Do not run tasks when their prerequisites fail

[ptarjan]

If tasks A and B both depend on C, they race to grab the invocation
lock on C. If A wins, it sets @already_invoked and executes C while
holding the lock. When C completes, A releases the lock, B acquires it
and immediately returns since C is @already_invoked. Unfortunately,
this does not distinguish failed execution of C. Instead, if executing
C raises an exception for A, we want to raise the same exception in B
so it is aborted.

[drbrain]

Can you add a test for this as well?

I think something like this will adequately simulate cross-thread exceptions (but I didn't try it):

  def test_cross_thread_prerequisite_failures
    failed = false

    task :fail_once do
      fail_now = !failed
      failed = true
      raise 'failing once' if fail_now
    end

    task a: :fail_once
    task b: :fail_once

    assert_raises RuntimeError do
      Rake::Task[:a].invoke
    end

    assert_raises RuntimeError do
      Rake::Task[:b].invoke
    end
  end

[ptarjan]

@drbrain added. Thanks for the starting point. I had to make one of them a multitask to run my code