Make SetUpCodePairer only browse for nodes that are in commissioning …

…mode. (#17356)

* Make SetUpCodePairer only browse for nodes that are in commissioning mode.

If we don't browse for the _CM subtype, we might find (and ignore) a
node that's not yet in commissioning mode (but has extended
commissioning discovery turned on) and when it enters commissioning
mode we won't notice because there won't be any new service instances
of the sort we are looking for, just TXT record changes.

* Address review comments

src/controller/SetUpCodePairer.cpp

@@ -138,9 +138,26 @@ CHIP_ERROR SetUpCodePairer::StartDiscoverOverIP(SetupPayload & payload)
                                                       : Dnssd::DiscoveryFilterType::kLongDiscriminator;
     currentFilter.code =
         payload.isShortDiscriminator ? static_cast<uint16_t>((payload.discriminator >> 8) & 0x0F) : payload.discriminator;
+
+    // We're going to ensure that anything we discover matches currentFilter
+    // before we use it, which will do our discriminator checks for us.
+    //
+    // We are using an mdns continuous query for some PTR record to discover
+    // devices.  If the PTR record we use is for one of the discriminator-based
+    // subtypes (based on currentFilter), then we can run into a problem where
+    // we discover a (possibly stale) advertisement for a non-commissionable
+    // (CM=0) node and ignore it, and then when it becomes commissionable we
+    // don't notice because that just updates the TXT record to CM=1 and does
+    // not touch the PTR record we are querying for.
+    //
+    // So instead we query the PTR record for the "_CM" subtype, which will get
+    // added when a node enters commissioning mode.
+    Dnssd::DiscoveryFilter filter;
+    filter.type = Dnssd::DiscoveryFilterType::kCommissioningMode;
+
     // Handle possibly-sync callbacks.
     mWaitingForDiscovery[kIPTransport] = true;
-    CHIP_ERROR err                     = mCommissioner->DiscoverCommissionableNodes(currentFilter);
+    CHIP_ERROR err                     = mCommissioner->DiscoverCommissionableNodes(filter);
     if (err != CHIP_NO_ERROR)
     {
         mWaitingForDiscovery[kIPTransport] = false;