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Push the base-branch merge before asking for conflict resolution
When updating a descendant after a parent squash-merge, the parent-branch merge (step 1) is clean far more often than the pre-squash target merge (step 2). When step 1 is clean and only step 2 conflicts, push the step-1 result to the branch before posting the comment and label. The user then resolves only the genuine conflict (and records the squash with -s ours) on top of a branch that already contains the base merge, instead of being handed the whole sequence to redo. Order matters: GitHub silently skips pull_request workflows on a PR that conflicts with its base, so pushing the base merge (which keeps the head a descendant of its base) before adding the label is what keeps the resume trigger alive. If step 1 itself conflicts the action can't pre-merge, so it falls back to handing over the full recipe. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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‎tests/test_push_base_merge_on_conflict.sh‎

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#!/bin/bash
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#
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# Test for the "push the base-branch merge before asking for help" behaviour.
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#
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# When updating a descendant branch after a parent squash-merge, the parent
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# branch is merged first (step 1), then the target's pre-squash state (step 2),
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# then the squash is recorded with -s ours (step 0). When step 1 is clean but
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# step 2 conflicts, the action pushes the step-1 result to the branch BEFORE
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# posting the comment and label. That way:
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# - the user resolves on top of the base merge (they can't strand the PR by
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# resolving in the wrong order), and
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# - the head stays a descendant of its base, so the PR stays mergeable and the
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# synchronize event that resumes the action still fires. GitHub does not run
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# pull_request workflows on a PR that conflicts with its base.
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#
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# This test reproduces that topology and asserts the action pushed the base
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# merge, that the comment omits the (already-done) base merge, and that
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# following the comment leaves the branch mergeable.
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#
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# Runs fully offline. Unlike the other unit tests, its mock git APPLIES pushes
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# to local origin refs, so the simulated user picks up what the action pushed.
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set -e
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SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
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source "$SCRIPT_DIR/../command_utils.sh"
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simulate_push() {
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log_cmd git update-ref "refs/remotes/origin/$1" "$1"
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}
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TEST_REPO=$(mktemp -d)
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cd "$TEST_REPO"
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echo "Created test repo at $TEST_REPO"
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# Mock git that applies pushes to local origin refs and passes everything else
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# through. This lets the action's internal push actually move origin/feature2.
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cat > "$TEST_REPO/mock_git.sh" <<'MOCK_GIT'
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#!/bin/bash
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if [[ "$1" == "push" ]]; then
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shift
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remote=""
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specs=()
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for a in "$@"; do
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case "$a" in
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-*) ;; # ignore flags like --force-with-lease
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*) if [[ -z "$remote" ]]; then remote="$a"; else specs+=("$a"); fi ;;
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esac
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done
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for s in "${specs[@]}"; do
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if [[ "$s" == :* ]]; then
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git update-ref -d "refs/remotes/origin/${s#:}" 2>/dev/null || true
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else
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git update-ref "refs/remotes/origin/$s" "$s"
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fi
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done
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printf "Executing (mock push applied):" >&2
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printf " %q" git push "$@" >&2
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printf "\n" >&2
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exit 0
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fi
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exec git "$@"
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MOCK_GIT
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chmod +x "$TEST_REPO/mock_git.sh"
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# Mock gh that records the conflict comment and otherwise stays quiet.
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COMMENT_FILE="$TEST_REPO/conflict_comment.md"
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cat > "$TEST_REPO/mock_gh.sh" <<MOCK_GH
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#!/bin/bash
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if [[ "\$1" == "pr" && "\$2" == "list" ]]; then
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base=""
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for ((i=1; i<=\$#; i++)); do
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if [[ "\${!i}" == "--base" ]]; then next=\$((i+1)); base="\${!next}"; fi
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done
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[[ "\$base" == "feature1" ]] && echo "feature2"
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elif [[ "\$1" == "pr" && "\$2" == "comment" ]]; then
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cat > "$COMMENT_FILE"
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elif [[ "\$1" == "label" || ( "\$1" == "pr" && "\$2" == "edit" ) ]]; then
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: # ignore label creation / edits
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else
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echo "Unknown gh command: \$@" >&2
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exit 1
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fi
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MOCK_GH
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chmod +x "$TEST_REPO/mock_gh.sh"
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# Replaying merges may create non-ff merge commits; never open an editor.
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export GIT_EDITOR=true
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log_cmd git init -b main
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log_cmd git config user.email "test@example.com"
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log_cmd git config user.name "Test User"
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for i in $(seq 1 12); do echo "line $i" >> file.txt; done
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log_cmd git add file.txt
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log_cmd git commit -m "Initial commit"
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simulate_push main
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# feature1 (parent PR): first commit on line 2.
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log_cmd git checkout -b feature1
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sed -i '2s/.*/f1 commit 1/' file.txt
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log_cmd git add file.txt
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log_cmd git commit -m "f1: commit 1"
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simulate_push feature1
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# feature2 (child PR): branched off feature1, changes line 9.
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log_cmd git checkout -b feature2
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sed -i '9s/.*/f2 content line 9/' file.txt
