Revert "Fix checking cancelled connections back into the connection pool (go-sql-driver#1095)"

This reverts commit 6313f20.

Author: jsha

AUTHORS

@@ -114,4 +114,3 @@ Multiplay Ltd.
 Percona LLC
 Pivotal Inc.
 Stripe Inc.
-Zendesk Inc.

connection.go

@@ -472,10 +472,6 @@ func (mc *mysqlConn) Ping(ctx context.Context) (err error) {
 
 // BeginTx implements driver.ConnBeginTx interface
 func (mc *mysqlConn) BeginTx(ctx context.Context, opts driver.TxOptions) (driver.Tx, error) {
-	if mc.closed.IsSet() {
-		return nil, driver.ErrBadConn
-	}
-
 	if err := mc.watchCancel(ctx); err != nil {
 		return nil, err
 	}

driver_test.go

@@ -2621,12 +2621,7 @@ func TestContextCancelBegin(t *testing.T) {
 	runTests(t, dsn, func(dbt *DBTest) {
 		dbt.mustExec("CREATE TABLE test (v INTEGER)")
 		ctx, cancel := context.WithCancel(context.Background())
-		conn, err := dbt.db.Conn(ctx)
-		if err != nil {
-			dbt.Fatal(err)
-		}
-		defer conn.Close()
-		tx, err := conn.BeginTx(ctx, nil)
+		tx, err := dbt.db.BeginTx(ctx, nil)
 		if err != nil {
 			dbt.Fatal(err)
 		}
@@ -2656,17 +2651,7 @@ func TestContextCancelBegin(t *testing.T) {
 			dbt.Errorf("expected sql.ErrTxDone or context.Canceled, got %v", err)
 		}
 
-		// The connection is now in an inoperable state - so performing other
-		// operations should fail with ErrBadConn
-		// Important to exercise isolation level too - it runs SET TRANSACTION ISOLATION
-		// LEVEL XXX first, which needs to return ErrBadConn if the connection's context
-		// is cancelled
-		_, err = conn.BeginTx(context.Background(), &sql.TxOptions{Isolation: sql.LevelReadCommitted})
-		if err != driver.ErrBadConn {
-			dbt.Errorf("expected driver.ErrBadConn, got %v", err)
-		}
-
-		// cannot begin a transaction (on a different conn) with a canceled context
+		// Context is canceled, so cannot begin a transaction.
 		if _, err := dbt.db.BeginTx(ctx, nil); err != context.Canceled {
 			dbt.Errorf("expected context.Canceled, got %v", err)
 		}