start tweaking things, mention mockingbirdnest#1441

ksp_plugin/interface.cpp

@@ -177,7 +177,7 @@ void principia__AdvanceTime(Plugin* const plugin,
 void principia__AdvanceParts(Plugin* const plugin, double const t) {
   journal::Method<journal::AdvanceParts> m({plugin, t});
   CHECK_NOTNULL(plugin);
-  plugin->AdvanceParts(FromGameTime(*plugin, t));
+  plugin->CatchUpLaggingPileUps(FromGameTime(*plugin, t));
   return m.Return();
 }
 

ksp_plugin/pile_up.cpp

@@ -218,6 +218,10 @@ void PileUp::NudgeParts() const {
   }
 }
 
+Instant const& PileUp::time() {
+  return psychohistory_->last().time();
+}
+
 void PileUp::WriteToMessage(not_null<serialization::PileUp*> message) const {
   for (auto const part : parts_) {
     message->add_part_id(part->part_id());

ksp_plugin/pile_up.hpp

@@ -88,6 +88,8 @@ class PileUp {
   // |DeformPileUpIfNeeded|.
   void NudgeParts() const;
 
+  Instant const& time();
+
   void WriteToMessage(not_null<serialization::PileUp*> message) const;
   static PileUp ReadFromMessage(
       serialization::PileUp const& message,

ksp_plugin/plugin.cpp

@@ -506,7 +506,7 @@ void Plugin::FreeVesselsAndPartsAndCollectPileUps() {
     vessel.ForSomePart([this](Part& first_part) {
       Subset<Part>::Find(first_part).mutable_properties().Collect(
           &pile_ups_,
-          current_time_,
+          previous_time_,
           DefaultProlongationParameters(),
           DefaultHistoryParameters(),
           ephemeris_.get());
@@ -533,17 +533,17 @@ void Plugin::SetPartApparentDegreesOfFreedom(
       part, world_to_apparent_bubble(degrees_of_freedom));
 }
 
-void Plugin::AdvanceParts(Instant const& t) {
+void Plugin::CatchUpLaggingPileUps() {
   CHECK(!initializing_);
-  CHECK_GT(t, current_time_);
 
-  ephemeris_->Prolong(t);
   for (PileUp& pile_up : pile_ups_) {
-    pile_up.DeformPileUpIfNeeded();
-    pile_up.AdvanceTime(t);
-    // TODO(egg): now that |NudgeParts| doesn't need the bubble barycentre
-    // anymore, it could be part of |PileUp::AdvanceTime|.
-    pile_up.NudgeParts();
+    if (pile_up.time() < current_time_) {
+      pile_up.DeformPileUpIfNeeded();
+      pile_up.AdvanceTime(current_time_);
+      // TODO(egg): now that |NudgeParts| doesn't need the bubble barycentre
+      // anymore, it could be part of |PileUp::AdvanceTime|.
+      pile_up.NudgeParts();
+    }
   }
 }
 
@@ -591,7 +591,7 @@ void Plugin::AdvanceTime(Instant const& t, Angle const& planetarium_rotation) {
   CHECK_GT(t, current_time_);
 
   // In some cases involving physics dewarping, the vessel may have an
-  // authoritative point in the future.  Bail out early.
+  // authoritative point in the future, see #1441.  Bail out early.
   Instant last_authoritative_time = Instant() - Infinity<Time>();
   for (auto const& pair : vessels_) {
     Vessel const& vessel = *pair.second;
@@ -608,7 +608,7 @@ void Plugin::AdvanceTime(Instant const& t, Angle const& planetarium_rotation) {
       tails_are_empty = part.tail().Empty();
     });
     if (tails_are_empty) {
-      AdvanceParts(t);
+      CatchUpLaggingPileUps(t);
     }
   }
 

ksp_plugin/plugin.hpp

@@ -213,7 +213,7 @@ class Plugin {
   // Advances time on the pile ups to |t|, filling the tails of all parts up to
   // instant |t|.  The vessels are unaffected, and |current_time_| remains
   // unchanged.
-  virtual void AdvanceParts(Instant const& t);
+  virtual void CatchUpLaggingPileUps();
 
   // Returns the degrees of freedom of the given part in |World|, assuming that
   // the origin of |World| is fixed at the centre of mass of the
@@ -231,7 +231,7 @@ class Plugin {
 
   // Simulates the system until instant |t|.  Sets |current_time_| to |t|.
   // Must be called after initialization.
-  // Calls |AdvanceParts| if it has not been called; otherwise the vessels are
+  // Calls |CatchUpLaggingPileUps| if it has not been called; otherwise the vessels are
   // advanced using the tails previously computed during that call.
   // Clears the intrinsic force on all loaded parts.
   // |t| must be greater than |current_time_|.  |planetarium_rotation| is the