net: LookupHost sometimes returns slice which when modified ruins cache state.

[suharshs]

To reproduce:
Machine must have localhost specified in the /etc/hosts file.
Machine must NOT have localhost entry in its DNS Server.

Running this code:

package main

import (
    "fmt"
    "net"
)

func main() {
    runClient()
    modifyAddrs("localhost")
    runClient()
}

func modifyAddrs(host string) []string {
    addrs, err := net.LookupHost(host)
    if err != nil {
            panic(err)
    }
    for i := range addrs {
            addrs[i] += "junk"
    }
    return addrs
}

func runClient() {
    _, err := net.Dial("tcp", "localhost:23000")
    fmt.Printf("err = %v\n", err)
}

Will output:

err = dial tcp 127.0.0.1:23000: getsockopt: connection refused
err = dial tcp: lookup localhost on 169.254.169.254:53: no such host

The first error message is able to resolve localhost but after modifying the returned addr slice, it no longer can.
I traced it down to the localStaticHost function returns a slice directly from the cache, rather than returning a copy of it.

I have a fix here: https://go-review.googlesource.com/#/c/19201/

[gopherbot]

CL https://golang.org/cl/19201 mentions this issue.

[bradfitz]

Is this a regression from Go 1.5?

[suharshs]

I observed it in Go 1.5.3 and it also exists at HEAD.

[bradfitz]

In that case (since it's not a regression and lived with it in Go 1.5) we can fix it when the Go 1.7 dev tree opens up.

/cc @mikioh

[mikioh]

Looks like this is one of long standing issues, actually since r56?

d6054fc
a4f6ed3

PS: Please change the test function names from "runTestSubject" to "testSubject" once the tree opens.