[6.1] Fix rare multi-packet string corruption

[cheenamalhotra]

Ports #3505 to fix #3504 by @Wraith2

[Copilot]

Pull Request Overview

This PR fixes a rare issue with multi-packet string corruption by correcting the logic that determines when to store and retrieve snapshot buffers during TDS protocol operations. The fix addresses cases where strings could become corrupted when they span multiple network packets.

Key changes:

• Replaces complex condition (isStarting || isContinuing) with simpler canContinue check
• Adds debug assertions to ensure buffer consistency when continuing operations
• Removes unnecessary nested conditional blocks for improved code clarity

Reviewed Changes

Copilot reviewed 3 out of 4 changed files in this pull request and generated no comments.

File	Description
TdsParserStateObject.cs	Updates buffer management logic in byte array and string reading methods
netfx/TdsParser.cs	Fixes Unicode character reading logic for .NET Framework implementation
netcore/TdsParser.cs	Fixes Unicode character reading logic for .NET Core implementation

[Wraith2]

Any chance of a preview 3 that contains this?

[ErikEJ]

@Wraith2 Looks like 6.1.0 is planned for tomorrow.

[Wraith2]

Oh. Well that's interesting. I thought my weekend would be free but now it'll be filled with worrying that an unknown bug might be making it out to the general public and breaking all sorts of things.

[ErikEJ]

@Wraith2 preview 2 has been out for a month, is that not sufficient?

[Wraith2]

Not when we found two fairly important bugs in it. No. But it's not my choice.

[cheenamalhotra]

Due to schedule constraints another preview is not planned right now. You can build, and test the NuGet package off the main branch if needed and propose a fix for any other issue for the next Hotfix.

Since this change mainly affects the large string read usecases, we've considered bringing the fix in.

Do you think leaving the edge case identified in #3504 open in 6.1 is safer than fixing it with this PR?

[Wraith2]

Absolutely not, better with the fix.

[codecov]

Codecov Report

Attention: Patch coverage is 36.36364% with 7 lines in your changes missing coverage. Please review.

Project coverage is 66.06%. Comparing base (6a37506) to head (08b8db9).
Report is 3 commits behind head on release/6.1.

Files with missing lines	Patch %	Lines
...nt/netfx/src/Microsoft/Data/SqlClient/TdsParser.cs	0.00%	4 Missing ⚠️
.../netcore/src/Microsoft/Data/SqlClient/TdsParser.cs	0.00%	3 Missing ⚠️

Additional details and impacted files

@@               Coverage Diff               @@
##           release/6.1    #3513      +/-   ##
===============================================
+ Coverage        66.04%   66.06%   +0.02%     
===============================================
  Files              281      281              
  Lines            62416    62401      -15     
===============================================
+ Hits             41221    41225       +4     
+ Misses           21195    21176      -19     

Flag	Coverage Δ
addons	90.82% <ø> (-0.23%)	⬇️
netcore	68.95% <57.14%> (+<0.01%)	⬆️
netfx	68.15% <50.00%> (+0.12%)	⬆️

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