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log_cmd git add file.txt
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log_cmd git commit -m "f2: change line 9"
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simulate_push feature2
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ORIG_FEATURE2=$(git rev-parse feature2)
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# feature1 advances AFTER feature2 branched (line 4): merging origin/feature1
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# into feature2 is clean and substantive.
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log_cmd git checkout feature1
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sed -i '4s/.*/f1 commit 2/' file.txt
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log_cmd git add file.txt
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log_cmd git commit -m "f1: commit 2"
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simulate_push feature1
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# main advances on line 9 so the pre-squash target conflicts with feature2.
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log_cmd git checkout main
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sed -i '9s/.*/main conflicting line 9/' file.txt
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log_cmd git add file.txt
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log_cmd git commit -m "main: conflicting change on line 9"
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simulate_push main
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# Squash-merge feature1 into main.
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log_cmd git checkout main
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log_cmd git merge --squash feature1
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log_cmd git commit -m "Squash merge feature1"
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SQUASH_COMMIT=$(git rev-parse HEAD)
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simulate_push main
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echo "Squash commit: $SQUASH_COMMIT"
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# Run the action: clean step-1 merge, conflicting step-2 merge.
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log_cmd \
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env \
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SQUASH_COMMIT="$SQUASH_COMMIT" \
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MERGED_BRANCH=feature1 \
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TARGET_BRANCH=main \
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GH="$TEST_REPO/mock_gh.sh" \
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GIT="$TEST_REPO/mock_git.sh" \
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"$SCRIPT_DIR/../update-pr-stack.sh"
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if [[ ! -f "$COMMENT_FILE" ]]; then
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echo "❌ No conflict comment was posted; scenario did not trigger a conflict"
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exit 1
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fi
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echo ""
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echo "=== Conflict comment posted to the PR ==="
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cat "$COMMENT_FILE"
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echo ""
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FAILED=0
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# The action should have pushed the base-branch merge to origin/feature2.
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if log_cmd git merge-base --is-ancestor origin/feature1 origin/feature2; then
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echo "✅ origin/feature2 now contains origin/feature1 (action pushed the base merge)"
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else
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echo "❌ origin/feature2 does not contain origin/feature1; the base merge was not pushed"
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FAILED=1
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fi
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# ...and that push must be a fast-forward on top of the original branch (so the
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# PR stays mergeable into its base and the synchronize event still fires).
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if log_cmd git merge-base --is-ancestor "$ORIG_FEATURE2" origin/feature2; then
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echo "✅ origin/feature2 is a descendant of the original branch (mergeable into its base)"
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else
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echo "❌ origin/feature2 is not a descendant of the original branch"
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FAILED=1
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fi
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# The comment should NOT ask the user to redo the base merge the action did.
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if grep -q '^git merge origin/feature1' "$COMMENT_FILE"; then
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echo "❌ Comment still asks the user to merge origin/feature1, which the action already did"
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FAILED=1
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else
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echo "✅ Comment omits the base merge the action already pushed"
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fi
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[[ "$FAILED" -ne 0 ]] && exit 1
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# Simulate the user: start from the branch as it is on the remote (which now
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# includes the action's base merge), then run the comment's `git merge` lines,
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# resolving any conflict by keeping our side.
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log_cmd git checkout feature2
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log_cmd git reset --hard origin/feature2
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MERGE_CMDS=$(grep -E '^git merge' "$COMMENT_FILE" || true)
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if [[ -z "$MERGE_CMDS" ]]; then
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echo "❌ Comment lists no 'git merge' commands to follow"
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exit 1
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fi
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while IFS= read -r cmd; do
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echo "Human runs: $cmd"
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if ! log_cmd bash -c "$cmd"; then
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echo "Resolving conflict by keeping our (feature2) side..."
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log_cmd git checkout --ours -- file.txt
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log_cmd git add file.txt
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log_cmd git commit --no-edit
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fi
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done <<< "$MERGE_CMDS"
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simulate_push feature2
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echo ""
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echo "=== feature2 after following the comment ==="
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log_cmd git log --graph --oneline --all
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echo ""
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# Following the comment must leave the branch mergeable into its new base (main):
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# the squash commit and the parent's advanced content are both present.
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if log_cmd git merge-base --is-ancestor "$SQUASH_COMMIT" feature2; then
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echo "✅ feature2 includes the squash commit (mergeable into main)"
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else
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echo "❌ feature2 is missing the squash commit after following the comment"
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FAILED=1
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fi
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if log_cmd git merge-base --is-ancestor origin/feature1 feature2; then
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echo "✅ feature2 includes the parent branch's advanced content"
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else
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echo "❌ feature2 is missing origin/feature1's content"
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FAILED=1
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fi
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[[ "$FAILED" -ne 0 ]] && exit 1
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echo ""
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echo "All push-base-merge-on-conflict tests passed! 🎉"
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echo "Test repository remains at: $TEST_REPO for inspection"

