Support rebase merges (#60)

Stacked on #59.

A rebase merge lands the branch's content as new commits without merging
its history, same as a squash spread over several commits. The old
refusal reasons were specific to the merge dance, which merged
`SQUASH_COMMIT~` and so could conflict against intermediate copies; the
single-merge re-parent (#56) only reads the landed tip's tree, and with
`--absorbed` (#59) the recorded parent is the merged branch's tip in
both cases. So the squash path handles rebase merges as-is.

This deletes the squash-vs-rebase detection (`is_rebase_merge` and its
commit-PR association polling, the flakiest part of the action: the
association is computed asynchronously and had to be waited for). The
only case left to detect is a merge-commit merge, where the children
already contain their parent's history and the action only retargets.
`tests/test_rebase_merge_skip.sh`, which asserted the old refusal,
becomes `tests/test_rebase_merge.sh` and asserts the update.

---
_Generated by [Claude
Code](https://claude.ai/code/session_01PHUVU3Ek3U9j3LrdjPnyAy)_

---------

Co-authored-by: Claude <noreply@anthropic.com>
Co-authored-by: github-actions <github-actions@github.com>

Author: 3 people

.github/workflows/tests.yml

@@ -20,7 +20,7 @@ jobs:
           bash tests/test_conflict_resolution_resume.sh
           bash tests/test_conflict_absorbed_resolution.sh
           bash tests/test_merge_commit_merge.sh
-          bash tests/test_rebase_merge_skip.sh
+          bash tests/test_rebase_merge.sh
 
   e2e-tests:
     name: E2E Tests

README.md

@@ -134,7 +134,7 @@ jobs:
 
 ### Notes
 
-* Built for squash merges. A PR merged with a merge commit keeps its history, so the action only retargets its children and deletes the branch. Rebase merges are not supported: the action detects them through GitHub's commit-PR association (the merge method itself is recorded nowhere) and comments on each child PR instead of acting.
+* Works with squash merges and rebase merges: both land the branch's content as new commits without merging its history, so the children are re-parented onto the landed tip (the squash commit, or the last copied commit of a rebase merge). A PR merged with a merge commit keeps its history, so the action only retargets its children and deletes the branch.
 * If a merge hits a conflict, you'll need to resolve it manually; pushing the resolution automatically continues the stack update
 * Very large stacks might hit GitHub rate limits
 * After retargeting, GitHub sometimes keeps rendering a PR's diff against its old, deleted base, so the PR appears to contain already-merged changes. The branch itself is correct (`git diff <base>...HEAD` shows the real diff). Pushing any commit to the PR usually makes GitHub recompute. Sometimes it doesn't. Tough luck.

action.yml

@@ -1,5 +1,5 @@
 name: 'Stacked PR Update Action'
-description: 'Automatically fixes stacked PRs after a base PR is squash-merged'
+description: 'Automatically fixes stacked PRs after a base PR is squash- or rebase-merged'
 author: 'GitHub Actions'
 
 inputs:

tests/mock_gh.sh

@@ -29,16 +29,6 @@ elif [[ "$1" == "pr" && "$2" == "comment" ]]; then
     fi
     # Just log the comment command
     echo "Mock: gh pr comment $3"
-elif [[ "$1" == "api" && "$2" == repos/*/commits/*/pulls ]]; then
-    # Which PRs introduced this trunk commit (already --jq filtered to bare
-    # numbers). The merge commit belongs to the merged PR, and so does any sha
-    # listed in MOCK_REBASE_COPIES (space-separated); anything else was not
-    # introduced by a PR. SQUASH_COMMIT and PR_NUMBER come from the test's env.
-    sha="${2#*/commits/}"
-    sha="${sha%/pulls}"
-    if [[ "$sha" == "$SQUASH_COMMIT" || " ${MOCK_REBASE_COPIES:-} " == *" $sha "* ]]; then
-        echo "$PR_NUMBER"
-    fi
 else
     echo "Unknown gh command: $@" >&2
     exit 1

tests/test_rebase_merge_skip.sh tests/test_rebase_merge.shtests/test_rebase_merge_skip.sh renamed to tests/test_rebase_merge.sh

@@ -1,10 +1,11 @@
 #!/bin/bash
 #
-# A PR merged with "Rebase and merge" is not supported: the commits on the
-# target are new copies, so neither retargeting children as-is nor the squash
-# sequence gives a correct result. The action must detect the rebase (the
-# commit below the merge sha was also introduced by this PR, per GitHub's
-# commit-PR association), comment on the children, and leave the stack alone.
+# A PR merged with "Rebase and merge" lands its commits as new copies on the
+# target, exactly like a squash spread over several commits: content without
+# history. The action treats it like a squash: SQUASH_COMMIT (the PR's
+# merge_commit_sha) is the last copy, and the child is re-parented onto it in
+# one merge. The intermediate copies never enter the 3-way merge, so they can
+# raise no spurious conflicts.
 
