[cheenamalhotra]

Description

Backport #4945 from commit 8dae63509ca17fa5b345596f7f3752bea7a4defd to release/7.0, persisted as 944419b61280237996f4ba4b58cca594380e1c2e.

Fix corrupted sequential nvarchar reads on big-endian platforms: decode raw UTF-16LE code units without mutating consumed or unconsumed source bytes. Preserve offsets, partial reads, zero-capacity conversions, and isolated surrogate code units. The production change is unchanged from the original fix.

Release-specific adaptations:

• Both active legacy implementation projects (netcore/src and netfx/src) compile the single shared decoder source; no duplicate implementation needs patching.
• Keep the existing driver targets, dependencies, and project structure unchanged. The unit regression file is identical to the source PR.
• This branch lacks main's globalization test file. Add only the relevant Unicode round-trip coverage, using the existing database-object fixture, and explicitly register it in manual test set 3. Do not import unrelated UTF-8 tests.
• This branch has no TESTGUIDE.md; document release-specific commands and the architecture limitation in BUILDGUIDE.md instead.

No public API changes, deprecations, localization changes, or intentional breaking changes. Suggested release note: Fix corrupted Unicode text returned by sequential SqlDataReader.GetTextReader reads on big-endian platforms.

The remote base was refreshed before implementation and again before opening this PR; it remains 0620c4cd441ceef4c83f75f190be4b321c6a2a3d, with no intervening fix. No existing PR matched this head branch.

Issues

Backports #4945. This PR does not close the original issue independently of the source fix.

Testing

Windows x64/little-endian: 44 decoder cases and 16 SQL round-trip cases passed, with no skipped cases in the successful final runs. Both active legacy implementation projects built with zero warnings and errors.

Decoder coverage includes raw code-unit preservation, offsets, capacity limits, odd trailing bytes, flush variants, repeated partial reads, unchanged input bytes, and empty conversion. SQL coverage exercises all four sync/async and buffered/streamed parameter combinations, packet-spanning nvarchar(max), single-character and mixed-size reads, destination sentinels, zero-length reads, and repeated EOF.

Environment: the repository-root invocation initially failed because global.json pins unavailable SDK 10.0.109. Successful commands below were run from C:\Users\cmalhotra\.copilot\session-state\87991f1b-e700-4ad8-a764-6e011c4f4a4f\files, using installed SDK 10.0.403, without modifying the SDK pin.

Exact validation commands and results:

# PASS: 11 matching unit test cases discovered.
dotnet test D:\copilot\copilot-worktrees\SqlClient\dev-cheena-curly-dollop\src\Microsoft.Data.SqlClient\tests\UnitTests\Microsoft.Data.SqlClient.UnitTests.csproj --framework net8.0 --list-tests --filter "FullyQualifiedName~SqlUnicodeEncodingTests" --verbosity quiet

# PASS: 11/11 on EACH net8.0, net9.0, net10.0, net462 (44 passed, 0 skipped).
dotnet test D:\copilot\copilot-worktrees\SqlClient\dev-cheena-curly-dollop\src\Microsoft.Data.SqlClient\tests\UnitTests\Microsoft.Data.SqlClient.UnitTests.csproj --filter "FullyQualifiedName~SqlUnicodeEncodingTests" --blame-hang-timeout 10m --verbosity quiet

# PASS: both builds, 0 warnings and 0 errors.
dotnet build D:\copilot\copilot-worktrees\SqlClient\dev-cheena-curly-dollop\src\Microsoft.Data.SqlClient\netcore\src\Microsoft.Data.SqlClient.csproj --verbosity quiet
dotnet build D:\copilot\copilot-worktrees\SqlClient\dev-cheena-curly-dollop\src\Microsoft.Data.SqlClient\netfx\src\Microsoft.Data.SqlClient.csproj --verbosity quiet

# PASS: target SQL regression discovered; initial run without config skipped the conditional theory.
dotnet test D:\copilot\copilot-worktrees\SqlClient\dev-cheena-curly-dollop\src\Microsoft.Data.SqlClient\tests\ManualTests\Microsoft.Data.SqlClient.ManualTests.csproj --framework net8.0 -p:TestSet=3 --list-tests --filter "FullyQualifiedName~UnicodeParameterAndReaderRoundTrip_PreservesWorldwideText" --verbosity quiet

# All framework builds succeeded; execution FAILED 4/framework opening default Northwind (SQL error 4060).
dotnet test D:\copilot\copilot-worktrees\SqlClient\dev-cheena-curly-dollop\src\Microsoft.Data.SqlClient\tests\ManualTests\Microsoft.Data.SqlClient.ManualTests.csproj -p:TestSet=3 --no-restore --filter "FullyQualifiedName~UnicodeParameterAndReaderRoundTrip_PreservesWorldwideText" --blame-hang-timeout 10m --verbosity quiet

