Keep file chooser responsive during stalled filesystem reads

[nerdislb]

Summary

• Keep chooser navigation responsive when a read-only listing backend stalls on FUSE/network I/O.
• Replace and reap only chooser-owned listing workers; discard obsolete replies and keep selection validation separate.
• Persist the portal reply before shutting down both read-only children, including failed-start/cancellation races. Normal file-manager write draining is unchanged.

Verification

• Six real Quickshell process lifecycle cases, including blocked listing/validation, stale output, cancellation and failed startup; controlled signal-order regression fails with the previous guard.
• 68 picker JavaScript checks pass.
• Actual release binary + complete picker UI selects and accepts a local fixture.
• Native desktop acceptance: Space/Enter selection, Backspace navigation away from a controlled stalled reader, Escape cancellation, and window-manager close all produce the expected portal reply and reap owned helpers.
• Small warm local navigation sample: replacement median 3 ms versus reused-worker median 0.5 ms (10 samples each, 1 ms clock resolution); not a general performance benchmark.
• Independent Claude Fable review; startup race corrected and re-reviewed.

Scope / baseline

Independent of #172 and based on current main. No production mount restart or configuration changes required. SIGKILL cannot guarantee completion for genuinely uninterruptible kernel I/O.

Release build passes. The file-budget check passes on the actual main baseline. No Rust source changes. The existing signal-arity gate fails on both base and candidate after the prior Messages.js extraction; no full-suite-green claim.

The local tracking ref initially lagged because this checkout fetched only a tag. This branch now explicitly includes actual main (f738261); the PR diff remains limited to ten picker files.

Summary by CodeRabbit

• Bug Fixes
  • Improved file chooser reliability when directory listings stall or return incomplete data.
  • Newer navigation requests now supersede outdated requests, helping the chooser recover without displaying stale results.
  • Improved cancellation and shutdown handling, including failed or interrupted background operations.
  • Prevented file-manager write activity during read-only chooser listing operations.
• Tests
  • Added coverage for stalled listings, navigation changes, cancellation, missing resources, and shutdown ordering.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Advanced

Run ID: cbc85759-3c68-4d60-bd3a-ee7249a17db1

📥 Commits

Reviewing files that changed from the base of the PR and between c8231b4 and 7d49c70.

📒 Files selected for processing (3)

• tests/picker-stall.sh
• ui/Backend.qml
• ui/qmldir

Included review availability: Your plan provides up to 4 included reviews per hour; 1 remains after this review.

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📝 Walkthrough

Walkthrough

The picker now uses a dedicated listing worker, a picker-only backend mode, and coordinated shutdown. Integration tests cover stalled reads, request supersession, cancellation, failed starts, and process cleanup.

Changes

Picker worker lifecycle

Layer / File(s)	Summary
Dedicated listing worker and routing ui/PickerListing.qml, ui/picker.qml, ui/qmldir	Listing requests use a separate worker. Obsolete workers are terminated, responses are parsed, and current messages are forwarded to the picker.
Picker-only backend and coordinated shutdown ui/Backend.qml, ui/PickerLifecycle.qml, ui/picker.qml	The chooser backend accepts only picker and format requests. Shutdown waits for both read processes after reply persistence.
Stall and cancellation validation tests/picker-stall-helper.py, tests/picker-stall.qml, tests/picker-stall.sh, tests/run-all.sh, AGENTS.md	Tests cover blocked reads, navigation supersession, cancellation, failed starts, process reaping, and test registration. Documentation describes the new behavior.

Priority: ➖ Normal

Estimated code review effort: 3 (Moderate) | ~25 minutes

Change: Bug fix

Sequence Diagram(s)

sequenceDiagram
  participant PickerList
  participant PickerListing
  participant FleaBackend as flea --backend
  participant PickerLifecycle
  participant Backend
  PickerList->>PickerListing: request listing
  PickerListing->>FleaBackend: run listing request
  FleaBackend-->>PickerListing: return JSON response
  PickerListing-->>PickerList: emit listing message
  PickerLifecycle->>PickerListing: quit
  PickerLifecycle->>Backend: quit
  PickerListing-->>PickerLifecycle: quitReady
  Backend-->>PickerLifecycle: quitReady

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Suggested reviewers: thisisgm

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 1 functions across 3 files. (2 skipped: 2 …	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
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.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly summarizes the primary change: keeping the file chooser responsive when filesystem reads stall.

Full details: Docstring Coverage

Explanation

Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 1 functions across 3 files. (2 skipped: 2 unsupported.)

• Fix all pre-merge checks with AI

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create a new PR

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[thisisgm]

Merged in aba90892 (v0.3.2): the chooser stays responsive through a stalled listing, and Cancel completes even when its backend never starts.