expfmt: TextDecoder infinite bufio Read recursion

[abursavich]

Problem

If the io.Reader passed to expfmt.NewDecoder with the expfmt.FmtText format is a *bufio.Reader, then Decoder.Decode will infinitely recurse in the bufio.(*Reader).Read function.

Explanation

expfmt.(*textDecoder).Decode calls expfmt.(*TextParser).TextToMetricFamilies passing its same io.Reader each time, which is then passed to expfmt.(*TextParser).reset, which contains the following snippet:

	if p.buf == nil {
		p.buf = bufio.NewReader(in)
	} else {
		p.buf.Reset(in)
	}

The problem is that bufio.NewReader will return the given io.Reader without wrapping it, if it happens to already be a *bufio.Reader of sufficient size. All good on the first call from Decode, but the second call ends up setting the *bufio.Reader as its own underlying io.Reader and we get an infinite recursion.

A reasonable person might argue that this is a bug in bufio, but we can still fix it in expfmt.

Repro

package main

import (
	"bufio"
	"fmt"
	"io"
	"os"
	"strings"

	dto "github.com/prometheus/client_model/go"
	"github.com/prometheus/common/expfmt"
)

const example = `
# TYPE foo gauge
foo 0
`

func main() {
	r := bufio.NewReader(strings.NewReader(example))
	dec := expfmt.NewDecoder(r, expfmt.FmtText)
	for {
		var mf dto.MetricFamily
		if err := dec.Decode(&mf); err != nil {
			if err == io.EOF {
				return
			}
			fmt.Fprintln(os.Stderr, err)
			os.Exit(1)
		}
	}
}

[abursavich]

Repro of the underlying issue with just the standard library: https://go.dev/play/p/2UTNBLVR1J__E