add async reads

src/SharpCompress/Archives/Rar/RarArchiveEntry.cs

@@ -70,26 +70,44 @@ public class RarArchiveEntry : RarEntry, IArchiveEntry
 
     public Stream OpenEntryStream()
     {
+        RarStream stream;
         if (IsRarV3)
         {
-            return new RarStream(
+            stream = new RarStream(
                 archive.UnpackV1.Value,
                 FileHeader,
                 new MultiVolumeReadOnlyStream(Parts.Cast<RarFilePart>(), archive)
             );
         }
 
-        return new RarStream(
+        stream = new RarStream(
             archive.UnpackV2017.Value,
             FileHeader,
             new MultiVolumeReadOnlyStream(Parts.Cast<RarFilePart>(), archive)
         );
+        stream.Initialize();
+        return stream;
     }
 
-    public Task<Stream> OpenEntryStreamAsync(CancellationToken cancellationToken = default)
+    public async Task<Stream> OpenEntryStreamAsync(CancellationToken cancellationToken = default)
     {
-        cancellationToken.ThrowIfCancellationRequested();
-        return Task.FromResult(OpenEntryStream());
+        RarStream stream;
+        if (IsRarV3)
+        {
+            stream = new RarStream(
+                archive.UnpackV1.Value,
+                FileHeader,
+                new MultiVolumeReadOnlyStream(Parts.Cast<RarFilePart>(), archive)
+            );
+        }
+
+        stream = new RarStream(
+            archive.UnpackV2017.Value,
+            FileHeader,
+            new MultiVolumeReadOnlyStream(Parts.Cast<RarFilePart>(), archive)
+        );
+        await stream.InitializeAsync(cancellationToken);
+        return stream;
     }
 
     public bool IsComplete

src/SharpCompress/Compressors/Rar/RarCrcStream.cs

@@ -1,5 +1,7 @@
 using System;
 using System.IO;
+using System.Threading;
+using System.Threading.Tasks;
 using SharpCompress.Common;
 using SharpCompress.Common.Rar.Headers;
 using SharpCompress.IO;
@@ -31,7 +33,7 @@ internal class RarCrcStream : RarStream, IStreamStack
     private uint currentCrc;
     private readonly bool disableCRC;
 
-    public RarCrcStream(
+    private RarCrcStream(
         IRarUnpack unpack,
         FileHeader fileHeader,
         MultiVolumeReadOnlyStream readStream
@@ -46,6 +48,26 @@ internal class RarCrcStream : RarStream, IStreamStack
         ResetCrc();
     }
 
+    public static RarCrcStream Create(
+        IRarUnpack unpack,
+        FileHeader fileHeader,
+        MultiVolumeReadOnlyStream readStream)
+    {
+        var stream = new RarCrcStream(unpack, fileHeader, readStream);
+        stream.Initialize();
+        return stream;
+    }
+
+    public static async Task<RarCrcStream> CreateAsync(
+        IRarUnpack unpack,
+        FileHeader fileHeader,
+        MultiVolumeReadOnlyStream readStream, CancellationToken cancellationToken = default)
+    {
+        var stream = new RarCrcStream(unpack, fileHeader, readStream);
+        await stream.InitializeAsync(cancellationToken);
+        return stream;
+    }
+
     protected override void Dispose(bool disposing)
     {
 #if DEBUG_STREAMS

src/SharpCompress/Compressors/Rar/RarStream.cs

@@ -3,6 +3,8 @@
 using System;
 using System.Buffers;
 using System.IO;
+using System.Threading;
+using System.Threading.Tasks;
 using SharpCompress.Common.Rar.Headers;
 using SharpCompress.IO;
 
@@ -56,13 +58,24 @@ internal class RarStream : Stream, IStreamStack
 #if DEBUG_STREAMS
         this.DebugConstruct(typeof(RarStream));
 #endif
+    }
 
+    public void Initialize()
+    {
         fetch = true;
         unpack.DoUnpack(fileHeader, readStream, this);
         fetch = false;
         _position = 0;
     }
 
+    public async Task InitializeAsync(CancellationToken cancellationToken = default)
+    {
+        fetch = true;
+        await unpack.DoUnpackAsync(fileHeader, readStream, this, cancellationToken);
+        fetch = false;
+        _position = 0;
+    }
+
     protected override void Dispose(bool disposing)
     {
         if (!isDisposed)

src/SharpCompress/Readers/Rar/RarReader.cs

@@ -113,7 +113,7 @@ public abstract class RarReader : AbstractReader<RarReaderEntry, RarVolume>
         );
         if (Entry.IsRarV3)
         {
-            return CreateEntryStream(new RarCrcStream(UnpackV1.Value, Entry.FileHeader, stream));
+            return CreateEntryStream( RarCrcStream.Create(UnpackV1.Value, Entry.FileHeader, stream));
         }
 
         if (Entry.FileHeader.FileCrc?.Length > 5)
@@ -123,6 +123,6 @@ public abstract class RarReader : AbstractReader<RarReaderEntry, RarVolume>
             );
         }
 
-        return CreateEntryStream(new RarCrcStream(UnpackV2017.Value, Entry.FileHeader, stream));
+        return CreateEntryStream(RarCrcStream.Create(UnpackV2017.Value, Entry.FileHeader, stream));
     }
 }