bufio: make zero value of Reader useful

[dsnet]

Consider the following code pattern:

var br bufio.Reader
for _, r := range readers {
    br.Reset(r)
    ... // make use of br
}

Currently, this pattern results in an infinite loop since the internal buffer of bufio.Reader is empty (resulting in no progress being made). The bufio.Reader.Reset method should initialize the internal buffer with defaultBufSize if it is empty.

this pattern results in an infinite loop

How so? The program below doesn't hang.

package main

import (
	"bufio"
	"io"
	"os"
	"strings"
)

func main() {
	var br bufio.Reader
	readers := []io.Reader{strings.NewReader("data\n")}
	for _, r := range readers {
		br.Reset(r)
		io.Copy(os.Stdout, &br)
	}
}

[dsnet]

Interesting, this program does hang:

package main

import (
	"bufio"
	"io"
	"strings"
)

func main() {
	var br bufio.Reader
	readers := []io.Reader{strings.NewReader("data\n")}
	for _, r := range readers {
		br.Reset(r)
		br.ReadBytes('\n')
	}
}

[ahmetakturk]

this pattern results in an infinite loop

How so? The program below doesn't hang.

It does not hang, when calling io.Copy. Because io.Copy calls src.WriteTo, bufio.Reader.WriteTo here, and in turn, bufio.Reader.WriteTo calls the internal reader's (strings.Reader here) WriteTo function.

[ahmetakturk]

Isn't it OK just doing this:

func (b *Reader) Reset(r io.Reader) {
        // add this if statement
	if len(b.buf) < minReadBufferSize {
		b.buf = make([]byte, minReadBufferSize)
	}
	b.reset(b.buf, r)
}

[yunnian]

Isn't it OK just doing this:

func (b *Reader) Reset(r io.Reader) {
        // add this if statement
	if len(b.buf) < minReadBufferSize {
		b.buf = make([]byte, minReadBufferSize)
	}
	b.reset(b.buf, r)
}

This will not save memory

[dsnet]

Isn't it OK just doing this:

Yea, that's more or less what's proposed with saying "bufio.Reader.Reset method should initialize the internal buffer with defaultBufSize if it is empty."

This will not save memory

I'm not sure what you're getting at. This issue isn't about saving memory, but making the zero value of bufio.Reader useful.

[gopherbot]

Change https://golang.org/cl/307991 mentions this issue: bufio : Solve the problem of infinite loop after reset

[gopherbot]

Change https://golang.org/cl/345570 mentions this issue: bufio: make Reader.Reset and Writer.Reset work on the zero value