Summary

Fixed #115

Related to #114 .

Added 3 attempts with backoff. I borrowed ideas from LinusU/node-appdmg#190 and modified the number of attempts from 5 to 3 per @aonez's suggestion (#115 (comment)).

Programming

set ±e

I think a temporary set +e may be required since you set set -e on the top of the script.
#115 (comment)

A temporary set +e is not required since the exit code of the statement executed in the {until... do} block is used to determine whether the {do... done} block will be executed. In other words, a non-zero exit code in the {until... do} block will not terminate the execution of a script with set -e . Therefore, we can safely test the exit code of hdiutil detach .

You can search "Exit immediately" in bash's manual page to see the official description.

how does it work

A small mock script:

#!/bin/bash


set -e

MAXIMUM_ATTEMPTS=3

attempts=0

until
    date --utc
    (( attempts++ ))
    bash -c "exit $1"
    exit_code=$?
    (( exit_code ==  0 )) && echo 'Nothing goes wrong' && break
    (( exit_code != 16 )) && echo 'Error (not 16)'     && exit $exit_code
do
    (( attempts == MAXIMUM_ATTEMPTS )) && echo 'Error 16' && exit 16
    echo 'wait';
    sleep $(( 1 * (2 ** attempts) ))
done

Execute it:

$ bash ./.sh 0
Fri Apr 30 08:19:22 UTC 2021
Nothing goes wrong
$ echo $?
0

$ bash ./.sh 1
Fri Apr 30 08:19:25 UTC 2021
Error (not 16)
$ echo $?
1

$ bash ./.sh 16
Fri Apr 30 08:19:27 UTC 2021
wait
Fri Apr 30 08:19:29 UTC 2021
wait
Fri Apr 30 08:19:33 UTC 2021
Error 16
$ echo $?
16

Test

I tested this on GitHub Actions. In the workflow, I tried to package Firefox like this:

...
      - uses: actions/checkout@v2
        with:
          ref: 'blahblah'

      - name: Catch the fox
        run: |
          bash ./create-dmg \
            ./dmg.dmg \
            /Applications/Firefox.app

And it works!

Log:

...
Fri, 30 Apr 2021 05:40:27 GMT | Unmounting disk image...
Fri, 30 Apr 2021 05:40:30 GMT | hdiutil: couldn't eject "disk2" - Resource busy
Fri, 30 Apr 2021 05:40:30 GMT | Wait a moment...
Fri, 30 Apr 2021 05:40:32 GMT | Unmounting disk image...
Fri, 30 Apr 2021 05:40:33 GMT | "disk2" ejected.
Fri, 30 Apr 2021 05:40:33 GMT | Compressing disk image...
...

Statistics:

How to do statistics: By adding echo "::warning EBUSY" , I'm able to know how many instances retried the unmounting operation without checking the logs of all instances one by one.

Please review my code, thank you.