fix(export): send HTTP headers before serializing large exports

[TimeToBuildBob]

Why

HTTP JSON exports still wait until serialization finishes before sending headers (#677 writes a tempfile first). A ~500k-event export then hits the web UI's 30s axios timeout and looks like a no-op. Same on desktop.

ActivityWatch/aw-android#228

Companion PRs:

• fix(export): stream large bucket exports without JSON.parse aw-webui#993
• fix(export): stream large JSON downloads to a cache file aw-android#304

Change

• 404 missing buckets and pick the download filename up front
• Return 200 + Content-Disposition immediately
• Serialize into an anonymous pipe while the client reads the body

Mid-stream failures truncate the download instead of hanging the connection. Event buffering stays bounded (same serializer as #677).

Tests

cargo test -p aw-server --test api export — existing round-trip/header tests plus empty all-buckets filename.

[TimeToBuildBob]

Review guide (auto-posted)

Structured warm-up for reviewers — what changed, what to run, where to look.

Key files

• aw-server/src/endpoints/util.rs (+75/-21, Δ96)
• aw-server/tests/api.rs (+18/-0, Δ18)
• Cargo.lock (+0/-1, Δ1)
• aw-server/Cargo.toml (+0/-1, Δ1)

Suggested verification

• Run the repo's usual CI-equivalent checks locally

Known risks / watch points

• No automatic high-risk tags; use file list + diff for judgment.

Suggested review focus

• Confirm behavior matches the PR description acceptance criteria.
• Skim the largest diffs first (listed above).
• If CI is green, spot-check the highest-risk paths called out here.

Generated by scripts/github/pr-warmup-review-guide.py for #721.

[greptile-apps]

Confidence Score: 5/5

The PR appears safe to merge; the prior datastore-worker blocking defect is fixed and no new actionable failure remains.

Findings

1.  Slow clients block datastore ▶
2.  Performance change lacks measurement ▶

Summary

This PR changes JSON exports so HTTP success headers and the download filename are determined before serialization begins.

• Validates requested buckets before constructing the response, preserving a 404 for missing buckets.
• Serializes exports into a private tempfile on the datastore worker, then copies the staged file to the response pipe from a separate thread.
• Adds coverage for empty all-bucket exports and verifies that an unread response body no longer blocks datastore operations.
• The earlier datastore-blocking finding is fully addressed: client-paced writes no longer occur on the shared datastore worker.
• TimeToBuildBob declined the performance-measurement finding because this addresses a reported timeout and incorrect response timing rather than a speculative optimization; they accepted that concern with this rationale.

Diagram

sequenceDiagram
    participant Client
    participant Endpoint
    participant ExportThread
    participant DatastoreWorker
    participant Tempfile

    Client->>Endpoint: GET export
    Endpoint->>Endpoint: Validate bucket and choose filename
    Endpoint-->>Client: 200 + Content-Disposition headers
    Endpoint->>ExportThread: Spawn export stream
    ExportThread->>Tempfile: Create private staging file
    ExportThread->>DatastoreWorker: Export JSON into staging file
    DatastoreWorker->>Tempfile: Serialize export
    DatastoreWorker-->>ExportThread: Completed file
    ExportThread->>Tempfile: Rewind
    ExportThread-->>Client: Copy staged JSON through pipe

Loading

_{Reviews (2) · Last reviewed commit: "fix(export): don't let a slow client sta..."}

[codecov]

Codecov Report

❌ Patch coverage is 92.22222% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 79.85%. Comparing base (656f3c9) to head (e6e245f).
⚠️ Report is 134 commits behind head on master.

Files with missing lines	Patch %	Lines
aw-server/src/endpoints/util.rs	82.50%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #721      +/-   ##
==========================================
+ Coverage   70.81%   79.85%   +9.03%     
==========================================
  Files          51       75      +24     
  Lines        2916     8404    +5488     
==========================================
+ Hits         2065     6711    +4646     
- Misses        851     1693     +842     

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[TimeToBuildBob]

@greptileai review

[TimeToBuildBob]

CI-green and mergeable (Greptile 5/5) — waiting only on a maintainer click.

This PR is ready to merge, but the bot has pull-only access to this repo and can't self-merge — surfacing it here so it isn't lost. The monitoring loop will stop re-flagging it now that this note is posted.