photocraft/docs/decisions.md

# Implementation decisions

## 2026-10-10: Smart Object contents identity (#2163)

Smart Object instances carry a document-scoped contents ID separately from their layer ID. Normal duplicates preserve it; New Smart Object via Copy creates a new one. Contents edits update the matching instances together, retaining their individual placement and filters. Copies into another document remap contents IDs as a batch, preserving sharing within the copied layers without joining existing destination objects.

Native files persist this ID. Legacy embedded objects without an ID remain independent: identical file bytes do not prove that two objects were intended to share edits. Legacy nonempty linked paths and PSD source identifiers retain their explicit sharing relationship. PSD export keys embedded sources by contents identity as well as bytes, so independent equal-content objects stay independent.

## 2026-10-17: keeping a replaced file's ACL and named streams (#7073, #7094, #7096)

Saving replaces a file by renaming a new one over it, which drops everything the old file
carried. Carrying the security-relevant parts across needs platform calls that have no safe API
in `std`, and the workspace forbids `unsafe`, so each platform took a dependency rather than an
exception.

**macOS ACLs: `exacl`, not `fclonefileat`.** The two routes that avoid `unsafe` are a macOS-only
`exacl` dependency (a safe wrapper over `acl_get_file`/`acl_set_file`) and `fclonefileat` with
the undocumented-in-`rustix` `CLONE_ACL` bit. The clone keeps the ACL, but only on APFS, and it
means writing every save as clone-then-truncate-then-write with a fallback for every other file
system — restructuring the one write path the whole app's durability rests on, to preserve an
ACL. `exacl` is a best-effort step beside the existing permission copy, it works wherever ACLs
do, and a failure cannot affect the bytes.

**Windows: `ReplaceFileW` via `winsafe`, with a backup name.** Copying the `Zone.Identifier`
stream by hand with `std` would cover the Mark of the Web but not the DACL, and enumerating every
stream needs `FindFirstStreamW`. `ReplaceFileW` is the call Microsoft documents for the
save-through-a-temporary-file pattern and preserves the DACL, named streams, creation time and
object identifier in one step; `winsafe`'s `kernel` feature wraps it safely and pulls in nothing.

The backup-file argument is a correctness requirement, not an option: `ReplaceFileW` without one
documents `ERROR_UNABLE_TO_MOVE_REPLACEMENT` as leaving the replaced file deleted and the
replacement under its temporary name — a failed save that loses the user's document. With a
backup name the same failure leaves both files alone, and the one failure that does move things
(`ERROR_UNABLE_TO_MOVE_REPLACEMENT_2`) is unwound in `atomic/meta.rs`.