os: MkdirAll error has junk characters on Plan 9

[petter9]

What version of Go are you using (go version)?

go version go1.10.1 plan9/amd64

Does this issue reproduce with the latest release?

Yes.

What operating system and processor architecture are you using (go env)?

GOARCH='amd64'
GOBIN=''
GOCACHE='/usr/p/lib/cache/go-build'
GOEXE=''
GOHOSTARCH='amd64'
GOHOSTOS='plan9'
GOOS='plan9'
GOPATH='/usr/p/go'
GORACE=''
GOROOT='/sys/go/1.10.1'
GOTMPDIR=''
GOTOOLDIR='/sys/go/1.10.1/pkg/tool/plan9_amd64'
GCCGO='gccgo'
CC='6c'
CXX='g++'
CGO_ENABLED='0'
CGO_CFLAGS='-g -O2'
CGO_CPPFLAGS=''
CGO_CXXFLAGS='-g -O2'
CGO_FFLAGS='-g -O2'
CGO_LDFLAGS='-g -O2'
PKG_CONFIG='pkg-config'
GOGCCFLAGS='-fPIC -m64 -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build510394034=/tmp/go-build -gno-record-gcc-switches'

What did you do?

package main

import (
        "fmt"
        "os"
)

func main() {
        fmt.Printf("%s\n", os.MkdirAll("foo/bar/baz", 0000))
        fmt.Printf("%s\n", os.MkdirAll("foo/bar/baz", 0000))
        fmt.Printf("%s\n", os.MkdirAll("foo/bar/baz", 0000))
        fmt.Printf("%s\n", os.MkdirAll("foo/bar/baz", 0000))
        fmt.Printf("%s\n", os.MkdirAll("foo/bar/baz", 0000))
}

What did you expect to see?

mkdir foo/bar: 'foo/bar' does not exist
mkdir foo/bar: 'foo/bar' does not exist
mkdir foo/bar: 'foo/bar' does not exist
mkdir foo/bar: 'foo/bar' does not exist
mkdir foo/bar: 'foo/bar' does not exist

What did you see instead?

mkdir foo/bar: 'foo/bar' does not exist existst
mkdir foo/bar: 'foo/bar' does not existes not ex
mkdir foo/bar: 'foo/bar' does not existon denied
mkdir foo/bar: 'foo/bar' does not existon denied
mkdir foo/bar: 'foo/bar' does not existon denied

[ianlancetaylor]

CC @0intro

[0intro]

Thanks for reporting. I think this is related to issue #24767

[millerresearch]

In fact this is another case of issue #13770, where all syscall error strings in Plan 9 are aliased to the same underlying byte array. The symptom in MkdirAll occurs when a Mkdir syscall fails, followed by an Lstat which fails and overwrites the Mkdir error string before it's used.
A local workaround would be to save a copy of the first error string. But really we should get rid of the aliasing behaviour, which is contrary to Go string semantics, by reworking this revision which introduced it.
I had a tentative correction a while ago, but I wasn't entirely happy with it. I'll make another attempt now.

[gopherbot]

Change https://golang.org/cl/111195 mentions this issue: syscall: eliminate aliasing of syscall error strings in Plan 9