I'm having an issue that I can only assume is a 32-bit integer overflow issue. I'm running on 64-bit Linux boxes with the 64-bit builds of MongoDB v1.6.3.

Here's the deal:

I have a collection used for logging and aggregate counters. I'm using an upsert with $inc to increment these counters. When the counters get over 2^31, it rolls over to negative.

I've been able to reproduce it with the ruby driver:

require 'rubygems'
require 'mongo'

mongo = Mongo::Connection.new "localhost"
db = mongo.db "test"

db.collection('junk').drop
(0..20).each do |i|
db.collection('junk').update({:name=>'luke'}, {"$inc"=>{:total=>123456789}}, {:upsert=>true})
puts db.collection('junk').find({}, {:fields=>{:_id=>0}}).to_a.inspect
end

and that outputs: (I've bolded the line where it flips negative)

[

{"name"=>"luke", "total"=>123456789}

]
[

{"name"=>"luke", "total"=>246913578}

]
[

{"name"=>"luke", "total"=>370370367}

]
[

{"name"=>"luke", "total"=>493827156}

]
[

{"name"=>"luke", "total"=>617283945}

]
[

{"name"=>"luke", "total"=>740740734}

]
[

{"name"=>"luke", "total"=>864197523}

]
[

{"name"=>"luke", "total"=>987654312}

]
[

{"name"=>"luke", "total"=>1111111101}

]
[

{"name"=>"luke", "total"=>1234567890}

]
[

{"name"=>"luke", "total"=>1358024679}

]
[

{"name"=>"luke", "total"=>1481481468}

]
[

{"name"=>"luke", "total"=>1604938257}

]
[

{"name"=>"luke", "total"=>1728395046}

]
[

{"name"=>"luke", "total"=>1851851835}

]
[

{"name"=>"luke", "total"=>1975308624}

]
[

{"name"=>"luke", "total"=>2098765413}

]
[

{"name"=>"luke", "total"=>-2072745094}

]
[

{"name"=>"luke", "total"=>-1949288305}

]
[

{"name"=>"luke", "total"=>-1825831516}

]
[

{"name"=>"luke", "total"=>-1702374727}

]

I suspect that initially MongoDB is storing the integer using the 4-byte integer data type, but when my upsert increments it, it isn't converted to the 8-byte integer.

Just for fun, I tried upserting a large number and it indeed is stored just fine (I suspect it is created as an 8-byte integer):

db.collection('junk').drop
db.collection('junk').update({:name=>'luke'}, {"$inc"=>{:total=>9999999999999}}, {:upsert=>true})
puts db.collection('junk').find({}, {:fields=>{:_id=>0}}).to_a.inspect

and that outputs (which is clearly an 8-byte integer):

[

{"name"=>"luke", "total"=>9999999999999}

]