git merge-stack — Cascade-merge a PR stack from bottom to top

git-merge-stack

#!/usr/bin/env bash
#
# git-merge-stack — Cascade-merge a PR stack from bottom to top.
#
# Setup (run once):
#   mkdir -p ~/.local/bin
#   cp this-file ~/.local/bin/git-merge-stack
#   chmod +x ~/.local/bin/git-merge-stack
#   Ensure ~/.local/bin is on your PATH (add to ~/.zshrc if needed):
#     export PATH="$HOME/.local/bin:$PATH"
#
# Usage: git merge-stack
#   Run from any branch in the stack. Discovers the full chain via `gh`,
#   merges each base into its child, and pushes. Aborts on any conflict.
#
set -euo pipefail

ORIGINAL_BRANCH=$(git rev-parse --abbrev-ref HEAD)
trap 'git checkout "$ORIGINAL_BRANCH" 2>/dev/null' EXIT

PR_JSON=$(gh pr list --author @me --state open --json number,headRefName,baseRefName)

if [ "$(echo "$PR_JSON" | jq length)" -eq 0 ]; then
  echo "No open PRs found for you."
  exit 1
fi

# --- Walk down to the root of the stack containing the current branch ---

current="$ORIGINAL_BRANCH"
while true; do
  base=$(echo "$PR_JSON" | jq -r --arg h "$current" \
    '[.[] | select(.headRefName == $h)][0].baseRefName // empty')

  if [ -z "$base" ]; then
    echo "Error: '$current' is not the head of any of your open PRs."
    exit 1
  fi

  parent_head=$(echo "$PR_JSON" | jq -r --arg h "$base" \
    '[.[] | select(.headRefName == $h)][0].headRefName // empty')

  if [ -z "$parent_head" ]; then
    stack_base="$base"
    break
  fi
  current="$base"
done

# --- Build the ordered chain (bottom → top) ---

chain=()
cursor="$current"
while [ -n "$cursor" ]; do
  chain+=("$cursor")

  # Ensure the stack is linear (no forks)
  children=$(echo "$PR_JSON" | jq --arg b "$cursor" \
    '[.[] | select(.baseRefName == $b)] | length')
  if [ "$children" -gt 1 ]; then
    echo "Error: '$cursor' has $children PRs targeting it — not a linear stack."
    exit 1
  fi

  cursor=$(echo "$PR_JSON" | jq -r --arg b "$cursor" \
    '[.[] | select(.baseRefName == $b)][0].headRefName // empty')
done

# --- Print the stack ---

echo "Stack:"
echo "  $stack_base"
for b in "${chain[@]}"; do
  pr=$(echo "$PR_JSON" | jq -r --arg h "$b" \
    '[.[] | select(.headRefName == $h)][0].number')
  echo "  └─ #$pr $b"
done
echo ""

# --- Back up each branch before any changes ---

TIMESTAMP=$(date +%Y%m%d-%H%M%S)
echo "Creating backups (timestamp: $TIMESTAMP) ..."
for branch in "${chain[@]}"; do
  safe_name=$(echo "$branch" | tr '/' '-')
  backup="backup/${safe_name}_${TIMESTAMP}"
  git branch "$backup" "$branch" 2>/dev/null || git branch "$backup" "origin/$branch"
  echo "  $branch → $backup"
done
echo ""

# --- Fetch once, then merge + push each level ---

git fetch origin

prev_ref="origin/$stack_base"
for branch in "${chain[@]}"; do
  pr=$(echo "$PR_JSON" | jq -r --arg h "$branch" \
    '[.[] | select(.headRefName == $h)][0].number')

  git checkout "$branch" 2>/dev/null

  # Fast-forward local to match remote (fail if diverged)
  if git rev-parse --verify "origin/$branch" >/dev/null 2>&1; then
    git merge "origin/$branch" --ff-only 2>/dev/null || {
      echo "Error: local '$branch' has diverged from origin — resolve manually."
      exit 1
    }
  fi

  echo "Merging into #$pr $branch ..."
  git merge "$prev_ref" --no-edit || {
    echo "CONFLICT merging into $branch — aborting."
    git merge --abort
    exit 1
  }
  git push origin "$branch"

  # Use the local ref going forward (it's now ahead of origin/)
  prev_ref="$branch"
done

echo ""
echo "Done — all branches in the stack are up to date."