 set -ueo pipefail
 
@@ -28,7 +29,8 @@ log_cmd git add file.txt
 log_cmd git commit -m "Initial commit"
 simulate_push main
 
-# feature1: two commits, so the rebase leaves a copy below the merge sha
+# feature1: two commits, so the rebase leaves an intermediate copy below the
+# merge sha
 log_cmd git checkout -b feature1
 sed -i '2s/.*/Feature 1 change A/' file.txt
 log_cmd git add file.txt
@@ -54,38 +56,47 @@ log_cmd git cherry-pick feature1~1 feature1
 MERGE_COMMIT=$(git rev-parse HEAD)
 simulate_push main
 
-FEATURE2_BEFORE=$(git rev-parse feature2)
-
 OUT=$(env \
     SQUASH_COMMIT="$MERGE_COMMIT" \
     MERGED_BRANCH=feature1 \
     PR_NUMBER=1 \
     TARGET_BRANCH=main \
-    MOCK_REBASE_COPIES="$(git rev-parse "$MERGE_COMMIT~")" \
     GH="$SCRIPT_DIR/mock_gh.sh" \
     GIT="$SCRIPT_DIR/mock_git.sh" \
     "$SCRIPT_DIR/../update-pr-stack.sh" 2>&1)
 echo "$OUT"
 
-if ! grep -q "rebase merges are not supported" <<<"$OUT"; then
-    echo "❌ Expected the rebase-merge skip message"
-    exit 1
-fi
-if ! grep -q "Mock: gh pr comment 2" <<<"$OUT"; then
-    echo "❌ The child PR must be told the stack was not updated"
+if ! git merge-base --is-ancestor "$MERGE_COMMIT" feature2; then
+    echo "❌ feature2 must be re-parented onto the rebased tip"
+    log_cmd git log --graph --oneline --all
     exit 1
 fi
-if grep -q "pr edit" <<<"$OUT"; then
-    echo "❌ No PR must be retargeted"
+if ! grep -q "Mock: gh pr edit 2 --base main" <<<"$OUT"; then
+    echo "❌ The child PR must be retargeted to main"
     exit 1
 fi
-if grep -q "push origin :feature1" <<<"$OUT"; then
-    echo "❌ The merged branch must be kept"
+if ! grep -q "push origin :feature1" <<<"$OUT"; then
+    echo "❌ The merged branch must be deleted"
     exit 1
 fi
-if [[ "$(git rev-parse feature2)" != "$FEATURE2_BEFORE" ]]; then
-    echo "❌ feature2's head must not be rewritten"
+
+# The child's PR diff against its new base shows only its own change: the
+# parent's changes arrive through the rebased copies on main, not the diff.
+EXPECTED_DIFF=$(cat <<'EOF'
+-line 14
++Feature 2 content
+EOF
+)
+ACTUAL_DIFF=$(log_cmd git diff main...feature2 | grep '^[+-]' | grep -v '^[+-][+-][+-]')
+if [[ "$ACTUAL_DIFF" == "$EXPECTED_DIFF" ]]; then
+    echo "✅ Triple-dot diff main...feature2 shows only feature2's unique change"
+else
+    echo "❌ Triple-dot diff is wrong"
+    echo "Expected:"
+    echo "$EXPECTED_DIFF"
+    echo "Actual:"
+    echo "$ACTUAL_DIFF"
     exit 1
 fi
 
-echo "✅ Rebase merge detected: children warned, stack left alone"
+echo "✅ Rebase merge handled like a squash: child re-parented, retargeted, branch deleted"

update-pr-stack.sh

@@ -3,7 +3,8 @@
 # Updates PR stack after merging a PR
 #
 # Required environment variables (squash-merge mode):
-# SQUASH_COMMIT - The hash of the squash commit that was merged
+# SQUASH_COMMIT - The merged PR's merge_commit_sha: the squash commit, or the
+#                 last copied commit of a rebase merge
 # MERGED_BRANCH - The name of the branch that was merged and will be deleted
 # TARGET_BRANCH - The name of the branch that the PR was merged into
 # PR_NUMBER - The number of the PR that was merged
@@ -99,58 +100,6 @@ has_squash_commit() {
         && git merge-base --is-ancestor SQUASH_COMMIT "$BRANCH"
 }
 
