectionForRead ctx(txn, config.ns);
       
                  Collection* collection = ctx.getCollection();
                  if (collection) {
                      rangePreserver.reset(new RangePreserver(collection));
                  }
       
                  // Get metadata before we check our version, to make sure it doesn't increment
                  // in the meantime.  Need to do this in the same lock scope as the block.
                  if (ShardingState::get(txn)->needCollectionMetadata(txn, config.ns)) {
                      collMetadata = CollectionShardingState::get(txn, config.ns)->getMetadata();
                  }
              }
       
              // Ensure that the RangePreserver is freed under the lock. This is necessary since the
              // RangePreserver's destructor unpins a ClientCursor, and access to the CursorManager must
              // be done under the lock.
              ON_BLOCK_EXIT([txn, &config, &rangePreserver] {
                  if (rangePreserver) {
                      AutoGetCollectionForRead ctx(txn, config.ns);
                      rangePreserver.reset();
                  }
              });
      

      This almost works, but AutoGetCollectionForRead does more than just acquire locks, it also can throw an exception. When this happens, rangePreserver is not reset in the ON_BLOCK_EXIT block, and instead the unique_ptr is destroyed, deleting the ClientCursorPin without a lock.

      This can be seen in this backtrace from one of my patch builds. In this patch, I did some work to allow the ClientCursorPin to hold a reference to an OperationContext, and then I added this dassert inside ClientCursorPin::deregisterCursor():

      @@ -236,8 +241,11 @@ void ClientCursorPin::deleteUnderlying() {
           //   we can't simply unpin with the cursor manager lock here, since we need to guarantee
           //   exclusive ownership of the cursor when we are deleting it).
           if (_cursor->cursorManager()) {
      +        dassert(_cursor->cursorManager()->getNss().ns().empty() ||
      +                _opCtx->lockState()->isCollectionLockedForMode(
      +                    _cursor->cursorManager()->getNss().ns(), MODE_IS));
               _cursor->cursorManager()->deregisterCursor(_cursor);
      -        _cursor->kill();
      +        _cursor->kill(_opCtx, "cursor exhausted");
           }
           _cursor->unsetPinned();
           delete _cursor;
      

      With this applied, the tests jstests/sharding/mapReduce_inSharded_outSharded and jstests/sharding/zBigMapReduce.js both failed on debug builds with this stack trace:

