fix(memory): join an in-flight fetch instead of starting a duplicate - #503
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📝 WalkthroughWalkthrough
ChangesBackground fetch deduplication
Estimated code review effort: 4 (Complex) | ~45 minutes Merge Risk: ⚪ Minimal · up to The change joins duplicate in-flight fetches while preserving callback and cancellation behavior. No actionable merge-blocking risk remains; a localized test improvement for cancellation-time failures can follow up separately. Sequence Diagram(s)sequenceDiagram
participant Caller
participant BackgroundDataService
participant FetchWorker
participant Cache
participant Callbacks
Caller->>BackgroundDataService: submit fetch for cache_key
BackgroundDataService->>FetchWorker: start or join active request
Caller->>BackgroundDataService: submit same cache_key
BackgroundDataService-->>Caller: return shared request_id
FetchWorker->>BackgroundDataService: report response
BackgroundDataService->>Cache: commit response after commit authorization
BackgroundDataService->>Callbacks: invoke callbacks when completion is not cancelled
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✅ Passed checks (4 passed)
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Up to standards ✅🟢 Issues
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| Metric | Results |
|---|---|
| Complexity | 16 |
| Duplication | 0 |
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Inline comments:
In `@src/background_data_service.py`:
- Around line 489-493: Update cancel_request and _fetch_data_worker to propagate
cancellation and prevent a cancelled worker from writing to the cache or
invoking callbacks, keeping the in-flight key protected until those side effects
are suppressed. Add a regression test that cancels a blocked request, submits a
replacement for the same cache key, and verifies the cancelled worker cannot
overwrite the replacement result.
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src/background_data_service.pytest/test_background_fetch_dedupe.py
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Inline comments:
In `@src/background_data_service.py`:
- Around line 316-337: Make cancellation terminal across the request worker and
cancel_request flow: prevent queued work from changing CANCELLED back to
IN_PROGRESS, and serialize the final cancellation check with cache commit,
status transition, and in-flight cleanup. Preserve CANCELLED on HTTP failure and
suppress callbacks for cancelled requests. Add deterministic tests covering
cancellation before worker start, during finalization, and during HTTP failure.
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src/background_data_service.pytest/test_background_fetch_dedupe.py
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Inline comments:
In `@test/test_background_fetch_dedupe.py`:
- Around line 399-415: Update test_a_failure_after_cancelling_stays_cancelled so
the worker begins the HTTP call before cancellation: block the session request,
cancel the in-flight request, then release the blocking session to raise the
configured requests.RequestException. Keep the assertions verifying cancellation
remains effective and no spurious failure callback is delivered.
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src/background_data_service.pytest/test_background_fetch_dedupe.py
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submit_fetch_request() had no notion of "already fetching this". request_id
embeds a millisecond timestamp, so every submit looked new, and
active_requests is keyed by that id rather than by what is being fetched.
Two submits for the same cache_key therefore started two identical fetches.
It is not a rare race. _fetch_data in the sports managers branches: the Live
manager fetches only today's games, but Recent and Upcoming both pull the
full season schedule under the SAME cache_key. On a cache miss both miss,
both submit, and nothing stops the second. On a running 512x64 board:
138 background fetches in 24 hours, arriving in pairs at identical
millisecond timestamps, roughly hourly:
2 2026-08-25 11:47:26.612
2 2026-08-25 10:46:28.064
2 2026-08-25 08:01:55.962
Half of them redundant. Each duplicate costs a second download, a second
JSON parse -- the expensive part on a Pi -- and a second parsed copy
resident at the same time. Schedules on that board run 256KB to 20MB, 106MB
across all sports. Because the pairs land in the same millisecond they also
occupy two of the three executor slots with identical work, which is what
makes two large parses peak simultaneously.
A submit for a cache_key already in flight now joins that request: its
callback is added to the existing one and the existing request_id is
returned, so get_result() works for both. Different keys are untouched, and
dedupe applies only while a fetch is in flight -- a submit after completion
fetches again, because this is not a second cache layer.
Three details:
- The in-flight entry is dropped and the callback list snapshotted in the
SAME critical section as filing the result. Otherwise a submitter could
join a fetch whose callbacks had already run and never be called back.
- Cancellation is the other way a request leaves active_requests, so it
releases the key too. And the join path looks the request up rather than
trusting the id, so an entry stranded any other way cannot wedge a key
permanently -- it is dropped and a fresh fetch starts.
- One callback raising no longer prevents the others being delivered.
Previously there was only ever one.
Interaction with #499, whichever merges second: that PR releases the payload
after the callback runs. With several callbacks the release must happen
after ALL of them, and must not happen at all if a joined submitter passed
no callback, since polling get_result() would then be its only delivery
path. The callback list built here is the hook for that.
test_background_fetch_dedupe.py -- 8 tests, covering the join, callback
delivery to both submitters, one callback raising, distinct keys not being
coalesced, a post-completion submit fetching again, cancellation releasing
the key, a stranded entry not wedging one, and the reported count. Verified
non-vacuous by removing only the join branch: 3 fail. The callback tests
assert the ids coalesced, without which they would pass trivially on two
independent requests.
Full suite: 3698 passed, 60 skipped, 1 failure that reproduces identically
on unmodified main (test_install_lowmem, environment-dependent: /var/tmp is
disk-backed on this machine).
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01STMbQE4YctTacQXfbYqKuW
Review follow-up on the dedupe.
Cancelling releases the cache_key, so a replacement fetch for that key can
start immediately. But _fetch_data_worker() had no cancellation check: the
cancelled worker still wrote its response to the cache, flipped its own
status from CANCELLED to COMPLETED, and ran its callbacks. The stale
response could therefore land on top of the replacement's fresher data.
