Exception on GridFSBucket upload stream

[Andris Reinman]

I have a relatively simple code to upload files to GridFS with mon...@2.2.27 (Node.js v6.10.3):

```

let store = gridstore.openUploadStreamWithId(id, null, opts);

store.once('error', err => {

    callback(err);

});

store.once('finish', () => {

    callback(null, id);

});

store.end(data);

```

At least once there has been an exception while uploading. Unfortunately I don't have the data available that was trying to get uploaded. The exception that was thrown looks like this:

```

/path/to/app/node_modules/mongodb/lib/utils.js:123

    process.nextTick(function() { throw err; });

                                   ^

Error: Digest already called

    at Error (native)

    at Hash.digest (crypto.js:81:23)

    at checkDone (/path/to/app/node_modules/mongodb/lib/gridfs-stream/upload.js:259:17)

    at /path/to/app/node_modules/mongodb/lib/gridfs-stream/upload.js:427:11

    at /path/to/app/node_modules/mongodb/lib/collection.js:536:5

    at /path/to/app/node_modules/mongodb/lib/collection.js:714:5

    at handleCallback (/path/to/app/node_modules/mongodb/lib/utils.js:120:56)

    at executeCommands (/path/to/app/node_modules/mongodb/lib/bulk/ordered.js:398:12)

    at resultHandler (/path/to/app/node_modules/mongodb/lib/bulk/ordered.js:425:5)

    at /path/to/app/node_modules/mongodb-core/lib/connection/pool.js:469:18

```

The app has uploaded more than a million files and I've seen this exception only once so far. It caused the process to crash though as it is uncatchable (in normal ways).

Regards,

Andris Reinman

[christkv]

I suspect there was a race condition where checkDone is called twice (thus calling digest on the md5 hash twice). I added a ticket and fix that should be out in 2.2.28

https://jira.mongodb.org/browse/NODE-1000?jql=project%20%3D%20NODE%20AND%20created%3E%3D-1w%20ORDER%20BY%20created%20DESC