strings: Builder ReadFrom permits creation of mutable strings

[fweimer]

This program allows the creation of mutable strings:

package main

import (
        "fmt"
        "io"
        "strings"
)

var global_buf []byte

type Reader struct{}

func (Reader) Read(buf []byte) (count int, err error) {
        global_buf = buf
        return 0, io.EOF
}

func main() {
        var b strings.Builder
        b.ReadFrom(Reader{})
        fmt.Printf("len(global_buf): %d\n", len(global_buf))
        b.WriteString("foo")
        s := b.String()
        fmt.Printf("string before patching: %#v\n", s)
        copy(global_buf[:3], "bar")
        fmt.Printf("string after patching: %#v\n", s)
}

Output is:

len(global_buf): 512
string before patching: "foo"
string after patching: "bar"

The reason why this works is that the io.Reader Read method is passed a slice which refers to the internal buffer of the builder.

[fweimer]

Issue #23084 is a different bug resulting in mutable strings.

[bradfitz]

Also bad. Thanks for the reports! Will fix.

[gopherbot]

Change https://golang.org/cl/83255 mentions this issue: strings: fix two Builder bugs allowing mutation of strings, remove ReadFrom