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fix(8.0): byte-copy _id_mapper/* in the pre-upgrade backup (cor's write-new nuance)

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cor confirmed the migration write-new / tmp+rename invariant the hard-link backup
relies on — with one exception: the native shared mmap `_id_mapper/*` is mutated
IN PLACE (msync + a rebuild truncate+re-inject), the same category as the tx-log.
A hard-linked snapshot of it would be corrupted by a live in-place mutation
reaching through the shared inode, so the backup would not be a faithful
pre-upgrade copy.

Add a SNAPSHOT_BYTE_COPY_DIRS set (`_id_mapper`) so snapshotToDirectory byte-copies
every file under it (the directory analogue of SNAPSHOT_BYTE_COPY_PATHS); the id
map is bounded, so the copy cost is small. Everything else (SSTables / .dkann /
manifests / objects, all tmp+rename) still hard-links. Test: `_id_mapper/*` gets
an independent inode + faithful content; a regular file stays hard-linked.
This commit is contained in:
David Snelling 2026-07-02 13:35:43 -07:00
parent 79e8709c35
commit a30ed72dc3
2 changed files with 49 additions and 1 deletions
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27
tests/integration/migration-backup.test.ts
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@ -54,6 +54,33 @@ async function buildBrainWithData(dir: string, staleAfter = false): Promise<stri
}
describe('Pre-migration backup (#42)', () => {
it('byte-copies _id_mapper/* (mmap, mutated in place) but hard-links the rest', async () => {
// cor confirmed the native _id_mapper mmap is msync/truncate-in-place, so a
// hard-linked snapshot of it would be corrupted by a live mutation reaching
// through the shared inode — it must be byte-copied. Everything else (written
// tmp+rename) hard-links safely.
const dir = mkTmp()
await buildBrainWithData(dir)
fs.mkdirSync(path.join(dir, '_id_mapper'), { recursive: true })
fs.writeFileSync(path.join(dir, '_id_mapper', 'map.bin'), Buffer.from([1, 2, 3, 4]))
fs.writeFileSync(path.join(dir, 'regular.bin'), Buffer.from([9, 9, 9, 9]))
const storage: any = new FileSystemStorage(dir)
await storage.init()
const backup = await storage.createMigrationBackup()
// _id_mapper file → byte-copied: DIFFERENT inode (a live msync can't reach it) + faithful content
const mapperLive = fs.statSync(path.join(dir, '_id_mapper', 'map.bin')).ino
const mapperBackup = fs.statSync(path.join(backup, '_id_mapper', 'map.bin')).ino
expect(mapperBackup).not.toBe(mapperLive)
expect(fs.readFileSync(path.join(backup, '_id_mapper', 'map.bin'))).toEqual(Buffer.from([1, 2, 3, 4]))
// a regular (tmp+rename) file → hard-linked: SAME inode (zero-copy)
const regLive = fs.statSync(path.join(dir, 'regular.bin')).ino
const regBackup = fs.statSync(path.join(backup, 'regular.bin')).ino
expect(regBackup).toBe(regLive)
})
it('FileSystemStorage.createMigrationBackup hard-links the store; removeMigrationBackup deletes it without touching live data', async () => {
const dir = mkTmp()
const id = await buildBrainWithData(dir)