53300: sorry, too many clients already

Randomly started getting this error when running tests using testcontainers...

Npgsql.PostgresException: '53300: sorry, too many clients already'

Always the same test would fail - initially just returning a System.NullReferenceException with no indication of what was happening. After a bit of digging around found that the underlying problem was using up all the available connections.

After a bit more digging around found that there's a command line parameter you can use to increase the maximum number of connection.

So updated the test container from this...

private readonly PostgreSqlContainer _postgresSqlContainer = new PostgreSqlBuilder()
    .WithImage("postgres:16")
    .Build();

To this...

private readonly PostgreSqlContainer _postgresSqlContainer = new PostgreSqlBuilder()
    .WithImage("postgres:16")
    .WithEntrypoint("docker-entrypoint.sh")
    .WithCommand("-c", "max_connections=20000", "-c", "shared_buffers=256MB")
    .Build();

and no more errors (for now!)