Fix marshal data too short error for default gems

[sakakibara]

This fixes #150 which happens when there's no version specified for gems in $HOME/.default-gems file.

[Stratus3D]

Can you explain what this change does? I'm not sure I understand what it is doing.

[sakakibara]

@Stratus3D
Sorry I should have explained a little more.

I'm experiencing #150 on bash 3.2.57 which is default version of bash installed on macOS Mojave. And that is the version I tested this pull request with.

When trying to find the culprit by trial and error I figured out the error won't happen when I have the gem version specified in $HOME/.default-gems.

I then tested without the gem version specified after I made the following modification to line 106 of bin/install.

    # if output=$($gem install "$gem_name" "${args[@]:-}" 2>&1); then
    if output=$($gem install "$gem_name" 2>&1); then

Then it caused no error.

Using the syntax "${args[@]:-}" array expands into empty string.
Where as "${args[@]+"args[@]}"}" expands into nothing, as right hand side of + won't be expanded if args array is unset.

You can see the difference in two using the following script.

#!/usr/bin/env bash

echo_size() {
  echo "size: $#"
  echo "content: '${args[*]}'"
}

main() {
  local args=()
  local echoed_size

  echoed_size="$(echo_size "${args[@]:-}")"
  echo "${echoed_size}"

  echoed_size="$(echo_size "${args[@]+"${args[@]}"}")"
  echo "${echoed_size}"
}

main

The output is

size: 1
content: ''
size: 0
content: ''

[Stratus3D]

Excellent! Thanks for the detailed explanation and the script!

I also reproduced this by running:

$ gem install bundler '' # fails with the marshal data too short error
$ gem install bundler # succeeds