A struct where two tags specify the same column name causes records to silently be not written

[andruwm]

I embarrassingly came across this with a careless copy + paste and more time debugging than I care to admit. The generic writer does not error, and the count returned from Write even equals the count of objects passed in. The result however is a parquet file with no objects.

A simple reporduction:

package main

import (
	"bytes"
	"io"
	"log"
	"strings"

	"github.com/segmentio/parquet-go"
)

type ExampleWorks struct {
	Foo string `parquet:"foo"`
	Bar string `parquet:"bar"`
}

type ExampleDoesntWork struct {
	Foo string `parquet:"foo"`
	Bar string `parquet:"foo"`
}

func main() {
	readWritePrint([]ExampleWorks{{Foo: "Hello", Bar: "World"}})
	readWritePrint([]ExampleDoesntWork{{Foo: "Hello", Bar: "World"}})
}

func readWritePrint[T any](input []T) {
	log.Printf("testing type %T", input)

	buf := bytes.NewBuffer([]byte{})
	w := parquet.NewGenericWriter[T](buf)

	n, err := w.Write(input)
	if err != nil {
		log.Fatal(err)
	}
	if n != len(input) {
		log.Fatal("expected write but got none")
	}

	if err = w.Close(); err != nil {
		log.Fatal(err)
	}

	sr := strings.NewReader(buf.String())
	r := parquet.NewGenericReader[T](sr)

	objs := make([]T, len(input))
	read, err := r.Read(objs)
	if err != nil && err != io.EOF {
		log.Fatal(err)
	}
	if read < len(input) {
		log.Println("expected read rows but got none")
	}

	log.Printf("%+v", objs)
}

$ go run ./main
# 2023/03/16 12:51:34 testing type []main.ExampleWorks
# 2023/03/16 12:51:34 [{Foo:Hello Bar:World}]
# 2023/03/16 12:51:34 testing type []main.ExampleDoesntWork
# 2023/03/16 12:51:34 expected read rows but got none
# 2023/03/16 12:51:34 [{Foo: Bar:}]