fix(): Fix Cross-Device Bug — Use Instead of Raw

[Arielpetit]

PR: Fix OpExchange Cross-Device Bug — Use swapAny Instead of Raw os.Rename

Issue

The OpExchange handler used three raw os.Rename calls to swap files back during undo operations.

os.Rename ultimately calls rename(2), which fails with EXDEV when the session store and working files are located on different filesystems. As a result, undo operations could silently fail, leaving files in an inconsistent state.

Fix

Replaced the three manual os.Rename calls with:

swapAny(field(0), field(1))

This is the same helper already used by OpMod.

swapAny:

• Attempts os.Rename first for same-filesystem moves.
• Falls back to a copy-and-delete implementation when EXDEV occurs.
• Provides atomic behavior using a temporary file with rollback on failure.

---

Before

The undo path performed three sequential renames:

A ──os.Rename──▶ tmp      ❌ EXDEV if A and tmp are on different devices
B ──os.Rename──▶ A        never reached
tmp ──os.Rename──▶ B      never reached

Problems:

• Three system calls
• Three potential failure points
• No rollback if the second rename fails
• Could silently leave files corrupted on cross-device setups

---

After

The undo path now performs a single swap operation:

A ──swapAny──▶ B          ✅ rename first, copy+delete fallback

Benefits:

• Single, well-tested helper used elsewhere in the codebase
• Handles cross-device moves transparently
• Atomic via temporary file
• Rolls back cleanly on failure

---

Code Diff

Before

tmp := field(0) + ".undo-xchg"
if err = os.Rename(field(0), tmp); err == nil {
    if err = os.Rename(field(1), field(0)); err == nil {
        err = os.Rename(tmp, field(1))
    }
}

After

err = swapAny(field(0), field(1))

---

Files Changed

File	Changes
internal/restore/restore.go	Replaced three os.Rename calls with swapAny(field(0), field(1)).
internal/restore/restore_test.go	Added TestExchange to verify cross-device-safe exchange behavior.

---

Verification

• ✅ 4/4 unit test packages passing
• ✅ 20/20 end-to-end tests passing
• ✅ Project builds successfully
• ✅ Cross-device behavior verified with:

UNDO_DATA_DIR=/dev/shm/undo-test go test ./internal/restore/ -run TestExchange

[Arielpetit]

@edaywalid can you please review this?