XZBlock accessing Position for non seakable streams throws exception

[SKProCH]

Hello. Previously I used this code:

using var responseMessage = await Utils.HttpClient.GetAsync(fileLink,
    HttpCompletionOption.ResponseHeadersRead);
await using var responseStream = await responseMessage.Content.ReadAsStreamAsync();
await using var xzStream = new XZStream(responseStream);
await using var reader = new TarReader(xzStream, true);

To download the tar.xz file and uncompress it on the fly in memory. After updating to 0.46.2 if starts throwing exception:

System.NotSupportedException: Specified method is not supported.
   at System.Net.Http.HttpBaseStream.get_Position()
   at SharpCompress.Compressors.Xz.XZBlock..ctor(Stream stream, CheckType checkType, Int32 checkSize) in D:\a\sharpcompress\sharpcompress\src\SharpCompress\Compressors\Xz\XZBlock.cs:line 38
   at SharpCompress.Compressors.Xz.XZStream.NextBlock() in D:\a\sharpcompress\sharpcompress\src\SharpCompress\Compressors\Xz\XZStream.cs:line 133
   at SharpCompress.Compressors.Xz.XZStream.ReadBlocksAsync(Byte[] buffer, Int32 offset, Int32 count, CancellationToken cancellationToken) in D:\a\sharpcompress\sharpcompress\src\SharpCompress\Compressors\Xz\XZStream.Async.cs:line 104
   at SharpCompress.Compressors.Xz.XZStream.ReadAsync(Byte[] buffer, Int32 offset, Int32 count, CancellationToken cancellationToken) in D:\a\sharpcompress\sharpcompress\src\SharpCompress\Compressors\Xz\XZStream.Async.cs:line 52
   at System.IO.Stream.ReadAtLeastAsyncCore(Memory`1 buffer, Int32 minimumBytes, Boolean throwOnEndOfStream, CancellationToken cancellationToken)
   at System.Runtime.CompilerServices.PoolingAsyncValueTaskMethodBuilder`1.StateMachineBox`1.System.Threading.Tasks.Sources.IValueTaskSource.GetResult(Int16 token)
   at System.Formats.Tar.TarHeader.TryGetNextHeaderAsync(Stream archiveStream, Boolean copyData, TarEntryFormat initialFormat, Boolean processDataBlock, CancellationToken cancellationToken)
   at System.Formats.Tar.TarReader.TryGetNextEntryHeaderAsync(Boolean copyData, CancellationToken cancellationToken)
   at System.Formats.Tar.TarReader.GetNextEntryInternalAsync(Boolean copyData, CancellationToken cancellationToken)
...

This is caused by accessing the stream position

sharpcompress/src/SharpCompress/Compressors/Xz/XZBlock.cs

Line 36 in aec0fca

_startPosition = stream.Position;

which will throw in HttpBaseStream implementation. This is introduced in #878

I suppose that we need firstly check if the stream supports CanSeek and only after that accesing this variable, assuming padding is fine. Or just surround this by trycatch.

[adamhathcock]

If you Wrap it with SharpCompressStream.Create does it fix it for you? There should be a rolling buffer that allows for some rewinding as I believe XZ requires that.

[SKProCH]

SharpCompressStream is internal, so I suppose I can't test it, at least directly.

[adamhathcock]

You're right. Making it public seems like the best thing as having to wrap it myself seems worse?

#1233

[adamhathcock]

Did a patch to make scstream public.

[SKProCH]

Yeah, it works fine now with

await using var responseStream = await responseMessage.Content.ReadAsStreamAsync();
await using var sharpCompressStream = SharpCompressStream.Create(responseStream);
await using var xzStream = new XZStream(sharpCompressStream);
await using var reader = new TarReader(xzStream, true);

Probably we can auto create SharpCompressStream in XZBlock/XZStream, if it required anyway?

[adamhathcock]

Yeah, it works fine now with

await using var responseStream = await responseMessage.Content.ReadAsStreamAsync();
await using var sharpCompressStream = SharpCompressStream.Create(responseStream);
await using var xzStream = new XZStream(sharpCompressStream);
await using var reader = new TarReader(xzStream, true);

Probably we can auto create SharpCompressStream in XZBlock/XZStream, if it required anyway?

I'm on the fence about that. I was scared to have to put another abstract in the way.

[SKProCH]

Yeah, but I suppose direct support for using the HttpClient streams would be nice. Probably create the stream only if CanSeek is false?