fix: preserve FDv2 protocol state across error events

Author: beekld

libs/internal/src/fdv2_protocol_handler.cpp

@@ -163,7 +163,9 @@ FDv2ProtocolHandler::Result FDv2ProtocolHandler::HandleError(
     boost::json::value const& data) {
     auto result = boost::json::value_to<
         tl::expected<std::optional<FDv2Error>, JsonError>>(data);
-    Reset();
+    // Discard any partial-payload accumulation but keep state intact so
+    // the next put-object/payload-transferred cycle continues normally.
+    changes_.clear();
     if (!result) {
         return Error::JsonParseError(result.error(),
                                      "could not deserialize error event");

libs/internal/tests/fdv2_protocol_handler_test.cpp

@@ -235,6 +235,29 @@ TEST(FDv2ProtocolHandlerTest,
     EXPECT_TRUE(cs->changes.empty());
 }
 
+TEST(FDv2ProtocolHandlerTest, ErrorMidPayloadDiscardsPartialAcceptsSubsequent) {
+    FDv2ProtocolHandler handler;
+
+    handler.HandleEvent("server-intent", MakeServerIntent("xfer-full"));
+    handler.HandleEvent("put-object",
+                        MakePutObject("flag", "abandoned", kFlagJson));
+    handler.HandleEvent(
+        "error", boost::json::parse(R"({"reason":"something went wrong"})"));
+
+    // After the error, a fresh put + payload-transferred (without an
+    // intervening server-intent) emits a changeset containing only the
+    // post-error put.
+    handler.HandleEvent("put-object",
+                        MakePutObject("flag", "fresh", kFlagJson));
+    auto result = handler.HandleEvent("payload-transferred",
+                                      MakePayloadTransferred("s", 1));
+
+    auto* cs = std::get_if<data_model::FDv2ChangeSet>(&result);
+    ASSERT_NE(cs, nullptr);
+    ASSERT_EQ(cs->changes.size(), 1u);
+    EXPECT_EQ(cs->changes[0].key, "fresh");
+}
+
 TEST(FDv2ProtocolHandlerTest, ErrorEventWithIdSetsServerId) {
     FDv2ProtocolHandler handler;