PrefixedFileSystem: Forward all paths unchanged when prefix is "/"

src/Ramstack.FileSystem.Prefixed/PrefixedFileSystem.cs

@@ -113,11 +113,15 @@ internal string WrapWithPrefix(string underlyingPath)
     {
         Debug.Assert(VirtualPath.IsNormalized(path));
 
+        if (prefix == "/")
+            return path;
+
         if (path == prefix)
             return "/";
 
-        if (path.StartsWith(prefix, StringComparison.Ordinal) && path[prefix.Length] == '/')
-            return new string(path.AsSpan(prefix.Length));
+        if ((uint)prefix.Length < (uint)path.Length)
+            if (path.StartsWith(prefix, StringComparison.Ordinal) && path[prefix.Length] == '/')
+                return new string(path.AsSpan(prefix.Length));
 
         return null;
     }

tests/Ramstack.FileSystem.Prefixed.Tests/PrefixedFileSystemTests.cs

@@ -1,3 +1,5 @@
+using System.Reflection;
+
 using Ramstack.FileSystem.Physical;
 using Ramstack.FileSystem.Specification.Tests;
 using Ramstack.FileSystem.Specification.Tests.Utilities;
@@ -7,8 +9,59 @@ namespace Ramstack.FileSystem.Prefixed;
 [TestFixture]
 public class PrefixedFileSystemTests : VirtualFileSystemSpecificationTests
 {
+    private static readonly Func<string, string, string?> s_unwrapPrefix =
+        typeof(PrefixedFileSystem)
+            .GetMethod("TryUnwrapPrefix", BindingFlags.Static | BindingFlags.NonPublic)!
+            .CreateDelegate<Func<string, string, string?>>();
+
     private readonly TempFileStorage _storage = new TempFileStorage();
 
+    [TestCase("/", "/",            ExpectedResult = "/")]
+    [TestCase("/", "/foo",         ExpectedResult = "/foo")]
+    [TestCase("/", "/a/b/c",       ExpectedResult = "/a/b/c")]
+
+    [TestCase("/a/b", "/a/b",      ExpectedResult = "/")]
+
+    [TestCase("/a/b", "/a/b/c",    ExpectedResult = "/c")]
+    [TestCase("/a/b", "/a/b/c/d",  ExpectedResult = "/c/d")]
+
+    [TestCase("/a/b", "/a/bc",     ExpectedResult = null)]
+
+    [TestCase("/a/b", "/a/c",      ExpectedResult = null)]
+    [TestCase("/a/b", "/a",        ExpectedResult = null)]
+    public string? UnwrapPrefix(string prefix, string path) =>
+        s_unwrapPrefix(path, prefix);
+
+    [Test]
+    public async Task File_RootPrefix_DelegatesToInnerProvider()
+    {
+        using var fs = new PrefixedFileSystem("/",
+            new PhysicalFileSystem(_storage.Root));
+
+        var file = fs.GetFile("/project/README.md");
+        Assert.That(await file.ExistsAsync(), Is.True);
+    }
+
+    [Test]
+    public async Task File_RootPrefix_MissingFile_ReturnsNotFound()
+    {
+        using var fs = new PrefixedFileSystem("/",
+            new PhysicalFileSystem(_storage.Root));
+
+        var file = fs.GetFile("/project/nonexistent.txt");
+        Assert.That(await file.ExistsAsync(), Is.False);
+    }
+
+    [Test]
+    public async Task Directory_RootPrefix_DelegatesToInnerProvider()
+    {
+        using var fs = new PrefixedFileSystem("/",
+            new PhysicalFileSystem(_storage.Root));
+
+        var file = fs.GetDirectory("/project");
+        Assert.That(await file.ExistsAsync(), Is.True);
+    }
+
     [Test]
     public async Task File_Create_InsideArtificialDirectory_ThrowsException()
     {