deepseek-harness/packages/util/atomic-write/README.md

dsh-atomic-write

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Zero-dependency atomic file replacement shared by file-backed stores that must never leave partial, symlink-hijacked, or wider-than-intended content on disk — the user-settings document (dsh-settings-local) and the credentials store (dsh-credentials-local).

Surface

import { writeFileAtomic } from '@deepseek-ai/dsh-atomic-write'

await writeFileAtomic('/home/u/.dsh/settings.yaml', text, { mode: 0o600 })

One export. The contract, in the order failures would exploit it:

• Exclusive-create temp (wx, random suffix): the open refuses to follow a symlink planted at a guessable temp path.
• The fresh inode carries mode through the rename: replacing a wider-permission file narrows it without a chmod race. mode is required so the permission decision stays visible at every call site (subject to the process umask, like every fresh inode).
• rename replaces a symlinked target itself, never writing through to its referent.
• Same-directory sibling keeps the rename on one filesystem, so the swap stays atomic.
• Parent directories are created; on any failure the temp is removed and the failure rethrown; readers observe either the old or the new complete content.

Model Experience

None, as this is a pure filesystem primitive; nothing here reaches a model request.

Known Limitations and Deferred Work

• Atomic, not durable — no fsync of the file or its directory, so after a crash the rename may be observed unwound. The file-backed stores here re-read and republish on boot, keeping durability the caller's policy.
• String content only — no Buffer or stream form until a consumer needs one.