implement warn on dnssd name collisions

entrypoint.sh

@@ -15,8 +15,11 @@
 #   PRINTER_IP           IP address / host name of the network printer.
 #   PRINTER_URI          Full device URI; overrides PRINTER_IP when set.
 #   PRINTER_PROTOCOL     socket (default), lpd or ipp - used with PRINTER_IP.
-#   PRINTER_NAME         CUPS queue name and advertised (DNS-SD) name.
-#   PRINTER_DESCRIPTION  CUPS Description (lpadmin -D) shown in the web UI.
+#   PRINTER_NAME         CUPS queue name; base of the advertised (DNS-SD)
+#                        name "<name> @ <host>" while no Description is set.
+#   PRINTER_DESCRIPTION  CUPS Description (lpadmin -D), shown in the web UI;
+#                        takes precedence over PRINTER_NAME as the advertised
+#                        (DNS-SD) name.
 #   PPD_FILE             PPD used for the queue.
 #   PRINTER_OPTIONS      Space-separated PPD option assignments (key=value)
 #                        applied as queue defaults on every start, e.g.
@@ -400,6 +403,84 @@ if ! curl -fsS --noproxy '*' --max-time 2 -o /dev/null http://127.0.0.1:631/ 2>/
     exit 1
 fi
 
+# --- DNS-SD name collision check --------------------------------------------
+# CUPS advertises every shared queue as the DNS-SD instance
+# "<Description> @ <host>" - the queue name when no Description is set.
+# Avahi can register only one instance per name, and CUPS 2.4 does not rename
+# on collision (scheduler/dnssd.c: "TODO: Handle collisions with
+# avahi_alternate_service_name"), so with duplicate derived names one queue is
+# silently never advertised. Clients that pinned the missing name - macOS
+# stores the exact instance name in the queue's dnssd:// device URI - then
+# report the printer offline. Detection only; a collision is never fatal
+# (see README, "macOS shows the printer offline after some time").
+check_dnssd_name_collisions() {
+    # Queue names and Descriptions straight from cupsd, forced to the C locale
+    # so the "Description:" detail line parses regardless of the container
+    # locale. An empty or failed result skips the check silently.
+    lpstat_out="$(LC_ALL=C lpstat -l -p 2>/dev/null || true)"
+    [ -n "${lpstat_out}" ] || return 0
+
+    # One "base<TAB>queue1, queue2" line per colliding name. The Shared flags
+    # come from printers.conf (both <Printer> and <DefaultPrinter> blocks,
+    # "Shared Yes/No"); a queue that cannot be read there (file not flushed
+    # after this run's lpadmin, no Shared line yet) counts as shared: a false
+    # warning is actionable, a missed one reproduces the incident. Every parse
+    # failure degrades to "no collision found" so that set -eu never trips
+    # inside this diagnostic.
+    collisions="$(printf '%s\n' "${lpstat_out}" | awk -v conf=/etc/cups/printers.conf '
+        BEGIN {
+            while ((getline line < conf) > 0) {
+                if (line ~ /^<(Default)?Printer /) {
+                    cur = line
+                    sub(/^<(Default)?Printer /, "", cur)
+                    sub(/>$/, "", cur)
+                } else if (line ~ /^<\/(Default)?Printer>$/) {
+                    cur = ""
+                } else if (cur != "" && line ~ /^Shared /) {
+                    flag[cur] = tolower(substr(line, 8))
+                }
+            }
+            close(conf)
+        }
+        /^[ \t]*printer / {
+            curq = $2
+            order[++n] = curq
+            next
+        }
+        /^[ \t]*Description: / {
+            sub(/^[ \t]*Description: /, "")
+            if (curq != "") desc[curq] = $0
+        }
+        END {
+            for (i = 1; i <= n; i++) {
+                q = order[i]
+                if ((q in flag) && flag[q] == "no") continue
+                base = ((q in desc) && desc[q] != "") ? desc[q] : q
+                count[base]++
+                list[base] = list[base] (list[base] == "" ? "" : ", ") q
+            }
+            for (b in count)
+                if (count[b] > 1) print b "\t" list[b]
+        }' || true)"
+    [ -n "${collisions}" ] || return 0
+
+    dnssd_host=" @ $(hostname -s 2>/dev/null || hostname 2>/dev/null || printf host)"
+    printf "%s\n" "${collisions}" \
+        | while IFS="$(printf '\t')" read -r base queues; do
+            log "warning: queues ${queues} would all be advertised as the DNS-SD name"
+            log "         '${base}${dnssd_host}' - Avahi keeps only one of them, so every"
+            log "         other queue is silently NOT advertised, and clients pinned to the"
+            log "         missing name (macOS dnssd:// device URI) show the printer offline."
+            log "         fix: give each queue a unique Description (lpadmin -p <queue> -D \"...\")"
+            log "              or remove the redundant queue (lpadmin -x <queue>); then remove"
+            log "              and re-add the printer on the client - see README, \"macOS shows"
+            log "              the printer offline after some time\"."
+        done
+    return 0
+}
+
+check_dnssd_name_collisions
+
 log "ready: CUPS on 631/tcp, mDNS on 5353/udp"
 
 # Keep the container running by waiting on cupsd.