Fix cross-filesystem operations in MountFileSystem

src/Zio.Tests/FileSystems/TestMemoryFileSystem.cs

@@ -93,6 +93,23 @@ public void TestMoveFileCross()
         Assert.Equal(TriggerMemoryFileSystem.TriggerType.Move, fs.Triggered);
     }
 
+    [Fact]
+    public void TestMoveFileCrossMount()
+    {
+        var fs = new TriggerMemoryFileSystem();
+        fs.CreateDirectory("/sub1");
+        fs.CreateDirectory("/sub2");
+        var mount = new MountFileSystem();
+        var sub1 = new SubFileSystem(fs, "/sub1");
+        var sub2 = new SubFileSystem(fs, "/sub2");
+        mount.Mount("/sub2-mount", sub2);
+        sub1.WriteAllText("/file.txt", "test");
+        sub1.MoveFileCross("/file.txt", mount, "/sub2-mount/file.txt");
+        Assert.Equal("test", sub2.ReadAllText("/file.txt"));
+        Assert.False(sub1.FileExists("/file.txt"));
+        Assert.Equal(TriggerMemoryFileSystem.TriggerType.Move, fs.Triggered);
+    }
+
     private sealed class TriggerMemoryFileSystem : MemoryFileSystem
     {
         public enum TriggerType

src/Zio/FileSystems/ComposeFileSystem.cs

@@ -276,5 +276,5 @@ protected override UPath ConvertPathFromInternalImpl(string innerPath)
     protected abstract UPath ConvertPathFromDelegate(UPath path);
 
     protected override (IFileSystem FileSystem, UPath Path) ResolvePathImpl(UPath path)
-        => FallbackSafe.ResolvePath(ConvertPathToDelegate(path));
+        => Fallback?.ResolvePath(ConvertPathToDelegate(path)) ?? base.ResolvePathImpl(path);
 }

src/Zio/FileSystems/MountFileSystem.cs

@@ -593,6 +593,19 @@ protected override bool TryResolveLinkTargetImpl(UPath linkPath, out UPath resol
         return true;
     }
 
+    /// <inheritdoc />
+    protected override (IFileSystem FileSystem, UPath Path) ResolvePathImpl(UPath path)
+    {
+        var mountfs = TryGetMountOrNext(ref path);
+
+        if (mountfs is null)
+        {
+            return base.ResolvePathImpl(path);
+        }
+
+        return mountfs.ResolvePath(path);
+    }
+
     /// <inheritdoc />
     protected override IEnumerable<UPath> EnumeratePathsImpl(UPath path, string searchPattern, SearchOption searchOption, SearchTarget searchTarget)
     {