database: Handle FindAncestryAndRollback datastore.Begin() error

[peacocb]

Do not call rollback() if transaction is not created.

Fixes #828

[jzelinskie]

Hey there! Great working tracking this down; this is a super common mistake in Go.

I only have one tiny nitpick to fix which is to put the defer statement in the same block as the error check:

tx, err := datastore.Begin()
if err != nil {
  return Ancestry{}, false, err
}
defer tx.Rollback()

return tx.FindAncestry(name)

[peacocb]

No problem with nitpicks :-) It takes a while to get the feel for others coding styles etc.

(I had to choose between the func above and the func below to determine where to put the defer and I chose the wrong one !)

[KeyboardNerd]

is this change also related?

[peacocb]

This change isn't related to the actual issue per se, it's just that my editor will not save this file without making this change and discussing it within my group we thought we'd leave it in as it helps with code correctness.
However, I am happy to remove it, just trying to get a feel for coding style etc.

[mirskifa]

Just for reference the editor is just making it more explicit that the mapset package is actually provided by golang-set as it's not necessarily obvious from the name (from 2014: deckarep/golang-set#11). Personally having the declaration there makes the code feel a little more readable, but I can see that it might not want to be associated with this PR.