Bigchaindb: double spend incorrectly reported

[sbellem]

From jmduque on gitter (with small edits to help clarifying):

This is an issue with the implementation of the get_spent() query for MongoDB.
For the RethinkDB implementation the issue does NOT arise.

How to duplicate the bug?

1. Create a divisible asset (outputs: tx1-0)
2. Transfer 2 amount to user A and 2 amount to user B (input: tx1-0, outpus: tx2-0/tx2-1)
3. Transfer another 1 Amount to A (inputs: tx1-0, ouputs: tx3-0/tx3-1)
4. Transfer 3 Amount from user A to user B (inputs: tx2-1/tx3-0, outputs: tx4-0/tx4-1)
5. Transfer 2 Amount from user B to user A (inputs: tx2-0, outputs: tx5-0)

See report by jmduque on gitter: https://gitter.im/bigchaindb/bigchaindb?at=58c12bf200c00c3d4fe02c88 for all details.

/cc @jmduque

[ttmc]

The bug arises when you ask for all of A's unspent outputs. The answer should be tx5-0 (i.e. only one unspent output), but the response is an error about invalid blockchain because of double-spend of output tx2-0.

[ttmc]

I drew a little diagram to understand how all the transactions are related:

[sbellem]

@jmduque a test that attempts to reproduce the issue is at https://github.com/sbellem/bigchaindb/blob/bug-1271-false-double-spend/tests/test_core.py#L331

It must be noted that the test should fail but does not right now. There are two possibilities:

1. The test is incorrect.
2. There's no bug.