fix airprint discovery

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

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+# Proposal
+
+## Why
+
+macOS adds the queue with *Use:* = **Generic PostScript Printer** instead of the
+driverless **AirPrint Printer**, so duplex, colour and tray 2 stay hidden — the
+generic PPD gates duplex behind `APOptionalDuplexer False`. The certificate is
+no longer the cause: `curl https://podp.local:631/` succeeds, CUPS serves the
+expected certificate, and every IPP query the Mac makes against the queue
+succeeds (7/7 `HTTP_STATUS_OK, ipp status 0`). iOS, which queries the printer
+live, already offers every option.
+
+The remaining cause is a Bonjour sub-type. Disassembling
+`AddPrinter.app/Contents/Frameworks/PMBrowser.framework` (macOS 27.0.1) shows the
+gate for the AirPrint driver entry:
+
+- `-[DNSSDPrinter isShared]` returns YES as soon as any of the printer's
+  registration types ends in `,_cups`; the registration types are the `regType`
+  strings the Bonjour browse callback reports.
+- `-[DNSSDPrinter airprintMatch]` returns nothing when `isShared` is YES, and
+  only builds the AirPrint driver when `isAirPrint` is YES — that is, when the
+  service was browsed under `_ipp._tcp,_universal` or `_ipps._tcp,_universal`.
+- `-[DNSSDBrowser startBrowsing]` browses `_ipp._tcp,_cups` **and**
+  `_ipp._tcp,_universal` (and the `_ipps` equivalents), so a printer that
+  carries both sub-types is classified as *shared* and never gets the AirPrint
+  driver.
+
+CUPS registers `DNSSDSubTypes`, whose default is `_cups,_print,_universal`
+(`scheduler/conf.c`), which is exactly that combination. The queue is therefore
+advertised as a shared CUPS printer and macOS falls back to its generic driver.
+The sub-types are configurable with the `BrowseDNSSDSubTypes` directive.
+
+## What Changes
+
+- Set `BrowseDNSSDSubTypes _print,_universal` in `cupsd.conf`, so the queue is
+  advertised with the AirPrint (`_universal`) sub-type and without the CUPS
+  (`_cups`) one. `_print` is kept for clients that look for it.
+- Keep CUPS' own advertising otherwise (the unconditional `_printer._tcp`
+  registration, the `printer-type` TXT key and the resulting `/cups` device
+  URI), so CUPS clients keep resolving the queue exactly as before.
+- Correct the README: the "macOS adds the printer as a Generic PostScript
+  printer" section blames the certificate alone and recommends adding the queue
+  from the command line; it now documents the sub-type cause first and how to
+  confirm what is advertised.
+
+Non-goals: hand-written Avahi service files, and patching CUPS' TXT record.
+Neither is needed unless the sub-type alone turns out to be insufficient.
+
+## Capabilities
+
+### New Capabilities
+<!-- None: this fixes the advertisement required by an existing capability. -->
+
+### Modified Capabilities
+- `printserver/cups`: the "Printer is shared and discoverable" requirement gains
+  a scenario that fixes which Bonjour sub-types are advertised, because the
+  advertised sub-types decide whether macOS offers the driverless AirPrint
+  driver.
+
+## Impact
+
+- `cupsd.conf`: one directive plus an explanatory comment.
+- `README.md`: troubleshooting section corrected.
+- `openspec/specs/printserver/cups/spec.md`: one scenario added.
+- Clients: macOS should offer *AirPrint Printer* under *Use:*; the `_ipp`/`_ipps`
+  services, the `/cups` device URI and Linux/Windows clients are unaffected.
+  macOS caches the old classification per printer entry, so an existing entry
+  has to be removed and added again.