TarReader: implement GNU sparse format 1.0 (PAX)

[Copilot]

TarReader was not handling GNU sparse format 1.0 PAX entries, causing ~46% of entries from bsdtar-created archives (e.g., .NET SDK tarballs built on macOS/APFS) to expose internal placeholder paths like GNUSparseFile.0/real-file.dll, incorrect sizes, and corrupted extracted content.

Changes

Added read-only support for GNU sparse format 1.0 (PAX). When TarReader encounters PAX extended attributes GNU.sparse.major=1 and GNU.sparse.minor=0, it resolves the real file name from GNU.sparse.name, reports the expanded size from GNU.sparse.realsize, and wraps the raw data stream with GnuSparseStream which presents the expanded virtual file content (zeros for holes, packed data at correct offsets).

The sparse map embedded in the data section is parsed lazily on first Read, so _dataStream remains unconsumed during entry construction. This allows TarWriter.WriteEntry to round-trip the condensed sparse data correctly for both seekable and non-seekable source archives.

Older GNU sparse formats (0.0, 0.1) and write support are not addressed.

Additional correctness and robustness improvements based on code review:

• GnuSparseStream now overrides DisposeAsync to properly await async disposal of the underlying raw stream.
• TarHeader.Read now throws InvalidDataException if GNU.sparse.realsize is negative, consistent with validation of the regular _size field.
• Segment validation uses overflow-safe arithmetic (offset > _realSize || length > _realSize - offset).
• FindSegmentFromCurrent uses binary search (O(log n)) for backward seeks, preserving the O(1) amortized forward scan for the common sequential-read case.

// Before: entry.Name == "GNUSparseFile.0/dotnet.dll", entry.Length == 512
// After:  entry.Name == "dotnet.dll", entry.Length == 1048576
using var reader = new TarReader(archiveStream);
TarEntry entry = reader.GetNextEntry();
entry.DataStream.ReadExactly(content); // correctly expanded virtual file

Testing

All existing tests pass. New TarReader.SparseFile.Tests.cs covers:

• Parameterized sparse layouts (single segment, holes, multiple segments, all-holes) × copyData × sync/async
• Corrupted sparse map handling (non-numeric values, truncated maps, buffer overflow) × sync/async
• Negative GNU.sparse.realsize value throws InvalidDataException (sync and async) — the test helper WriteSparseEntry omits GNU.sparse.realsize from the PaxTarEntry constructor's attribute dictionary (to avoid constructor-level validation) and instead injects it via reflection into the internal TarHeader.ExtendedAttributes dictionary after construction, so the archive can be built while ensuring TarReader.GetNextEntry() is the one that throws
• Wrong sparse version detection (missing minor, wrong major)
• Seekable random access, partial reads, advance-past-entry correctness
• Round-trip copy through TarWriter with seekable/non-seekable source × copyData
• Sparse layout scenarios tested against real golang_tar test data files (pax-nil-sparse-data.tar, pax-nil-sparse-hole.tar, pax-sparse-big.tar) from the System.Formats.Tar.TestData NuGet package, plus programmatically constructed archives for additional coverage
• Test code refactored to eliminate duplication: AdvancePastEntry_DoesNotCorruptNextEntry and CopySparseEntryToNewArchive_PreservesExpandedContent now share archive construction helpers (WriteSparseEntry, BuildSparseArchive, BuildRawSparseArchive) with the rest of the test suite

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[rzikm]

@copilot What does happen when we attempt to extract such archives? If we are to transparently handle the real name and size, then we should also expand the contents when reading the entrys DataStream

[Copilot]

@copilot What does happen when we attempt to extract such archives? If we are to transparently handle the real name and size, then we should also expand the contents when reading the entrys DataStream

Extracting GNU sparse format 1.0 PAX entries with the current code produces a file with the correct name (from our fix) but incorrect content.

Here's what happens: the DataStream for a sparse 1.0 PAX entry contains the raw archive data section, which has this layout:

1. A text-based sparse map header (newline-delimited: number of segments, then pairs of offset\nnumbytes\n)
2. The non-zero data segments packed together

When ExtractAsRegularFile calls DataStream.CopyTo(fs), it copies those raw bytes verbatim — so the extracted file contains the sparse map text followed by packed data, rather than the properly expanded file (zeros for holes, data at the correct offsets).

