Fix UF_HIDDEN causing st_flags to be dropped in ConvertFileStatus; add PAL_UF_HIDDEN static assert

src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/FileInfo/GetSetAttributes.Unix.cs

@@ -0,0 +1,43 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using Xunit;
+
+namespace System.IO.Tests
+{
+    public partial class FileInfo_GetSetAttributes
+    {
+        [Fact]
+        [PlatformSpecific(TestPlatforms.OSX)]
+        public void TogglingHiddenAttribute_PreservesOtherUserFlags()
+        {
+            // UF_NODUMP (0x01) is a harmless BSD user flag that any file owner can set/clear.
+            const uint UF_NODUMP = 0x01;
+            const uint UF_HIDDEN = (uint)Interop.Sys.UserFlags.UF_HIDDEN;
+
+            string path = GetTestFilePath();
+            File.Create(path).Dispose();
+
+            // Set UF_NODUMP on the file directly via lchflags.
+            Assert.Equal(0, Interop.Sys.LChflags(path, UF_NODUMP));
+            Assert.Equal(0, Interop.Sys.Stat(path, out Interop.Sys.FileStatus before));
+            Assert.NotEqual(0u, before.UserFlags & UF_NODUMP);
+
+            // Toggle Hidden ON via the public API — this must preserve UF_NODUMP.
+            var fi = new FileInfo(path);
+            fi.Attributes |= FileAttributes.Hidden;
+
+            Assert.Equal(0, Interop.Sys.Stat(path, out Interop.Sys.FileStatus afterSet));
+            Assert.NotEqual(0u, afterSet.UserFlags & UF_HIDDEN);
+            Assert.NotEqual(0u, afterSet.UserFlags & UF_NODUMP);
+
+            // Toggle Hidden OFF — UF_NODUMP must still survive.
+            fi.Refresh();
+            fi.Attributes &= ~FileAttributes.Hidden;
+
+            Assert.Equal(0, Interop.Sys.Stat(path, out Interop.Sys.FileStatus afterClear));
+            Assert.Equal(0u, afterClear.UserFlags & UF_HIDDEN);
+            Assert.NotEqual(0u, afterClear.UserFlags & UF_NODUMP);
+        }
+    }
+}

src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/FileInfo/GetSetAttributes.cs

@@ -5,7 +5,7 @@
 
 namespace System.IO.Tests
 {
-    public class FileInfo_GetSetAttributes : InfoGetSetAttributes<FileInfo>
+    public partial class FileInfo_GetSetAttributes : InfoGetSetAttributes<FileInfo>
     {
         protected override bool CanBeReadOnly => true;
         protected override FileAttributes GetAttributes(string path) => new FileInfo(path).Attributes;

src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/System.IO.FileSystem.Tests.csproj

@@ -107,6 +107,11 @@
              Link="Common\Interop\Unix\Interop.OpenFlags.cs" />
     <Compile Include="$(CommonPath)Interop\Unix\System.Native\Interop.MkFifo.cs"
              Link="Common\Interop\Unix\Interop.MkFifo.cs" />
+    <Compile Include="$(CommonPath)Interop\Unix\System.Native\Interop.Stat.cs"
+             Link="Common\Interop\Unix\Interop.Stat.cs" />
+    <Compile Include="$(CommonPath)Interop\Unix\System.Native\Interop.LChflags.cs"
+             Link="Common\Interop\Unix\Interop.LChflags.cs" />
+    <Compile Include="FileInfo\GetSetAttributes.Unix.cs" />
   </ItemGroup>
   <ItemGroup Condition="'$(TargetPlatformIdentifier)' == 'windows'">
     <Compile Include="Directory\Delete.Windows.cs" />

src/native/libs/System.Native/pal_io.c

@@ -201,6 +201,12 @@ c_static_assert(PAL_IN_EXCL_UNLINK == IN_EXCL_UNLINK);
 c_static_assert(PAL_IN_ISDIR == IN_ISDIR);
 #endif // HAVE_INOTIFY
 
+// Validate that our UserFlags enum values match the platform, since
+// SystemNative_LChflags and SystemNative_FChflags pass them directly to the OS.
+#if HAVE_STAT_FLAGS && defined(UF_HIDDEN)
+c_static_assert(PAL_UF_HIDDEN == UF_HIDDEN);
+#endif
+
 static void ConvertFileStatus(const struct stat_* src, FileStatus* dst)
 {
     dst->Dev = (int64_t)src->st_dev;
@@ -231,7 +237,7 @@ static void ConvertFileStatus(const struct stat_* src, FileStatus* dst)
 #endif
 
 #if HAVE_STAT_FLAGS && defined(UF_HIDDEN)
-    dst->UserFlags = ((src->st_flags & UF_HIDDEN) == UF_HIDDEN) ? PAL_UF_HIDDEN : 0;
+    dst->UserFlags = (uint32_t)src->st_flags;
 #else
     dst->UserFlags = 0;
 #endif