fix(storage): guarantee forward progress for visits on transient flush failures

[vringar]

Problem

In production parquet->S3 crawls, some visits were observed to get claimed and
perpetually marked in-progress by the work-queue client while their data
never got committed or pushed out. Records were produced and cached in the
StorageController, but the cache never flushed and the visit never reached a
terminal state -- neither committed nor explicitly marked incomplete. The visit
was stranded forever and its data lost. This is a liveness (forward-progress)
failure.

Root cause

A single transient structured-storage write error (e.g. an S3 PutObject
failure) was fatal to forward progress:

1. ArrowProvider.flush_cache was not atomic. It wrote batches
   table-by-table and only afterwards cleared _batches and set the per-finalize
   flush_events. If a write_table call raised partway through, the cached
   records were stranded and the finalize tokens for those visits could never
   resolve, so those visits stayed perpetually in-progress.

2. save_batch_if_past_timeout -- the only periodic flush driver while a crawl
   is live -- had no error handling around flush_cache. One transient error
   killed the coroutine permanently (it is a NoReturn loop), so finalized
   visits were never flushed again and stayed pending forever.

3. shutdown's final drain flush was likewise unguarded, so a transient
   error there aborted the drain and left finalized visits out of the completion
   queue (and could deadlock on the never-resolving token).

The work-queue client never observes completion for a stranded visit, so it
keeps the site claimed indefinitely.

Fix

• ArrowProvider.flush_cache is now atomic: write all tables first; only on
  full success clear _batches and set flush_events. On failure, keep both
  intact and re-raise so a later flush retries cleanly -- no data loss, no
  stranded tokens.
• save_batch_if_past_timeout survives transient flush errors: it logs and
  retries on the next tick instead of dying.
• The shutdown drain flush is retried a bounded number of times
  (SHUTDOWN_FLUSH_RETRIES) so a transient error no longer aborts the drain. The
  bound guarantees shutdown can never hang on a permanently-failing backend.

Tests

New property-based (Hypothesis) liveness tests in
test/storage/test_storage_controller_liveness.py drive the controller in-process
(no socket, no subprocess, no Firefox) with arbitrary interleavings of
start / finalize(success) / finalize(failure) / never-finalize / concurrent
visits and transient write failures, asserting the forward-progress invariants:

• I1 every visit finalized(success) with records is fully committed;
• I2 every visit the controller observed reaches a terminal state (in the
  completion queue / recorded incomplete), never silently stranded;
• I3 shutdown drains all pending committable data (nothing left only in
  cache);
• I4 per-visit bookkeeping stays bounded.

test_forward_progress_survives_transient_write_failures reproduces the original
stranding bug (fails on the pre-fix code, green with this fix).

Documented contract boundary

A visit that emits zero records and is never finalized is invisible to
the StorageController (it has nothing to key on); driving such a
claimed-but-silent visit to a terminal state is the work-queue client's
responsibility (e.g. a visit-claim timeout). The tests encode this boundary
explicitly.

🤖 Generated with Claude Code

[vringar]

Follow-up: the branch was CI-red on all 7 tests shards with ModuleNotFoundError: No module named 'hypothesis' at collection — the new liveness tests import hypothesis but the dependency was absent from the CI environment.

Added hypothesis=6.155.6 to environment.yaml and hypothesis to scripts/environment-unpinned-dev.yaml (the "environment.yaml pins match unpinned sources" pre-commit hook passes). All 4 liveness invariants pass locally (4 passed, hypothesis 6.155.6); CI re-triggered.

[codecov]

Codecov Report

❌ Patch coverage is 90.74074% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 62.72%. Comparing base (25a7827) to head (1dba4a9).

Files with missing lines	Patch %	Lines
openwpm/storage/storage_controller.py	88.63%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1197      +/-   ##
==========================================
+ Coverage   62.36%   62.72%   +0.35%     
==========================================
  Files          40       40              
  Lines        3930     3965      +35     
==========================================
+ Hits         2451     2487      +36     
+ Misses       1479     1478       -1     

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