The worker cannot abort an HTTP call in flight, so the response is discarded
on return instead: no cache write, no callbacks, status left CANCELLED. The
check sits immediately before the cache write, which is the first
side effect.
Also fixed, found by the new test rather than by reading:
request_id was f"{sport}_{year}_{milliseconds}", which is not unique.
Two submits inside the same millisecond produced the SAME id -- the
test's two sequential fetches collided on a fast mocked response, and
one request silently replaced the other in active_requests and
completed_requests. Rare before this PR; load-bearing now, because
dedupe hands that id back to every joiner as their handle for
get_result(). A per-service counter is appended.
Two test problems of my own, both fixed here rather than left to flake:
- The cancellation test synchronised with time.sleep(0.4). A slow worker
would have made it pass for the wrong reason. It now waits for the
request to be filed in completed_requests.
- The id-uniqueness test patched session.get, but submits are async: the
50 workers outlived the patch and made real DNS calls to the dummy host.
It stubs the executor instead, which is what a submit-time test should
exercise.
20 consecutive runs of the dedupe file: 0 failures. Full suite: 3700 passed,
60 skipped, 1 failure that reproduces identically on unmodified main
(test_install_lowmem, environment-dependent).
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01STMbQE4YctTacQXfbYqKuW
Three paths wrote request.status without checking whether the request had already been cancelled, so a cancel could be silently undone and the work it was meant to stop went ahead anyway. - A request cancelled while queued had CANCELLED overwritten with IN_PROGRESS the moment its worker started, defeating the discard check entirely: it downloaded, cached and called back for work the caller had withdrawn. It now skips the fetch outright, which is also the cheapest possible cancel. - The cancelled-check and the cache write were separate critical sections, so a cancel landing between them left the payload in the cache with the callbacks suppressed -- every submitter that joined the fetch waited for a call that never came. The worker now claims the commit in the same critical section that reads the status, and cancel_request refuses once claimed. The write stays outside the lock: it serialises a multi-megabyte payload to the SD card, and holding the service lock across that would stall every submit, status query and cancel behind it. - A cancelled request that then failed was relabelled FAILED, which slipped past the CANCELLED-only callback gate and delivered a spurious error callback. The except path now leaves CANCELLED alone. get_request_status() also reported a cancelled request as FAILED, since it inferred status from result.success; the final status is now recorded on the result. Both early returns assign to `result` so completed_requests files the outcome that was reported rather than the untouched placeholder. Tests cover cancellation before worker start, during the commit, and during an HTTP failure; all three fail against the unfixed code. Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01STMbQE4YctTacQXfbYqKuW
test_a_failure_after_cancelling_stays_cancelled cancelled the request while its worker was still queued, so once the pre-start branch landed the worker returned there and never reached the exception handler the test is named for. It passed against the unfixed code only because that branch did not exist yet; with it, the test passed for the wrong reason and reverting the except-path guard did not fail it. Cancel while the worker is parked inside the HTTP call instead, and assert the fetch actually started so the test cannot silently degrade into the pre-start case again. Reverting each of the three guards now fails exactly one test. Also read the payload inside the callback rather than off the FetchResult afterwards: #499 releases result.data once delivery is done, so the later read saw the released object and not what the caller was handed. Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01STMbQE4YctTacQXfbYqKuW
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submit_fetch_request()had no notion of "already fetching this".request_idembeds a millisecond timestamp so every submit looked new, andactive_requestsis keyed by that id rather than by what is being fetched. Two submits for the samecache_keystarted two identical fetches.It isn't a rare race
_fetch_datain the sports managers branches: Live fetches only today's games, but Recent and Upcoming both pull the full season schedule under the samecache_key. On a cache miss both miss, both submit, and nothing stops the second.Measured on a running 512×64 board — 138 background fetches in 24 hours, in pairs at identical millisecond timestamps, roughly hourly:
Half redundant. Each duplicate costs a second download, a second JSON parse (the expensive part on a Pi), and a second parsed copy resident simultaneously. Schedules on that board run 256 KB to 20 MB, 106 MB across all sports.
Because the pairs land in the same millisecond they also occupy two of the three executor slots with identical work — which is what makes two large parses peak at once.
What changes
A submit for a
cache_keyalready in flight joins that request: its callback is added to the existing one and the existingrequest_idis returned, soget_result()works for both.Scope is deliberately narrow — different keys are untouched, and dedupe applies only while a fetch is in flight. A submit after completion fetches again; this is not a second cache layer.
Three details
active_requests, so it releases the key too. And the join path looks the request up rather than trusting the id, so an entry stranded any other way cannot wedge a key permanently.Interaction with #499
Whichever merges second needs a small edit. #499 releases the payload after the callback runs; with several callbacks the release must happen after all of them, and must not happen at all if a joined submitter passed no callback — polling
get_result()would then be its only delivery path. The callback list built here is the hook for that.Testing
test_background_fetch_dedupe.py, 8 tests: the join, callback delivery to both submitters, one callback raising, distinct keys not coalesced, a post-completion submit fetching again, cancellation releasing the key, a stranded entry not wedging one, and the reported count.Verified non-vacuous by removing only the join branch: 3 fail. The callback tests assert the ids coalesced — without that they'd pass trivially on two independent requests.
Full suite: 3698 passed, 60 skipped, 1 failure that reproduces identically on unmodified
main(test_install_lowmem, environment-dependent:/var/tmpis disk-backed on this machine).Not included
max_workers=3is left alone deliberately. It bounds the transient peak rather than what stays resident, and removing the duplicates may well remove the pressure — worth re-measuring before tuning it.Summary by CodeRabbit
Bug Fixes
Tests