# PASS: isolated existing MDS_TEST_CONFIG override used accessible local tempdb instead of unavailable Northwind.
$env:MDS_TEST_CONFIG = 'C:\Users\cmalhotra\.copilot\session-state\87991f1b-e700-4ad8-a764-6e011c4f4a4f\files\unicode-test-config.json'
# PASS: 4/4 on EACH net8.0, net9.0, net10.0, net462 (16 passed, 0 skipped).
dotnet test D:\copilot\copilot-worktrees\SqlClient\dev-cheena-curly-dollop\src\Microsoft.Data.SqlClient\tests\ManualTests\Microsoft.Data.SqlClient.ManualTests.csproj -p:TestSet=3 --no-build --filter "FullyQualifiedName~UnicodeParameterAndReaderRoundTrip_PreservesWorldwideText" --blame-hang-timeout 10m --verbosity quiet

# PASS: no whitespace errors before commit.
git diff --cached --check

The isolated JSON used Data Source=tcp:localhost;Database=tempdb;Integrated Security=true;Encrypt=false; for TCP, the equivalent np:localhost string for named pipes, SupportsIntegratedSecurity=true, and UseManagedSNIOnWindows=false. It was removed after validation. Transient tables were cleaned by the existing RAII fixture; tracked configuration and server setup were unchanged.

Outstanding coverage: no real big-endian runtime is available. These successful x64 runs validate only the little-endian decoder path; running the client on a real big-endian runtime such as s390x remains required to validate the original platform-specific symptom. Full test suites were not run.

• Tests added or updated
• Public API changes documented (not applicable; no API changes)
• Verified against customer repro (original big-endian platform unavailable)
• Ensure no breaking changes introduced

Guidelines

Please review the contribution guidelines before submitting a pull request:

• Contributing
• Code of Conduct
• Best Practices
• Coding Style
• Review Process

[Copilot]

🔵 Needs a closer look

Runtime validation covers only x64/little-endian, leaving the decoder fix and sync/async SQL sequential-read regressions unverified on a real big-endian client.

No open issues 🎉

What changed in this PR

Backports #4945 to release/7.0, fixing corrupted sequential Unicode reads on big-endian platforms by decoding UTF-16LE code units without modifying source buffers.

Changes:

• Replaces in-place byte swapping with non-mutating decoding while preserving partial-read behavior.
• Adds decoder unit tests and sync/async SQL round-trip tests, registered in manual test set 3.
• Documents regression commands and platform limitations without changing public APIs or driver targets.

Reported validation passed on Windows x64; the big-endian path still requires validation on a real big-endian runtime.

File	Description
src/​Microsoft.Data.SqlClient/​tests/​UnitTests/​SqlUnicodeEncodingTests.cs	Tests code-unit preservation, partial reads, offsets, and unchanged input buffers.
src/​Microsoft.Data.SqlClient/​tests/​ManualTests/​SQL/​GlobalizationTest/​GlobalizationEncodingTests.cs	Adds Unicode round-trip coverage for buffered and streamed sync/async operations.
src/​Microsoft.Data.SqlClient/​tests/​ManualTests/​Microsoft.Data.SqlClient.ManualTests.csproj	Registers the integration tests in test set 3.
src/​Microsoft.Data.SqlClient/​src/​Microsoft/​Data/​SqlClient/​SqlSequentialTextReader.cs	Decodes consumed UTF-16LE bytes without mutating the source.
BUILDGUIDE.md	Documents focused regression commands and the big-endian validation requirement.

_{Review effort: Balanced}

[codecov]

Codecov Report

❌ Patch coverage is 62.50000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 66.30%. Comparing base (0620c4c) to head (6e2bad2).

Files with missing lines	Patch %	Lines
...icrosoft/Data/SqlClient/SqlSequentialTextReader.cs	62.50%	3 Missing ⚠️

❗ There is a different number of reports uploaded between BASE (0620c4c) and HEAD (6e2bad2). Click for more details.

HEAD has 1 upload less than BASE

Flag	BASE (0620c4c)	HEAD (6e2bad2)
CI-SqlClient	1	0

Additional details and impacted files

@@               Coverage Diff               @@
##           release/7.0    #4962      +/-   ##
===============================================
- Coverage        73.21%   66.30%   -6.91%     
===============================================
  Files              296      291       -5     
  Lines            43453    66316   +22863     
===============================================
+ Hits             31813    43970   +12157     
- Misses           11640    22346   +10706     

Flag	Coverage Δ
CI-SqlClient	?
PR-SqlClient-Project	66.30% <62.50%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[Copilot]

🔵 Needs a closer look

The corrected corruption path runs only on big-endian runtimes, while reported validation covered only little-endian x64, leaving the affected branch unverified and requiring maintainer review and real big-endian regression testing.

No open issues 🎉

_{Review effort: Balanced}