strings: Copying Builders allows string mutation

[fweimer]

This program:

package main

import (
        "fmt"
        "strings"
)

func main() {
        var b1 strings.Builder
        b1.Grow(3)
        b2 := b1
        b1.WriteString("foo")
        s := b1.String()
        fmt.Printf("string before patching: %#v\n", s)
        b2.WriteString("bar")
        fmt.Printf("string after patching: %#v\n", s)
}

Prints:

string before patching: "foo"
string after patching: "bar"

I think another level of indirection is needed in the implementation, so that the internal buffer is shared even after a copy has been made.

[bradfitz]

Ouch, thanks. Will fix.

[gopherbot]

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