Use target branch tip as base for new independent branches by mtsgrd · Pull Request #13626 · gitbutlerapp/gitbutler

mtsgrd · 2026-05-05T00:18:57Z

Problem

When creating a new independent branch, we used the workspace's lower bound — the lowest common ancestor of all stacks. If stacks have different merge bases with the target, this places the new branch on an unnecessarily old commit:

                    target tip
                      ↓
M1 ← M2 ← M3 ← M4   (origin/main)
      ↑    ↑
  Stack A  Stack B

lower_bound = M2   ← new branch landed here (old!)

Solution

Use the stored target commit (target.sha) as the base for new independent branches. If unavailable, fall back to the target ref tip, then to the workspace lower bound (for local-only workflows without a target).

new branch base = M4   ← now uses target commit ✓

Key changes

but-graph/.../workspace/api.rs
- resolved_target_commit_id() — returns the stored target_commit ID, falling back to the target_ref segment tip
- target_segment_index() — extracted shared helper to deduplicate target resolution with merge_base_with_target_branch()
but-workspace/.../create_reference.rs — use resolved_target_commit_id().or(lower_bound) instead of just lower_bound

Test plan

Five tests in resolved_target_commit_id.rs:

Two stacks with different bases — returns the target tip
One stack above the target — still returns the target tip
target_commit preferred over target_ref when both are set
No target configured — returns None
Only extra_target set — returns None (not sufficient)

Copilot

Pull request overview

This PR adjusts how “independent” branches are based in a workspace by preferring the newest (closest-to-target) merge base among all stacks, instead of always using the workspace lower bound. This better aligns new independent branches with the most recent common history when stacks diverge at different points from the target.

Changes:

Added Workspace::newest_base_among_stacks() to compute a newer merge base across stacks vs. the target.
Updated independent-branch creation to prefer newest_base_among_stacks() and fall back to lower_bound.
Added workspace tests covering expected “newest base” selection and target configuration edge cases.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
crates/but-workspace/src/branch/create_reference.rs	Uses the new workspace API to choose a more recent base for independent branches.
crates/but-graph/src/projection/workspace/api.rs	Introduces `newest_base_among_stacks()` to compute a newer merge base among stacks and the target.
crates/but-graph/tests/graph/workspace/newest_base_among_stacks.rs	Adds tests validating newest-base selection and behavior without a configured target.
crates/but-graph/tests/graph/workspace/mod.rs	Registers the new test module.

Caleb-T-Owens

https://discord.com/channels/1060193121130000425/1501294321897312426

Copilot

Pull request overview

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

Copilot

Pull request overview

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

Copilot

Pull request overview

Copilot reviewed 5 out of 5 changed files in this pull request and generated 3 comments.

Byron · 2026-05-07T00:43:09Z

@mtsgrd This PR reads like it's trying to optimize the location of newly created refs, such that more recent locations are preferred. While this makes sense, I don't know where this is coming from:

Creating an independent branch without any target is now an error.

In theory, that independent branch can still be created on the lower bound of the workspace. Needing a target, always, goes against supporting all states that Git can legitimately be in, like local-only workflows.

Maybe it's just the description that is somewhat stale, I didn't look at the implementation in detail.

Copilot

Pull request overview

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

Previously, new independent branches were based on the workspace's lower bound (the lowest common ancestor of all stacks). When stacks have different merge bases with the target, this placed the new branch on an unnecessarily old commit. Now we prefer the stored target commit (target.sha) via a new `Workspace::target_tip_commit_id()` helper, which checks `target_commit` first, then `target_ref`. Falls back to the workspace lower bound for local-only workflows without a target. Also extracts `target_segment_index()` to deduplicate the target resolution logic shared with `merge_base_with_target_branch()`. Tested with four cases in `target_tip_commit_id.rs`: - Two stacks with different bases — returns the target tip - One stack above the target — still returns the target tip - No target configured — returns None - Only extra_target set — returns None (not sufficient)

mtsgrd · 2026-05-07T10:11:57Z

@Byron could you have another look? I'd like to get this merged asap so we can build a nightly. 🙏

Copilot

Pull request overview

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

Byron · 2026-05-08T01:01:19Z

@Byron could you have another look? I'd like to get this merged asap so we can build a nightly. 🙏

I think we are past that, I am many PRs behind.

github-actions Bot added the rust Pull requests that update Rust code label May 5, 2026

mtsgrd force-pushed the newest-base-for-new-branches branch 3 times, most recently from dcef811 to 31a10a1 Compare May 5, 2026 00:34

mtsgrd marked this pull request as ready for review May 5, 2026 00:53

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Comment thread crates/but-graph/src/projection/workspace/api.rs Outdated

Comment thread crates/but-graph/tests/graph/workspace/newest_base_among_stacks.rs Outdated

mtsgrd force-pushed the newest-base-for-new-branches branch from 31a10a1 to 0590302 Compare May 5, 2026 17:52

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mtsgrd changed the title ~~Use newest stack merge base when creating independent branches~~ Use target branch tip as base for new independent branches May 5, 2026

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mtsgrd force-pushed the newest-base-for-new-branches branch from 6e87b22 to b6dd81e Compare May 6, 2026 06:41

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Comment thread crates/but-graph/src/projection/workspace/api.rs

mtsgrd force-pushed the newest-base-for-new-branches branch 2 times, most recently from 6cac6cd to 4718902 Compare May 6, 2026 12:21

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Comment thread crates/but-graph/src/projection/workspace/api.rs

Comment thread crates/but-graph/src/projection/workspace/api.rs

Comment thread crates/but-graph/tests/graph/workspace/resolved_target_commit_id.rs

mtsgrd force-pushed the newest-base-for-new-branches branch 2 times, most recently from 620682c to b9be514 Compare May 6, 2026 13:54

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Comment thread crates/but-graph/tests/graph/workspace/resolved_target_commit_id.rs

mtsgrd force-pushed the newest-base-for-new-branches branch from 12d229a to c3789dd Compare May 7, 2026 09:47

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Comment thread crates/but-graph/src/projection/workspace/api.rs

Comment thread crates/but-workspace/src/branch/create_reference.rs

Caleb-T-Owens approved these changes May 7, 2026

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mtsgrd merged commit b8de772 into master May 7, 2026
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mtsgrd deleted the newest-base-for-new-branches branch May 7, 2026 10:44

This was referenced May 15, 2026

Simpler Workspace API discovery to help with legacy-code updates #13821

Merged

follow-up on but-graph updates #13858

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