43 lines
2.3 KiB
Markdown
43 lines
2.3 KiB
Markdown
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# ADR-008: First-Parent Ordering in Reconciliation Merges
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**Status:** Accepted
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**Date:** 2026-02
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## Context
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Josh-proxy uses **first-parent traversal** when mapping subrepo history back to the monorepo. When you push a commit through josh-proxy, josh walks the first-parent chain to find a commit it can map to a monorepo commit. If the first parent leads to unmappable history, josh cannot reconstruct the monorepo-side branch correctly.
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This became critical when the reconciliation merge (ADR-007) initially had the wrong parent order: old subrepo history as parent 1, josh-filtered as parent 2. Josh followed parent 1, couldn't find any mappable commit, and created a monorepo branch containing only the subrepo subfolder content — effectively deleting 1280 files from the rest of the monorepo.
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## Decision
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In reconciliation merge commits, the josh-filtered HEAD **must be parent 1** (first parent). The old subrepo HEAD is parent 2.
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```bash
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git commit-tree "$josh_tree" \
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-p "$josh_head" \ # parent 1: josh-filtered — josh follows this
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-p "$subrepo_head" \ # parent 2: old history — side branch, ignored by josh
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-m "..."
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```
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### Why this is safe
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- The old subrepo HEAD (`subrepo_head`) is still an ancestor of the merge commit regardless of parent order — push succeeds either way
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- `--ancestry-path` in reverse sync still follows `B → M → C` regardless of parent order (it traces all paths, not just first-parent)
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- Josh follows first-parent and finds the josh-filtered commit, which maps cleanly back to the monorepo
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## Consequences
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**Positive:**
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- Josh can map the reconciliation merge back to the monorepo correctly
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- Reverse sync through josh produces correct diffs (only subrepo-scoped changes)
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- `git log --first-parent` on the subrepo shows the clean josh-filtered lineage
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**Negative:**
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- This is a subtle invariant — future changes to merge commit creation must preserve parent order
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- The constraint is undocumented in josh-proxy's own documentation (discovered empirically)
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- No automated test can verify this without a running josh-proxy instance
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**Lesson learned:**
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Parent order in `git commit-tree -p` is not cosmetic. For tools that rely on first-parent traversal (josh-proxy, `git log --first-parent`), parent 1 must be the "mainline" that the tool should follow.
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