Fix writeUtf8Lines evaluates leading effects in the input stream twice

[ChenCMD]

Fixes #3488.

There was a problem with writeUtf8Lines executing in.pull.uncons and evaluating leading effects, but discarding the result and using in.

(Sorry if it is difficult to read due to the use of translation software.)

[mpilquist]

Can we just remove the in.pull.uncons.flatMap entirely?

[ChenCMD]

If we keep the “don't add lineSeparator if the stream is empty” specification, I don't think we can remove in.pull.uncons.flatMap since we cannot determine if the stream is empty until we evaluate the first element.

[mpilquist]

Right, ok... can we test a bit with scopes? I worry, perhaps unnecessarily, that the proposed fix might mess with resource finalization -- i.e. some resources might get finalized early.

[ChenCMD]

It looks like any resource that was to be released at the end of the input stream also gets released at the end of the unconsed-and-flatMaped stream.

I was chatting with @kory33 about this potential corner case. His opinion was that uncons is not specified enough, and that we may want to have additional test-cases on uncons to ensure such early finalizations do not occur.

Code

val program = Stream
  .bracketWeak(IO { println("Resource 1 acquired"); "1" })(_ => IO.println("Resource 1 released"))
  .append(
    Stream
      .bracketWeak(IO { println("Resource 2 acquired"); "2" })(_ => IO.println("Resource 2 released"))
      .append(Stream.emit("3"))
      .append(Stream.exec(IO.println(s"test 1")))
      .scope
  )
  .append(Stream.exec(IO.println(s"test 2")))
  .through(Files.forIO.writeUtf8Lines(Path("test.txt")))
  .compile
  .drain
program.unsafeRunSync()

Output

Resource 1 acquired
Resource 2 acquired
test 1
Resource 2 released
test 2
Resource 1 released