Improve PaxTarEntry construction performance

[rzikm]

Summary

• capture known PAX attributes while validating and inserting them
• avoid repeated dictionary lookups when applying attributes to standard header fields
• preserve validation, duplicate-key, parsing, exception-ordering, and GNU sparse behavior

Fixes #126154.

Performance

Local Windows x64 BenchmarkDotNet results:

Benchmark	Baseline	Updated	Difference
PaxTarEntry construction	955.6 ns	728.5 ns	-24%
Write preconstructed entry	1,512.6 ns	1,471.7 ns	statistically unchanged

Allocations were unchanged. The full PaxTarEntry_WriteEntry benchmark was statistically unchanged on the local Windows machine because writer cost and measurement variance dominate the recovered ~227 ns constructor cost. EgorBot is being requested for cross-platform validation of the tracked benchmark.

Validation

• dotnet.cmd build src\libraries\System.Formats.Tar\src\System.Formats.Tar.csproj -c Release
• PaxTarEntry_ExtendedAttributes_Tests: 17 passed
• TarReader_SparseFileTests: 60 passed

Note

This pull request description was generated with GitHub Copilot.

[rzikm]

@EgorBot -linux_arm64 -linux_amd

using System.Collections.Generic;
using System.Formats.Tar;
using System.IO;
using BenchmarkDotNet.Attributes;
using BenchmarkDotNet.Running;

BenchmarkSwitcher.FromAssembly(typeof(Perf_TarWriter).Assembly).Run(args);

public class Perf_TarWriter
{
    private static readonly string s_fileName = "file.txt";
    private Dictionary<string, string> _extendedAttributes = null!;
    private MemoryStream _memoryStream = null!;

    [GlobalSetup]
    public void Setup()
    {
        _memoryStream = new MemoryStream();
        _extendedAttributes = new Dictionary<string, string>
        {
            { "uname", "username" },
            { "gname", "groupname" },
            { "uid", "483745" },
            { "gid", "193783" },
            { "mtime", "1409547224" },
            { "ctime", "1409547225" },
            { "atime", "1409547226" },
            { "MSWINDOWS.rawsd", "AQAAgBQAAAAkAAAAAAAAAAAAAAABAgAAAAAABSAAAAAhAgAAAQIAAAAAAAUgAAAAIQIAAA==" }
        };
    }

    [GlobalCleanup]
    public void Cleanup() => _memoryStream.Dispose();

    [Benchmark]
    public void PaxTarEntry_WriteEntry()
    {
        PaxTarEntry entry = new(TarEntryType.RegularFile, s_fileName, _extendedAttributes);
        _memoryStream.Position = 0;
        using TarWriter writer = new(_memoryStream, leaveOpen: true);
        writer.WriteEntry(entry);
    }
}

Note

This benchmark request was generated with GitHub Copilot.

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[Copilot]

Pull request overview

This PR refactors PAX extended-attribute processing to reduce overhead during PaxTarEntry construction by capturing frequently-used attributes during the initial validation/insertion pass, then applying them to standard header fields without repeated dictionary lookups.

Changes:

• Refactors TarHeader.ReplaceNormalAttributesWithExtended to validate + insert extended attributes and capture known PAX/GNU-sparse attribute values in a single enumeration pass.
• Adds string?-based overloads in TarHelpers to parse timestamps / base-10 numbers without an intermediate dictionary lookup at call sites.
• Extracts extended-attribute key/value validation into a shared helper (ValidateExtendedAttribute) reused by both read-path and construction helpers.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
src/libraries/System.Formats.Tar/src/System/Formats/Tar/TarHelpers.cs	Adds string? overloads for timestamp and base-10 numeric parsing so callers can avoid extra dictionary probes.
src/libraries/System.Formats.Tar/src/System/Formats/Tar/TarHeader.Read.cs	Reworks extended-attribute application to capture known keys while inserting, reducing repeated TryGetValue calls when applying to standard fields.
src/libraries/System.Formats.Tar/src/System/Formats/Tar/TarHeader.cs	Factors extended-attribute validation into a helper method and reuses it from both insertion paths.

Suppressed comments (1)

src/libraries/System.Formats.Tar/src/System/Formats/Tar/TarHelpers.cs:177

• Same as above: this comment says "valid" but the implementation uses long.Parse and will throw on invalid input. Update the comment so it doesn't imply validation/try-parse semantics.

        // If the specified fieldName is found in the provided dictionary and is a valid string representation of a number, returns true and sets the value in 'baseTenLong'.

[alinpahontu2912]

Missing test: no case constructs PaxTarEntry from a duplicate-key List to verify the new single-pass loop in ReplaceNormalAttributesWithExtended still throws correctly (existing tests use Dictionary, which can't hold duplicates).