Writing

The Root handle exposes a tight set of mutation verbs. Replacement writes (write, writeJson, and copyIn) publish with a sibling-temp commit so no half-written replacement appears at the destination. Create-only writes (create, createJson, and write with overwrite: false) use sibling-temp staging with an atomic no-replace rename only on backends that provide one — the native binding, which require mode guarantees and auto mode uses when the binding loads. Ordinary buffered creation in the pure-JavaScript fallback claims the final name exclusively with O_EXCL and writes content in place, so a concurrent observer can see the new file before its content is complete. Buffered create and createJson accept atomic: true to stage complete content on this fallback too. Streamed creation already stages before publication. append and openWritable intentionally modify an opened file in place; move, remove, and mkdir mutate directory entries rather than file bytes. Each verb applies the boundary checks appropriate to its operation.

await fs.write("state.json", body);
await fs.create("seed.json", initial);   // throws if exists
await fs.writeJson("config.json", state);
await fs.append("logs/today.log", line);
await fs.copyIn("inbox/upload.bin", "/tmp/upload.bin");
await fs.move("notes/draft.md", "notes/published.md");
await fs.remove("logs/yesterday.log");
await fs.mkdir("snapshots/2026/05");

#What replacement writes do

1. Resolve the relative target against the canonical root and reject anything that escapes (outside-workspace).
2. If mkdir: true, create missing parent directories relative to a pinned parent fd in the native path, or with per-component identity guards in the JavaScript fallback. When denyMutations or an explicit mutationSymlinks policy is present, each missing POSIX-native component is authorized before creation and its opened descriptor is authorized again before descent.
3. Pin or guard the parent directory for the selected mechanism. Native operations use a parent fd and reapply configured mutation policy to the actual canonical destination selected by that descriptor; the pinned JavaScript fallback verifies directory identity before and after mutation and performs the same policy revalidation before its pathname dispatch. The JavaScript check cannot make the intervening pathname syscall atomic, so a same-privilege peer that can replace the parent may cause an out-of-root side effect before detection. Use native require mode for that threat model; see the security model.
4. Write data to a sibling temp file in the same directory.
5. Atomically rename the temp file over the destination.
6. Stat the resulting fd and verify identity.

Private sibling temporary names are independent of the destination basename, so staging does not add a suffix to an otherwise valid long filename.

Write paths reject path-alias when symlinks and parent components select a different target before and after lexical normalization, such as link/../file where link points into a deeper directory. This avoids silently modifying the wrong file. Resolve an intended alias explicitly with Root.resolve() before passing its canonical path to a mutation.

A failure before the final rename leaves the destination at its previous contents. A successful rename publishes the complete replacement. This old-or-new guarantee does not apply to append() or openWritable(), which write in place, or to lower-level atomic helpers when their explicitly non-atomic permission-error copy fallback is enabled.

Post-publication verification can still reject after a complete replacement has been committed. Rejection does not promise that a successful rename was rolled back; the published file or a raced replacement may remain at the destination.

Failed-write cleanup compares exact parent and file identities, including large Windows file indexes. Replaced paths and paths whose ownership cannot be verified are preserved.

#Denying mutations

All mutation verbs accept denyMutations?: DenyMutationPolicy, either as a root default or per-call option:

const fs = await root("/srv/workspace", {
  denyMutations: {
    paths: ["/srv/workspace/.env"],
    prefixes: ["/srv/workspace/.ssh"],
  },
});

await fs.write(".env", "x");       // throws FsSafeError code "denied-path"
await fs.remove(".ssh/id_rsa");    // throws FsSafeError code "denied-path"

paths blocks exact absolute paths. prefixes blocks absolute paths and everything below them. For POSIX pinned write, create, and copyIn, an existing exact-path directory does not implicitly deny a mutation to its descendants, but creating a missing directory at that exact path is itself denied. fs-safe preserves path strings exactly and canonicalizes through existing ancestors before comparing, so a mutation through a symlinked ancestor to a denied path is still denied. Root-level and per-call policies are additive; per-call policy can add denies, but cannot clear root defaults. Those POSIX pinned operations snapshot the merged policy before their first path observation, then authorize the actual descriptor-selected parent before staging or publication. A caller mutation of the original arrays, or a contained parent redirect after preflight, cannot clear that admission check.

#Write verbs

#fs.write(rel, data, options?)

