Can this introduce unwanted side-effects with our existing scripts that are called afterwards?

Another option would be, to just use it in the _setup function.

Edit: After a quick grep, we don't seem to use '**/' anywhere else.

I was wondering about this too. But I left it up to the tests. Seems there is one place we need to adjust. I will do this later.

Nevermind, was a sporadic BATS issue.

I decided against this for two reasons:

1. The effects of shopt are not scoped to a function, i.e.

$ cat test.sh
#!/bin/bash

shopt -u globstar
shopt | grep "globstar"

function a() {
  shopt -s globstar
  shopt | grep "globstar"
}

a
shopt | grep "globstar"

$ bash test.sh
globstar     off
globstar     on
globstar     on

Hence, setting this inside the whole setup, i.e. changing the behavior at a certain point, is undesirable. We could turn it off again, but actually setting it as fine I think; we may be able to use it in the future and it does not hurt at all.
2. I want Bash adjustments like shopt or set to be used at the very beginning of scripts so everyone immediately notices them. I plan on introducing set -u and shopt -s inherit_errexit too, and these should go together at the beginning start-mailserver.sh.

I am aware of that. I didn't express it clearly enough. I meant something like this:

function foo() {
shopt -s globstar
for i in /foo/**/*; do
bar
done
shopt -u globstar
}

Setting globstar in the function and revert it after usage.

I actually think that this feature is useful in general, and having it available everywhere is a nice thing :)