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Fix writeUtf8Lines evaluates leading effects in the input stream twice #3489

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Fix writeUtf8Lines evaluates leading effects in the input stream twice #3489

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8874319
Add writeUtf8Lines with side-effect test
ChenCMD Oct 8, 2024
14bdb4d
Fix writeUtf8Lines evaluates leading effects in the input stream twice
ChenCMD Oct 8, 2024
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9 changes: 7 additions & 2 deletions io/shared/src/main/scala/fs2/io/file/Files.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -464,8 +464,13 @@ sealed trait Files[F[_]] extends FilesPlatform[F] {
def writeUtf8Lines(path: Path, flags: Flags): Pipe[F, String, Nothing] = in =>
in.pull.uncons
.flatMap {
case Some(_) =>
in.intersperse(lineSeparator).append(Stream[F, String](lineSeparator)).underlying
case Some((next, rest)) =>
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Can we just remove the in.pull.uncons.flatMap entirely?

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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.

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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.

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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

Stream
.chunk(next)
.append(rest)
.intersperse(lineSeparator)
.append(Stream[F, String](lineSeparator))
.underlying
case None => Pull.done
}
.stream
Expand Down
19 changes: 18 additions & 1 deletion io/shared/src/test/scala/fs2/io/file/FilesSuite.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -23,7 +23,7 @@ package fs2
package io
package file

import cats.effect.{IO, Resource}
import cats.effect.{IO, Resource, Ref}
import cats.kernel.Order
import cats.syntax.all._

Expand Down Expand Up @@ -170,6 +170,23 @@ class FilesSuite extends Fs2IoSuite with BaseFileSuite {
|""".stripMargin)
}

test("writeUtf8Lines - side effect") {
Stream
.resource(tempFile)
.flatMap { path =>
Stream.eval(Ref[IO].of(0)).flatMap { counter =>
Stream
.eval(counter.update(_ + 1).as(""))
.append(Stream.eval(counter.update(_ + 1).as("")))
.through(Files[IO].writeUtf8Lines(path)) ++
Stream.eval(counter.get)
}
}
.compile
.foldMonoid
.assertEquals(2)
}

test("writeUtf8Lines - empty stream") {
Stream
.resource(tempFile)
Expand Down