Transaction Commit/Rollback returns conn's cached error, if present (#1691)

If a transaction connection has a cached error, return it
instead of ErrInvalidConn during Commit/Rollback operations.
Fix #1690

Author: brad-defined

AUTHORS

@@ -25,6 +25,7 @@ Asta Xie <xiemengjun at gmail.com>
 B Lamarche <blam413 at gmail.com>
 Bes Dollma <bdollma@thousandeyes.com>
 Bogdan Constantinescu <bog.con.bc at gmail.com>
+Brad Higgins <brad at defined.net>
 Brian Hendriks <brian at dolthub.com>
 Bulat Gaifullin <gaifullinbf at gmail.com>
 Caine Jette <jette at alum.mit.edu>
@@ -135,6 +136,7 @@ Ziheng Lyu <zihenglv at gmail.com>
 
 Barracuda Networks, Inc.
 Counting Ltd.
+Defined Networking Inc.
 DigitalOcean Inc.
 Dolthub Inc.
 dyves labs AG

transaction.go

@@ -13,18 +13,32 @@ type mysqlTx struct {
 }
 
 func (tx *mysqlTx) Commit() (err error) {
-	if tx.mc == nil || tx.mc.closed.Load() {
+	if tx.mc == nil {
 		return ErrInvalidConn
 	}
+	if tx.mc.closed.Load() {
+		err = tx.mc.error()
+		if err == nil {
+			err = ErrInvalidConn
+		}
+		return
+	}
 	err = tx.mc.exec("COMMIT")
 	tx.mc = nil
 	return
 }
 
 func (tx *mysqlTx) Rollback() (err error) {
-	if tx.mc == nil || tx.mc.closed.Load() {
+	if tx.mc == nil {
 		return ErrInvalidConn
 	}
+	if tx.mc.closed.Load() {
+		err = tx.mc.error()
+		if err == nil {
+			err = ErrInvalidConn
+		}
+		return
+	}
 	err = tx.mc.exec("ROLLBACK")
 	tx.mc = nil
 	return