Overwrite or create. Always atomic.

await fs.write("state/last-run.json", JSON.stringify(run));
await fs.write("notes/today.txt", "hello\n", { encoding: "utf8" });

data accepts string | Buffer. mode sets the file's POSIX mode. If neither the call nor RootDefaults supplies it, a replacement preserves the existing file mode and a new file uses 0o600. mkdir and overwrite both default to true; set overwrite: false for the same no-clobber behavior as create().

#Write options

write, create, writeJson, createJson, append, and copyIn accept durable. Precedence is per-call option, then Root.defaults.durable, then true; an explicitly undefined call option preserves the root default. durable: false keeps the sibling-temp replace/rename behavior of replacement writes but skips file and parent-directory fsync calls. Create-only and append publication behavior, permissions, identity checks, and error codes are unchanged. Use it only for reconstructible data: a crash may lose the write or leave the previous file. move and streaming openWritable do not use this option. Native and pure-JavaScript Windows writers honor the option. Replacement writes sync staged content before rename and the final mode through the retained file handle. Directory sync remains best-effort.

For create and createJson, durable: "file" requires each file sync to succeed, including on EPERM; it overrides a disabled Root durability default. Parent directory synchronization retains the existing best-effort policy. This also applies to streamed creation and is independent of publication strategy. Boolean durability options keep their existing behavior, including compatibility paths that tolerate EPERM. A failed file sync before staged publication prevents publication; a failure after publication can leave the complete file present.

POSIX modes without read permission, including 0o000 and 0o200, succeed: final verification uses a descriptor retained by the writer rather than reopening the published file. The requested mode is not relaxed for verification. Publication verification compares exact bigint descriptor and pathname identities, including large file indexes that cannot be represented by a JavaScript number. With durability enabled (the default), native publication syncs content before rename and syncs the parent directory. Modes that retain owner read/write skip the extra mode-only file sync: after a crash, the file may retain staged mode 0o600 instead of the wider requested mode. Modes that remove owner read or write, and corrections of observed wider staging permissions, keep the post-chmod file sync. Later reads still obey OS permissions, and access checks on a pre-existing destination are unchanged. The explicit FUSE compatibility policy still requires a readable destination to prove matching content when rename changes its identity.

When Windows cannot report a pathname's identity, the publication verifier reopens the name and compares that descriptor's exact identity with the original retained file, without reading bytes. It also rechecks links and root/parent containment. This write-specific proof does not relax ordinary reads: unknown pathname metadata alone is never proof that the name still refers to the expected file.

#fs.create(rel, data, options?)

Don't-clobber variant of write(). Throws already-exists if the target is there. Create-only preflight preserves boundary, alias, hardlink, and type checks without opening an existing target to inherit its mode; a fresh file uses the requested mode or the normal new-file default. When the native binding is in use (require mode, or auto mode with a successfully loaded binding), content is staged privately and published with an atomic no-replace rename, so the name never appears before its bytes. In the pure-JavaScript fallback the name is claimed exclusively first and content is written afterward, so observers can briefly see an empty file; failure cleanup removes a claimed file only when its identity is unchanged.

try {
  await fs.create("config/seed.json", initial);
} catch (err) {
  if (err instanceof FsSafeError && err.code !== "already-exists") throw err;
}

#Atomic buffered creation

await fs.create("config/seed.json", initial, { atomic: true });
await fs.createJson("config/settings.json", { enabled: true }, { atomic: true });
await fs.create("config/flushed.json", initial, { atomic: true, durable: "file" });

atomic: true keeps the destination absent until all bytes have been written. The native backend uses its no-replace rename; the JavaScript fallback hardlinks the completed stage and unlinks its temporary name in the same JavaScript turn. The fallback requires hardlink support and fails without publishing partial bytes when that mechanism is unavailable. Other processes can briefly observe both names. Existing and raced entries are preserved, including dangling symlinks; ordinary confinement, type, hardlink, and symlink-policy rejections still apply. assertBeforeMutation retains its live checks through content writes and publication.

Omitted or false preserves the existing buffered behavior. The option belongs to buffered create and createJson, not replacement writes or Root defaults. Streamed creation has no atomic opt-out. atomic changes visibility, not the existing durable file/directory synchronization policy; it does not turn best-effort synchronization into a strict crash-durability guarantee or strengthen JavaScript pathname containment.

