Do not read position when tar archive stream is unseekable

src/libraries/System.Formats.Tar/src/System/Formats/Tar/TarWriter.cs

@@ -311,7 +311,6 @@ private void WriteFinalRecords()
             byte[] emptyRecord = new byte[TarHelpers.RecordSize];
             _archiveStream.Write(emptyRecord);
             _archiveStream.Write(emptyRecord);
-            _archiveStream.SetLength(_archiveStream.Position);
         }
 
         // Partial method for reading an entry from disk and writing it into the archive stream.

src/libraries/System.Formats.Tar/tests/CompressedTar.Tests.cs

@@ -0,0 +1,80 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System.IO.Compression;
+using System.IO;
+using Xunit;
+
+namespace System.Formats.Tar.Tests
+{
+    public class CompressedTar_Tests : TarTestsBase
+    {
+        [Fact]
+        public void TarGz_TarWriter_TarReader()
+        {
+            using TempDirectory root = new TempDirectory();
+
+            string archivePath = Path.Join(root.Path, "compressed.tar.gz");
+
+            string fileName = "file.txt";
+            string filePath = Path.Join(root.Path, fileName);
+            File.Create(filePath).Dispose();
+
+            // Create tar.gz archive
+            using (FileStream streamToCompress = File.Create(archivePath))
+            {
+                using GZipStream compressorStream = new GZipStream(streamToCompress, CompressionMode.Compress);
+                using TarWriter writer = new TarWriter(compressorStream);
+                writer.WriteEntry(fileName: filePath, entryName: fileName);
+            }
+            FileInfo fileInfo = new FileInfo(archivePath);
+            Assert.True(fileInfo.Exists);
+            Assert.True(fileInfo.Length > 0);
+
+            // Verify tar.gz archive contents
+            using (FileStream streamToDecompress = File.OpenRead(archivePath))
+            {
+                using GZipStream decompressorStream = new GZipStream(streamToDecompress, CompressionMode.Decompress);
+                using TarReader reader = new TarReader(decompressorStream);
+                TarEntry entry = reader.GetNextEntry();
+                Assert.Equal(TarFormat.Pax, reader.Format);
+                Assert.Equal(fileName, entry.Name);
+                Assert.Null(reader.GetNextEntry());
+            }
+        }
+
+        [Fact]
+        public void TarGz_TarFile_CreateFromDir_ExtractToDir()
+        {
+            using TempDirectory root = new TempDirectory();
+
+            string archivePath = Path.Join(root.Path, "compressed.tar.gz");
+
+            string sourceDirectory = Path.Join(root.Path, "source");
+            Directory.CreateDirectory(sourceDirectory);
+
+            string destinationDirectory = Path.Join(root.Path, "destination");
+            Directory.CreateDirectory(destinationDirectory);
+
+            string fileName = "file.txt";
+            string filePath = Path.Join(sourceDirectory, fileName);
+            File.Create(filePath).Dispose();
+
+            using (FileStream streamToCompress = File.Create(archivePath))
+            {
+                using GZipStream compressorStream = new GZipStream(streamToCompress, CompressionMode.Compress);
+                TarFile.CreateFromDirectory(sourceDirectory, compressorStream, includeBaseDirectory: false);
+            }
+            FileInfo fileInfo = new FileInfo(archivePath);
+            Assert.True(fileInfo.Exists);
+            Assert.True(fileInfo.Length > 0);
+
+            using (FileStream streamToDecompress = File.OpenRead(archivePath))
+            {
+                using GZipStream decompressorStream = new GZipStream(streamToDecompress, CompressionMode.Decompress);
+                TarFile.ExtractToDirectory(decompressorStream, destinationDirectory, overwriteFiles: true);
+                Assert.True(File.Exists(filePath));
+            }
+        }
+    }
+}

src/libraries/System.Formats.Tar/tests/System.Formats.Tar.Tests.csproj

@@ -8,6 +8,7 @@
   </PropertyGroup>
   <ItemGroup>
     <Compile Include="$(CommonTestPath)System\IO\TempDirectory.cs" Link="Common\System\IO\TempDirectory.cs" />
+    <Compile Include="CompressedTar.Tests.cs" />
     <Compile Include="TarEntry\TarEntryV7.Tests.cs" />
     <Compile Include="TarEntry\TarEntryUstar.Tests.cs" />
     <Compile Include="TarEntry\TarEntryPax.Tests.cs" />

src/libraries/System.Formats.Tar/tests/TarWriter/TarWriter.Tests.cs

@@ -79,6 +79,29 @@ public void Constructor_NoEntryInsertion_WritesNothing()
             Assert.Equal(0, archiveStream.Length);
         }
 
+        [Fact]
+        public void Write_To_UnseekableStream()
+        {
+            using MemoryStream inner = new MemoryStream();
+            using WrappedStream wrapped = new WrappedStream(inner, canRead: true, canWrite: true, canSeek: false);
+
+            using (TarWriter writer = new TarWriter(wrapped, leaveOpen: true))
+            {
+                PaxTarEntry paxEntry = new PaxTarEntry(TarEntryType.RegularFile, "file.txt");
+                writer.WriteEntry(paxEntry);
+            } // The final records should get written, and the length should not be set because position cannot be read
+
+            inner.Seek(0, SeekOrigin.Begin); // Rewind the base stream (wrapped cannot be rewound)
+
+            using (TarReader reader = new TarReader(wrapped))
+            {
+                TarEntry entry = reader.GetNextEntry();
+                Assert.Equal(TarFormat.Pax, reader.Format);
+                Assert.Equal(TarEntryType.RegularFile, entry.EntryType);
+                Assert.Null(reader.GetNextEntry());
+            }
+        }
+
         [Fact]
         public void VerifyChecksumV7()
         {