Major - P3 The balancing loop for draining and zone correction should ensure that a shard which has been used as a destination once is not considered as a source as well, because this would result in one of the requests sent to that shard to fail.
The manifestation of this bug is an invariant with the following message:
2016-10-06T10:20:59.779-0400 I - [main] Invariant failure usedShards.insert(stat.shardId).second src\mongo\db\s\balancer\balancer_policy.cpp 380 2016-10-06T10:20:59.780-0400 I - [main] ***aborting after invariant() failure ... 2016-10-06T10:21:00.366-0400 I CONTROL [main] ucrtbased.dll raise+0x468 2016-10-06T10:21:00.366-0400 I CONTROL [main] ucrtbased.dll abort+0x39 2016-10-06T10:21:00.366-0400 I CONTROL [main] balancer_test.exe ...\src\mongo\util\assert_util.cpp(154) mongo::invariantFailed+0x110 2016-10-06T10:21:00.366-0400 I CONTROL [main] balancer_test.exe ...\src\mongo\db\s\balancer\balancer_policy.cpp(380) mongo::BalancerPolicy::balance+0xc0b