SERVER-37344's ticket description says:

a shard that receives 'recoverTransaction' returns NoSuchTransaction if the shard does not know about the transaction. otherwise, if the decision has been made, returns the decision; *if the decision has not been made, decides to abort.*

And the server design also says:

If the client is unable to reach the original router after having attempted to send commitTransaction to the original router, the client can send commitTransaction to a different router.
Doing so will never initiate committing the transaction. *Instead, the recovery token in the request will be used to try to abort the transaction if a decision to commit has not already been made*, otherwise to recover the transaction's outcome.

However the implementation of SERVER-37344 says:

commit recovery is best effort. If coordinateCommit was never sent to the coordinator, the recovery commit will timeout waiting for it.

So I think the current implementation is incomplete. The abort optimization is important because it prevents applications from blocking for 60 seconds (or transactionLifetimeLimitSeconds) when the original commit attempt is lost.

CC: renctan.