Git pull rebase trap

git-pull-rebase-trap

The trap

git pull --no-ff does not override pull.rebase=true. The --no-ff flag applies to merge operations; when the effective pull strategy is rebase, --no-ff is silently ignored and the rebase proceeds anyway.

When local has diverged from origin (e.g., 22 local commits + N remote commits), this rewrites every local commit with a new SHA. Any artifact that referenced the old SHAs — state files, gate-event ledgers, retrospection logs, release notes — is now stale and must be backfilled.

When this fires

Any of these conditions:

• git config pull.rebase returns true (globally or per-repo)
• git config branch.<current>.rebase is true
• A [pull] rebase = true block exists in .git/config or ~/.gitconfig

The trap fires whether you pass --no-ff or no flag at all: the configured pull strategy is rebase, so a merge-side flag is simply ignored and your commits are rebased. --ff-only is the exception — on diverged history it refuses outright (git aborts with a "cannot fast-forward" message; the exact wording varies by git version) instead of silently rebasing. That makes --ff-only a loud guard rail rather than a silent SHA rewrite — but not a usable everyday pull, since it also refuses every legitimate diverged sync.

Pre-flight (before git pull)

git config --get pull.rebase
git config --get branch.$(git branch --show-current).rebase

If either returns true, do NOT use git pull for divergence resolution. Use the explicit two-step:

git fetch origin <branch>
git merge --no-ff origin/<branch>   # produces a real merge commit

Or, if a linear history is actually wanted:

git fetch origin <branch>
git rebase origin/<branch>   # explicit rebase — at least the intent is recorded

Recovery (if you already pulled and triggered a rebase)

1. Identify SHAs that need backfilling. Grep state files / gate ledgers / release notes for the OLD SHAs. The reflog has both:

   git reflog --pretty='%h %s' | head -40
   

2. Map old → new. The new SHAs are HEAD-relative; pair them with old SHAs in order from the reflog.
3. Backfill in a single commit. Stage the state-file and audit-trail updates together; commit message should explicitly call out "post-rebase SHA backfill" so future audit-state checks don't flag the changes as drift.
4. Authorize the force-push explicitly before pushing. Never force-push to a protected branch without explicit user authorization.

Why this is non-obvious

• The Git documentation for pull --no-ff doesn't mention that the flag is a no-op under pull.rebase=true.
• The rebase happens silently — there's no warning that --no-ff was ignored.
• pull.rebase=true is a common Git-config recommendation for "clean history" workflows, and many repos inherit it from team .gitconfig templates without the operator realizing it's set.
• The blast radius (22 SHA rewrites + 5 state-file backfills + force-push pressure) is visible only after the fact.

Anti-patterns

• ❌ git pull --no-ff as a "safe default" — only safe if you've verified pull.rebase is unset or false.
• ❌ Running git pull to "investigate divergence." Use git fetch + git log HEAD..origin/<branch> for read-only divergence inspection.
• ❌ Trusting that --no-ff documentation describes the full behavior. It describes git merge --no-ff behavior; git pull --no-ff behaves differently under rebase.