signing-preflight

On real projects: an agent committed unsigned without asking to unlock the key; a signing call on a locked SSH key hung the session; 121 commits had to be re-signed after the fact — and the key turned out to have no passphrase at all. Each was a two-second question at the start of the session.

Check (read-only, bounded)

git config --get commit.gpgsign          # true → signing is expected
git config --get gpg.format              # ssh | openpgp (empty = openpgp)
git config --get user.signingkey

SSH signing — does a test signature finish? (user.signingkey may be a key file or a literal key::ssh-…; the test signature is the check that works for both.)

K="$(git config --get user.signingkey)"; case "$K" in key::*) printf '%s\n' "${K#key::}" > /tmp/sigpre.pub; K=/tmp/sigpre.pub ;; esac
echo preflight | perl -e 'alarm 5; exec @ARGV' ssh-keygen -Y sign -n git -f "$K" >/dev/null 2>&1 && echo signs || echo CANNOT-SIGN

OpenPGP — a batch signature that refuses to prompt:

echo preflight | perl -e 'alarm 5; exec @ARGV' gpg --batch --pinentry-mode error --clearsign -u "$(git config --get user.signingkey)" >/dev/null 2>&1 && echo signs || echo CANNOT-SIGN

Always bound the test with a timeout: an unbounded signing call on a locked key waits for a passphrase nobody will type, and the session stalls.

Then

• signs → proceed; nothing to say.
• CANNOT-SIGN → one question to the operator, before any work: "Commit signing is on and the key is locked / not loaded. Unlock it (ssh-add / gpg-agent) and I will continue — or tell me to commit unsigned for this session." Wait for the answer. This is the one question worth stopping for at the start.

Never, without being told

• git -c commit.gpgsign=false commit … or --no-gpg-sign — an unsigned commit on a branch that requires signatures is a push that will be refused, or worse, accepted.
• Re-signing history (rebase --exec 'git commit --amend -S') — it rewrites every hash after the first commit it touches; on a pushed branch that is a force-push.

Report unsigned work

If commits were made unsigned anyway, say so with the list:

git log --format='%h %G? %s' @{upstream}..HEAD | awk '$2!="G"'