      [js_test:mapReduce_inSharded_outSharded] 2016-12-08T04:55:20.645+0000 d22261| 2016-12-08T04:55:20.646+0000 I -        [conn6] Invariant failure _cursor->cursorManager()->getNss().ns().empty() || _opCtx->lockState()->isCollectionLockedForMode( _cursor->cursorManager()->getNss().ns(), MODE_IS) src\mongo\db\clientcursor.cpp 246
      [js_test:mapReduce_inSharded_outSharded] 2016-12-08T04:55:20.645+0000 d22261| 2016-12-08T04:55:20.646+0000 I -        [conn6]
      [js_test:mapReduce_inSharded_outSharded] 2016-12-08T04:55:20.645+0000 d22261|
      [js_test:mapReduce_inSharded_outSharded] 2016-12-08T04:55:20.645+0000 d22261| ***aborting after invariant() failure
      [js_test:mapReduce_inSharded_outSharded] 2016-12-08T04:55:20.645+0000 d22261|
      [js_test:mapReduce_inSharded_outSharded] 2016-12-08T04:55:20.645+0000 d22261|
      [js_test:mapReduce_inSharded_outSharded] 2016-12-08T04:55:20.684+0000 d22261| 2016-12-08T04:55:20.684+0000 I SHARDING [conn5] MetadataLoader loading chunks for mrShard.srcSharded based on: (empty)
      [js_test:mapReduce_inSharded_outSharded] 2016-12-08T04:55:20.688+0000 d22261| 2016-12-08T04:55:20.689+0000 I SHARDING [conn5] Refreshing metadata for collection mrShard.srcSharded from collection version: 4|1||5848e79d4d151577c1e50900, shard version: 4|1||5848e79d4d151577c1e50900 to collection version: 5|1||5848e79d4d151577c1e50900, shard version: 5|1||5848e79d4d151577c1e50900
      [js_test:mapReduce_inSharded_outSharded] 2016-12-08T04:55:20.688+0000 d22261| 2016-12-08T04:55:20.689+0000 I SHARDING [conn5] MetadataLoader took 4 ms and found version 5|1||5848e79d4d151577c1e50900
      [js_test:mapReduce_inSharded_outSharded] 2016-12-08T04:55:20.688+0000 d22261| 2016-12-08T04:55:20.689+0000 I SHARDING [conn5] Migration succeeded and updated collection version to 5|1||5848e79d4d151577c1e50900
      [js_test:mapReduce_inSharded_outSharded] 2016-12-08T04:55:20.690+0000 d22261| 2016-12-08T04:55:20.690+0000 I SHARDING [conn5] about to log metadata event into changelog: { _id: "WIN-K6DURTQEFVN-2016-12-08T04:55:20.690+0000-5848e7b831d7d08e8550c61b", server: "WIN-K6DURTQEFVN", clientAddr: "10.63.93.217:50717", time: new Date(1481172920690), what: "moveChunk.commit", ns: "mrShard.srcSharded", details: { min: { _id: ObjectId('5848e79fadaa73f5c48b9144') }, max: { _id: ObjectId('5848e79fadaa73f5c48b930f') }, from: "shard0001", to: "shard0000" } }
      [js_test:mapReduce_inSharded_outSharded] 2016-12-08T04:55:20.709+0000 d22261| 2016-12-08T04:55:20.708+0000 I SHARDING [conn5] forking for cleanup of chunk data
      [js_test:mapReduce_inSharded_outSharded] 2016-12-08T04:55:20.709+0000 d22261| 2016-12-08T04:55:20.709+0000 I SHARDING [conn5] waiting for open cursors before removing range [{ _id: ObjectId('5848e79fadaa73f5c48b9144') }, { _id: ObjectId('5848e79fadaa73f5c48b930f') }) in mrShard.srcSharded, cursor ids: [46841715673]
      [js_test:mapReduce_inSharded_outSharded] 2016-12-08T04:55:20.710+0000 d22261| 2016-12-08T04:55:20.709+0000 I SHARDING [conn5] about to log metadata event into changelog: { _id: "WIN-K6DURTQEFVN-2016-12-08T04:55:20.709+0000-5848e7b831d7d08e8550c61d", server: "WIN-K6DURTQEFVN", clientAddr: "10.63.93.217:50717", time: new Date(1481172920709), what: "moveChunk.from", ns: "mrShard.srcSharded", details: { min: { _id: ObjectId('5848e79fadaa73f5c48b9144') }, max: { _id: ObjectId('5848e79fadaa73f5c48b930f') }, step 1 of 7: 0, step 2 of 7: 7, step 3 of 7: 32, step 4 of 7: 258, step 5 of 7: 105, step 6 of 7: 51, step 7 of 7: 0, to: "shard0000", from: "shard0001", note: "success" } }
