# Design

## Context

See `proposal.md` — Why.

The relevant facts, all from the running system (macOS 27.0.1 build 26A434,
CUPS 2.4.2 in the container) and the CUPS sources:

- The container registers the queue with CUPS' own DNS-SD code
  (`scheduler/dirsvc.c`): `_printer._tcp` unconditionally, and `_ipp._tcp` /
  `_ipps._tcp` with the sub-type list from `DNSSDSubTypes`. The list is passed
  to one `DNSServiceRegister` call as `<type>,<subtypes>` (`snprintf(temp, "%s,%s",
  type, subtypes)`), so `DNSSDSubTypes` decides the whole sub-type set.
- `DNSSDSubTypes` defaults to `_cups,_print,_universal` (`scheduler/conf.c`) and
  is settable with the `BrowseDNSSDSubTypes` directive.
- The queue's TXT record already carries what a driverless client needs: `URF`,
  `pdl=…,image/urf`, `TLS=1.2`, `Color=T`, `Duplex=T`, `mopria-certified=1.3`.
- macOS' Add Printer browser (`AddPrinter.app`'s `PMBrowser.framework`) browses
  `_ipp._tcp,_cups` **and** `_ipp._tcp,_universal` (plus the `_ipps` forms) and
  keys a printer's registration types by the `regType` the browse callback
  reports. `-[DNSSDPrinter isShared]` is YES when any of them ends in `,_cups`,
  and `-[DNSSDPrinter airprintMatch]` returns nothing when `isShared` is YES.

## Goals / Non-Goals

**Goals:**
- The queue is advertised so that macOS classifies it as an AirPrint printer and
  offers the driverless driver, without changing anything else a client sees.

**Non-Goals:**
- Replacing CUPS' DNS-SD advertising with hand-written Avahi service files, or
  changing the TXT record CUPS generates. CUPS' advertisement works for every
  other client; only the sub-type set is wrong.
- Changing the certificate, the queue's PPD or the device URI.

## Decisions

**Drop `_cups` from `DNSSDSubTypes` instead of suppressing CUPS' registration.**

`BrowseDNSSDSubTypes _print,_universal` removes exactly the marker that makes
macOS treat the queue as shared, while keeping the `_universal` sub-type that
makes `isAirPrint` YES. `_print` is kept because CUPS ships it by default and
dropping it would remove information some clients use, with no benefit here.

Alternatives considered:

- *Advertise the queue with a hand-written Avahi service definition and turn
  CUPS' own advertising off.* Gives full control over sub-types and TXT, but
  duplicates what CUPS already generates (including the UUID and `printer-type`
  TXT that CUPS clients resolve the queue with) and has to be kept in sync with
  the queues. Only worth it if the sub-type turns out not to be sufficient.
- *Keep `_cups` and patch macOS' classification* — not possible; it is the
  client's behaviour.
- *Stop sharing the printer (`printer-is-shared=false`)* — would remove the
  queue from mDNS entirely.

**Keep CUPS' TXT record and the `/cups` device URI.**

The `/cups` path marker and the `printer-type` TXT key come from CUPS' own
advertisement and tell CUPS clients that this is a CUPS server; they are what
makes `dnssd://…/cups?uuid=…` resolve. macOS' sub-type-based `isShared` is a
separate mechanism from that marker, so removing the sub-type keeps CUPS
clients working while fixing macOS' driver choice.

**Rely on `airprintMatch`'s remaining requirements being satisfied by the
existing advertisement.**

After the sub-type fix, `airprintMatch` also needs a device-ID entry with the
TXT-derived `URF:` key and a `dnssd servicetype:` entry for `_ipp._tcp` /
`_ipps._tcp`, and a locally built generic URF PPD. Those come from the printer's
TXT record, which already carries `URF` and `pdl` (PMBrowser normalises TXT keys
to its `KEY:` form) and from the service type in the browse result — the same
inputs a real AirPrint printer provides. If the GUI still offers only the
generic driver, the fallback is the Avahi service file mentioned above.

## Risks / Trade-offs

- [The queue is still classified as shared because the `/cups` device URI or
  `printer-type` TXT matter in a code path not covered by the disassembly] →
  Small risk; the disassembly of `isShared`/`isAirPrint`/`airprintMatch` covers
  the driver-choice gate. Verify on the client; fall back to an Avahi service
  file if needed.
- [Some client relies on the `_cups` sub-type to identify the server] → CUPS'
  `dnssd` backend uses the TXT `printer-type`/`cups_shared` marker for that, not
  the sub-type, so the `/cups` URI is unchanged. Verify with `lpinfo -v` on a
  CUPS client.
- [`cupsctl`'s rewrite of `/etc/cups/cupsd.conf` drops the directive] →
  `cupsAdminSetServerSettings()` copies unrecognised directives through
  verbatim (`cups/adminutil.c`), so `BrowseDNSSDSubTypes` survives the
  `cupsctl --share-printers --remote-admin` the entrypoint runs after cupsd has
  started. Confirmed by reading the write loop, not by a live run.
- [macOS caches the previous classification for an existing printer entry] →
  Nothing the server can do; the printer has to be removed and added again. It
  is documented in the README.

## Migration Plan

Rebuild and redeploy the image; no volume reset. `cupsd` re-registers the
queue's services on start and drops the `_cups` sub-type, so the change takes
effect immediately (mDNS caching on clients aside). Rollback is redeploying the
previous image; the extra sub-type reappears and macOS falls back to the generic
driver as before.