PgReader.GetTextReader with size > 65536 fails

[carlokok]

Steps to reproduce

Query:

select d.data from pubdata.mt_doc_displaytype as d where ((d.data ->> 'Public' is not null and CAST(d.data ->> 'Public' as boolean) = True) and CAST(d.data ->> 'RoomId' as uuid) = 'c04785c8-6f3b-4446-9a78-3b9546e42273'
) order by CAST(d.data ->> 'SortOrder' as integer);

Code:

using var conn = new Npgsql.NpgsqlConnection("....");
conn.Open();
using var cmd = conn.CreateCommand();
cmd.CommandText = "select d.data from pubdata.mt_doc_displaytype as d where ((d.data ->> 'Public' is not null and CAST(d.data ->> 'Public' as boolean) = True) and CAST(d.data ->> 'RoomId' as uuid) = 'c04785c8-6f3b-4446-9a78-3b9546e42273'\r\n) order by CAST(d.data ->> 'SortOrder' as integer)";
using var read = cmd.ExecuteReader();
while (read.Read())
{
    read.GetTextReader(0);
}

against attached sql dump.
dump.zip

The issue

When using .GetTextReader() it calls PgReader.cs GetTextReader() which returns a streaming reader (StreamReader) if the column size is > 65536 (const int maxPreparedSize = 1024 * 64;) however EndRead fails if the FieldOffset does not exactly match the FieldSize:

		if (FieldOffset != FieldSize)
		{
			ThrowNotConsumedExactly();
		}

Even before returning from the outer GetTextReader

Exception:

>	Npgsql.dll!Npgsql.Internal.PgReader.ThrowNotConsumedExactly() Line 802	C#
 	Npgsql.dll!Npgsql.Internal.PgReader.EndRead() Line 492	C#
 	Npgsql.dll!Npgsql.NpgsqlDataReader.GetFieldValueCore<System.IO.TextReader>(int ordinal) Line 1629	C#
 	Npgsql.dll!Npgsql.NpgsqlDataReader.GetTextReader(int ordinal) Line 1521	C#
 	ConsoleApp6.dll!ConsoleApp6.Program.Main(string[] args) Line 14	C#

'```


### Further technical details

Npgsql version: 8.0.0
PostgreSQL version: 16.0
Operating system: Windows

[NinoFloris]

Thanks for your report.

I was able to reproduce the issue, #5454 fixes the underlying problem and it will be part of Npgsql 8.0.1 shortly.