Atomic and streamed creates settle owned cleanup and close operations before returning. Failed or unverifiable cleanup is reported rather than silently discarded. Errors after publication and incomplete-settlement errors carry the existing StagedFileFailureDetails publication/cleanup receipts where the writer can establish them; native disposal can retain them inside a SuppressedError cause. Preserve those details when handling errors: a rejection can follow complete publication, and an indeterminate link or native rename must preserve names for recovery. A cleanup or close failure also retains the original operation failure. No later verification, mode, or synchronization failure authorizes deleting an already published complete destination. See receipt meanings.

#Streamed creation

Pass an AsyncIterable<Uint8Array> to create() when bytes come from a database, network response, or another incremental producer. Buffers are accepted chunks. The writer consumes each chunk completely before requesting the next one; it does not collect the full input in memory or expose a writable descriptor.

async function* snapshotChunks(): AsyncGenerator<Uint8Array> {
  yield Buffer.from("first stored chunk\n");
  yield Buffer.from("second stored chunk\n");
}

await fs.create("restored/config.txt", snapshotChunks(), {
  mode: 0o600,
  maxBytes: 8 * 1024 * 1024,
  durable: false,
  signal: AbortSignal.timeout(30_000),
});

RootCreateStreamOptions keeps mkdir, mode, durable, assertBeforeMutation, denyMutations, and mutationSymlinks from buffered creation and adds maxBytes and signal. Byte chunks have no encoding option; streamed creation uses strict publication identity and does not support renameIdentity: "verify-content-with-lock". Existing Root defaults apply, including explicit maxBytes; with no byte cap at either level, input size is unlimited. Zero permits an empty input only. Invalid limits reject before I/O.

Unlike buffered creation's JavaScript fallback, streamed creation stages all chunks before publishing the final name. Native mode uses no-replace rename; the JavaScript fallback hardlinks the completed stage and removes its temporary name in the same JavaScript turn. That fallback requires a filesystem supporting hardlinks; other processes may briefly observe both names. An existing or concurrently created destination is preserved. Existing-target preflight does not consume the input. The final mode and durability policy use the same guarded writer as other Root operations.

Cancellation checks run before and after producer pulls, before content writes, and before publication. assertBeforeMutation also rechecks current application authority after producer waits and before each partial write. The operation waits for any pending producer pull or filesystem write, then awaits the producer's return() and cleans only the owned unpublished stage. Pass the same signal into a producer that may stall: an arbitrary async iterator cannot be forcibly interrupted, so cancellation waits for its pending work and cleanup to settle. Do not mutate a yielded chunk until the next pull. Producer errors retain their original value when cleanup succeeds.

Streamed creation retains the signal and assertBeforeMutation callback selected when the call starts. Replacing or deleting those options during a producer wait does not change the in-flight operation. Abort the original signal or update the live authority state checked by the original callback to revoke it; the callback continues to receive the original options object as this.

An aborted or failed operation can leave created parent directories. If a stage's identity or parent cannot be verified during cleanup, the existing guarded cleanup preserves it. After publication, later verification or cleanup failures preserve the destination; rejection does not prove that no file was created. Cancellation arriving after publication does not undo the completed file. Application recovery remains caller-owned.

#fs.writeJson(rel, value, options?)

JSON.stringify(value, replacer, space) + atomic write. Adds a trailing newline by default.

await fs.writeJson("config.json", state, { space: 2 });
await fs.writeJson("compact.json", state, { trailingNewline: false });

Options:

type RootWriteJsonOptions = RootWriteOptions & {
  replacer?: (this: any, key: string, value: any) => any | (number | string)[];
  space?: number | string;
  trailingNewline?: boolean; // default true
};

createJson(rel, value, options?) is the don't-clobber variant.

#fs.append(rel, data, options?)

Open in append mode, write, sync the file handle, and close. Honors mkdir for the parent directory and syncs the parent directory when the append creates the file. durable: false skips both syncs. Pass prependNewlineIfNeeded: true to insert a \n if the file does not already end in one.

await fs.append("logs/today.log", `[${ts}] ${line}\n`);
await fs.append("notes/scratch.md", "* new bullet", { prependNewlineIfNeeded: true });

For high-volume logging, consider openWritable and a long-lived append handle. Direct append-mode writes preserve kernel append semantics, but they are not atomic against external rotators that rename or unlink the target.

#fs.copyIn(rel, sourceAbsPath, options?)

