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JRuby starts executing Enumerator code too soon #4583
Description
Activity
The problem is that the thread started to power Enumerator#next does not pause after yielding the nil result to the yielder. It doesn't pause until the next iteration, because there's nobody waiting for that value. As a result, it executes past the yield.
I believe I already fixed this for Fiber. What I didn't do is reimplement Enumerator#next in terms of Fiber, as MRI does.
If you are really just using this yielder pattern, you could swap in Fiber and it would work ok.
fiber = Fiber.new { Fiber.yield nil puts "EXECUTED" } fiber.resume
This will not output anything, because you only resume and get the first value, after which the Fiber properly transfers control back and waits.
More on reimplementing Enumerator#next in terms of Fiber...
I have prototyped this before, and the amount of code required is very small (all implemented in Ruby). The piece that's missing is our short-circuited logic for when the data being enumerated comes from an Array.
RubyEnumerator, implemented in Java, has at least two kinds of
nextlogic: one that uses a pseudo-fiber to actually drive the nexting like a fiber, and one that just acts as a cursor into the given collection. The latter is significantly lighter-weight, both because it doesn't have to transfer values out of a fiber and because it doesn't have to spin up a native thread.If (when?) we move Enumerator#next to use a fiber, we will still want to keep the optimized versions for known collections.
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on May 19, 2017 - added a commit that references this issue
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on Aug 18, 2017
Environment
Expected Behavior
For the given code
MRI 2.4.1 doesn't output anything. I expected JRuby to behave the same, to pause and not to execute any code after
yield, since we requested only the first element of the Enumerator.Actual Behavior
However, JRuby outputs
In other words, JRuby started executing the code after
yield, even though we only asked for the first element.