      [js_test:mapReduce_inSharded_outSharded] 2016-12-08T04:55:20.729+0000 d22261| 2016-12-08T04:55:20.729+0000 I COMMAND  [conn5] command admin.$cmd appName: "MongoDB Shell" command: moveChunk { moveChunk: "mrShard.srcSharded", shardVersion: [ Timestamp 4000|1, ObjectId('5848e79d4d151577c1e50900') ], epoch: ObjectId('5848e79d4d151577c1e50900'), configdb: "test-configRS/WIN-K6DURTQEFVN:22262,WIN-K6DURTQEFVN:22263,WIN-K6DURTQEFVN:22264", fromShard: "shard0001", toShard: "shard0000", min: { _id: ObjectId('5848e79fadaa73f5c48b9144') }, max: { _id: ObjectId('5848e79fadaa73f5c48b930f') }, chunkVersion: [ Timestamp 4000|1, ObjectId('5848e79d4d151577c1e50900') ], maxChunkSizeBytes: 1048576, waitForDelete: false, takeDistLock: false } numYields:3 reslen:74 locks:{ Global: { acquireCount: { r: 23, w: 7 } }, MMAPV1Journal: { acquireCount: { r: 8, w: 11 } }, Database: { acquireCount: { r: 8, w: 5, W: 2 }, acquireWaitCount: { w: 2 }, timeAcquiringMicros: { w: 3458 } }, Collection: { acquireCount: { R: 8, W: 5 } } } protocol:op_command 478ms
      [js_test:mapReduce_inSharded_outSharded] 2016-12-08T04:55:20.888+0000 d22261| 2016-12-08T04:55:20.889+0000 I CONTROL  [conn6] mongod.exe    ...\src\mongo\util\stacktrace_windows.cpp(239)                                   mongo::printStackTrace+0x43
      [js_test:mapReduce_inSharded_outSharded] 2016-12-08T04:55:20.888+0000 d22261| 2016-12-08T04:55:20.889+0000 I CONTROL  [conn6] mongod.exe    ...\src\mongo\util\signal_handlers_synchronous.cpp(180)                          mongo::`anonymous namespace'::printSignalAndBacktrace+0x73
      [js_test:mapReduce_inSharded_outSharded] 2016-12-08T04:55:20.888+0000 d22261| 2016-12-08T04:55:20.889+0000 I CONTROL  [conn6] mongod.exe    ...\src\mongo\util\signal_handlers_synchronous.cpp(236)                          mongo::`anonymous namespace'::abruptQuit+0x83
      [js_test:mapReduce_inSharded_outSharded] 2016-12-08T04:55:20.888+0000 d22261| 2016-12-08T04:55:20.889+0000 I CONTROL  [conn6] mongod.exe    d:\th\minkernel\crts\ucrt\src\appcrt\misc\signal.cpp(522)                        raise+0x468
      [js_test:mapReduce_inSharded_outSharded] 2016-12-08T04:55:20.888+0000 d22261| 2016-12-08T04:55:20.889+0000 I CONTROL  [conn6] mongod.exe    d:\th\minkernel\crts\ucrt\src\appcrt\startup\abort.cpp(71)                       abort+0x39
      [js_test:mapReduce_inSharded_outSharded] 2016-12-08T04:55:20.890+0000 d22261| 2016-12-08T04:55:20.889+0000 I CONTROL  [conn6] mongod.exe    ...\src\mongo\util\assert_util.cpp(154)                                          mongo::invariantFailed+0x20c
      [js_test:mapReduce_inSharded_outSharded] 2016-12-08T04:55:20.890+0000 d22261| 2016-12-08T04:55:20.889+0000 I CONTROL  [conn6] mongod.exe    ...\src\mongo\db\clientcursor.cpp(244)                                           mongo::ClientCursorPin::deleteUnderlying+0x13c
      [js_test:mapReduce_inSharded_outSharded] 2016-12-08T04:55:20.890+0000 d22261| 2016-12-08T04:55:20.889+0000 I CONTROL  [conn6] mongod.exe    c:\program files (x86)\microsoft visual studio 14.0\vc\include\memory(1397)      std::unique_ptr<mongo::RangePreserver,std::default_delete<mongo::RangePreserver> >::~unique_ptr<mongo::RangePreserver,std::default_delete<mongo::RangePreserver> >+0x29
      [js_test:mapReduce_inSharded_outSharded] 2016-12-08T04:55:20.890+0000 d22261| 2016-12-08T04:55:20.889+0000 I CONTROL  [conn6] mongod.exe    ...\src\mongo\db\commands\mr.cpp(1659)                                           mongo::mr::MapReduceCommand::run+0x1b45
      [js_test:mapReduce_inSharded_outSharded] 2016-12-08T04:55:20.890+0000 d22261| 2016-12-08T04:55:20.889+0000 I CONTROL  [conn6] mongod.exe    ...\src\mongo\db\commands\dbcommands.cpp(1528)                                   mongo::Command::run+0x6db
      [js_test:mapReduce_inSharded_outSharded] 2016-12-08T04:55:20.890+0000 d22261| 2016-12-08T04:55:20.889+0000 I CONTROL  [conn6] mongod.exe    ...\src\mongo\db\commands\dbcommands.cpp(1438)                                   mongo::Command::execCommand+0xc80