Bring a file from outside the root into the root, atomically. The source path must be absolute. The library streams the source through the boundary, writes to a sibling temp, and renames over the destination.

await fs.copyIn("inbox/upload.bin", "/tmp/incoming.bin", {
  maxBytes: 64 * 1024 * 1024,
});

Options are { denyMutations?, durable?, maxBytes?, mkdir?, mode?, sourceHardlinks? }. durable follows the root default and is true when omitted at both levels; set it to false to skip file and parent-directory syncs for reconstructible data. Use sourceHardlinks: "reject" to refuse if the source itself is a hardlinked alias. There is no encoding option: copying preserves source bytes.

#fs.move(from, to, options?)

Rename one path inside the root to another. Defaults to no clobber:

await fs.move("incoming/foo.txt", "archive/foo.txt");
await fs.move("incoming/foo.txt", "archive/foo.txt", { overwrite: true });

Both from and to are bounded; .. in either is rejected.

Mutation policy is captured at call start; changes to caller-owned denial arrays apply to later moves. For live cancellation or revocation, throw from assertBeforeMutation immediately before dispatch.

The default no-clobber mode requires the native helper. It admits both parent directory descriptors and performs a descriptor-relative no-replace rename, so a competitor that creates the target first is preserved and the source remains in place. If the helper or safe parent admission is unavailable, the call fails with helper-unavailable; it never falls back to a check followed by a replacing rename. After dispatch it rechecks both parent identities, so a post-operation rejection can mean the no-replace rename completed. Directory moves continue to require overwrite: true.

Both selected canonical endpoints are admitted inside the retained Root after native parent admission. With mutationSymlinks: "reject", both full operation paths are rechecked after the live mutation-authority callback and before dispatch. The Root and retained parents are fenced again after any such callback. These checks retain the documented final check-to-syscall race.

For { overwrite: true }, the JavaScript path checks both parent directories before and after the rename. A failed post-operation check rejects even though the rename may already have completed; rejection does not imply rollback.

#fs.remove(rel)

Unlink a file or rmdir an empty directory. Non-empty directories throw not-empty. For atomic directory replacement, use replaceDirectoryAtomic.

Before a nonrecursive JavaScript fallback removal, fs-safe retains exact Root-to-parent directory identities, with canonical endpoint checks at Root and the immediate parent. It rejects a parent redirected through a symlink or junction before unlink or rmdir, even with force: true, and rechecks the retained ancestry after the operation settles. The entry may already have been removed when that final verification rejects.

await fs.remove("logs/yesterday.log");
await fs.remove("snapshots/empty-dir"); // ok
await fs.remove("snapshots/full-dir");  // throws not-empty

For a tree, opt into bounded recursive removal:

await fs.remove("scratch/finished-job", {
  recursive: true,
  force: true,
  maxEntries: 20_000,
  maxDepth: 32,
  mutationSymlinks: "reject",
  signal: AbortSignal.timeout(30_000),
  assertBeforeMutation: () => assertJobLeaseCurrent(),
});

Budgets must be non-negative safe integers or explicit Infinity, and require recursive: true. Omitted budgets retain their finite defaults. An entry or depth limit throws too-large before processing the over-budget entry.

The default filesystem order streams each directory once, using memory and open handles proportional to depth rather than directory width. Sorted order also enumerates each directory once, but collects its names before descending and deletes directories after their children. It uses JavaScript's default string sort, not locale collation. Collected names consume the shared entry budget, including sibling names still pending while an earlier directory is traversed. With a finite budget, collection overflow rejects before processing that directory's children; one extra name distinguishes an exact limit from overflow. An Infinity entry budget uses a bulk name read while retaining the opened directory handle and identity checks. Sorted mode retains collected names, so unlimited budgets also permit unlimited name storage.

Sorted traversal preserves lexicographic processing, not the incidental syscall timing of a caller that repeatedly rescans parent directories. Each child is inspected when visited. Newly added entries can make the final rmdir fail with not-empty; the operation does not retry indefinitely.

Recursive removal never follows a discovered symlink or junction. With an omitted mutationSymlinks policy it unlinks that entry, preserving the existing nonrecursive behavior. Both explicit mutation policies reject discovered links. follow-parents-within-root permits aliases only in the requested target's parents, and still rejects a final link. Root and per-call denyMutations policies remain additive; a denied descendant prevents removal of the enclosing requested tree before any entry is removed.

