Anachronaut
ce8fb721fe
Saving something that already exists writes a temporary, deletes the original and gives the temporary its name, so that nothing is lost if the writing fails. The temporary was told apart from a real file by being called sbfs.part or sbfs.out - and those are legal names. Starting a save deleted whatever answered to one as stale scratch, so saving anything at all in a directory destroyed your own file of that name there, silently. Flag bit 0x04 now says it. The property is not in the contents - the same bytes become the finished file the instant the rename lands - so it belongs in the entry, which is the thing the commit changes. sbfsCreateTempAt is the door temporaries come in by, the commit writes the flags flat along with the name, and cleanup wipes what it finds only if the entry says it is ours. Anything else stops the save instead. The bit is also the recovery. Both listings show an unfinished write rather than sizing it, because the size in the entry is the room that was asked for and not what was written: "<unfinished>" from dir, and a line from SplitDisk saying the blocks are held and a rename brings the data back. That was the gap in what the last commit documented - the data survived a crash and nothing would show you where it was. Four new agreement checks, three of which fail with the guards removed. The fourth needed rebuilding first: both tests started on one disk, and the first save ate the sbfs.part that was the second test's SOURCE, so the copy failed for want of a file, never opened a stream, and passed while reporting on nothing. A disk each. The fifth check forges the wreckage by setting the flag on a finished file, since nothing here can crash a save half way through. No version bump: a committed file never carries the bit, so a disk this writes is byte for byte the disk the old code wrote, which the whole-image comparisons confirm. Only the wreckage differs, and older code reads that as an ordinary file - which is what it did before.
139 lines
6.0 KiB
C
139 lines
6.0 KiB
C
// sbfs.h
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// The SplitBit Filesystem, version two.
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//
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// This is the host side's copy of the format. The other implementation is SplitBit
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// assembly running on the machine itself, so nothing can be shared between them except
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// the specification: the two have to be kept honest by a document rather than by a
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// header. Everything here follows that document exactly, and anything that changes here
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// has to change there in the same breath.
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//
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// All multi byte numbers are most significant byte first, the same as every other number
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// SplitBit stores: addresses, the SPBT boot image header, and the vector table.
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//
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// Written by Anachronaut
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#ifndef SBFS_H
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#define SBFS_H
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#include <stdint.h>
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#define SBFS_MAGIC "SBFS"
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#define SBFS_MAGIC_BYTES 4
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// ---- Versions ----
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//
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// Version two adds directories, and adds them without moving a single byte that version
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// one had defined: the parent lives in two of the four bytes each entry already reserved,
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// and a directory is a flag bit in a byte that was using one of its eight.
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//
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// A VERSION ONE DISK IS ALREADY A VALID VERSION TWO DISK. The parent field is written as
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// the entry's index PLUS ONE, so that the zero a version one disk has in those reserved
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// bytes reads as "in the root" - which is exactly what every file on a flat disk is in.
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// There is nothing to convert and no tool to convert it with.
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//
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// Compatibility therefore runs one way, which is the ordinary shape of it: this reads
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// either version, and version one code reading a version two disk would list directories
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// as strange empty files. A disk is written back at the version it was read at, and is
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// only raised to two by the thing that makes the difference real - the first directory
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// created on it.
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#define SBFS_VERSION_FLAT 1
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#define SBFS_VERSION_TREE 2
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#define SBFS_VERSION SBFS_VERSION_TREE
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#define SBFS_BLOCK_BYTES 256
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// ---- Block 0, the superblock ----
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//
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// 0 4 "SBFS"
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// 4 1 Version
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// 5 1 Reserved
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// 6 2 Blocks on the disk
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// 8 2 First directory block
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// 10 2 Blocks the directory occupies
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// 12 2 Free blocks, a cache rather than the authority
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// 14 Reserved to the end of the block
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#define SBFS_SUPER_VERSION 4
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#define SBFS_SUPER_DISK 6
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#define SBFS_SUPER_DIRSTART 8
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#define SBFS_SUPER_DIRBLOCKS 10
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#define SBFS_SUPER_FREE 12
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// ---- Directory entries ----
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//
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// 0 1 Flags
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// 1 2 First block of the file's data
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// 3 2 Whole blocks the file occupies
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// 5 1 Bytes in the trailing part block, or zero if there is not one
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// 6 22 Name, padded with zeroes
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// 28 2 Parent, as an entry index plus one. Zero is the root. (Version two.)
