Auto-switch push_to_pull_request_branch to bundle transport when merge commits are detected

[Copilot]

Long-running PR branches that merge their base branch locally produce patches git am --3way cannot apply, blocking every subsequent push (e.g. autoloop on tsessebe). The bundle transport already handles this but is opt-in.

Changes

• safe_outputs_handlers.cjs — In pushToPullRequestBranchHandler, when patch-format is not explicitly configured and origin/<branch>..<branch> contains ≥1 merge commit, flip useBundle = true. Explicit patch-format: am is still honored. The bundle code path now also computes the incremental net diff_size via computeIncrementalDiffSize(baseCommit..branch) and records it on the safe-output entry, so the downstream push step validates max_patch_size against the actual net change rather than the on-disk bundle artifact (which on long-running branches accumulates packed git objects and per-commit metadata, and can exceed the limit even when each iteration only changes a few KB).
• git_helpers.cjs — New hasMergeCommitsInRange(baseRef, headRef, { cwd }) wrapping git rev-list --merges --count. Best-effort: returns false on any git error so detection failures fall through to existing behavior. Uses local refs only (no extra fetch).
• Tests — Two cases in safe_outputs_handlers.test.cjs: (a) merge-commit scenario produces a bundle, emits the auto-switch debug line, and records a non-negative diff_size on the entry; (b) explicit patch_format: am is respected even with merges in range.
• Changeset — Patch-level entry documenting the new behavior, including the diff_size recording for bundle transport.

Detection logic

const patchFormatExplicit =
  pushConfig["patch_format"] !== undefined || config["patch_format"] !== undefined;
let useBundle = pushPatchFormat === "bundle";

if (!useBundle && !patchFormatExplicit && entry.branch) {
  if (hasMergeCommitsInRange(`refs/remotes/origin/${entry.branch}`, entry.branch, { cwd: repoCwd || undefined })) {
    useBundle = true; // git am can't apply merge commits
  }
}

Notes for review

• Scoped to push_to_pull_request_branch (incremental). create_pull_request (full mode) is not auto-switched here — that path rarely produces merge commits since the branch is typically new, and the range computation is more involved. Can extend if needed.
• Detection range matches what would be sent: refs/remotes/origin/<branch>..<branch> against the local checkout, identical to what the patch generator uses in incremental mode.
• Users can still force am via explicit patch-format: am if they want the prior failure mode (e.g. to surface a configuration mistake loudly rather than silently switching transports).
• diff_size recording for bundle transport applies whether the bundle was selected explicitly via patch-format: bundle or auto-selected by the new merge-commit detection — so existing bundle users also benefit from accurate max_patch_size validation.

[Copilot]

Pull request overview

Automatically switches push_to_pull_request_branch from patch transport to bundle transport when the incremental range contains merge commits, to avoid git am --3way failures on long-running PR branches.

Changes:

• Added merge-commit detection in pushToPullRequestBranchHandler and auto-fallback to bundle unless patch_format was explicitly set.
• Introduced hasMergeCommitsInRange() in git_helpers.cjs using git rev-list --merges --count.
• Added tests covering auto-switch behavior and explicit patch_format: am, plus a changeset entry documenting the behavior.

Show a summary per file

File	Description
actions/setup/js/safe_outputs_handlers.test.cjs	Adds regression tests for merge-commit auto-switching and explicit am override behavior.
actions/setup/js/safe_outputs_handlers.cjs	Implements merge-commit detection and conditional auto-switch from patch to bundle transport.
actions/setup/js/git_helpers.cjs	Adds a helper to detect whether a ref range contains merge commits.
.changeset/patch-auto-bundle-on-merge-commits.md	Documents the new auto-fallback behavior for release notes.

Copilot's findings

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Comments suppressed due to low confidence (2)

actions/setup/js/safe_outputs_handlers.cjs:591

• Switching to bundle here bypasses the excluded_files filtering that this handler only applies on the patch path (pushPatchOptions.excludedFiles). generateGitBundle has no equivalent exclusion support, so a branch with merge commits can end up pushing files that the workflow explicitly configured to omit. The auto-switch should be disabled when excluded_files is set, or bundle generation needs matching exclusion logic first.

    if (!useBundle && !patchFormatExplicit && entry.branch) {
      const hasMerges = hasMergeCommitsInRange(`refs/remotes/origin/${entry.branch}`, entry.branch, { cwd: repoCwd || undefined });
      if (hasMerges) {
        server.debug(`push_to_pull_request_branch: detected merge commit(s) in incremental range origin/${entry.branch}..${entry.branch}; auto-switching to bundle transport (set patch-format: am to override).`);
        useBundle = true;

actions/setup/js/safe_outputs_handlers.cjs:591

• Auto-switching to bundle here also skips the protected-file / allowed-files enforcement in the actual push step. That code inspects patchContent, but bundle pushes have no patch content, so checkFileProtection("") treats the change as touching no files (actions/setup/js/push_to_pull_request_branch.cjs:247-265 and actions/setup/js/manifest_file_helpers.cjs:202-229). As a result, any branch with a merge commit can bypass those safeguards entirely once this fallback fires.

    if (!useBundle && !patchFormatExplicit && entry.branch) {
      const hasMerges = hasMergeCommitsInRange(`refs/remotes/origin/${entry.branch}`, entry.branch, { cwd: repoCwd || undefined });
      if (hasMerges) {
        server.debug(`push_to_pull_request_branch: detected merge commit(s) in incremental range origin/${entry.branch}..${entry.branch}; auto-switching to bundle transport (set patch-format: am to override).`);
        useBundle = true;

• Files reviewed: 4/4 changed files
• Comments generated: 1

[pelikhan]

@copilot review all comments

[Copilot]

@copilot review all comments

Addressed in 63532bd: the bundle code path now computes the incremental net diff_size via computeIncrementalDiffSize(baseCommit..branch) and records it on the safe-output entry, so the downstream push step validates max_patch_size against the actual change rather than the on-disk bundle artifact (which on long-running branches accumulates packed git objects). The existing patch-path behavior is unchanged. Test updated to assert diff_size is set on the auto-switched entry.