From a4530cdaee4eee1b6a7d50a05bcc6291d0e256b6 Mon Sep 17 00:00:00 2001
From: i10416 <ito.yo16uh90616@gmail.com>
Date: Tue, 21 Jul 2026 02:42:38 +0900
Subject: [PATCH] fix: avoid NPE when a relative path is provided to
 Watcher::watch

Before this commit, `Watcher::watch` throws NPE when a path is relative path
without a parent.
This commit updates watchFile methods to defensively convert a path into absolute path
and adds a test to confirm it won't throw NPE.

It supersedes https://github.com/typelevel/fs2/pull/3553 by SanjayUG
---
 .../main/scala/fs2/io/DeprecatedWatcher.scala    |  2 +-
 .../src/main/scala/fs2/io/file/Watcher.scala     |  2 +-
 .../test/scala/fs2/io/file/WatcherSuite.scala    | 16 ++++++++++++++++
 3 files changed, 18 insertions(+), 2 deletions(-)

diff --git a/io/jvm-native/src/main/scala/fs2/io/DeprecatedWatcher.scala b/io/jvm-native/src/main/scala/fs2/io/DeprecatedWatcher.scala
index 500dd8fb47..ccf8ed3abd 100644
--- a/io/jvm-native/src/main/scala/fs2/io/DeprecatedWatcher.scala
+++ b/io/jvm-native/src/main/scala/fs2/io/DeprecatedWatcher.scala
@@ -263,7 +263,7 @@ object Watcher {
         types: Seq[Watcher.EventType],
         modifiers: Seq[WatchEvent.Modifier]
     ): F[F[Unit]] =
-      registerUntracked(path.getParent, types, modifiers).flatMap(key =>
+      registerUntracked(path.toAbsolutePath.getParent, types, modifiers).flatMap(key =>
         track(
           Registration(
             path,
diff --git a/io/jvm-native/src/main/scala/fs2/io/file/Watcher.scala b/io/jvm-native/src/main/scala/fs2/io/file/Watcher.scala
index f688d4e212..353383b30b 100644
--- a/io/jvm-native/src/main/scala/fs2/io/file/Watcher.scala
+++ b/io/jvm-native/src/main/scala/fs2/io/file/Watcher.scala
@@ -288,7 +288,7 @@ object Watcher {
         types: Seq[Watcher.EventType],
         modifiers: Seq[WatchEvent.Modifier]
     ): F[F[Unit]] =
-      registerUntracked(path.toNioPath.getParent, types, modifiers).flatMap(key =>
+      registerUntracked(path.absolute.toNioPath.getParent(), types, modifiers).flatMap(key =>
         track(
           Registration(
             path.toNioPath,
diff --git a/io/jvm/src/test/scala/fs2/io/file/WatcherSuite.scala b/io/jvm/src/test/scala/fs2/io/file/WatcherSuite.scala
index 93af3c29f5..3d5b92cafb 100644
--- a/io/jvm/src/test/scala/fs2/io/file/WatcherSuite.scala
+++ b/io/jvm/src/test/scala/fs2/io/file/WatcherSuite.scala
@@ -33,6 +33,22 @@ import java.nio.file.WatchEvent
 class WatcherSuite extends Fs2Suite with BaseFileSuite {
   override def munitIOTimeout = 1.minute
 
+  group("support watching a file") {
+    test("for relative paths without throwing NPE") {
+      val mkRelativePath = for {
+        cwd <- Files[IO].currentWorkingDirectory.toResource
+        file <- Files[IO].tempFile(Some(cwd), "", ".tmp", None)
+        relPath = cwd.relativize(file)
+      } yield relPath
+      mkRelativePath
+        .both(Watcher.default[IO])
+        .use { case (path, watcher) =>
+          watcher.watch(path)
+        }
+        .void
+    }
+  }
+
   group("supports watching a file") {
     test("for modifications") {
       Stream