-# Args: a commit sha. Echoes the numbers of the pull requests that introduced
-# the commit to the repository, one per line.
-commit_pull_numbers() {
-    gh api "repos/{owner}/{repo}/commits/$1/pulls" --jq '.[].number' \
-        || { echo "❌ Could not list the pull requests that introduced commit $1" >&2; return 1; }
-}
-
-# Args: the merge commit sha, the merged PR's number. The association is
-# computed asynchronously, some time after the merge. The merge commit always
-# belongs to the merged PR, so once it shows up the index has caught up with
-# this merge; until then, an empty answer for any commit of the merge means
-# nothing. Exits if the association never appears.
-wait_for_pull_association() {
-    local MERGE_SHA="$1" PR_NUMBER="$2"
-    local NUMBERS
-    for _ in $(seq 1 24); do
-        NUMBERS=$(commit_pull_numbers "$MERGE_SHA") || exit 1
-        if grep -qx "$PR_NUMBER" <<<"$NUMBERS"; then
-            return 0
-        fi
-        sleep "${ASSOCIATION_POLL_SECONDS:-5}"
-    done
-    echo "❌ GitHub never associated $MERGE_SHA with PR #$PR_NUMBER; cannot tell a squash from a rebase" >&2
-    exit 1
-}
-
-# Args: the merged PR's number. The event payload does not say which merge
-# method was used (GitHub records it nowhere), but GitHub associates every
-# trunk commit with the PR that introduced it. A squash introduces a single
-# commit, so the commit below SQUASH_COMMIT belongs to an older PR or to
-# none; a rebase introduces a copy of each PR commit, so with two or more
-# commits the one below SQUASH_COMMIT still belongs to this PR. A
-# single-commit PR merges identically under rebase and squash and correctly
-# reads as a squash here.
-is_rebase_merge() {
-    local PR_NUMBER="$1"
-    local MERGE_SHA PARENT_SHA NUMBERS
-    MERGE_SHA=$(git rev-parse SQUASH_COMMIT)
-    PARENT_SHA=$(git rev-parse SQUASH_COMMIT~)
-
-    NUMBERS=$(commit_pull_numbers "$PARENT_SHA") || exit 1
-    if [[ -z "$NUMBERS" ]]; then
-        # Ambiguous: "no PR introduced this commit" (a squash on top of a
-        # direct push) and "not indexed yet" (a rebase copy) both come back
-        # empty. Wait until the index has caught up with this merge, then ask
-        # again; this time empty really means no PR.
-        wait_for_pull_association "$MERGE_SHA" "$PR_NUMBER"
-        NUMBERS=$(commit_pull_numbers "$PARENT_SHA") || exit 1
-    fi
-    grep -qx "$PR_NUMBER" <<<"$NUMBERS"
-}
-
 # Echoes "<number> <head branch>" for each open PR based on the merged branch.
 # try, not run: callers run this in a command substitution, where a die would
 # only leave the subshell, so they capture the output and die themselves. An
@@ -425,19 +374,11 @@ main() {
         return 0
     fi
 
-    # Rebase merges are not supported: the copies on the target are new
-    # commits, so a child retargeted as-is would show its parent's changes in
-    # its diff, and the squash sequence can raise spurious conflicts against
-    # the intermediate copies. Tell the children and leave everything alone.
-    if is_rebase_merge "$PR_NUMBER"; then
-        echo "⚠️ '$MERGED_BRANCH' looks rebase-merged; rebase merges are not supported, leaving the stack alone"
-        CHILDREN=$(list_child_prs) || die "could not list the PRs based on $MERGED_BRANCH"
-        while read -r NUMBER BRANCH; do
-            [[ -n "$BRANCH" ]] || continue
-            run gh pr comment "$NUMBER" --body "ℹ️ The base branch \`$MERGED_BRANCH\` of this PR was merged with \"Rebase and merge\", which autorestack does not support. Update this PR manually. \`$MERGED_BRANCH\` was kept so this PR stays open."
-        done <<<"$CHILDREN"
-        return 0
-    fi
+    # A squash merge and a rebase merge both land the branch's content as new
+    # commits without merging its history, and both take the path below.
+    # SQUASH_COMMIT (the PR's merge_commit_sha) is the squash commit or the
+    # last rebased copy; either way its tree carries the merged branch's full
+    # content, which is all the single-merge re-parent reads from it.
 
     # Find all PRs directly targeting the merged PR's head
     CHILDREN=$(list_child_prs) || die "could not list the PRs based on $MERGED_BRANCH"