      [js_test:mapReduce_inSharded_outSharded] 2016-12-08T04:55:20.890+0000 d22261| 2016-12-08T04:55:20.889+0000 I CONTROL  [conn6] mongod.exe    ...\src\mongo\db\run_commands.cpp(73)                                            mongo::runCommands+0x510
      [js_test:mapReduce_inSharded_outSharded] 2016-12-08T04:55:20.891+0000 d22261| 2016-12-08T04:55:20.889+0000 I CONTROL  [conn6] mongod.exe    ...\src\mongo\db\instance.cpp(273)                                               mongo::`anonymous namespace'::receivedRpc+0x1e7
      [js_test:mapReduce_inSharded_outSharded] 2016-12-08T04:55:20.891+0000 d22261| 2016-12-08T04:55:20.889+0000 I CONTROL  [conn6] mongod.exe    ...\src\mongo\db\instance.cpp(618)                                               mongo::assembleResponse+0x815
      [js_test:mapReduce_inSharded_outSharded] 2016-12-08T04:55:20.891+0000 d22261| 2016-12-08T04:55:20.889+0000 I CONTROL  [conn6] mongod.exe    ...\src\mongo\db\service_entry_point_mongod.cpp(135)                             mongo::ServiceEntryPointMongod::_sessionLoop+0x195
      [js_test:mapReduce_inSharded_outSharded] 2016-12-08T04:55:20.891+0000 d22261| 2016-12-08T04:55:20.889+0000 I CONTROL  [conn6] mongod.exe    c:\program files (x86)\microsoft visual studio 14.0\vc\include\functional(212)   std::_Func_impl<<lambda_0476eb5123845e42a2f452f2efde6866>,std::allocator<int>,void,std::shared_ptr<mongo::transport::Session> const & __ptr64>::_Do_call+0x43
      [js_test:mapReduce_inSharded_outSharded] 2016-12-08T04:55:20.891+0000 d22261| 2016-12-08T04:55:20.889+0000 I CONTROL  [conn6] mongod.exe    ...\src\mongo\transport\service_entry_point_utils.cpp(78)                        mongo::`anonymous namespace'::runFunc+0x25e
      [js_test:mapReduce_inSharded_outSharded] 2016-12-08T04:55:20.891+0000 d22261| 2016-12-08T04:55:20.890+0000 I CONTROL  [conn6] mongod.exe    c:\program files (x86)\microsoft visual studio 14.0\vc\include\thr\xthread(247)  std::_LaunchPad<std::unique_ptr<std::tuple<std::_Binder<std::_Unforced,void * __ptr64 (__cdecl&)(void * __ptr64),mongo::`anonymous namespace'::Context * __ptr64> >,std::default_delete<std::tuple<std::_Binder<std::_Unforced,void * __ptr64 (__cdecl&)(void * __ptr64),mongo::`anonymous namespace'::Context * __ptr64> > > > >::_Run+0x6f
      [js_test:mapReduce_inSharded_outSharded] 2016-12-08T04:55:20.891+0000 d22261| 2016-12-08T04:55:20.890+0000 I CONTROL  [conn6] mongod.exe    c:\program files (x86)\microsoft visual studio 14.0\vc\include\thr\xthread(210)  std::_Pad::_Call_func+0x9
      [js_test:mapReduce_inSharded_outSharded] 2016-12-08T04:55:20.891+0000 d22261| 2016-12-08T04:55:20.890+0000 I CONTROL  [conn6] mongod.exe    d:\th\minkernel\crts\ucrt\src\appcrt\startup\thread.cpp(92)                      invoke_thread_procedure+0x2c
      [js_test:mapReduce_inSharded_outSharded] 2016-12-08T04:55:20.891+0000 d22261| 2016-12-08T04:55:20.890+0000 I CONTROL  [conn6] mongod.exe    d:\th\minkernel\crts\ucrt\src\appcrt\startup\thread.cpp(115)                     thread_start<unsigned int (__cdecl*)(void * __ptr64)>+0x93
      [js_test:mapReduce_inSharded_outSharded] 2016-12-08T04:55:20.891+0000 d22261| 2016-12-08T04:55:20.890+0000 I CONTROL  [conn6] kernel32.dll                                                                                   BaseThreadInitThunk+0xd
      [js_test:mapReduce_inSharded_outSharded] 2016-12-08T04:55:20.891+0000 d22261| 2016-12-08T04:55:20.890+0000 F -        [conn6] Got signal: 22 (SIGABRT).
      [js_test:mapReduce_inSharded_outSharded] 2016-12-08T04:55:20.891+0000 d22261| 2016-12-08T04:55:20.890+0000 I CONTROL  [conn6] *** unhandled exception 0x0000000E at 0x000007FEFCEBA06D, terminating
      [js_test:mapReduce_inSharded_outSharded] 2016-12-08T04:55:20.891+0000 d22261| 2016-12-08T04:55:20.890+0000 I CONTROL  [conn6] *** stack trace for unhandled exception:
