[BUG] Closing an already closed connection kills the Erlang VM

[mindreframer]

Code to reproduce:

Interactive Elixir (1.12.2) - press Ctrl+C to exit (type h() ENTER for help)
iex(1)>
nil
iex(2)> alias Exqlite.Sqlite3
Exqlite.Sqlite3
iex(3)> {:ok, conn} = Sqlite3.open(":memory:")
{:ok, #Reference<0.3373779577.804651032.117242>}
iex(4)> Sqlite3.close(conn)
:ok
iex(5)>  Sqlite3.close(conn)
fish: Job 1, 'iex -S mix' terminated by signal SIGSEGV (Address boundary error)

It would be nice to handle this situation more gracefully.

[warmwaffles]

In this

exqlite/c_src/sqlite3_nif.c

Lines 165 to 172 in 096e0c2

	// note: _v2 may not fully close the connection, hence why we check if
	// any transaction is open above, to make sure other connections aren't blocked.
	// v1 is guaranteed to close or error, but will return error if any
	// unfinalized statements, which we likely have, as we rely on the destructors
	// to later run to clean those up
	sqlite3_close_v2(conn->db);
	conn->db = NULL;

I think we can just wrap it with

    if (conn->db) {
        // note: _v2 may not fully close the connection, hence why we check if
        // any transaction is open above, to make sure other connections aren't blocked.
        // v1 is guaranteed to close or error, but will return error if any
        // unfinalized statements, which we likely have, as we rely on the destructors
        // to later run to clean those up
        sqlite3_close_v2(conn->db);

        conn->db = NULL;
    }

I don't really have time at the moment to test this closely but it should in theory work. And we also need a test case for a double close. I thought we had one.

[mindreframer]

@warmwaffles Thanks for the hint. This particular code did not work, because we try to access a NULL-ed DB before in this line:

int autocommit = sqlite3_get_autocommit(conn->db);

I just return early, if conn->db is NULL before doing anything further with it.