fix airprint discovery

openspec/changes/fix-airprint-discovery/design.md

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+# 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.