The operation retains exact identities for the traversal directories and each observed target. Swapped or missing ancestors reject even with force: true; an abort reason or authority refusal carrying ENOENT is not treated as absence. Authority is rechecked immediately before each direct unlink or rmdir dispatch. Directory handles close before their directories are removed, including on Windows. If both traversal and close fail, SuppressedError retains both failures. Directory-stream filesystem errors use the same removal codes as unlink and rmdir, with the original error in cause; caller abort and authority refusals retain their original values.

When force: true encounters a missing directory during an opendir or read, it closes any open stream and reaches the ordinary final target check. The surviving ancestors and original target identity are checked again; a replacement is never accepted as a missing directory. An actually vanished child does not prevent processing later siblings.

Filesystem failures normalized by the recursive removal owner and its direct symlink rejections carry additional FsSafeError.details context:

type RemoveFailureDetails = {
  operation: "remove";
  phase: "enumerate" | "inspect" | "remove";
  relativePath: string;
};

relativePath is relative to the requested removal target, using host path separators: "" identifies that target and "nested/link" identifies a child on POSIX. It does not replace caller spelling with the canonical Root path. enumerate covers directory stream operations, inspect covers initial child observations, and remove covers final identity checks and unlink/rmdir failures. The existing error codes, messages, and causes remain unchanged. Errors that already have their own identity, including caller authority and cancellation reasons, are propagated without adding or changing their details. Context is diagnostic; it is not permission to retry or mutate an entry.

Removal is incremental, not atomic. A later budget, cancellation, identity, or filesystem failure does not restore already removed entries. As with existing remove, this is a guarded JavaScript operation in every native mode: pathname checks are best-effort against a hostile concurrent process and do not create an atomic check-and-delete syscall. Use OS isolation for that threat model.

#fs.mkdir(rel)

mkdir -p. Creates missing parents.

await fs.mkdir("snapshots/2026/05");

Pass { private: true } to create missing components with private permissions. An existing requested directory must already satisfy that policy; fs-safe does not repair it or change existing ancestor permissions. Concurrent creators may reuse the winner only after it passes the same checks.

Buffered, streamed, and JSON create calls also accept private: true. New POSIX directories default to 0700 and files to 0600; conflicting group/world or privilege bits are rejected before creation. Restrictive owner-only file modes remain available through mode. On Windows, creation uses protected ACLs rather than interpreting POSIX mode bits as access rules. This does not change create's no-overwrite behavior or select its durability policy. See creation for supported backends and owned descriptors.

#fs.ensureRoot()

Treats "" / "." as the root itself. Useful when a generic helper computes a relative directory and might end up at the root.

const targetRel = path.relative(fs.rootReal, candidateAbs); // could be "" if candidateAbs === root
await fs.ensureRoot(); // accepts "" without throwing

#openWritable() for streaming

When write doesn't fit (very large outputs, slow producers), open a writable handle:

const opened = await fs.openWritable("logs/current.log", { writeMode: "append" });
try {
  for await (const chunk of source) {
    await opened.handle.appendFile(chunk);
  }
} finally {
  await opened.handle.close();
}

Options are { denyMutations?, mkdir?, mode?, writeMode? }, where writeMode is "replace" (default), "append", or "update". replace truncates existing files; update keeps existing contents. Streaming writes go directly to the destination — there is no atomic-rename step. For exclusive publication of a complete stream, use create(). For streamed replacement, the atomic helpers provide a staged writer.

On POSIX, existing-target opens use O_NONBLOCK as an admission safeguard so a no-reader FIFO cannot stall regular-file validation. This does not change ordinary regular-file write semantics. replace and update remain write-only opens, including for mode 0o200 files; replacement truncation happens only after type, identity, and boundary checks pass. Rejected existing paths are never cleanup-owned or unlinked.

Identity admission uses bigint descriptor and pathname receipts even though the public stat field remains a numeric Node Stats object. Windows retries an unknown device or file index once while retaining known components, then rejects persistent ambiguity. If admission of a newly created file fails, cleanup only unlinks a pathname that still has the exact created identity; rounded aliases, symlinks, and unknown identities are preserved.

