net: TestLookupNullByte is failing on macOS (and maybe other non-Windows ports)

[dmitshur]

CL 173362 was a part of Go 1.14 changes and fixed a crash in net.LookupHost on Windows. It added the test TestLookupNullByte. A part of that test is not compatible with non-Windows ports such as darwin/amd64, causing the test to fail:

$ go test net -run=TestLookupNullByte
--- FAIL: TestLookupNullByte (0.00s)
    lookup_test.go:1188: unexpected success
FAIL
FAIL	net	0.234s

Marking as release-blocker because it's a regression, and the test failure is very reproducible. The fix will be small.

---

The test's primary goal is was to ensure net.LookupHost("foo\x00bar") doesn't crash on Windows. Additionally, it happens to check that err != nil on Windows:

// Issue 31586: don't crash on null byte in name
func TestLookupNullByte(t *testing.T) {
	testenv.MustHaveExternalNetwork(t)
	testenv.SkipFlakyNet(t)
	_, err := LookupHost("foo\x00bar") // used to crash on Windows
	if err == nil {
		t.Errorf("unexpected success")
	}
}

On darwin/amd64, net.LookupHost("foo\x00bar") on my machine/network returns a nil error even with Go 1.13.7:

package main

import (
	"fmt"
	"net"
	"runtime"
)

func main() {
	fmt.Println(runtime.Version())
	_, err := net.LookupHost("foo\x00bar")
	fmt.Println(err)
}

// Output:
// go1.13.7
// <nil>

/cc @bradfitz @golang/osp-team I plan to update the test so it either runs only on Windows, or relax the err == nil check. Let me know if there are preferences between those two fixes.

/cc @bradfitz Is it normal that net.LookupHost("foo\x00bar") can succeed on macOS? If it's not expected, I believe a separate issue should be filed for that.

[ianlancetaylor]

What is the net.Lookup call returning for the non-error result?

[dmitshur]

It's returning the same IP that I get if I do ping foo on my machine.

This was using the Cgo DNS resolver, which was picked by default:

$ echo "1.1.1.1 foo" >> /etc/hosts
$ GODEBUG=netdns=1 go run try.go
go1.13.7
go package net: using cgo DNS resolver
[... 1.1.1.1] <nil>

[dmitshur]

This might be related to the network I'm on more than to the choice of operating system. If that's the case, a better fix is to remove the "unexpected success" check rather than make the test Windows-only.

Edit: That said, this test has passed for two other people on a similar network who were using Linux. /cc @cagedmantis @toothrot

[gopherbot]

Change https://golang.org/cl/217729 mentions this issue: net: don't check LookupHost error in TestLookupNullByte