IndexedDB VFS xClose doesn't flush the cache

[rhashimoto]

Right now the IndexedDB VFS cache only writes dirty blocks in xSync, not in xClose. xSync should always be called for database files, but I'm not certain whether that is true for journal files - I think it might depend on xDeviceCharacteristics flags (maybe SQLITE_IOCAP_SEQUENTIAL?).

[rhashimoto]

xSync on journal files is indeed skipped when xDeviceCharacteristics returns SQLITE_IOCAP_SEQUENTIAL, e.g. here.

The current IndexedDB VFS does not set this flag so it should work correctly, but defensive programming would avoid injecting this bug in the future, e.g. by:

• Adding a comment to xDeviceCharacteristics to warn about this, or
• Checking the flag in xClose and syncing if present