distinguish between timeouts and errors

Author: beekld

libs/server-sdk/src/data_interfaces/source/fdv2_source_result.hpp

@@ -56,8 +56,13 @@ struct FDv2SourceResult {
         bool fdv1_fallback;
     };
 
-    using Value =
-        std::variant<ChangeSet, Interrupted, TerminalError, Shutdown, Goodbye>;
+    /**
+     * Next() returned because the timeout expired before a result arrived.
+     */
+    struct Timeout {};
+
+    using Value = std::variant<ChangeSet, Interrupted, TerminalError, Shutdown,
+                               Goodbye, Timeout>;
 
     Value value;
 };

libs/server-sdk/src/data_interfaces/source/ifdv2_synchronizer.hpp

@@ -25,7 +25,7 @@ class IFDv2Synchronizer {
      * Subsequent calls continue reading from the same connection.
      *
      * If the timeout expires before a result arrives, returns
-     * FDv2SourceResult::Interrupted. The orchestrator uses this to evaluate
+     * FDv2SourceResult::Timeout. The orchestrator uses this to evaluate
      * fallback conditions.
      *
      * Close() may be called from another thread to unblock Next(), in which