Safer FAT cache commit ordering and mirror handling#2
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Add dc_commit_range() to flush dirty cache entries within a sector range, and use it in cache_commit() to flush data/directory sectors before FAT sectors so a crash mid-flush leaves only lost clusters rather than cross-linked chains. In dc_writeback_callback(), write the FAT2 mirror before FAT1 (so FAT1 remains the last-known-good copy on power loss) and retry failed sector writes up to 3 times before panicking. In cache_sector(), fall back to reading FAT2 if FAT1 is unreadable.
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Add dc_commit_range() to flush dirty cache entries within a sector range, and use it in cache_commit() to flush data/directory sectors before FAT sectors so a crash mid-flush leaves only lost clusters rather than cross-linked chains.
In dc_writeback_callback(), write the FAT2 mirror before FAT1 (so FAT1 remains the last-known-good copy on power loss) and retry failed sector writes up to 3 times before panicking. In cache_sector(), fall back to reading FAT2 if FAT1 is unreadable.