‎update-pr-stack.sh‎

Lines changed: 70 additions & 51 deletions
Original file line numberDiff line numberDiff line change
@@ -45,15 +45,50 @@ has_squash_commit() {
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&& git merge-base --is-ancestor SQUASH_COMMIT "$BRANCH"
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}
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48-
format_branch_list_for_text() {
49-
for ((i=1; i<=$#; i++)); do
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case $i in
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1) format='`%s`';;
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$#) format=', and `%s`';;
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*) format=', `%s`';;
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esac
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printf "$format" "${!i}"
56-
done
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# Post the manual-resolution comment and add the conflict label.
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#
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# $2 ("true"/"false") says whether we already pushed the parent-branch merge.
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# When true, the branch the user fetches already contains it, so the recipe
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# skips that step and the user only resolves the genuine conflict.
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#
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# The recipe is otherwise always the same sequence the clean path runs, so we
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# print it verbatim rather than tracking which merge failed.
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report_conflict() {
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local BRANCH="$1"
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local BASE_ALREADY_MERGED="$2"
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{
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echo "### ⚠️ Automatic update blocked by a merge conflict"
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echo
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if [[ "$BASE_ALREADY_MERGED" == "true" ]]; then
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echo "I merged the base branch into this pull request, but hit a conflict bringing in the squashed changes from the target branch. Please finish by hand:"
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else
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echo "I hit a merge conflict while updating this pull request after its base branch was squash-merged. Please resolve it by hand:"
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fi
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echo
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echo "#### How to resolve"
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echo
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echo "Run these in order, resolving any conflicts as they come up:"
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echo '```bash'
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echo "git fetch origin"
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echo "git switch $BRANCH"
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if [[ "$BASE_ALREADY_MERGED" == "true" ]]; then
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echo "git pull --ff-only # pick up the base-branch merge I pushed"
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else
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echo "git merge origin/$MERGED_BRANCH"
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fi
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echo "git merge $(git rev-parse SQUASH_COMMIT~)"
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echo "# record the squash commit as merged without re-applying its changes:"
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echo "git merge -s ours $(git rev-parse SQUASH_COMMIT)"
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echo "git push"
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echo '```'
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echo
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echo 'If a merge stops with conflicts, fix them (e.g. with `git mergetool`), `git commit`, then run the next command.'
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echo
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echo "Once you push, this action will resume and finish updating this pull request."
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} | log_cmd gh pr comment "$BRANCH" -F -
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# Create the label if it doesn't exist, then add it to the PR
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gh label create "$CONFLICT_LABEL" --description "PR needs manual conflict resolution" --color "d73a4a" 2>/dev/null || true
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log_cmd gh pr edit "$BRANCH" --add-label "$CONFLICT_LABEL"
5792
}
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5994
update_direct_target() {
@@ -72,61 +107,45 @@ update_direct_target() {
72107

73108
echo "Updating direct target $BRANCH (from $MERGED_BRANCH to $BASE_BRANCH)"
74109

