How to fix fatal: cannot create worktree: read-only filesystem in CI in Git

# Check if current directory is writable
touch .write-test && rm .write-test && echo "Writable" || echo "Read-only"

# Check mount options for the filesystem
mount | grep $(df . --output=source | tail -1)

# List common writable directories
for dir in /tmp /workspace $HOME $CI_PROJECT_DIR; do
  if [ -d "$dir" ]; then
    touch "$dir/.test" 2>/dev/null && rm "$dir/.test" && echo "$dir: writable" || echo "$dir: read-only"
  fi
done

# Use /tmp which is typically writable
git worktree add /tmp/my-worktree feature-branch

# Or use the CI's designated workspace directory
# GitHub Actions
git worktree add "$GITHUB_WORKSPACE/../worktree" feature-branch

# GitLab CI
git worktree add "$CI_PROJECT_DIR/../worktree" feature-branch

# Jenkins
git worktree add "$WORKSPACE/../worktree" feature-branch

# docker-compose.yml
services:
  build:
    image: your-image
    volumes:
      - .:/app:rw  # Explicitly set read-write (default, but clear)
      # NOT: - .:/app:ro (read-only)

# Docker run command
docker run -v "$(pwd):/app:rw" your-image

# If you need the source read-only but want writable worktrees,
# mount a separate writable volume
docker run \
  -v "$(pwd):/app:ro" \
  -v "/tmp/worktrees:/worktrees:rw" \
  your-image

git worktree add /worktrees/feature feature-branch

name: Build
on: [push, pull_request]

jobs:
  build:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
        with:
          fetch-depth: 0  # Needed for worktrees with full history

      - name: Create worktree
        run: |
          # GitHub Actions workspace is writable
          # Create worktree in a sibling directory
          git worktree add ../feature-worktree origin/feature-branch

          # Or use runner temp directory
          git worktree add "$RUNNER_TEMP/worktree" origin/feature-branch

# .gitlab-ci.yml
build:
  script:
    # GitLab CI's build directory is writable
    - git worktree add "$CI_BUILDS_DIR/worktree-$CI_JOB_ID" origin/feature

    # Or use the project directory's parent
    - git worktree add "$CI_PROJECT_DIR/../feature-worktree" origin/feature

    # Clean up after use
    - git worktree remove "$CI_BUILDS_DIR/worktree-$CI_JOB_ID" || true

build:
  image: your-image
  variables:
    # Some images require this
    GIT_STRATEGY: clone

# Pod spec
apiVersion: v1
kind: Pod
spec:
  containers:
    - name: build
      securityContext:
        readOnlyRootFilesystem: true  # Security requirement
      volumeMounts:
        - name: workspace
          mountPath: /workspace
        - name: tmp
          mountPath: /tmp
        - name: worktrees
          mountPath: /worktrees
  volumes:
    - name: workspace
      emptyDir: {}
    - name: tmp
      emptyDir: {}
    - name: worktrees
      emptyDir: {}

# Clone to writable workspace
git clone https://github.com/org/repo /workspace/repo
cd /workspace/repo

# Create worktree in writable volume
git worktree add /worktrees/feature origin/feature

# Instead of worktree, use a shallow clone of the specific branch
git clone --depth 1 --branch feature-branch \
  "$(git remote get-url origin)" /tmp/feature-checkout

# Or for local changes, use git archive
git archive feature-branch | tar -x -C /tmp/feature-checkout

# For comparing branches, use git diff directly
git diff main...feature-branch

# Check kernel messages for filesystem errors
dmesg | grep -i "read-only\|error\|ext4\|xfs" | tail -20

# Check if filesystem was remounted due to errors
mount | grep "ro,"

# Attempt to remount as read-write (requires privileges)
# This is typically done by CI admins, not in build scripts
sudo mount -o remount,rw /path/to/mount

EXT4-fs error (device sda1): ext4_lookup:1604: inode #12345: comm git: deleted inode referenced
EXT4-fs (sda1): Remounting filesystem read-only

# Enable sparse checkout
git sparse-checkout init --cone

# Define which directories to include
git sparse-checkout set src/ tests/

# Switch to the branch you need
git checkout feature-branch

# Check out specific files from another branch
git checkout feature-branch -- path/to/file.txt

# Or restore from another branch to the working directory
git restore --source=feature-branch -- path/to/file.txt