Make BatchResults.Close safe to be called multiple times

https://github.com/jackc/pgx/issues/1138
https://github.com/jackc/pgx/issues/938

batch.go

@@ -3,6 +3,7 @@ package pgx
 import (
 	"context"
 	"errors"
+	"fmt"
 
 	"github.com/jackc/pgconn"
 )
@@ -46,17 +47,18 @@ type BatchResults interface {
 
 	// Close closes the batch operation. This must be called before the underlying connection can be used again. Any error
 	// that occurred during a batch operation may have made it impossible to resyncronize the connection with the server.
-	// In this case the underlying connection will have been closed.
+	// In this case the underlying connection will have been closed. Close is safe to call multiple times.
 	Close() error
 }
 
 type batchResults struct {
-	ctx  context.Context
-	conn *Conn
-	mrr  *pgconn.MultiResultReader
-	err  error
-	b    *Batch
-	ix   int
+	ctx    context.Context
+	conn   *Conn
+	mrr    *pgconn.MultiResultReader
+	err    error
+	b      *Batch
+	ix     int
+	closed bool
 }
 
 // Exec reads the results from the next query in the batch as if the query has been sent with Exec.
@@ -64,6 +66,9 @@ func (br *batchResults) Exec() (pgconn.CommandTag, error) {
 	if br.err != nil {
 		return nil, br.err
 	}
+	if br.closed {
+		return nil, fmt.Errorf("batch already closed")
+	}
 
 	query, arguments, _ := br.nextQueryAndArgs()
 
@@ -114,6 +119,11 @@ func (br *batchResults) Query() (Rows, error) {
 		return &connRows{err: br.err, closed: true}, br.err
 	}
 
+	if br.closed {
+		alreadyClosedErr := fmt.Errorf("batch already closed")
+		return &connRows{err: alreadyClosedErr, closed: true}, alreadyClosedErr
+	}
+
 	rows := br.conn.getRows(br.ctx, query, arguments)
 
 	if !br.mrr.NextResult() {
@@ -140,6 +150,10 @@ func (br *batchResults) Query() (Rows, error) {
 
 // QueryFunc reads the results from the next query in the batch as if the query has been sent with Conn.QueryFunc.
 func (br *batchResults) QueryFunc(scans []interface{}, f func(QueryFuncRow) error) (pgconn.CommandTag, error) {
+	if br.closed {
+		return nil, fmt.Errorf("batch already closed")
+	}
+
 	rows, err := br.Query()
 	if err != nil {
 		return nil, err
@@ -179,6 +193,11 @@ func (br *batchResults) Close() error {
 		return br.err
 	}
 
+	if br.closed {
+		return nil
+	}
+	br.closed = true
+
 	// log any queries that haven't yet been logged by Exec or Query
 	for {
 		query, args, ok := br.nextQueryAndArgs()