75-
CONFLICTS=()
76110
log_cmd git update-ref BEFORE_MERGE HEAD
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112+
# Merge the parent branch. If this conflicts we can make no progress without
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# the user, so hand off the whole recipe.
77114
if ! log_cmd git merge --no-edit "origin/$MERGED_BRANCH"; then
78-
CONFLICTS+=("origin/$MERGED_BRANCH")
79115
log_cmd git merge --abort
116+
report_conflict "$BRANCH" false
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return 1
80118
fi
81-
# Only try merging the pre-squash target state if it's not already
82-
# included in the merged branch — otherwise the first merge covers it.
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# The parent-branch merge is clean. Merge the target's pre-squash state
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# (skipped when the merged branch already contains it). If THAT conflicts,
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# push the clean parent-branch merge first so the user resolves on top of
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# it: that keeps the branch a descendant of its base, so the PR stays
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# mergeable and the synchronize event that resumes this action still fires.
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# GitHub does not run pull_request workflows on a PR that conflicts with its
126+
# base, so leaving the branch in that state would strand it for good.
83127
if ! git merge-base --is-ancestor SQUASH_COMMIT~ "origin/$MERGED_BRANCH"; then
84128
if ! log_cmd git merge --no-edit SQUASH_COMMIT~; then
85-
CONFLICTS+=( "$(git rev-parse SQUASH_COMMIT~)" )
86129
log_cmd git merge --abort
130+
log_cmd git push origin "$BRANCH"
131+
report_conflict "$BRANCH" true
132+
return 1
87133
fi
88134
fi
89135

90-
if [[ "${#CONFLICTS[@]}" -gt 0 ]]; then
91-
{
92-
echo "### ⚠️ Automatic update blocked by merge conflicts"
93-
echo
94-
echo -n "I tried to merge "
95-
format_branch_list_for_text "${CONFLICTS[@]}"
96-
echo " into this branch while updating the pull request stack and hit conflicts."
97-
echo
98-
echo "#### How to resolve"
99-
echo '```bash'
100-
echo "git fetch origin"
101-
echo "git switch $BRANCH"
102-
for conflict in "${CONFLICTS[@]}"; do
103-
echo "git merge $conflict"
104-
echo "# ..."
105-
echo '# fix conflicts, for instance with `git mergetool`'
106-
echo "# ..."
107-
echo "git commit"
108-
done
109-
echo "git push"
110-
echo '```'
111-
echo
112-
echo "Once you push, this action will resume and finish updating this pull request."
113-
} | log_cmd gh pr comment "$BRANCH" -F -
114-
# Create the label if it doesn't exist, then add it to the PR
115-
gh label create "$CONFLICT_LABEL" --description "PR needs manual conflict resolution" --color "d73a4a" 2>/dev/null || true
116-
log_cmd gh pr edit "$BRANCH" --add-label "$CONFLICT_LABEL"
117-
return 1
118-
else
119-
log_cmd git merge --no-edit -s ours SQUASH_COMMIT
120-
log_cmd git update-ref MERGE_RESULT "HEAD^{tree}"
121-
COMMIT_MSG="Merge updates from $BASE_BRANCH and squash commit"
122-
if [[ "${GITHUB_ACTIONS:-}" == "true" ]]; then
123-
COMMIT_MSG="$COMMIT_MSG
136+
# Record the squash commit as merged without re-applying its tree, then bake
137+
# the result into a synthetic commit whose parents preserve the branch's
138+
# history.
139+
log_cmd git merge --no-edit -s ours SQUASH_COMMIT
140+
log_cmd git update-ref MERGE_RESULT "HEAD^{tree}"
141+
COMMIT_MSG="Merge updates from $BASE_BRANCH and squash commit"
142+
if [[ "${GITHUB_ACTIONS:-}" == "true" ]]; then
143+
COMMIT_MSG="$COMMIT_MSG
124144
125145
See $GITHUB_SERVER_URL/$GITHUB_REPOSITORY/actions/runs/$GITHUB_RUN_ID"
126-
fi
127-
CUSTOM_COMMIT=$(log_cmd git commit-tree MERGE_RESULT -p BEFORE_MERGE -p "origin/$MERGED_BRANCH" -p SQUASH_COMMIT -m "$COMMIT_MSG")
128-
log_cmd git reset --hard "$CUSTOM_COMMIT"
129146
fi
147+
CUSTOM_COMMIT=$(log_cmd git commit-tree MERGE_RESULT -p BEFORE_MERGE -p "origin/$MERGED_BRANCH" -p SQUASH_COMMIT -m "$COMMIT_MSG")
148+
log_cmd git reset --hard "$CUSTOM_COMMIT"
130149

131150
return 0
132151
}

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