Fix: re-open connection between migration-retry attempts (PostgresqlMigrator)

[jeremydmiller]

What

PostgresqlMigrator.executeWithConcurrencyRetryAsync (added in #294 / closes #293) re-invokes cmd.ExecuteNonQueryAsync on the same DbCommand after a transient PostgresException, but doesn't account for Npgsql moving the underlying connection to Closed or Broken when the server-side error has aborted the session — typically 40P01 deadlock_detected and XX000 \"tuple concurrently updated\".

The next call then throws InvalidOperationException(\"Connection is not open\"), which isn't a PostgresException — so it slips past the catch filter and surfaces to the caller as a hard migration failure, defeating the whole point of the retry.

Why

Recurring intermittent failure across JasperFx/marten CI runs:

EventSourcingTests.end_to_end_event_capture_and_fetching_the_stream.
query_before_saving(tenancyStyle: Conjoined) [FAIL]
  Marten.Exceptions.MartenSchemaException : DDL Execution for 'All Configured Changes' Failed!
  ---- System.InvalidOperationException : Connection is not open
       at Npgsql.NpgsqlCommand.CheckAndGetConnection()
       at Weasel.Postgresql.PostgresqlMigrator.executeWithConcurrencyRetryAsync(DbCommand cmd, CancellationToken ct)
         in /_/src/Weasel.Postgresql/PostgresqlMigrator.cs:line 179

Hit on PRs JasperFx/marten#4576, #4578, #4582, #4584 — all the same stack frame.

Fix

After the backoff delay, call EnsureConnectionOpenAsync:

internal static async Task EnsureConnectionOpenAsync(DbCommand cmd, CancellationToken ct)
{
    if (cmd.Connection is null) return;
    if (cmd.Connection.State == ConnectionState.Open) return;

    // A Broken connection cannot be reopened without a Close first.
    if (cmd.Connection.State == ConnectionState.Broken)
    {
        await cmd.Connection.CloseAsync().ConfigureAwait(false);
    }

    await cmd.Connection.OpenAsync(ct).ConfigureAwait(false);
}

Pulled out as an internal static helper so the reopen rules are unit-testable against a fake DbCommand / DbConnection.

Tests

4 new unit tests in PostgresqlMigratorConcurrencyRetryTests:

Test	Pins
ensure_connection_open_is_a_noop_when_already_open	No call to Open/Close
ensure_connection_open_reopens_a_closed_connection	Open called once, no Close
ensure_connection_open_closes_then_reopens_a_broken_connection	Close then Open (in that order — OpenAsync on Broken throws)
ensure_connection_open_does_nothing_when_command_has_no_connection	Defensive guard, doesn't throw

The retry loop itself races on the catalog and can't be exercised deterministically (called out in the existing test class header comment); the reopen helper is a pure function over (DbCommand, ConnectionState) and is fully testable.

Weasel.Postgresql.Tests.PostgresqlMigratorConcurrencyRetryTests: 16/16 passing locally (12 original + 4 new) on both net9.0 and net10.0.

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