[ARC] Implement GetXattr.

Aaru.Archives/Arc/Unimplemented.cs

@@ -9,10 +9,6 @@ public sealed partial class Arc
 {
 #region IArchive Members
 
-    /// <inheritdoc />
-    public ErrorNumber GetXattr(int entryNumber, string xattr, ref byte[] buffer) =>
-        throw new NotImplementedException();
-
     /// <inheritdoc />
     public ErrorNumber Stat(int entryNumber, out FileEntryInfo stat) => throw new NotImplementedException();
 

Aaru.Archives/Arc/Xattrs.cs

@@ -1,4 +1,5 @@
 using System.Collections.Generic;
+using System.Text;
 using Aaru.CommonTypes.Enums;
 
 namespace Aaru.Archives;
@@ -23,5 +24,21 @@ public sealed partial class Arc
         return ErrorNumber.NoError;
     }
 
+    /// <inheritdoc />
+    public ErrorNumber GetXattr(int entryNumber, string xattr, ref byte[] buffer)
+    {
+        buffer = null;
+
+        if(!Opened) return ErrorNumber.NotOpened;
+
+        if(entryNumber < 0 || entryNumber >= _entries.Count) return ErrorNumber.OutOfRange;
+
+        if(xattr != "comment" || _entries[entryNumber].Comment is null) return ErrorNumber.NoSuchExtendedAttribute;
+
+        buffer = Encoding.UTF8.GetBytes(_entries[entryNumber].Comment);
+
+        return ErrorNumber.NoError;
+    }
+
 #endregion
 }