Merkle tree construction: single leaf is automatically promoted to root.

I would like to draw your attention to how Merkle tree is constructed when from a list of SecureHash-es in the case when this list contains a single element. In such a case any leaf automatically becomes the Merkle root.

Step by step:

1. getMerkleTree (here) receives a list of SecureHash-es.

2. padWithZeroes (here) ensures that the list contains such a number of elements which is a power of 2. Importantly 1 is a power of 2, i.e., 2^0 = 1. That is, no padding occurs.

3. buildMerkleTree (here) is executed with a single element list which means, that the only leaf automatically becomes the root of the Merkle tree, i.e.,

return if (lastNodesList.size == 1) {
   lastNodesList[0] // Root reached.
}

This behaviour clearly contradicts the documentation, in particular this image, see Attachment, Notary, TimeWindow groups.

A technically simple fix can align the code with the documentation and the actual behaviour: Change this conditional

if (isPow2(n)) return allLeavesHashes

to

if (isPow2(n) && n != 1) return allLeavesHashes

[r3jirabot]

Automatically created Jira issue: CORDA-4019

[mvdbos]

Fixed by #6895