#Write defaults vs per-call options

Set mkdir: true once on root(); pass text encodings per call when needed:

const fs = await root("/srv/workspace", {
  mkdir: true,
});

await fs.write("notes/today.txt", "ascii", { encoding: "utf8" });
await fs.write("data/blob.bin", buffer);     // mkdir true, no encoding because data is Buffer
await fs.write("data/blob.bin", buffer, { mkdir: false }); // override

#Errors you'll catch

Full list in Errors.

#Common patterns

#Replace if changed

const next = JSON.stringify(state);
const prev = await fs.readText("state.json").catch(() => "");
if (prev !== next) await fs.write("state.json", next);

#Stage many writes, then commit

const stagingDir = "snapshots/incoming";
await fs.mkdir(stagingDir);
for (const file of files) await fs.write(`${stagingDir}/${file.name}`, file.body);
await fs.move(stagingDir, "snapshots/2026-05-05", { overwrite: true });

For guarded whole-directory publication, use replaceDirectoryAtomic. Replacing an existing target is a two-rename protocol with a temporary target-absence interval and conditional no-replace rollback, not a transactional commit-or-rollback.

#Rotate logs

const today = `logs/${formatDate(new Date())}.log`;
try {
  await fs.create(today, "");
} catch (err) {
  if (!(err instanceof FsSafeError) || err.code !== "already-exists") throw err;
}
await fs.append(today, line);

#FUSE mounts and unstable inode numbers

Some FUSE mounts — rclone is a confirmed example — assign the destination a different inode number from the source temp file as a result of rename, even within a single process with zero concurrency. Repeated stats of an unchanged destination remain stable, but the source-to-destination (dev, ino) comparison always fails with path-mismatch.

Set renameIdentity: "verify-content-with-lock" on the root (or per call) to use a SHA-256 content comparison under a cooperative sidecar lock instead:

const fs = await root("/mnt/rclone-workspace", {
  renameIdentity: "verify-content-with-lock",
});

await fs.write("state.json", body); // succeeds on rclone FUSE

How it works. The full write runs under an exclusive per-target lock named .fs-safe-write-<sha256>.lock in the root. Keeping the lock in the already-canonical root avoids creating an unguarded lock path through a missing or raced target parent. The guarded Node fallback accepts the source-temp-to-destination inode mismatch only when the SHA-256 of the re-read bytes matches the SHA-256 of the bytes written. Subsequent path identity checks remain strict, so this mode requires an unchanged destination path to report stable identity. It deliberately stays on the guarded JavaScript path because content verification replaces the normal inode-preserving rename contract. The lock is released before the call returns.

Security note. verify-content-with-lock proves that the bytes observed after rename match the requested write and prevents cooperating writers from interleaving. It does not prove that the destination still names the temp-file object, retain fd-relative parent pinning, or stop a same-UID process that ignores the advisory lock. Do not use this option on directories writable by untrusted same-UID processes. Strict identity verification remains the default.

Windows Root.write() and Root.writeJson() honor this policy both as a Root default and as a per-call option. The Windows buffered writer acquires the same Root compatibility lock before creating parents, placeholders, or content. It keeps staging writable until publication, then applies the final mode through the retained destination descriptor. When content verification accepts a changed rename identity, that destination remains pinned through file sync, parent sync, and final strict identity checks.

The Windows buffered compatibility path resolves permitted in-root aliases before choosing its lock and binds publication to that effective destination. With the existing lock protocol, effective path components beneath the Root must contain only lower-case ASCII letters, digits, ., _, or -, with no trailing .. Unsupported spellings, including missing upper-case or non-ASCII names, fail with path-alias before mutation; no filesystem case-sensitivity or Unicode-folding behavior is guessed. This restriction does not apply to strict writes. Opaque Windows pathname identities still use strict verification against the retained original descriptor: they never, by themselves, authorize content-based acceptance of a replacement.

The library's own lock-destination check permits the writer to reuse parent admission within that operation. The lock key and destination checks remain unchanged, and evidence is revoked when the locked operation finishes. Supplying an assertBeforeMutation callback retains full parent admission because caller code can change the filesystem before dispatch.

Lock recovery is fail-closed. If a process crashes and leaves the root-level .fs-safe-write-<sha256>.lock, a later write reports the stale lock instead of deleting it based on a host-local PID. Recover only under external authority that excludes every competing writer; see File lock.

#See also

• Atomic writes — the lower-level replaceFileAtomic and friends.
• JSON files — standalone JSON helpers without going through root().
• Reading — companion read API.
• Errors — every code, when it fires.