Module cow 

Copy-on-write defense against package-manager hardlink farms.

Several package managers (pnpm, bazel mirrors, nix store overlays, npm linked workspaces) point multiple project trees at a single content-addressed inode via symlinks or hardlinks. A naive patch that opens the path in a workspace and rewrites it would mutate the shared inode — corrupting every other project that references the same package.

break_hardlink_if_needed is the pre-write hook that turns these shared-inode references into private file copies before any patch bytes touch disk. After the call, mutating the path is safe: only this project’s copy changes; the store entry and every other project’s link survive untouched.

The function is idempotent and fast on the common case (regular file with nlink == 1): a single symlink_metadata syscall, no I/O beyond that. CoW only runs when there is something to break.

Windows note: we always handle symlinks the same on Windows (replace with private regular file) but skip the nlink > 1 check — std::fs::Metadata on Windows does not expose the file information that carries it, and pnpm-on-Windows typically uses reflinks/copies rather than hardlinks. A follow-up could call GetFileInformationByHandle via windows-sys for full Windows parity.

Enums

CowAction

Outcome of break_hardlink_if_needed.

Functions

break_hardlink_if_needed

Ensure path (if it exists) points at a private inode this project alone owns, so a subsequent in-place write only mutates our copy.