sql.js has no incremental write path, so persist() rewrites the whole image on
every save. Going through fs.writeFileSync(filePath, ...) opened the destination
with O_TRUNC, leaving the on-disk database 0 bytes and then partial for the whole
write -- a window that scales with database size and recurs on every save.
Unlike better-sqlite3 / node:sqlite, that window is not covered by SQLite's
locking protocol, so it is visible to every other process reading the same file:
a backup job, a metrics exporter, an operator running sqlite3. Those readers get
SQLITE_CORRUPT ("database disk image is malformed") while PRAGMA
integrity_check passes moments later, which makes the failure look random and
blames the reader.
Now: temp file in the same directory, fsync, rename() over the destination.
rename is atomic on POSIX and on Windows for a same-volume replace, so a reader
sees either the previous image or the new one, never a truncated one. It also
closes a total-loss window: a crash mid-write used to leave the real database
truncated, and now only leaves a stale temp file behind.
The regression guard asserts the property that separates the two implementations
without racing a timer: a reader that opened the file before a save still reads a
complete, valid image afterwards, and the published file sits on a new inode.
It fails on the previous implementation and passes on this one.
Co-authored-by: Max <maxmad64@gmail.com>
Co-authored-by: adevwithpurpose <adevwithpurpose@users.noreply.github.com>
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