Fix PathUtils.IsSymlink throwing on common lstat failures (#183)

* Fix PathUtils.IsSymlink throwing on common lstat failures

IsSymlink previously threw an exception for any lstat failure, including
ENOENT (file not found) and EACCES (permission denied). These are expected
non-exceptional conditions — especially when walking directory trees to check
for symlinks via IsSymlinkOrHasParentSymlink.

Now returns false for ENOENT, EACCES, and ENOTDIR, and only throws for
genuinely unexpected errno values. Uses named constants for clarity.

Also adds InternalsVisibleTo for the test project and new PathUtilsTests
(5 tests covering non-existent, regular, symlink, parent-walk, and ENOTDIR).

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Address review feedback: remove unused using, add parent-symlink test

- Remove unused 'using System;' from PathUtilsTests.cs
- Add IsSymlinkOrHasParentSymlink_ReturnsTrue_WhenParentIsSymlink test
  that creates a symlink directory and verifies parent traversal

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

---------

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: rmarinho

Xamarin.MacDev/PathUtils.cs

@@ -48,6 +48,15 @@ static int lstat (string path, out Stat buf)
 			}
 		}
 
+		const int ENOENT = 2;
+		const int EACCES = 13;
+		const int ENOTDIR = 20;
+
+		/// <summary>
+		/// Returns whether the given path is a symlink.
+		/// Returns false (rather than throwing) when the path cannot be examined
+		/// due to non-existence, permissions, or a non-directory path component.
+		/// </summary>
 		public static bool IsSymlink (string file)
 		{
 			if (Environment.OSVersion.Platform == PlatformID.Win32NT) {
@@ -56,8 +65,12 @@ public static bool IsSymlink (string file)
 			}
 			Stat buf;
 			var rv = lstat (file, out buf);
-			if (rv != 0)
-				throw new Exception (string.Format ("Could not lstat '{0}': {1}", file, Marshal.GetLastWin32Error ()));
+			if (rv != 0) {
+				var errno = Marshal.GetLastWin32Error ();
+				if (errno == ENOENT || errno == EACCES || errno == ENOTDIR)
+					return false;
+				throw new Exception (string.Format ("Could not lstat '{0}': {1}", file, errno));
+			}
 			const int S_IFLNK = 40960;
 			return (buf.st_mode & S_IFLNK) == S_IFLNK;
 		}

Xamarin.MacDev/Xamarin.MacDev.csproj

@@ -38,6 +38,9 @@
   <ItemGroup Condition=" '$(TargetFramework)' != 'netstandard2.0' ">
     <Compile Remove="NullableAttributes.cs" />
   </ItemGroup>
+  <ItemGroup>
+    <InternalsVisibleTo Include="tests" />
+  </ItemGroup>
   <ItemGroup>
     <PackageReference Include="System.Text.Json" Version="8.0.5" Condition=" '$(TargetFramework)' == 'netstandard2.0' " />
   </ItemGroup>

tests/PathUtilsTests.cs

@@ -0,0 +1,105 @@
+// Copyright (c) Microsoft Corporation.
+// Licensed under the MIT License.
+
+#nullable enable
+
+using System.IO;
+using NUnit.Framework;
+using Xamarin.MacDev;
+
+namespace tests {
+
+	[TestFixture]
+	public class PathUtilsTests {
+
+		[Test]
+		[Platform ("MacOsX")]
+		public void IsSymlink_ReturnsFalse_ForNonExistentFile ()
+		{
+			var path = Path.Combine (Path.GetTempPath (), Path.GetRandomFileName ());
+			// Should not throw; returns false for ENOENT
+			Assert.That (PathUtils.IsSymlink (path), Is.False);
+		}
+
+		[Test]
+		[Platform ("MacOsX")]
+		public void IsSymlink_ReturnsFalse_ForRegularFile ()
+		{
+			var path = Path.GetTempFileName ();
+			try {
+				Assert.That (PathUtils.IsSymlink (path), Is.False);
+			} finally {
+				File.Delete (path);
+			}
+		}
+
+		[Test]
+		[Platform ("MacOsX")]
+		public void IsSymlink_ReturnsTrue_ForSymlink ()
+		{
+			var target = Path.GetTempFileName ();
+			var link = target + ".link";
+			try {
+#if NET
+				File.CreateSymbolicLink (link, target);
+#else
+				// File.CreateSymbolicLink is not available on net472.
+				// Use a shell command to create the symlink on macOS.
+				var psi = new System.Diagnostics.ProcessStartInfo ("ln", $"-s \"{target}\" \"{link}\"") {
+					UseShellExecute = false,
+				};
+				System.Diagnostics.Process.Start (psi)!.WaitForExit ();
+#endif
+				Assert.That (PathUtils.IsSymlink (link), Is.True);
+			} finally {
+				File.Delete (link);
+				File.Delete (target);
+			}
+		}
+
+		[Test]
+		[Platform ("MacOsX")]
+		public void IsSymlinkOrHasParentSymlink_ReturnsFalse_ForNonExistentPath ()
+		{
+			var path = Path.Combine (Path.GetTempPath (), Path.GetRandomFileName ());
+			Assert.That (PathUtils.IsSymlinkOrHasParentSymlink (path), Is.False);
+		}
+
+		[Test]
+		[Platform ("MacOsX")]
+		public void IsSymlink_ReturnsFalse_WhenPathComponentIsNotDirectory ()
+		{
+			// /etc/hosts is a file, so /etc/hosts/bogus triggers ENOTDIR
+			var path = Path.Combine ("/etc/hosts", "bogus");
+			Assert.That (PathUtils.IsSymlink (path), Is.False);
+		}
+
+		[Test]
+		[Platform ("MacOsX")]
+		public void IsSymlinkOrHasParentSymlink_ReturnsTrue_WhenParentIsSymlink ()
+		{
+			var realDir = Path.Combine (Path.GetTempPath (), Path.GetRandomFileName ());
+			Directory.CreateDirectory (realDir);
+			var childDir = Path.Combine (realDir, "subdir");
+			Directory.CreateDirectory (childDir);
+
+			var linkDir = Path.Combine (Path.GetTempPath (), Path.GetRandomFileName ());
+			try {
+#if NET
+				Directory.CreateSymbolicLink (linkDir, realDir);
+#else
+				var psi = new System.Diagnostics.ProcessStartInfo ("ln", $"-s \"{realDir}\" \"{linkDir}\"") {
+					UseShellExecute = false,
+				};
+				System.Diagnostics.Process.Start (psi)!.WaitForExit ();
+#endif
+				var childViaLink = Path.Combine (linkDir, "subdir");
+				Assert.That (PathUtils.IsSymlinkOrHasParentSymlink (childViaLink), Is.True);
+			} finally {
+				if (Directory.Exists (linkDir))
+					Directory.Delete (linkDir);
+				Directory.Delete (realDir, recursive: true);
+			}
+		}
+	}
+}