[6.1] Revert partial packet fixup and replay improvements

[mdaigle]

Description

This PR reverts several large changes made to improve partial packet detection, fixup, and replay functionality. It maintains all test coverage added along with the original changes. We have a handle on many of the issues discovered in 6.1.0, but out of an abundance of caution, we've made the decision to revert this functionality so that we can provide a stable 6.1.X release. We'll continue to evaluate the functionality on the main branch with the goal of including it in the next, non-patch release.

To simplify the diff with release/6.0, one additional piece of functionality dealing with rented buffs when reading plp data is also reverted: #2982

Buffer management is also added for reading vector data. This change needs close review.

Issues

#3536
#3519

Testing

New tests are maintained. Existing test coverage should help ensure that we've returned to baseline.
Stress tests have been run on this PR and show identical behavior to 6.0.2.

Guidelines

Please review the contribution guidelines before submitting a pull request:

• Contributing
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[Wraith2]

I've looked through the files and it looks like a complete reversion of the work to add continue.
I'm not convinced that it should have any affect on #3536 though. If it does it isn't by direct code change but by assumption invalidation.

[Copilot]

Pull Request Overview

This PR reverts large-scale changes made to improve partial packet detection, fixup, and replay functionality in the SQL Client library while maintaining test coverage. The changes are primarily focused on simplifying the packet processing logic and removing complex buffer management features. Additionally, buffer management for reading vector data is modified.

Key Changes:

• Reverts packet multiplexing and replay improvements for partial packet handling
• Removes complex snapshot state management for asynchronous operations
• Simplifies buffer reading operations by removing continue-capable multi-packet read functionality

Reviewed Changes

Copilot reviewed 22 out of 22 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
DataReaderTest.cs	Removes unused import and fixes test method name typo
DataReaderStreamsTest.cs	Fixes spelling in comment (minimum -> minimun)
TdsParserStateObjectHelper.cs	Removes unused import and adds whitespace formatting
AsyncCancelledConnectionsTest.cs	Removes complex tracking logic and simplifies async test execution
TdsParserStateObject.TestHarness.cs	Removes entire test harness file
MultiplexerTests.cs	Removes entire multiplexer test file
Microsoft.Data.SqlClient.FunctionalTests.csproj	Removes references to deleted test files
LocalAppContextSwitchesTests.cs	Removes compatibility switch test cases
LocalAppContextSwitchesHelper.cs	Removes compatibility switch helper properties
TdsParserStateObject.cs	Major simplification of packet processing, snapshot management, and buffer handling
TdsParserStateObject.Multiplexer.cs	Removes entire multiplexer implementation file
SqlDataReader.cs	Simplifies debug assertion
SqlCachedBuffer.cs	Simplifies buffer creation logic
Packet.cs	Removes entire packet utility class
LocalAppContextSwitches.cs	Removes compatibility switch properties
TdsParserStateObject.netfx.cs	Adds simplified ProcessSniPacket implementation
TdsParser.cs	Simplifies data reading operations and removes continue-capable methods
TdsParserStateObject.netcore.cs	Adds simplified ProcessSniPacket implementation
TdsParser.Windows.cs	Removes packet assertion
Microsoft.Data.SqlClient.csproj	Removes references to deleted source files

[codecov]

Codecov Report

❌ Patch coverage is 69.40639% with 67 lines in your changes missing coverage. Please review.
✅ Project coverage is 68.84%. Comparing base (7a7b54e) to head (9a6817f).
⚠️ Report is 5 commits behind head on release/6.1.

Files with missing lines	Patch %	Lines
...nt/netfx/src/Microsoft/Data/SqlClient/TdsParser.cs	6.81%	41 Missing ⚠️
.../netcore/src/Microsoft/Data/SqlClient/TdsParser.cs	78.18%	12 Missing ⚠️
...c/Microsoft/Data/SqlClient/TdsParserStateObject.cs	90.27%	7 Missing ⚠️
...osoft/Data/SqlClient/TdsParserStateObject.netfx.cs	76.19%	5 Missing ⚠️
...oft/Data/SqlClient/TdsParserStateObject.netcore.cs	90.90%	2 Missing ⚠️

Additional details and impacted files

@@               Coverage Diff               @@
##           release/6.1    #3556      +/-   ##
===============================================
- Coverage        69.69%   68.84%   -0.86%     
===============================================
  Files              281      279       -2     
  Lines            62413    61748     -665     
===============================================
- Hits             43500    42511     -989     
- Misses           18913    19237     +324     

Flag	Coverage Δ
addons	90.82% <ø> (ø)
netcore	72.71% <86.36%> (-0.05%)	⬇️
netfx	68.16% <62.67%> (-1.08%)	⬇️

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[paulmedynski]

I'll need a walkthrough of the TdsParser* files.

[Wraith2]

If the change I made in d38da1c does truly fix the gchandle problem then this PR will too because it removes the changed code that contained the mistake.

I'll need a walkthrough of the TdsParser* files.
From me?

[cheenamalhotra]

@Wraith2 can you share more info on how to reproduce the gchandle issue to confirm this?
Me and @mdaigle haven't been able to reproduce the issue as per repro here: #3519 (comment)

If the change I made in d38da1c does truly fix the gchandle problem then this PR will too because it removes the changed code that contained the mistake.

I'll need a walkthrough of the TdsParser* files.
From me?