From 887431967e7d74bafb77d65583ac25fa3bd0e143 Mon Sep 17 00:00:00 2001
From: ChenCMD <scnmed.g@gmail.com>
Date: Wed, 9 Oct 2024 03:02:23 +0900
Subject: [PATCH 1/2] Add writeUtf8Lines with side-effect test

---
 .../test/scala/fs2/io/file/FilesSuite.scala   | 19 ++++++++++++++++++-
 1 file changed, 18 insertions(+), 1 deletion(-)

diff --git a/io/shared/src/test/scala/fs2/io/file/FilesSuite.scala b/io/shared/src/test/scala/fs2/io/file/FilesSuite.scala
index bc1359373b..57014f76d6 100644
--- a/io/shared/src/test/scala/fs2/io/file/FilesSuite.scala
+++ b/io/shared/src/test/scala/fs2/io/file/FilesSuite.scala
@@ -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._
 
@@ -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)

From 14bdb4d77503e9ef196dd70443b3e25fde0bf947 Mon Sep 17 00:00:00 2001
From: ChenCMD <scnmed.g@gmail.com>
Date: Wed, 9 Oct 2024 03:18:16 +0900
Subject: [PATCH 2/2] Fix writeUtf8Lines evaluates leading effects in the input
 stream twice

Fix #3488
---
 io/shared/src/main/scala/fs2/io/file/Files.scala | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/io/shared/src/main/scala/fs2/io/file/Files.scala b/io/shared/src/main/scala/fs2/io/file/Files.scala
index ae618aba1d..bc4e307846 100644
--- a/io/shared/src/main/scala/fs2/io/file/Files.scala
+++ b/io/shared/src/main/scala/fs2/io/file/Files.scala
@@ -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)) =>
+          Stream
+            .chunk(next)
+            .append(rest)
+            .intersperse(lineSeparator)
+            .append(Stream[F, String](lineSeparator))
+            .underlying
         case None => Pull.done
       }
       .stream