Fix tests

Author: rzikm

src/libraries/System.Formats.Tar/tests/TarReader/TarReader.File.Async.Tests.cs

@@ -283,11 +283,20 @@ public async Task Throw_ArchiveIsShortAsync()
         }
 
         [Fact]
-        public async Task GarbageEntryChecksumZeroReturnNullAsync()
+        public async Task GarbageEntryChecksumZeroThrowsInvalidDataException()
         {
             await using MemoryStream archiveStream = GetTarMemoryStream(CompressionMethod.Uncompressed, "golang_tar", "issue12435");
             await using TarReader reader = new TarReader(archiveStream);
-            Assert.Null(await reader.GetNextEntryAsync());
+            await Assert.ThrowsAsync<InvalidDataException>(async () => await reader.GetNextEntryAsync());
+        }
+
+        [Fact]
+        public async Task InvalidChecksum_ThrowsInvalidDataException()
+        {
+            await using MemoryStream archiveStream = GetTarMemoryStream(CompressionMethod.Uncompressed, "node-tar", "bad-cksum");
+            await using TarReader reader = new TarReader(archiveStream);
+            await reader.GetNextEntryAsync(); // first entry is okay
+            await Assert.ThrowsAsync<InvalidDataException>(async () => await reader.GetNextEntryAsync());
         }
 
         [Theory]

src/libraries/System.Formats.Tar/tests/TarReader/TarReader.File.Tests.cs

@@ -291,11 +291,20 @@ public void Throw_ArchiveIsShort()
         }
 
         [Fact]
-        public void GarbageEntryChecksumZeroReturnNull()
+        public void GarbageEntryChecksumZeroThrowsInvalidDataException()
         {
             using MemoryStream archiveStream = GetTarMemoryStream(CompressionMethod.Uncompressed, "golang_tar", "issue12435");
             using TarReader reader = new TarReader(archiveStream);
-            Assert.Null(reader.GetNextEntry());
+            Assert.Throws<InvalidDataException>(() => reader.GetNextEntry());
+        }
+
+        [Fact]
+        public void InvalidChecksum_ThrowsInvalidDataException()
+        {
+            using MemoryStream archiveStream = GetTarMemoryStream(CompressionMethod.Uncompressed, "node-tar", "bad-cksum");
+            using TarReader reader = new TarReader(archiveStream);
+            reader.GetNextEntry(); // first entry is okay
+            Assert.Throws<InvalidDataException>(() => reader.GetNextEntry());
         }
 
         [Theory]

src/libraries/System.Formats.Tar/tests/TarTestsBase.cs

@@ -155,7 +155,6 @@ public abstract partial class TarTestsBase : FileCleanupTestBase
 
         private static readonly string[] NodeTarTestCaseNames = new[]
         {
-            "bad-cksum",
             "body-byte-counts",
             "dir",
             "emptypax",

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

@@ -233,6 +233,12 @@ public void Verify_Compatibility_RegularFile_EmptyFile_NoSizeStored()
             archiveStream.Seek(sizeLocation, SeekOrigin.Begin);
             archiveStream.Write(replacement);
 
+            // Also fixup checksum
+            int checksumLocation = 148;
+            ReadOnlySpan<byte> checksumReplacement = "04116\0\0\0"u8; // 2126 in octal
+            archiveStream.Seek(checksumLocation, SeekOrigin.Begin);
+            archiveStream.Write(checksumReplacement);
+
             archiveStream.Position = 0;
             using TarReader reader = new(archiveStream);
 
@@ -368,7 +374,7 @@ private int GetChecksumForCommonFields(TarEntry entry, TarEntryType entryType)
             if (entryType is TarEntryType.RegularFile or TarEntryType.V7RegularFile)
             {
                 entry.DataStream = new MemoryStream();
-                byte[] buffer = [ 72, 101, 108, 108, 111 ]; // values don't matter, only length (5)
+                byte[] buffer = [72, 101, 108, 108, 111]; // values don't matter, only length (5)
 
                 // '00000000005\0' = 48 + 48 + 48 + 48 + 48 + 48 + 48 + 48 + 48 + 48 + 53 + 0 = 533
                 entry.DataStream.Write(buffer);