Fix a bug in Part compatibility deserialization and add a test

ksp_plugin/part.cpp

@@ -337,10 +337,17 @@ not_null<std::unique_ptr<Part>> Part::ReadFromMessage(
       }
     }
   } else if (is_pre_ζήνων) {
+    DiscreteTrajectorySegmentIterator<Barycentric> history;
+    DiscreteTrajectorySegmentIterator<Barycentric> psychohistory;
     auto prehistory = DiscreteTraject0ry<Barycentric>::ReadFromMessage(
         message.prehistory(),
-        /*tracked=*/{&part->history_, &part->psychohistory_});
-    part->trajectory_ = prehistory.DetachSegments(part->history_);
+        /*tracked=*/{&history, &psychohistory});
+    // We want to get rid of the prehistory segment, so the easiest is to detach
+    // the history and psychohistory.  We must take care of the segment
+    // iterators as they get invalidated in this case.
+    part->trajectory_ = prehistory.DetachSegments(history);
+    part->history_ = part->trajectory_.segments().begin();
+    part->psychohistory_ = std::next(part->history_);
   } else {
     part->trajectory_ = DiscreteTraject0ry<Barycentric>::ReadFromMessage(
         message.prehistory(),

ksp_plugin_test/plugin_compatibility_test.cpp

@@ -42,7 +42,9 @@ using ksp_plugin::Barycentric;
 using ksp_plugin::Plugin;
 using physics::DiscreteTraject0ry;
 using quantities::Speed;
+using quantities::si::Degree;
 using quantities::si::Kilo;
+using quantities::si::Second;
 using testing_utilities::operator""_⑴;
 using testing_utilities::IsNear;
 using testing_utilities::ReadFromBinaryFile;
@@ -386,6 +388,28 @@ TEST_F(PluginCompatibilityTest, DISABLED_Lpg) {
   WriteAndReadBack(std::move(plugin));
 }
 
+TEST_F(PluginCompatibilityTest, DISABLED_Egg) {
+  StringLogSink log_warning(google::WARNING);
+  not_null<std::unique_ptr<Plugin const>> plugin = ReadPluginFromFile(
+      R"(P:\Public Mockingbird\Principia\Saves\3136\3136b.proto.b64)",
+      /*compressor=*/"gipfeli",
+      /*decoder=*/"base64");
+  EXPECT_THAT(log_warning.string(),
+              AllOf(HasSubstr(u8"pre-Ζήνων"), Not(HasSubstr("pre-Haar"))));
+
+  auto& mutable_plugin = const_cast<Plugin&>(*plugin);
+
+  // This would fail if segment iterators were invalidated during part
+  // deserialization.
+  mutable_plugin.AdvanceTime(
+      mutable_plugin.GameEpoch() + 133218.91123694609 * Second,
+      295.52698460805016 * Degree);
+  mutable_plugin.CatchUpVessel("1e07aaa2-d1f8-4f6d-8b32-495b46109d98");
+
+  // Make sure that we can upgrade, save, and reload.
+  WriteAndReadBack(std::move(plugin));
+}
+
 // Use for debugging saves given by users.
 TEST_F(PluginCompatibilityTest, DISABLED_SECULAR_Debug) {
   CheckSaveCompatibility(

physics/discrete_traject0ry.hpp

@@ -87,6 +87,8 @@ class DiscreteTraject0ry : public Trajectory<Frame> {
 
   SegmentIterator NewSegment();
 
+  // All the point and segment iterators in the attached/detached segments are
+  // invalidated.
   DiscreteTraject0ry DetachSegments(SegmentIterator begin);
   SegmentIterator AttachSegments(DiscreteTraject0ry&& trajectory);
   void DeleteSegments(SegmentIterator& begin);