Fix checking cancelled connections back into the connection pool

If
    * BeginTx is called with a non-default isolation level,
    * The context is canceled before SET TRANSACTION ISOLATION LEVEL
    completes,
then the connection:
    * has the cancelled property set to "context cancelled",
    * has the closed property set to true,
and,
    * BeginTx returns "context canceled"

Because of this, the connection gets put back into the connection pool.
When it is checked out again, if BeginTx is called on it again _without_
an isolation level,
    * then we fall into the mc.closed.IsSet() check in begin(),
    * so we return ErrBadConn,
    * so the driver kicks the broken connection out of the pool
    * (and transparently retries to get a new connection that isn't
    broken too).

However, if BeginTx is called on the connection _with_ an isolation
level, then we return a context canceled error from the SET TRANSACTION
ISOLATION LEVEL call.

That means the broken connection will stick around in the pool forever
(or until it's checked out for some other operation that correctly
returns ErrBadConn).

The fix is to check for the connection being closed before executing SET
TRANSACTION ISOLATION LEVEL.

Author: KJ Tsanaktsidis

connection.go

@@ -489,6 +489,10 @@ 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

@@ -2608,7 +2608,12 @@ func TestContextCancelBegin(t *testing.T) {
 	runTests(t, dsn, func(dbt *DBTest) {
 		dbt.mustExec("CREATE TABLE test (v INTEGER)")
 		ctx, cancel := context.WithCancel(context.Background())
-		tx, err := dbt.db.BeginTx(ctx, nil)
+		conn, err := dbt.db.Conn(ctx)
+		if err != nil {
+			dbt.Fatal(err)
+		}
+		defer conn.Close()
+		tx, err := conn.BeginTx(ctx, nil)
 		if err != nil {
 			dbt.Fatal(err)
 		}
@@ -2638,7 +2643,17 @@ func TestContextCancelBegin(t *testing.T) {
 			dbt.Errorf("expected sql.ErrTxDone or context.Canceled, got %v", err)
 		}
 
-		// Context is canceled, so cannot begin a transaction.
+		// 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
 		if _, err := dbt.db.BeginTx(ctx, nil); err != context.Canceled {
 			dbt.Errorf("expected context.Canceled, got %v", err)
 		}