proposal: os: required env variables with MustGetenv

[igorcafe]

In many application we use environment variables for things such as API keys, database connection credentials, and so on.

Many times we wish to get an environment variable and ensure it is not empty, otherwise the application immediately, but since we can't check that outside a function, we do something like this:

var dbURL = os.Getenv("DB_URL")

func init() {
  if dbURL == "" {
    panic("missing env variable DB_URL")
  }
}

Since many applications can't run without some env variables and would rather prefer to "fail first", sounds reasonable to me a function like this:

var dbURL = os.MustGetenv("DB_URL")

func main() {
  // at this point I know for sure that dbURL is filled
}

// implementation example
package os

func MustGetenv(key string) string {
  val := os.Getenv(key)
  if val == "" {
    panic(fmt.Sprintf("missing environment variable '%s'", key))
  }
  return val
}

I know it is something trivial to implement by your own in your projects, but Go stdlib has many functions that are trivial to implement but very useful at the same time.

[seankhliao]

Duplicate of #54297

[igorcafe]

@seankhliao Although they are similar, I wouldn't say it is a duplicate.
The other proposal is about "generic must", while this is a specific use case for environment variables.