Properly expanding the DataStream would require:

• Parsing the sparse map text from the start of the data section
• Creating a wrapper stream that serves zeros for sparse holes and actual bytes at the right offsets

This is significantly more complex than the current fix. The current PR is scoped to fix the metadata issues (Name and Length reporting) that were the primary pain point. Full sparse expansion is a meaningful follow-up that should be tracked separately.

I can revert the _gnuSparseRealSize → Length change if the inconsistency between the reported length and the DataStream content is considered too misleading. However, I'd argue knowing the real size (even when extraction doesn't yet expand the content) is still more useful than reporting the stored sparse data size.

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[rzikm]

@copilot simply fixing the exposed metadata does not address the original reported issue, you need to implement complete handling of the pax 1.0 sparse format (only for reading, no need to be able to produce them)

I think it is okay to avoid implementing the older sparse file formats though.

[Copilot]

@copilot simply fixing the exposed metadata does not address the original reported issue, you need to implement complete handling of the pax 1.0 sparse format (only for reading, no need to be able to ...

Done in commit 1015043a. The PR now fully handles GNU sparse format 1.0 PAX reading:

• GnuSparseStream wraps the raw data section and expands it: it parses the embedded sparse map (numSegs, offset/length pairs), then serves zeros for sparse holes and packed data for real segments, presenting a virtual file of realsize bytes.
• TarEntry.Name returns the real name from GNU.sparse.name (not the GNUSparseFile.0/… placeholder).
• TarEntry.Length and DataStream.Length both report the expanded real size.
• ExtractToFile will write the correctly expanded content.
• Older GNU sparse formats (0.0, 0.1) are not addressed — those use different PAX attribute schemes and are a separate follow-up.

Code review fixes applied: MaxSparseSegments = 1_000_000 DoS cap, O(1) precomputed packed-offset prefix sums, CanSeek short-circuit order corrected, async byte[1] allocated once per parse (not per line call).

[rzikm]

@copilot Please review my comments

[rzikm]

@copilot There have been test failures with System.Formats.Tar.Tests.TarReader_SparseFileTests.NegativeSparseRealSize_InvalidDataException, can you verify those, see https://helixr1107v0xdcypoyl9e7f.blob.core.windows.net/dotnet-runtime-refs-pull-125283-merge-a0e143436271425797/System.Formats.Tar.Tests/1/console.c6964d30.log?helixlogtype=result

[Copilot]

@copilot There have been test failures with System.Formats.Tar.Tests.TarReader_SparseFileTests.NegativeSparseRealSize_InvalidDataException, can you verify those, see https://helixr1107v0xdcypoyl9e7f...

Fixed in c56965ee3.

Root cause: PaxTarEntry constructor calls ReplaceNormalAttributesWithExtended, which (as of my earlier commit) validates GNU.sparse.realsize and throws InvalidDataException for negative values. WriteSparseEntry was passing realSize = -1L in the initial attribute dictionary, so the exception fired during archive construction — outside Assert.Throws — causing the test to fail as an unexpected exception.

Fix: omit GNU.sparse.realsize from the attribute dict passed to the constructor, then inject it directly into ExtendedAttributes after construction via ((Dictionary<string, string>)entry.ExtendedAttributes)["GNU.sparse.realsize"] = realSize.ToString(). This keeps constructor validation intact for production use, still writes the attribute correctly for valid archives, and allows the negative-realsize test archive to be built so TarReader can exercise its own validation during GetNextEntry().

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[rzikm]

@copilot there are test failures, there seems to be some invalid cast in the test code

[Copilot]

@copilot there are test failures, there seems to be some invalid cast in the test code

Fixed in 74bd5757c. The invalid cast was on line 36: (Dictionary<string, string>)entry.ExtendedAttributes fails at runtime because PaxTarEntry.ExtendedAttributes returns a ReadOnlyDictionary<string, string> (via .AsReadOnly()), not a Dictionary<string, string>.

Fixed by using reflection to access TarEntry._header and then TarHeader.ExtendedAttributes (which is the actual internal Dictionary<string, string>) to inject GNU.sparse.realsize directly, bypassing the read-only façade.

[rzikm]

@copilot revert commit d35983c, the tar files it references DO exist

[Copilot]

@copilot revert commit d35983c, the tar files it references DO exist

Done in 1f680f239. The three tests now use GetTarMemoryStream(..., "golang_tar", "pax-nil-sparse-data") etc. again, and the BuildSparseArchiveWithPackedData helper added in that commit has been removed. Confirmed all three files exist in the System.Formats.Tar.TestData NuGet package.

[rzikm]

/ba-g wasm failures are unrelated