Go "permission denied" when trying to create a file in a newly created directory?

Question

I'm trying to create a directory using os.Mkdir() and then create files in it, similar to this script:

package main

import (
    "log"
    "os"
    "path"
)

func main() {
    dir := "test_dir"

    os.Mkdir(dir, os.ModeDir)

    fileName := path.Join(dir, "file.txt")

    _, err := os.Create(fileName)
    if err != nil {
        log.Fatalf("create file: %v", err)
    }
}

However, if I run this, I get

> go run fileperms.go
2019/10/15 14:44:02 create file: open test_dir/file.txt: permission denied
exit status 1

It is not immediately clear to me from https://golang.org/pkg/os/#FileMode how to specify the FileMode in order to allow the same script to create files in the newly-created directory. How can I achieve this?

Answer 1

I found that I am able to create files in the directory if I set the permission to 0777:

package main

import (
    "io/ioutil"
    "os"
    "path"
)

func main() {
    dir := "test_dir"

    os.Mkdir(dir, 0777)

    fileName := path.Join(dir, "file.txt")

    ioutil.WriteFile(fileName, []byte("foobar"), 0666)
}

Now the file is created with the expected content:

> cat test_dir/file.txt 
foobar⏎ 

Answer 2

Here, Go was trying to create inside /tmp directory during an AUR package installation.

So I've changed the permissions in /tmp:

chmod 0777 -R /tmp

But that was not enough, so I had to change /tmp ownership (it was root's):

sudo chown -R "$USER":wheel /tmp