[akasakariko]

Writing an entry larger than 4 GiB to a non-seekable stream currently emits a ZIP32 local header followed by a Zip64 data descriptor, so readers using local headers cannot determine the descriptor width

Declare Zip64 before writing streamed entry data and always use 64-bit descriptor sizes for those entries, including small entries
Preserve the original local Zip64 declaration when Update mode rewrites metadata while retaining the existing descriptor
Extend the existing creation and metadata-update tests for both synchronous and asynchronous paths

Compatibility: streamed entries with data now require a Zip64-capable reader and small streamed entries gain 28 bytes of metadata
Applications requiring ZIP32 compatibility for small entries can write to a seekable stream
No public API changes

Local validation on macOS arm64:

• Four focused regression cases failed before the fix and passed afterward
• Actual 4 GiB + 1 MiB reproduction passed with the locally built runtime, including unzip -t and reading the following entry through piped tar input
• System.IO.Compression: 3026 regular tests and 92 OuterLoop tests passed
• System.IO.Compression.ZipFile: 379 regular tests and 16 OuterLoop tests passed
• Browser System.IO.Compression: 1283 tests passed, including all four regression cases
• Native and browser library builds passed, as did git diff --check
• Whitespace verification reports only the same two pre-existing diagnostics at zip_CreateTests.cs:82 as the unchanged baseline

Browser builds used /p:EnableHotReloadInRuntimeConfigDevFile=false for the SDK issue tracked in dotnet/sdk#56656, with no SDK or build-system changes in this PR
The browser host logs Aborted(Assertion failed) during teardown after WASM EXIT 0 in both the unmodified-product control and passing patched run, while XHarness and the build return 0

Resolves #127389

Note

This PR was prepared with AI assistance

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[dotnet-policy-service]

Tagging subscribers to this area: @karelz, @dotnet/area-system-io-compression
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[akasakariko]

@dotnet-policy-service agree

[alinpahontu2912]

Looks good to me, thanks for the contribution! Do you have anything to add @rzikm @iremyux ?

[Copilot]

🔵 Needs a closer look

Requiring ZIP64 for all non-empty entries on non-seekable streams, even small entries, needs maintainer confirmation of downstream-reader compatibility trade-offs and breaking-change handling.

No open issues 🎉

What changed in this PR

Fixes ZIP header and data-descriptor consistency in System.IO.Compression for non-seekable output, allowing readers to determine descriptor width before reading entry data.

Changes:

• Declares Zip64 before writing streamed entry data and uses 64-bit descriptor sizes, including for small entries.
• Preserves local Zip64 declarations during metadata-only updates that retain existing descriptors.
• Extends synchronous and asynchronous regression tests for creation and metadata updates.

Streamed entries containing data now require a Zip64-capable reader; small entries gain 28 bytes of metadata. No public API changes are introduced.

File	Description
src/​libraries/​System.IO.Compression/​tests/​ZipArchive/​zip_UpdateTests.cs	Verifies retained Zip64 headers and descriptors after metadata updates.
src/​libraries/​System.IO.Compression/​tests/​ZipArchive/​zip_CreateTests.cs	Checks streamed Zip64 structure and exercises asynchronous entry writes.
src/​libraries/​System.IO.Compression/​src/​System/​IO/​Compression/​ZipBlocks.cs	Reports whether parsed local extra fields contain a Zip64 declaration.
src/​libraries/​System.IO.Compression/​src/​System/​IO/​Compression/​ZipBlocks.Async.cs	Returns the same Zip64 declaration information during asynchronous parsing.
src/​libraries/​System.IO.Compression/​src/​System/​IO/​Compression/​ZipArchiveEntry.cs	Declares streamed Zip64 entries, selects descriptor width, and preserves retained declarations.
src/​libraries/​System.IO.Compression/​src/​System/​IO/​Compression/​ZipArchiveEntry.Async.cs	Records local Zip64 declarations during asynchronous loading.

_{Review effort: Balanced}

[iremyux]

LGTM

[dotnet-policy-service]

Added needs-breaking-change-doc-created label because this PR has the breaking-change label.

When you commit this breaking change:

1. Create and link to this PR and the issue a matching issue in the dotnet/docs repo using the breaking change documentation template, then remove this needs-breaking-change-doc-created label.
2. Ask a committer to mail the .NET Breaking Change Notification DL.

Tagging @dotnet/compat for awareness of the breaking change.