Somebody else got there first, and the waiter is told so

[sridharkalaibala]

The handset half of Posnic/POS#821.

A save sends the whole order and the till keeps only what arrives, which is how a cancelled dish gets cancelled. On a floor with several handsets it is also how food goes missing:

Another waiter adds a biryani while this screen is open. This screen saves a list that never had it. The till removes a dish the kitchen has already cooked.

The bill goes out short and nobody is told.

What the phone now does

Both saves that rewrite the item list - the edit sheet and the move sheet - say which version of the order they were looking at. It is the order's own timestamp, handed back exactly as it arrived, nothing guessed. The till refuses a save written against an older one.

A full cancel is deliberately untouched. Cancelling the whole order is an intentional act on the order as a whole, and blocking it on a conflict would stop a legitimate cancel.

Being refused is not an error message

The waiter did nothing wrong, and lost nothing: what they typed was never sent.

• They are told somebody else changed this order, not that something failed.
• The sheet closes, rather than sitting there looking ready for a second tap at the same refusal.
• The order is fetched again, so the screen tells the truth.

What happens next is theirs. Only a person knows whether the dish somebody else added was meant to go.

An order nobody has ever edited falls back to when it was opened, so a first edit is guarded too - which is the case a busy table hits most.

An ordinary failure still reads as a failure. A server that is down is a different problem, and a waiter must not be told somebody edited the order when nobody did.

Tests

✔ a move says which version of the order it was looking at
✔ an order nobody has edited falls back to when it was opened
✔ being refused reads as somebody else got there first
✔ the sheet closes on a refusal, rather than sitting there looking ready
✔ a refusal fetches the order again, so the screen tells the truth
✔ an ordinary failure still reads as a failure

Full suite: 302 passed.

One loose end, now closed

The new sentence shipped untranslated in the first commit, because #116 was rewriting the Tamil pack at the time and a line here would have collided with it. That merged, so the second commit adds it - rather than leaving a waiter who reads Tamil to meet one English sentence at the worst possible moment.

Note on rollout

POS#821 is on develop. Until it promotes to main, the handset sends seen_at and the live till ignores it. That is harmless by design - silence means yes - but two phones can still erase each other in production until that promotion happens.