[release/7.0] Fix XmlSqlBinaryReader and introduce a corpus of SqlXml tests (#81878)

[jeffhandley]

Backport of #81878 to release/7.0. See #82062 for the corresponding backport to release/6.0.

Customer Impact

As reported in #74852, a regression was introduced in .NET 6 with #43379 that prevents SqlXml.CreateReader() from processing SQL Binary XML content (see MS-BINXML).

The regression is caused by seeking beyond the end of the current token during token validation. This is due to a simple mistake of using _end instead of _pos when _end represents the end of the entire buffer and _pos represents the position 1 byte beyond the end of the current token.

We currently have 2 customers reporting that their migrations to .NET 6 are blocked by this regression. While we have not yet obtained their validation that the .NET 8 builds work for them, our collective confidence in the fix is high. We will get their validation as soon as possible, but we wanted to get this fix queued up for the March servicing releases rather than waiting for that validation.

Testing

While the root cause and fix for this regression were well understood since @WaynePaiMS submitted #74852, this issue surfaced a substantial test hole. We did not have any tests validating that SQL Binary XML content could be processed. Such tests existed in .NET Framework, but they had not been ported to .NET Core+.

The fix includes reintroduction of the same collection of tests that were used in .NET Framework, including regression tests that had been added over time. The test cases include collections of SQL Binary and Text XML files. A new unit test processes the test files in both forms, validating that the processed XML matches the results of processing the text form directly through XmlTextReader (without SqlXml.CreateReader()). The tests were reviewed for thoroughness and named to match the scenarios they cover.

The details of those test cases can be seen here:
https://github.com/dotnet/runtime-assets/tree/main/src/System.Data.Common.TestData

Regression

Yes. The regression categorically prevents SQL XML Binary data from being processed, and this is currently blocking customers from migrating from .NET Core 3.1 and .NET 5.0 to .NET 6 or .NET 7.

Risk

Low. SQL Binary XML processing is categorically broken in .NET 6 and .NET 7. The root cause and fix are understood, and the fix introduces a body of tests to ensure SqlXml.CreateReader() produces the same result for SQL Binary XML files as it does for Text XML files, and those tests also ensure that it's processing of Text XML files matches the results of using XmlTextReader directly.

[ghost]

Tagging subscribers to this area: @dotnet/area-system-xml
See info in area-owners.md if you want to be subscribed.

Issue Details

---

Backport of #81878 to release/7.0. See #82062 for the corresponding backport to release/6.0.

Customer Impact

As reported in #74852, a regression was introduced in .NET 6 with #43379 that prevents SqlXml.CreateReader() from processing SQL Binary XML content (see MS-BINXML).

The regression is caused by seeking beyond the end of the current token during token validation. This is due to a simple mistake of using _end instead of _pos when _end represents the end of the entire buffer and _pos represents the position 1 byte beyond the end of the current token.

We currently have 2 customers reporting that their migrations to .NET 6 are blocked by this regression. While we have not yet obtained their validation that the .NET 8 builds work for them, our collective confidence in the fix is high. We will get their validation as soon as possible, but we wanted to get this fix queued up for the March servicing releases rather than waiting for that validation.

Testing

While the root cause and fix for this regression were well understood since @WaynePaiMS submitted #74852, this issue surfaced a substantial test hole. We did not have any tests validating that SQL Binary XML content could be processed. Such tests existed in .NET Framework, but they had not been ported to .NET Core+.

The fix includes reintroduction of the same collection of tests that were used in .NET Framework, including regression tests that had been added over time. The test cases include collections of SQL Binary and Text XML files. A new unit test processes the test files in both forms, validating that the processed XML matches the results of processing the text form directly through XmlTextReader (without SqlXml.CreateReader()). The tests were reviewed for thoroughness and named to match the scenarios they cover.

The details of those test cases can be seen here:
https://github.com/dotnet/runtime-assets/tree/main/src/System.Data.Common.TestData

Regression

Yes. The regression categorically prevents SQL XML Binary data from being processed, and this is currently blocking customers from migrating from .NET Core 3.1 and .NET 5.0 to .NET 6 or .NET 7.

Risk

Low. SQL Binary XML processing is categorically broken in .NET 6 and .NET 7. The root cause and fix are understood, and the fix introduces a body of tests to ensure SqlXml.CreateReader() produces the same result for SQL Binary XML files as it does for Text XML files, and those tests also ensure that it's processing of Text XML files matches the results of using XmlTextReader directly.

Author:	jeffhandley
Assignees:	jeffhandley
Labels:	Servicing-consider, area-System.Xml
Milestone:	7.0.x

[carlossanlop]

Approved by Tactics via email for 7.0.4.
Signed-off by area owners.
CI failure is known, pre-existing and unrelated: #81914
No OOB changes needed.
Ready to merge.