Each engine now refuses the other's volume at mount(): the exFAT engine
rejects a FAT boot sector (a non-zero byte where exFAT keeps MustBeZero),
and the FAT engine rejects an exFAT one (MustBeZero reads as a zero
bytes-per-sector), each mounting its own as a control. The two engines
can never claim the same medium.
exfat-test is the mount round-trip client, cloned from fat-test: it waits
for /volumes/exfat-e0fa0001, reads the seeded HELLO.TXT, then exercises
mkdir + write + rename + read-back + remove through the VFS and reports
"exfat-test: ok". It ships as a lazy fixture in test builds (the
exfat-volume QEMU case, step 8, drives it). build + zig build test +
bounds green.
create / write / truncate / remove / mkdir / rename, completing the
engine's pub-fn contract with the shared harness. The allocation BITMAP
is the authority: setAllocated IS the allocation (a set bit), and the
32-bit FAT only records a fragmented chain's order — forgetting the bit
would hand a live cluster out twice, so the write path never touches the
FAT without also owning the bit.
This engine writes FAT-linked (no_fat_chain=0) files: createFile makes
an empty set; writeFile allocates+links+zeroes clusters (so a sparse gap
reads zero and valid_data_length can honestly equal data_length) and
rewrites the Stream entry; a contiguous file opened for growth is first
threaded through the FAT. truncate frees the chain; removeFile clears
each set entry's InUse bit and frees the chain (a non-empty directory is
refused); rename re-homes the same clusters under a new name set.
createDirectory allocates one zeroed cluster — exFAT directories carry no
"." / ".." entries. Every entry-set mutation recomputes the set
checksum. Offsets clamp to the vfs u32 surface.
Ten engine host tests now (read + write across clusters, truncate+reuse,
remove, subdir+inner file, rename); 17 total with on-disk. bounds green.
mount + resolve + list + read over a BlockDevice, host-tested against a
RAM-backed image the tests build with formatExfat. mount reads the VBR,
loads the geometry, and scans the root for the Allocation Bitmap (0x81)
and Up-case Table (0x82); the up-case prefix is decompressed (0xFFFF
identity runs) into a bounded table so names fold correctly.
A directory is read as consecutive 32-byte entries via readChain, so a
File/Stream/Name SET that straddles a sector or cluster boundary
assembles cleanly; each set's checksum is validated before it counts as
a file. A stream's no_fat_chain flag picks contiguous-arithmetic vs
FAT-follow cluster walking. reads honor valid_data_length (allocated-
but-unwritten tail reads as zero) and clamp data_length to the vfs u32
offset surface.
Tests cover mount + up-case fold, listing (skipping the metadata
entries), case-insensitive resolve, and reads across a cluster boundary
on both a contiguous and a FAT-fragmented file. Wired via engine.zig
(which imports on-disk.zig) into the host-test aggregate. 12/12, bounds
green. Write path is step 3.
Daniel Samson