migrate diff silently ignores repeated --field paths

[pablontiv]

[rootline-team/investigador] 2026-09-08T18:24:25Z

Problem and impact

migrate diff output accepts repeated --field flags but silently drops every path after the first, in both single-stem and batch mode. An invalid second path is also ignored: the command emits a plausible partial result and exits 0. Consumers cannot rely on the documented ordered-array or all-paths-must-resolve contract.

This is a reproduced residual bug, not a new selector design or a regression attributed to the most recent merge.

Accepted contract and dependencies

• PR #132, merged, selected one bare value for one path, a JSON array in flag order for multiple paths, and failure without partial output if any path cannot resolve.
• Its Known residual explicitly left the two migrate.go call sites untouched to avoid overlapping work on Bare os.WriteFile in migrate, init and templates can leave truncated .stem files #106, and requested a follow-up after Bare os.WriteFile in migrate, init and templates can leave truncated .stem files #106 landed.
• Bare os.WriteFile in migrate, init and templates can leave truncated .stem files #106 is now closed; current migrate.go uses fsx.WriteFileAtomic. No concurrent implementation PR touches this work: the only open PR at triage was chore(deps): bump github/codeql-action/upload-sarif from 4.37.7 to 4.37.9 #219 (Scorecard pin).
• docs/output.md publishes repeatability globally. CLAUDE.md still records this specific residual.

Priority: P2 — deterministic incorrect machine-readable output and hidden input errors, without observed writes or data loss. No unresolved implementation dependency.

Evidence and root cause

Examined and built current master 514e42823275222b6e14cd99b4a354661b4a139c.

• cmd/rootline/migrate.go: renderMigrateBatch and renderMigrateJSON call extractField(data, fieldPath[0]) (lines 255 and 270).
• cmd/rootline/validate.go: the working shared outputJSON uses effectiveFieldPaths() and extractFields, resolving all paths before emitting.
• migrate --rename and --scaffold already call the shared writer; the bounded defect is in diff rendering.

Runtime provenance: Linux/x86_64; freshly downloaded official Go 1.27.1, compatible with go.mod (Go 1.26). Archive SHA-256 matched current go.dev metadata: 63d339f0da5ab53635a56f2490a7984dfe12dfcff22ad749f63edaf590168445.

Build:

go build -mod=readonly -ldflags '-X main.version=dev' -o /absolute/rootline-repro ./cmd/rootline

Binary reported rootline version dev; SHA-256: be77e9658696380d340a3aa561a0b2f55b71e8afa1fbedd0efb0eacfac099edc. The checkout remained clean.

Minimal reproduction

Create these files in a disposable directory outside any Git repository.

Both single/.stem and batch/.stem:

version: 2
root: true
scope:
  match: "*.md"
schema:
  status:
    type: string

batch/child/.stem:

version: 2
schema:
  note:
    type: string

Run the fresh binary as an external process:

rootline-repro migrate single/.stem --from single/.stem
rootline-repro migrate single/.stem --from single/.stem --field kind
rootline-repro migrate single/.stem --from single/.stem --field kind --field version
rootline-repro migrate single/.stem --from single/.stem --field version --field kind
rootline-repro migrate single/.stem --from single/.stem --field kind --field nonexistent
rootline-repro migrate single/.stem --from single/.stem --field nonexistent
rootline-repro migrate batch
rootline-repro migrate batch --field kind --field version
rootline-repro migrate batch --field kind --field nonexistent
rootline-repro describe single --field kind --field version

The single-stem case explicitly compares a valid file against itself, so it requires neither Git nor a previous revision. For batch, absence of Git exercises the existing new-schema fallback; full output first confirmed two results and summary.stems_checked == 2. That fallback is not being changed.

Actual results

Case	Expected	Observed
Single, no projection	Existing v1 diff envelope, no changes	Correct; exit 0
Single, --field kind	"rootline/migrate-diff"	Correct; exit 0
Single, kind then version	["rootline/migrate-diff",1]	"rootline/migrate-diff"; exit 0
Single, version then kind	[1,"rootline/migrate-diff"]	1; exit 0
Single, valid then nonexistent	Nonzero; diagnostic; no partial stdout	"rootline/migrate-diff"; exit 0; empty stderr
Single, nonexistent alone	Nonzero; diagnostic; no stdout	Exit 1; Error: field "nonexistent": key "nonexistent" not found
Batch, kind then version	["rootline/migrate-batch",1]	"rootline/migrate-batch"; exit 0
Batch, valid then nonexistent	Nonzero; diagnostic; no partial stdout	"rootline/migrate-batch"; exit 0; empty stderr
Working control: describe, kind then version	["rootline/describe",1]	Correct; exit 0

All valid-path invocations had empty stderr. All three fixture files retained their bytes, mode 0644 and modification times throughout.

Scope and acceptance

Implement the already-selected #132 projection contract in both migrate diff renderers:

• No projection retains the current single/batch envelope, kind/version, values and newline behavior.
• One effective path retains the bare JSON value.
• Two or more effective paths produce one JSON array in flag order, including reversed-order and repeated-identical-path cases.
• A missing path in any position fails nonzero with a diagnostic and no partial stdout, for single and batch rendering.
• Reuse the shared effective-path/extraction behavior, including existing empty-path handling; do not invent a second selector implementation.
• Keep unsupported-format rejection and table output unchanged.
• Exercise the command boundary with valid single-stem and multi-stem fixtures, and verify the read-only calls do not write.
• Update the stale CLAUDE.md residual and relevant migration/output documentation or distributed reference if affected.

Exclusions: no migration diff algorithm, Git fallback, schema grammar, file mutation, atomicity, rename/scaffold/split semantics, new format or envelope-version changes. Do not reopen the other sub-defects of #63.

Compatibility note: callers relying on the known broken multi-field behavior will see an array instead of the first scalar, and an invalid later path will now fail. This is the specifically deferred application of #132's accepted contract, not authority to introduce other compatibility changes. Document the correction and follow the repository's Conventional Commit/release conventions.

Verification handoff and deduplication

Add focused command tests that fail before the correction; run current repository quality gates and leave independent QA to execute the above flows against the PR SHA. Researcher ran the build and the ten external-process cases above, not the full test/coverage/lint suite, and did not emit a QA verdict or modify production code.

Searched open and closed issues/PRs for migrate field, repeatable, and fieldPath; found #63 and its merged implementation #132 but no dedicated residual ticket. This ticket tracks the explicitly deferred follow-up rather than reopening the completed broad issue. #190 and #194 retain their needs-info decisions.

[pablontiv]

[rootline-team/desarrollador] 2026-09-08T20:44:20Z

Estado: implementación completa publicada en el PR borrador #250 desde rootline-team/issue-249.

SHA: 6fa8e2e46d4bc5a30c8ddad1bb939a656631a569.

Se aplicó por TDD el contrato ya aceptado de proyección múltiple en ambos renderizadores de migrate diff. Las verificaciones locales completas y la reproducción con binario real sin Git están documentadas en el PR. CI #864 terminó correctamente para el SHA exacto, incluidos build/race/cobertura, tidy, lint, vulnerabilidades, gitleaks, docs e instaladores Linux/macOS/Windows.

Pendientes únicos: QA independiente del SHA actual y revisión técnica. No queda implementación conocida por completar.

PR: #250
CI: https://github.com/pablontiv/rootline/actions/runs/34276421176