How to fix The command git fetch --depth=50 origin +refs/pull/123/merge: exited with 128 in Git

The most reliable prevention is to wait for all CI jobs to finish before clicking the merge button:

# In your .travis.yml, add branch protection awareness
branches:
  only:
    - main
    - master
    - /^v\d+\.\d+(\.\d+)?(-\S*)?$/

Best practices:
- Enable "Require status checks to pass before merging" in GitHub branch protection
- Wait for the green checkmark before merging
- Don't restart builds for already-merged PRs

If a PR was merged and you need to verify the build, trigger a build on the base branch instead:

Travis CI:
1. Go to your Travis CI dashboard
2. Navigate to the repository
3. Click "More options" > "Trigger build"
4. Select your base branch (main/master)
5. Click "Trigger custom build"

GitHub Actions:
If using GitHub Actions, the push event to main will automatically trigger a new build.

# Example workflow that builds on push to main
on:
  push:
    branches: [main, master]
  pull_request:
    branches: [main, master]

Sometimes increasing the clone depth helps when merge refs are further back in history:

# .travis.yml
git:
  depth: 100  # Increase from default 50

# Or disable shallow cloning entirely (slower but more reliable)
git:
  depth: false

For GitHub Actions:

- uses: actions/checkout@v4
  with:
    fetch-depth: 0  # Fetch all history

For GitLab CI:

variables:
  GIT_DEPTH: 0  # Full clone

Configure your CI to build the PR head commit instead of the merge commit:

# .travis.yml - build the PR branch directly
git:
  depth: 50

# Optionally, configure to not build merge commits
# This is controlled by Travis settings, not YAML

GitHub Actions approach:

- uses: actions/checkout@v4
  with:
    ref: ${{ github.event.pull_request.head.sha }}

This approach builds the PR branch tip rather than the merge result, which is more stable but doesn't test merge compatibility.

Before troubleshooting further, confirm the PR state:

# Check if the PR merge ref exists
gh pr view 123 --json state,merged

# Or using git directly
git ls-remote origin refs/pull/123/merge
git ls-remote origin refs/pull/123/head

If the PR is merged:
- The refs/pull/X/merge ref will not exist
- The refs/pull/X/head ref may still exist temporarily

If the PR is open and the ref doesn't exist, there may be a temporary GitHub issue.

Add error handling to your CI configuration for this specific scenario:

# .travis.yml with fallback
before_install:
  - |
    if ! git fetch origin +refs/pull/${TRAVIS_PULL_REQUEST}/merge: 2>/dev/null; then
      echo "Merge ref not found, PR may have been merged"
      if [ "${TRAVIS_PULL_REQUEST}" != "false" ]; then
        echo "Building head commit instead"
        git fetch origin +refs/pull/${TRAVIS_PULL_REQUEST}/head:
      fi
    fi

GitHub Actions with conditional:

- name: Checkout with fallback
  run: |
    git fetch origin +refs/pull/${{ github.event.number }}/merge: || \
    git fetch origin +refs/pull/${{ github.event.number }}/head:

If the PR is still open and the error persists, check for platform issues:

Check status pages:
- GitHub: https://www.githubstatus.com/
- Travis CI: https://www.traviscistatus.com/
- GitLab: https://status.gitlab.com/

Manual retry steps:
1. Wait 5-10 minutes for any temporary issues to resolve
2. Push an empty commit to trigger a new build:

git commit --allow-empty -m "Trigger CI rebuild"
   git push

3. Or restart the build from the CI platform's web interface

Set up GitHub branch protection to prevent merging before CI completes:

1. Go to Settings > Branches in your GitHub repository
2. Click Add rule or edit existing rule for your main branch
3. Enable these options:
- Require status checks to pass before merging
- Require branches to be up to date before merging
- Select your CI jobs as required status checks

This prevents the race condition where someone merges while CI is still running.

# You can also use GitHub CLI to configure this
gh api repos/{owner}/{repo}/branches/main/protection -X PUT \
  -F required_status_checks='{"strict":true,"contexts":["continuous-integration/travis-ci/pr"]}'