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// 30 2 Reserved
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//
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// Thirty two divides two hundred and fifty six, so an entry never straddles a block and
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// reading one never means handling a split. Version two did not change that, because it
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// spent bytes that were already inside the entry.
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//
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// A DIRECTORY IS AN ENTRY WITH NO BLOCKS. Its start, blocks and tail are all zero, and it
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// costs one entry and nothing else. That is what keeps the flat array of entries the
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// whole allocation map: with files laid down contiguously, every block is inside some
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// entry's range or it is not, and an entry with no range is in nobody's way.
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//
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// Because parents are entry indices, and because nothing ever compacts the directory,
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// those indices are stable for as long as an entry is in use. DELETING A DIRECTORY THAT
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// STILL HAS CHILDREN MUST BE REFUSED: the freed index would be handed to some unrelated
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// file later, and the orphans would reappear inside it.
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#define SBFS_ENTRY_BYTES 32
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#define SBFS_ENTRIES_PER_BLOCK (SBFS_BLOCK_BYTES / SBFS_ENTRY_BYTES)
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#define SBFS_ENTRY_FLAGS 0
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#define SBFS_ENTRY_START 1
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#define SBFS_ENTRY_BLOCKS 3
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#define SBFS_ENTRY_TAIL 5
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#define SBFS_ENTRY_NAME 6
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#define SBFS_NAME_BYTES 22
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#define SBFS_ENTRY_PARENT 28
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#define SBFS_FLAG_IN_USE 0x01
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#define SBFS_FLAG_DIRECTORY 0x02
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// A FILE BEING WRITTEN, WHICH IS NOT YET A FILE. Saving something that already exists is
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// done by writing a temporary, deleting the original and giving the temporary its name,
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// so that nothing is lost if the writing fails. The temporary has to be an ordinary entry
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// while that happens - it holds real blocks and needs a name - and the only thing that
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// distinguishes it from a finished file is that nobody has committed it yet.
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//
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// That is not a property of its contents. The same bytes become the real file the moment
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// the rename lands, so there is nothing to put inside it that would be true; it belongs
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// in the entry, which is the thing the commit changes. It was formerly told apart by
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// being called "sbfs.part" or "sbfs.out", and those are legal names a user may also
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// choose, so cleaning up by name could delete somebody's file.
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//
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// Cleared as part of committing. An entry still carrying it is the wreckage of a write
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// that stopped, and its blocks are spoken for until something clears it up.
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#define SBFS_FLAG_TEMPORARY 0x04
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// The root is not an entry. It is the absence of a parent, written as zero, which is why
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// the field is an index plus one and why a freshly zeroed entry is already in the root.
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#define SBFS_PARENT_ROOT 0
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#define SBFS_PARENT_OF(index) ((uint16_t)((index) + 1))
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#define SBFS_PARENT_INDEX(parent) ((int)(parent) - 1)
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// Paths are separated by this, and a leading one means "from the root". A name may not
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// contain it, which is what makes a path unambiguous without any quoting.
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#define SBFS_SEPARATOR '/'
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// The directory begins at block 1 and is this many blocks unless told otherwise, which
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// is sixty four files. The superblock carries the real number, so this is only what a
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// freshly formatted disk gets.
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#define SBFS_FIRST_DIRECTORY_BLOCK 1
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#define SBFS_DEFAULT_DIRECTORY_BLOCKS 8
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// A file of n bytes occupies n / 256 whole blocks and, if anything is left over, one more
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// for the tail. Zero means zero in both, so an empty file occupies nothing at all.
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#define SBFS_WHOLE_BLOCKS(bytes) ((bytes) / SBFS_BLOCK_BYTES)
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#define SBFS_TAIL_BYTES(bytes) ((bytes) % SBFS_BLOCK_BYTES)
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#define SBFS_BLOCKS_USED(blocks, tail) ((blocks) + ((tail) ? 1 : 0))
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#endif // SBFS_H
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