      [js_test:mapReduce_inSharded_outSharded] 2016-12-08T04:55:20.898+0000 d22261| 2016-12-08T04:55:20.899+0000 I CONTROL  [conn6] KERNELBASE.dll                                                                                   RaiseException+0x3d
      [js_test:mapReduce_inSharded_outSharded] 2016-12-08T04:55:20.898+0000 d22261| 2016-12-08T04:55:20.899+0000 I CONTROL  [conn6] mongod.exe      ...\src\mongo\util\signal_handlers_synchronous.cpp(237)                          mongo::`anonymous namespace'::abruptQuit+0x9b
      [js_test:mapReduce_inSharded_outSharded] 2016-12-08T04:55:20.898+0000 d22261| 2016-12-08T04:55:20.899+0000 I CONTROL  [conn6] mongod.exe      d:\th\minkernel\crts\ucrt\src\appcrt\misc\signal.cpp(522)                        raise+0x468
      [js_test:mapReduce_inSharded_outSharded] 2016-12-08T04:55:20.900+0000 d22261| 2016-12-08T04:55:20.899+0000 I CONTROL  [conn6] mongod.exe      d:\th\minkernel\crts\ucrt\src\appcrt\startup\abort.cpp(71)                       abort+0x39
      [js_test:mapReduce_inSharded_outSharded] 2016-12-08T04:55:20.900+0000 d22261| 2016-12-08T04:55:20.899+0000 I CONTROL  [conn6] mongod.exe      ...\src\mongo\util\assert_util.cpp(154)                                          mongo::invariantFailed+0x20c
      [js_test:mapReduce_inSharded_outSharded] 2016-12-08T04:55:20.900+0000 d22261| 2016-12-08T04:55:20.899+0000 I CONTROL  [conn6] mongod.exe      ...\src\mongo\db\clientcursor.cpp(244)                                           mongo::ClientCursorPin::deleteUnderlying+0x13c
      [js_test:mapReduce_inSharded_outSharded] 2016-12-08T04:55:20.900+0000 d22261| 2016-12-08T04:55:20.899+0000 I CONTROL  [conn6] mongod.exe      c:\program files (x86)\microsoft visual studio 14.0\vc\include\memory(1397)      std::unique_ptr<mongo::RangePreserver,std::default_delete<mongo::RangePreserver> >::~unique_ptr<mongo::RangePreserver,std::default_delete<mongo::RangePreserver> >+0x29
      [js_test:mapReduce_inSharded_outSharded] 2016-12-08T04:55:20.900+0000 d22261| 2016-12-08T04:55:20.899+0000 I CONTROL  [conn6] mongod.exe      ...\src\mongo\db\commands\mr.cpp(1659)                                           mongo::mr::MapReduceCommand::run+0x1b45
      [js_test:mapReduce_inSharded_outSharded] 2016-12-08T04:55:20.900+0000 d22261| 2016-12-08T04:55:20.899+0000 I CONTROL  [conn6] mongod.exe      ...\src\mongo\db\commands\dbcommands.cpp(1528)                                   mongo::Command::run+0x6db
      [js_test:mapReduce_inSharded_outSharded] 2016-12-08T04:55:20.900+0000 d22261| 2016-12-08T04:55:20.899+0000 I CONTROL  [conn6] mongod.exe      ...\src\mongo\db\commands\dbcommands.cpp(1438)                                   mongo::Command::execCommand+0xc80
      [js_test:mapReduce_inSharded_outSharded] 2016-12-08T04:55:20.901+0000 d22261| 2016-12-08T04:55:20.900+0000 I CONTROL  [conn6] mongod.exe      ...\src\mongo\db\run_commands.cpp(73)                                            mongo::runCommands+0x510
      [js_test:mapReduce_inSharded_outSharded] 2016-12-08T04:55:20.901+0000 d22261| 2016-12-08T04:55:20.900+0000 I CONTROL  [conn6] mongod.exe      ...\src\mongo\db\instance.cpp(273)                                               mongo::`anonymous namespace'::receivedRpc+0x1e7
      [js_test:mapReduce_inSharded_outSharded] 2016-12-08T04:55:20.901+0000 d22261| 2016-12-08T04:55:20.900+0000 I CONTROL  [conn6] mongod.exe      ...\src\mongo\db\instance.cpp(618)                                               mongo::assembleResponse+0x815
      [js_test:mapReduce_inSharded_outSharded] 2016-12-08T04:55:20.901+0000 d22261| 2016-12-08T04:55:20.900+0000 I CONTROL  [conn6] mongod.exe      ...\src\mongo\db\service_entry_point_mongod.cpp(135)                             mongo::ServiceEntryPointMongod::_sessionLoop+0x195
