On Darwin, synchronous=FULL (the WAL default) only issues a plain
fsync(), which Apple documents does NOT guarantee writes reach stable
storage or stay ordered. SQLite's WAL corruption-safety guarantee
assumes the OS honors the fsync barrier; macOS does not unless the app
uses F_FULLFSYNC. During a launchd *system* shutdown the page cache is
dropped (effectively power-loss for in-flight pages), so a WAL
checkpoint whose fsync 'reported' durable may never hit the platter —
corrupting state.db with a malformed image. That is the trigger in
#30636 ('SIGTERM during launchd shutdown under high load').
Apply PRAGMA checkpoint_fullfsync=1 (macOS-guarded) in
apply_wal_with_fallback. It forces the F_FULLFSYNC barrier only at
checkpoint boundaries (where WAL frames land in the main DB), so cost
amortizes to ~+0.1ms/commit vs ~+4ms for the broader fullfsync=1.
No-op off Darwin (F_FULLFSYNC is macOS-only).
Root-cause analysis by @catapreta on #30636. Supersedes #30654, whose
synchronous=FULL is a no-op (already FULL in WAL mode) and whose
TRUNCATE-on-close is already on main.
Co-authored-by: catapreta <catapreta@users.noreply.github.com>
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