Fix parsing of multiple files with different headers (#6318). (#8400)

plugins/parsers/csv/parser.go

@@ -32,6 +32,8 @@ type Config struct {
 	Timezone          string   `toml:"csv_timezone"`
 	TrimSpace         bool     `toml:"csv_trim_space"`
 
+	gotColumnNames bool
+
 	TimeFunc    func() time.Time
 	DefaultTags map[string]string
 }
@@ -64,6 +66,8 @@ func NewParser(c *Config) (*Parser, error) {
 		return nil, fmt.Errorf("csv_column_names field count doesn't match with csv_column_types")
 	}
 
+	c.gotColumnNames = len(c.ColumnNames) > 0
+
 	if c.TimeFunc == nil {
 		c.TimeFunc = time.Now
 	}
@@ -102,10 +106,13 @@ func (p *Parser) Parse(buf []byte) ([]telegraf.Metric, error) {
 			return nil, err
 		}
 	}
-	// if there is a header and nothing in DataColumns
+	// if there is a header and we did not get DataColumns
 	// set DataColumns to names extracted from the header
-	headerNames := make([]string, 0)
-	if len(p.ColumnNames) == 0 {
+	// we always reread the header to avoid side effects
+	// in cases where multiple files with different
+	// headers are read
+	if !p.gotColumnNames {
+		headerNames := make([]string, 0)
 		for i := 0; i < p.HeaderRowCount; i++ {
 			header, err := csvReader.Read()
 			if err != nil {