[ruby-core:98059] [Ruby master Bug#16816] Prematurely terminated Enumerator should stay terminated

Issue #16816 has been reported by headius (Charles Nutter).

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Bug #16816: Prematurely terminated Enumerator should stay terminated
https://bugs.ruby-lang.org/issues/16816

* Author: headius (Charles Nutter)
* Status: Open
* Priority: Normal
* ruby -v: all
* Backport: 2.5: UNKNOWN, 2.6: UNKNOWN, 2.7: UNKNOWN
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When iterating over an Enumerator, there are three different possible results of calling `next`:

1. The next item is returned and a cursor is advanced
2. There's no next item and the Enumerator will forever raise `StopIteration`
3. There's an error getting the next item which is raised out of `next`

This third case has some unexpected behavior that I discovered while working on https://github.com/jruby/jruby/issue/6157

It seems that when an Enumerator fails prematurely with an exception, any subsequent call to #next will cause it to restart.

This can be seen in a simple script I used to write a ruby/spec in https://github.com/jruby/jruby/pull/6190

```ruby
Enumerator.new {
  2.times {|i| raise i.to_s }
}.tap {|f|
  p 2.times.map { f.next rescue $!.message }
}
```

The output from this is `[0, 0]`. After the iteration fails, the second `next` call causes it to restart and it fails again.

Contrast this to the behavior of item 3 above; when an Enumerator finishes iterating without error, it remains "finished" forever and can't be restarted.

I believe the restarting behavior is at best undocumented behavior and at worst incorrect and unspected. Take this example:

```ruby
e = Enumerator.new { |y|
  c = new_database_cursor
  5.times { y.yield c.next_result }
}
```

If `next_result` here raises an error, a subsequent call to `next` on this enumerator will cause it to restart, re-acquire the cursor, and begin again.

As another example I ask a question: how do you indicate that an Enumerator failed due to an error, and *keep it failed* so it doesn't restart again?



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