Improve async string perf and fix reading chars with initial offset.

[Wraith2]

Description

For performance related changes see background information in #3274 (comment) and the preceeding conversation. Reading a string in async mode was being slower than expected so I investigated. I will annotate each change with an explanation.

Issues

fixes #3331
fixes #3274

Testing

Tests have been added which cover both issues that are being fixed.

[Wraith2]

/cc @Tornhoof, @AlexanderKot this should fix your issues.

@dotnet/sqlclientdevteam can I get a CI kick? I'd really really like to get this in to preview 2 because it fixes all the known issues with async continue.

[cheenamalhotra]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 2 pipeline(s).

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 12 lines in your changes missing coverage. Please review.

Project coverage is 59.71%. Comparing base (b8948f2) to head (e8df89e).
Report is 15 commits behind head on main.

Files with missing lines	Patch %	Lines
...nt/netfx/src/Microsoft/Data/SqlClient/TdsParser.cs	0.00%	6 Missing ⚠️
.../netcore/src/Microsoft/Data/SqlClient/TdsParser.cs	0.00%	5 Missing ⚠️
...c/Microsoft/Data/SqlClient/TdsParserStateObject.cs	0.00%	1 Missing ⚠️

❗ There is a different number of reports uploaded between BASE (b8948f2) and HEAD (e8df89e). Click for more details.

HEAD has 1 upload less than BASE

Flag	BASE (b8948f2)	HEAD (e8df89e)
addons	1	0

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3377      +/-   ##
==========================================
- Coverage   67.04%   59.71%   -7.33%     
==========================================
  Files         300      293       -7     
  Lines       65376    65308      -68     
==========================================
- Hits        43831    39000    -4831     
- Misses      21545    26308    +4763     

Flag	Coverage Δ
addons	?
netcore	63.02% <0.00%> (-9.17%)	⬇️
netfx	61.03% <0.00%> (-4.15%)	⬇️

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[Copilot]

Pull Request Overview

This PR targets performance improvements in asynchronous string processing and corrects an issue with reading characters using an initial offset.

• Added a new sequential character reading test ("CanGetCharsSequentially") to verify correct behavior.
• Introduced helper methods in tests to use pooled buffers, ensuring buffers are resized appropriately.
• Updated buffer management logic in TdsParser (both netfx and netcore) to use the correct offset and to improve buffer reallocation efficiency.

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

File	Description
src/Microsoft.Data.SqlClient/tests/ManualTests/SQL/DataReaderTest/DataReaderTest.cs	Adds a new test and helper method for sequential character reading using pooled buffers.
src/Microsoft.Data.SqlClient/src/Microsoft/Data/SqlClient/TdsParserStateObject.cs	Introduces a GetPacketSize() method and a property to calculate the current packet index.
src/Microsoft.Data.SqlClient/netfx/src/Microsoft/Data/SqlClient/TdsParser.cs	Adjusts buffer sizing logic and offset handling to improve performance and correctness.
src/Microsoft.Data.SqlClient/netcore/src/Microsoft/Data/SqlClient/TdsParser.cs	Mirrors changes in netfx for improved buffer management and offset accumulation.

[cheenamalhotra]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 2 pipeline(s).

[benrr101]

I can't say I know much about the internals here, so I appreciate the callouts on what changes are being made. However, I'm not understanding what the _longlen changes are really doing. Please take a look at my comments and I'll approve if you can explain it without code changes :)

[paulmedynski]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 2 pipeline(s).

[mdaigle]

@benrr101 this one needs an updated review from you

[benrr101]

Thanks for making the changes!