Fix executing connectionless command on NpgsqlDataSource with multiplexing (#4841)

Fixes #4840

(cherry picked from commit ad7dee8)

Author: vonzshik

src/Npgsql/MultiplexingDataSource.cs

@@ -98,7 +98,7 @@ async Task MultiplexingWriteLoop()
                     }
 
                     connector = await OpenNewConnector(
-                        command.Connection!,
+                        command.InternalConnection!,
                         new NpgsqlTimeout(TimeSpan.FromSeconds(Settings.Timeout)),
                         async: true,
                         CancellationToken.None);

test/Npgsql.Tests/DataSourceTests.cs

@@ -260,4 +260,24 @@ public async Task As_DbDataSource([Values] bool async)
             ? await command.ExecuteScalarAsync()
             : command.ExecuteScalar(), Is.EqualTo(1));
     }
+
+    [Test, IssueLink("https://github.com/npgsql/npgsql/issues/4840")]
+    public async Task Multiplexing_connectionless_command_open_connection()
+    {
+        var csb = new NpgsqlConnectionStringBuilder(ConnectionString)
+        {
+            Multiplexing = true
+        };
+        await using var dataSource = NpgsqlDataSource.Create(csb.ConnectionString);
+
+        await using var conn = await dataSource.OpenConnectionAsync();
+        await using var _ = await conn.BeginTransactionAsync();
+
+        await using var command = dataSource.CreateCommand();
+        command.CommandText = "SELECT 1";
+
+        await using var reader = await command.ExecuteReaderAsync();
+        Assert.True(reader.Read());
+        Assert.That(reader.GetInt32(0), Is.EqualTo(1));
+    }
 }