galaxy/specs/APP-4218/TECH.md

APP-4218: Git operations dialogs compare against the branch's actual parent — Tech Spec

Product spec: specs/APP-4218/PRODUCT.md

Context

Today the Push / Publish dialog, the Create PR dialog, and their AI helpers hard-code the repo's main branch as the comparison base whenever the current branch has no upstream. On a branch-off-a-branch, every commit inherited from the parent branch shows up as "included" in the push / PR. All of the offending code paths already route their base through one function call: detect_main_branch. The fix is to introduce a detect_parent_branch helper that returns the closest-ancestor branch (falling back to main), and swap the four callers that currently say detect_main_branch to say detect_parent_branch. No function signatures change. Relevant code:

• app/src/util/git.rs:316-359 — get_unpushed_commits: git log @{u}..HEAD with main..HEAD fallback.
• app/src/util/git.rs:602-634 — get_branch_diff_entries: main..<end>.
• app/src/util/git.rs:743-766 — get_diff_for_pr: main..<end>, feeds AI.
• app/src/util/git.rs:775-784 — get_branch_commit_messages: main..HEAD, feeds AI.
• app/src/util/git.rs:803-826 — create_pr: invokes gh pr create without --base.
• app/src/code_review/git_dialog/{push,pr}.rs — per-dialog state, unchanged in shape. The detected parent is used transparently through the four util helpers.

Proposed changes

1. detect_parent_branch

// app/src/util/git.rs
/// Returns the closest-ancestor branch of `HEAD`, or the main branch when
/// no candidate qualifies. Ties prefer the detected main branch, then local
/// over `origin/*`, then alphabetical for determinism.
pub async fn detect_parent_branch(repo_path: &Path) -> Result<String>;

Implementation (all upfront queries run in parallel via futures::join!):

1. git for-each-ref --merged HEAD --format='%(objectname) %(refname:short)' refs/heads refs/remotes to list ancestor refs with their commit SHAs. --merged HEAD filters out non-ancestors at the git level, avoiding per-candidate subprocess spawns.
2. git log HEAD --format=%H to walk HEAD's history once. A HashMap<&str, usize> of sha → position gives each candidate's distance from HEAD in O(1) lookups.
3. Resolve the actual upstream via git rev-parse --abbrev-ref --symbolic-full-name @{u} and exclude it (plus the current branch name) from candidates. Handles non-origin upstream configurations.
4. Rank candidates by (distance, !is_main, !is_local, name). Log the winner at debug.
5. If no candidate qualified, return detect_main_branch(repo_path). Return type is a plain String — either a local branch (feature-a) or a remote-tracking ref (origin/feature-a).

2. Swap detect_main_branch for detect_parent_branch inside the four helpers

No caller / signature changes. Each helper keeps its current shape; only the internal base-branch lookup changes:

• get_unpushed_commits: the no-upstream fallback branch becomes detect_parent_branch instead of detect_main_branch. The primary @{u}..HEAD path is unchanged (when an upstream exists, it's still the most accurate "what will be pushed"). When upstream is unset, the fallback now uses the closest ancestor.
• get_branch_diff_entries: let base = detect_parent_branch(repo_path).await?; in place of the current detect_main_branch call. The {base}..{end_ref} range logic is unchanged.
• get_diff_for_pr: same one-line swap.
• get_branch_commit_messages: same one-line swap.

3. create_pr passes --base

create_pr internally calls detect_parent_branch, strips any origin/ prefix, and passes --base <parent> to gh pr create. Signature unchanged:

pub async fn create_pr(
    repo_path: &Path,
    title: Option<&str>,
    body: Option<&str>,
) -> Result<PrInfo> {
    let base = detect_parent_branch(repo_path).await?;
    let base = base.strip_prefix("origin/").unwrap_or(&base).to_string();
    // ...existing gh pr create invocation, plus --base <base>...
}

If detection errors, propagate the error — the caller's existing user_facing_git_error path shows the generic failure toast.

4. Dialogs

No dialog-level plumbing. The four util helpers (get_unpushed_commits, get_branch_diff_entries, get_diff_for_pr, get_branch_commit_messages) already feed the Push and Create-PR dialogs; swapping them to detect_parent_branch internally is enough. The Commit dialog's CommitAndCreatePr chain inherits the fix via create_pr (§3). Surfacing the detected parent in the dialog chrome (e.g. a "Based on" row) was evaluated and dropped for now — it added visible latency waiting on detection to resolve, with limited user value. Tracked as a follow-up.

5. Feature flag gating

All changes live under FeatureFlag::GitOperationsInCodeReview (already gating the dialogs); no new flag.

Risks and mitigations

Heuristic picks the wrong branch

Two branches pointing at the same commit, deleted historical parents, etc. The parent isn't visible in the UI right now, so bad detections only manifest as a wrong commit list / wrong PR base. A follow-up can surface the parent or add a per-branch override.

Cost of repeated detection

detect_parent_branch runs inside each of the four helpers on every dialog open (and once more in create_pr). Each detection is 2–4 parallel subprocess calls (for-each-ref --merged HEAD, log HEAD --format=%H, rev-parse @{u}, detect_main_branch) regardless of branch count — typically sub-100ms even in repos with thousands of remote-tracking refs. For the common PR-create flow, that's ~4× the cost on top of the AI call, which is already the dominant latency. Acceptable. If large-repo latency becomes measurable, cache per-repo inside detect_parent_branch itself (follow-up).

PR targets an unpushed base

If the parent is a local-only branch, gh pr create --base <b> fails. Surfaces via the generic "Git operation failed." toast; we do not silently retry without --base.

Backwards compatibility

Fresh-feature-off-main shapes resolve to main, so today's behavior is preserved on the common path.

Testing and validation

References below are to specs/APP-4218/PRODUCT.md success criteria.

Manual validation

• feature-a (pushed), git checkout -b feature-b feature-a, 1 new commit, no push: Publish dialog shows 1 commit (SC 1). Create PR shows only those files; confirming runs gh pr create --base feature-a and the PR targets feature-a (SC 2).
• Fresh branch off main, no upstream: dialog shows main..HEAD (SC 3).
• Rebase feature-b onto main, reopen dialog: commits list reflects the rebased range (SC 4).
• Commit-and-create-PR on feature-b: PR targets feature-a (SC 6).
• Change the pane's diff-mode dropdown; reopen dialogs: previews are unchanged (SC 7).

Integration / screenshot coverage

None added. The git_dialog module ships without an integration harness today (see specs/APP-4125/TECH.md).

Follow-ups

• Surface the detected parent in the dialog chrome (a "Based on" row) once we have a way to populate it without visible latency — e.g. caching it on DiffMetadata so it's ready by the time the dialog opens.
• Per-branch override for mis-detected parents (stored in .git/config as branch.<name>.warpParent).
• Per-repo caching inside detect_parent_branch if repeated detection shows up in profiles.