os, x/build: only write permissions 0222 for file result in successful read for linux-amd64

[odeke-em]

Coming here from https://go-review.googlesource.com/c/go/+/229357/3#message-c7922ea647b703faa8e858428f47221a6d921275 and https://storage.googleapis.com/go-build-log/b49db1e9/linux-amd64_454ff41f.log in which we have an odd test failure

# go run run.go -- fixedbugs/issue36437.go
incorrect output
Expected a non-nil error, but got:
	""
want
	"open /workdir/tmp/33921360662258/main.go: permission denied\n"
FAIL	fixedbugs/issue36437.go	0.291s
2020/04/22 23:32:59 Failed: exit status 1
go tool dist: FAILED

in which the relevant test is

package main

import (
        "fmt"
        "io/ioutil"
        "os"
        "os/exec"
        "path/filepath"
)

func main() {
        tmpDir, err := ioutil.TempDir("", "issue36437")
        if err != nil {
                panic(err)
        }
        defer os.RemoveAll(tmpDir)

        msgOrErr := func(bmsg []byte, err error) string {
                msg := string(bmsg)
                if msg == "" && err != nil {
                        msg = err.Error()
                }
                return msg
        }

        // 2. Invoke the compiler with a file that we don't have read permissions to.
        path := filepath.Join(tmpDir, "main.go")
        if err := ioutil.WriteFile(path, []byte("package p"), 0222); err != nil {
                panic(err)
        }
        output, err = exec.Command("go", "tool", "compile", path).CombinedOutput()
        want := fmt.Sprintf("open %s: permission denied\n", path)
        if g, w := msgOrErr(output, err), want; g != w {
                panic(fmt.Sprintf("Expected a non-nil error, but got:\n\t%q\nwant\n\t%q", g, w))
        }
}

Is this perhaps just a problem with the builder? I'd expect that 0222 would make a file writable but would error on reading; other GOOS-GOARCH combinations work properly but not this one.

[odeke-em]

I can't test it out unfortunately on my home machine at Linux as it malfunctioned.

[bcmills]

I don't know why the other TryBots would have succeeded, but most of the linux builders run as root, so file permissions are often ignored. (See #10719.)

[gopherbot]

Change https://golang.org/cl/229357 mentions this issue: cmd/compile: omit file:pos for non-existent, permission errors

[odeke-em]

Thank you @bcmills! That explains it for me.

[gopherbot]

Change https://golang.org/cl/231044 mentions this issue: test/fixedbugs: skip issue36347 since builders seem to be running on root