fix(stdio): short-circuit semantic_search when index is being rebuilt

[aeneasr]

Summary

• When a background indexer holds the index flock, `ensureIndexed` sets `out.StaleWarning` and returns, but `handleSemanticSearch` still called `embedQuery`. On a single-instance embedding backend (e.g. LM Studio) saturated by the indexer's 32-chunk batches, the query embed queues behind those batches and the MCP call hangs well past Claude Code's timeout.
• Fix: short-circuit `handleSemanticSearch` when `StaleWarning` is set, returning the warning text immediately. The warning already instructs the caller to use standard tools for the next 10 tool calls before retrying, so doing the embed work would be pure waste — and on a busy backend, it hangs.
• Covers all four `StaleWarning`-setting paths in `ensureIndexed`: flock held by another process, in-process goroutine already running, background goroutine couldn't acquire the flock, and the 15s timeout firing.

Reproduction

Two parallel `semantic_search` MCP calls into different subdirectories of a monorepo (`/cloud/kratos` and `/cloud/keto`) while a background `lumen index /cloud` holds the shared-effective-root flock.

	Before fix	After fix
Parallel kratos+keto `tools/call`	timed out at 100s, no responses	id:2 at 1.375s, id:3 at 1.389s
Regression unit test	n/a	PASS in ~0.7s
Sanity revert of fix	n/a	unit test FAILs in 3.67s

Test plan

• `go test ./cmd -run TestHandleSemanticSearch_StaleWarningShortCircuits` passes
• Revert the short-circuit and confirm the regression test fails (catches the bug)
• End-to-end repro with parallel MCP `tools/call` against a held-flock monorepo returns both responses in <2s instead of hanging
• `go vet ./...` clean
• `golangci-lint run` clean

🤖 Generated with Claude Code

Summary by CodeRabbit

• Bug Fixes
  • Search query handling improved during index rebuilding. When the index is being rebuilt by background processes, searches now immediately return a stale-data warning instead of attempting to process the query, preventing unnecessary processing overhead and resource usage during this maintenance window.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro Plus

Run ID: 4b214827-6cd0-41fe-ada0-953931c80f24

📥 Commits

Reviewing files that changed from the base of the PR and between e310ed0 and d24beed.

📒 Files selected for processing (2)

• cmd/stdio.go
• cmd/stdio_concurrency_test.go

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📝 Walkthrough

Walkthrough

This PR adds a short-circuit optimization to handleSemanticSearch that returns early with a formatted warning when the search index is stale (being rebuilt), avoiding unnecessary query embedding and search execution. A concurrency regression test validates this behavior under lock contention scenarios.

Changes

Stale Warning Short-Circuit

Layer / File(s)	Summary
handleSemanticSearch stale-warning short-circuit cmd/stdio.go	When out.StaleWarning is set, handleSemanticSearch returns immediately with formatted results and the warning text instead of embedding the query and executing the search.
Regression test for stale-warning short-circuit cmd/stdio_concurrency_test.go	New imports and TestHandleSemanticSearch_StaleWarningShortCircuits verify the handler returns quickly with the stale-warning message when the index lock is held externally, and that the embedder is never called.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately describes the main change: adding short-circuit logic to semantic_search when the index is being rebuilt, which directly addresses the core fix implemented in the changeset.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix-hanging

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