Fix undefined behavior in getCabledPortTranceivers for backplane ports

Summary:
getCabledPortTranceivers() was  dereferencing the optional returned by getTranscieverIdx() without checking if it actually had a value. This worked fine when all cabled ports had transceivers, but breaks for xphy-backplane ports  which have no transceiver at all.

When getTranscieverIdx() returns nullopt for these ports, dereferencing it is undefined behavior. Tt was reading garbage memory that happened to look like TransceiverID(1). This garbage ID got added to the set of "expected transceivers", causing HW tests to fail with errors like "Transceivers that should be programmed but are not: 1".

The fix simply checks has_value() before dereferencing and skips ports that don't have transceivers.

Reviewed By: shiva-menta

Differential Revision: D91055041

fbshipit-source-id: 07a58a872700b3495e9d468f91396b0da122d6cc

Author: Protick Bhowmick

fboss/qsfp_service/test/hw_test/HwPortUtils.cpp

@@ -247,9 +247,14 @@ std::vector<TransceiverID> getCabledPortTranceivers(
     const HwQsfpEnsemble* ensemble) {
   std::unordered_set<TransceiverID> transceivers;
   // There could be multiple ports in a single transceiver. Therefore use a set
-  // to get unique cabled transceivers
+  // to get unique cabled transceivers.
+  // Note: Some ports (e.g., backplane ports with XPHY) don't have transceivers,
+  // so we skip them.
   for (auto port : getCabledPorts(ensemble)) {
-    transceivers.insert(*getTranscieverIdx(port, ensemble));
+    auto tcvrId = getTranscieverIdx(port, ensemble);
+    if (tcvrId.has_value()) {
+      transceivers.insert(*tcvrId);
+    }
   }
   return std::vector<TransceiverID>(transceivers.begin(), transceivers.end());
 }