fixing `gpg failed to sign data` error on macOS

gpg_fix.txt

For troubleshooting, two things to first try:

run `git config --global gpg.program gpg2`, to make sure git uses gpg2 and not gpg
run `echo "test" | gpg2 --clearsign`, to make sure gpg2 itself is working
If that all looks all right, one next thing to try:

run `brew install pinentry` to ensure you have a good tool installed for passphrase entry
If after that install and you re-try git commit and still get the "failed to sign the data" error:

run `gpgconf --kill gpg-agent` to kill any running agent that might be hung
If that says gpgconf isn’t installed or doesn’t have a `--kill` option, you might try this:

`cp ~/.gnupg ~/.gnupg-GOOD` to save a copy of your `~/.gnupg` to revert to later if needed
`brew install gnupg21` to install `GnuPG 2.1`
The reason for saving a copy of your `~/.gnupg` dir is that GnuPG 2.1 potentially creates/changes some key data in way that isn’t backward-compatible with GnuPG 2.0 and earlier, so if you want to go back later, you can do `mv ~/.gnupg ~/.gnupg21 && mv ~/.gnupg-GOOD ~/.gnupg`.

Otherwise there are some basic steps to run to check you’ve got a working GnuPG environment:

run `gpg2 -K --keyid-format SHORT`, to check that you have at least one key pair
If the output of that shows you have no secret key for GnuPG to use, then you need to create one:

run `gpg2 --gen-key`, to have GnuPG walk you through the steps for creating a key pair

source: https://stackoverflow.com/a/41054093/4902448