Reusing prepared statements throws runtime errors

[grzm]

When using prepared statements for queries, subsequent execution of the statement throws runtime errors.

	var res bool

	stmt, err := connDB.PrepareContext(ctx, "SELECT true AS res")
	if err != nil {
		os.Exit(1)
	}

        // first call
	rows, err := stmt.QueryContext(ctx)
	if err != nil {
		os.Exit(1)
	}

	for rows.Next() {
		testLogger.Info("first use of stmt: %t", res) // succeeds
	}

        // second call
	rows, err = stmt.QueryContext(ctx)
	if err != nil {
		os.Exit(1)
	}

	for rows.Next() {    // <- throws
		testLogger.Info("second use of stmt: %t", res)
	}

For comparison, this code works with PostgreSQL (using the lib/pq driver), and this pattern of reusing a prepared statement for a query is used in the Prometheus generic database free/sql_exporter exporter.

It looks like the row description might be getting lost on subsequent calls, as the number of columns returned in the second call (from rows.Columns()) is 0.

I've added tests which exhibit this behavior.

grzm@0cdacf8

[huebnerr]

Thank you for finding this, @grzm. We'll work on repro'ing the error, incorporating your test into the suite, and keep you updated. Hopefully this can be fixed in short order.

[huebnerr]

@grzm I've made a fix to the code that passes this test. Please verify that I've correctly translated your test into a new unit test. Note that in yours, the value was never Scan()'d, but I've made a change in this one to Scan and validate the cursor state.

func TestStmtReuseBug(t *testing.T) {
	connDB := openConnection(t)
	defer closeConnection(t, connDB)

	var res bool

	stmt, err := connDB.PrepareContext(ctx, "SELECT true AS res")
	assertNoErr(t, err)

	// first call
	rows, err := stmt.QueryContext(ctx)
	assertNoErr(t, err)

	defer rows.Close()

	assertNext(t, rows)
	assertNoErr(t, rows.Scan(&res))
	assertEqual(t, res, true)
	assertNoNext(t, rows)

	// second call
	rows, err = stmt.QueryContext(ctx)
	assertNoErr(t, err)

	defer rows.Close()

	assertNext(t, rows)
	assertNoErr(t, rows.Scan(&res))
	assertEqual(t, res, true)
	assertNoNext(t, rows)
}
`

[grzm]

@huebnerr Cheers! I applied your patch https://github.com/huebnerr/vertica-sql-go/commit/399816b3f198fb58f26dbab9cd135c59b0c7a72f and it resolved this particular issue.

Note that in yours, the value was never Scan()

Thanks for catching that. Copy/paste error. That snippet was just to demonstrate in the comment. I did have full tests in the patch I linked to (grzm@0cdacf8), where I did indeed call rows.Scan ;)

Thanks for the quick turnaround!

[huebnerr]

No problem at all. Thank you for catching it, @grzm ! I'll go ahead and close the issue. @sitingren will merge the PR. #18