      [js_test:mapReduce_inSharded_outSharded] 2016-12-08T04:55:20.901+0000 d22261| 2016-12-08T04:55:20.900+0000 I CONTROL  [conn6] mongod.exe      c:\program files (x86)\microsoft visual studio 14.0\vc\include\functional(212)   std::_Func_impl<<lambda_0476eb5123845e42a2f452f2efde6866>,std::allocator<int>,void,std::shared_ptr<mongo::transport::Session> const & __ptr64>::_Do_call+0x43
      [js_test:mapReduce_inSharded_outSharded] 2016-12-08T04:55:20.901+0000 d22261| 2016-12-08T04:55:20.900+0000 I CONTROL  [conn6] mongod.exe      ...\src\mongo\transport\service_entry_point_utils.cpp(78)                        mongo::`anonymous namespace'::runFunc+0x25e
      [js_test:mapReduce_inSharded_outSharded] 2016-12-08T04:55:20.901+0000 d22261| 2016-12-08T04:55:20.900+0000 I CONTROL  [conn6] mongod.exe      c:\program files (x86)\microsoft visual studio 14.0\vc\include\thr\xthread(247)  std::_LaunchPad<std::unique_ptr<std::tuple<std::_Binder<std::_Unforced,void * __ptr64 (__cdecl&)(void * __ptr64),mongo::`anonymous namespace'::Context * __ptr64> >,std::default_delete<std::tuple<std::_Binder<std::_Unforced,void * __ptr64 (__cdecl&)(void * __ptr64),mongo::`anonymous namespace'::Context * __ptr64> > > > >::_Run+0x6f
      [js_test:mapReduce_inSharded_outSharded] 2016-12-08T04:55:20.901+0000 d22261| 2016-12-08T04:55:20.900+0000 I CONTROL  [conn6] mongod.exe      c:\program files (x86)\microsoft visual studio 14.0\vc\include\thr\xthread(210)  std::_Pad::_Call_func+0x9
      [js_test:mapReduce_inSharded_outSharded] 2016-12-08T04:55:20.901+0000 d22261| 2016-12-08T04:55:20.900+0000 I CONTROL  [conn6] mongod.exe      d:\th\minkernel\crts\ucrt\src\appcrt\startup\thread.cpp(92)                      invoke_thread_procedure+0x2c
      [js_test:mapReduce_inSharded_outSharded] 2016-12-08T04:55:20.901+0000 d22261| 2016-12-08T04:55:20.900+0000 I CONTROL  [conn6] mongod.exe      d:\th\minkernel\crts\ucrt\src\appcrt\startup\thread.cpp(115)                     thread_start<unsigned int (__cdecl*)(void * __ptr64)>+0x93
      [js_test:mapReduce_inSharded_outSharded] 2016-12-08T04:55:20.901+0000 d22261| 2016-12-08T04:55:20.900+0000 I CONTROL  [conn6] kernel32.dll                                                                                     BaseThreadInitThunk+0xd
      [js_test:mapReduce_inSharded_outSharded] 2016-12-08T04:55:20.901+0000 d22261| 2016-12-08T04:55:20.900+0000 I -        [conn6]
      [js_test:mapReduce_inSharded_outSharded] 2016-12-08T04:55:20.901+0000 d22261| 2016-12-08T04:55:20.900+0000 I CONTROL  [conn6] writing minidump diagnostic file C:\data\mci\b4699d47a7958c4241a69935b1137ca0\src\mongod.2016-12-08T04-55-20.mdmp
      

      This patch seems to fix the issue, but causes this assertion in jstests/core/profile_mapReduce.js, I think because the removed AutoGetCollectionForRead was setting the CurOp's ns back to the source collection, even though the last operation was actually an insert into the temp collection that is eventually renamed into the output collection.

      diff --git a/src/mongo/db/commands/mr.cpp b/src/mongo/db/commands/mr.cpp
      index 3abd4a8..6c1afde 100644
      --- a/src/mongo/db/commands/mr.cpp
      +++ b/src/mongo/db/commands/mr.cpp
      @@ -1417,7 +1417,8 @@ public:
               // be done under the lock.
               ON_BLOCK_EXIT([txn, &config, &rangePreserver] {
                   if (rangePreserver) {
      -                AutoGetCollectionForRead ctx(txn, config.ns);
      +                // Be sure not to use AutoGetCollectionForRead here, as that may throw.
      +                AutoGetCollection ctx(txn, NamespaceString(config.ns), MODE_IS);
                       rangePreserver.reset();
                   }
               });
      

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              Assignee:
              charlie.swanson Charlie Swanson
              Reporter:
              charlie.swanson Charlie Swanson
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