In the Knapsack voting style, we can't add an investment if its cost is
greater than the money we've got left, but in other voting styles money
might not be the issue.
So we're introducing the term "resources" and adapting the code
accordingly.
We were using the same logic twice.
I've moved the logic to the Ballot model, which to me is a more natural
place to calculate whether there's enough money left than the Investment
model. After all, the remaining money is in the ballot, and not in the
investment.
With two concurrent requests, it's possible to create two ballot lines
when only one of them should be created.
The reason is the code validating the line is not thread safe:
```
if ballot.amount_available(investment.heading) < investment.price.to_i
errors.add(:money, "insufficient funds")
end
```
If the second request executes this code after the first request has
executed it but before the first request has saved the record to the
database, both records will pass this validation and both will be saved
to the database.
So we need to introduce a lock. Now when the second request tries to
lock the ballot, it finds it's already locked by the first request, and
will wait for the transaction of the first request to finish before
checking whether there are sufficient funds.
Note we need to disable transactions during the test; otherwise the
second thread will wait for the first one to finish.
Also note that we need to update a couple of tests because records are
reloaded when they're locked.
In one case, reloading the ballot causes `ballot.user` to be `nil`,
since the user is hidden. So we hide the user after creating all its
associated records (which is the scenario that would take place in real
life).
In the other case, reloading the ballot causes `ballot.user` to reload
as well. So we need to reload the user object used in the test too so it
gets the updates done on `ballot.user`.
I haven't been able to reproduce this behavior in a system test. The
following test works with Rails 5.0, but it stopped working when we
moved to system tests in commit 9427f014. After that commit, for reasons
I haven't been able to debug (reintroducing truncation with
DatabaseClaner didn't seem to affect this test, and neither did
increasing the number of threads in Puma), the two AJAX requests
executed here are no longer simultaneous; the second request waits for
the first one to finish.
scenario "Race conditions with simultaneous requests", :js do
allow_any_instance_of(Budget::Ballot::Line).to receive(:check_sufficient_funds) do |object|
allow(object).to receive(:check_sufficient_funds).and_call_original
object.check_sufficient_funds
sleep 0.3
end
["First", "Second"].each do |title|
create(:budget_investment, :selected,
heading: california,
price: california.price,
title: title
)
end
login_as(user)
visit budget_investments_path(budget, heading_id: california.id)
within(".budget-investment", text: "First") { click_link "Vote" }
within(".budget-investment", text: "Second") { click_link "Vote" }
expect(page).to have_link "Remove vote"
expect(Budget::Ballot::Line.count).to eq 1
end
We were inconsistent on this one. I consider it particularly useful when
a method starts with a `return` statement.
In other cases, we probably shouldn't have a guard rule in the middle of
a method in any case, but that's a different refactoring.
