[5.0.2] Migrations: Generate valid SQL when escaping new lines in seed data

[smitpatel]

Resolves #23518
Resolves #23459

Description

We fixed a bug in EF Core 5.0 to preserve the formatting of seed data with embedded new lines: #20662. The fix involves using the SQL concat function, but this was done in a way that creates an unbalanced tree in the SQL which then fails in both SQL Server and SQLite for large amounts of data.

Customer Impact

Migrations of EnsureCreated with large, multi-line strings in seed data throw. The only workarounds are to remove the seed data and get it to the database some other way, or to strip out the newlines.

How found

Customer reported on 5.0.0 for both SQL Server and SQLite

Test coverage

We have added test coverage to ensure the tree is created in balanced way.

Regression?

Yes, from 3.1.

Risk

Low. Doesn't fundamentally change anything except the balancing of the tree. Quirked.

[smitpatel]

Updated.

[ajcvickers]

Approved by Tactics for 5.0.2.

[saliksaly]

Using CONCAT for strings is not so good due to string truncation to 4000 chars in SQL server :-(
Now the migrations run without error (Microsoft.Data.SqlClient.SqlException (0x80131904): The concat function requires 2 to 254 arguments.) but my seeded strings are now truncated in the database. Silently!

Here are some resources for this SQL server behavior:
https://stackoverflow.com/questions/4547821/using-varcharmax-with-string-concatenation-in-sql-server-2005
https://stackoverflow.com/questions/4833549/nvarcharmax-still-being-truncated
https://sqlundercover.com/2017/09/27/concatenation-truncation-are-your-strings-being-truncated/

Is there any workaround for this? We have had CSHTML templates in seed data and now it would not be posible in new version of .net?