fix(mdns): accumulate probe records across responses; unit tests [#619]

Live proof: ecobee answers unicast mDNS with PTR only
(_hap._tcp.local -> "Main Floor._hap._tcp.local"), so the old
replace-on-probe wiped learned records every cycle. Merge by
(name, type) instead. CONFIG_PATH now env-overridable for tests.
Details: https://projects.knownelement.com/issues/619#note-5

💘 Generated with Crush

Assisted-by: Crush:glm-5.2

netinfra/mdns/hap-bridge.py

@@ -19,13 +19,14 @@ Design notes:
 """
 
 import json
+import os
 import logging
 import socket
 import struct
 import threading
 import time
 
-CONFIG_PATH = "/usr/local/etc/hap-bridge.conf"
+CONFIG_PATH = os.environ.get("CONFIG_PATH", "/usr/local/etc/hap-bridge.conf")
 MDNS_GROUP = "224.0.0.251"
 MDNS_PORT = 5353
 REFRESH_INTERVAL = 60
@@ -60,6 +61,24 @@ def encode_name(name):
     return out + b"\x00"
 
 
+def decode_rr_header(data, off):
+    """Return (rtype, rclass, ttl, rdata_len) for the RR header at offset."""
+    return struct.unpack(">HHIH", data[off:off + 10])
+
+
+def merge_records(prev_recs, new_recs):
+    """Union record sets by (name, type); new records replace same-key old ones.
+
+    Devices often answer a multi-question probe with partial record sets
+    (ecobee returns only the PTR, #619); accumulate across probes so a thin
+    response never discards previously learned SRV/TXT/A records.
+    """
+    merged = {(r["name"], r["type"]): r for r in prev_recs}
+    for r in new_recs:
+        merged[(r["name"], r["type"])] = r
+    return list(merged.values())
+
+
 def probe_device(ip, timeout=2.0):
     """Return normalized records from a device's mDNS response."""
     qname = encode_name(SERVICE)
@@ -83,7 +102,7 @@ def probe_device(ip, timeout=2.0):
             records = []
             for _ in range(an + ns):
                 name, name_end = read_name(data, off)
-                rtype, _rclass, ttl, rlen = struct.unpack(">HHIH", data[name_end:name_end + 10])
+                rtype, _rclass, ttl, rlen = decode_rr_header(data, name_end)
                 rec = {"name": name, "type": rtype, "ttl": min(ttl, 4500)}
                 if rtype == 12:  # PTR
                     rec["ptr_target"] = read_name(data, name_end + 10)[0]
@@ -147,7 +166,7 @@ def main():
             recs = probe_device(ip)
             with lock:
                 if recs:
-                    state[ip] = recs
+                    state[ip] = merge_records(state.get(ip, []), recs)
             log.info("probe %s: %s", ip, f"{len(recs)} records" if recs else "no response")
 
     def replay():