Clear local fork save points as they catch up

It pointlessly uses up memory to keep local fork savepoints in the
middle of strokes around, they're only useful if they're on or right
next to an undo point that may get rolled back to. So we now only keep
those useful states around and purge the ones that are in the middle of
nowhere and are very unlikely to serve as replay start points.

ChangeLog

@@ -7,6 +7,7 @@ Unreleased Version 2.2.0-pre
  * Fix: Properly disable blend mode combo box in layer properties dialog. Thanks xxxx for reporting.
  * Fix: Move chat status bar button into a legal position. Thanks Flz for reporting.
  * Fix: Correct various errors in text recording reading and writing.
+ * Fix: Don't keep locally forked canvas states around. This reduces memory usage by a bunch, especially when drawing locally.
 
 2023-07-31 Version 2.2.0-beta.6
  * Fix: Don't forget account password when entering a wrong session password.

src/drawdance/libengine/dpengine/canvas_history.c

@@ -83,6 +83,7 @@ struct DP_CanvasHistory {
     DP_AffectedIndirectAreas aia;
     bool mark_command_done;
     struct {
+        bool starts_at_undo_point;
         int start;
         int fallbehind;
         DP_Queue queue;
@@ -329,10 +330,11 @@ static void dump_snapshot(DP_CanvasHistory *ch, DP_CanvasState *cs)
 }
 
 
-static void set_fork_start(DP_CanvasHistory *ch)
+static void set_fork_start(DP_CanvasHistory *ch, bool starts_at_undo_point)
 {
     DP_ASSERT(ch->used > 0);
     DP_ASSERT(ch->offset >= 0);
+    ch->fork.starts_at_undo_point = starts_at_undo_point;
     ch->fork.start = ch->offset + ch->used - 1;
     HISTORY_DEBUG("Set fork start to %d", ch->fork.start);
 }
@@ -519,7 +521,7 @@ DP_CanvasHistory *DP_canvas_history_new_inc(
         DP_malloc(entries_size),
         {0},
         true,
-        {0, 0, DP_QUEUE_NULL},
+        {false, 0, 0, DP_QUEUE_NULL},
         {save_point_fn, save_point_user},
         {0, {0}},
         DP_ATOMIC_INIT(0),
@@ -1078,7 +1080,9 @@ static void replay_from_inc(DP_CanvasHistory *ch, DP_DrawContext *dc,
 
     if (have_local_fork(ch)) {
         DP_ASSERT(ch->fork.start >= start_index + ch->offset);
-        set_fork_start(ch);
+        bool starts_at_undo_point =
+            DP_message_type(peek_fork_entry_message(ch)) == DP_MSG_UNDO_POINT;
+        set_fork_start(ch, starts_at_undo_point);
         DP_queue_each(&ch->fork.queue, sizeof(DP_ForkEntry), replay_fork,
                       (void *[]){ch, &cs, dc});
     }
@@ -1332,6 +1336,19 @@ static bool handle_command(DP_CanvasHistory *ch, DP_DrawContext *dc,
     }
 }
 
+static void clear_fork_fallbehind(DP_CanvasHistory *ch)
+{
+    HISTORY_DEBUG("Fork fallbehind cleared");
+    ch->fork.fallbehind = 0;
+    int index = ch->fork.start - ch->offset;
+    DP_ASSERT(index >= 0 && index < ch->used);
+    DP_CanvasHistoryEntry *entry = &ch->entries[index];
+    if (!ch->fork.starts_at_undo_point && !is_undo_point_entry(entry)) {
+        DP_canvas_state_decref_nullable(entry->state);
+        entry->state = NULL;
+    }
+}
+
 static DP_ForkAction reconcile_remote_command(DP_CanvasHistory *ch,
                                               DP_Message *msg,
                                               DP_MessageType type,
@@ -1350,8 +1367,7 @@ static DP_ForkAction reconcile_remote_command(DP_CanvasHistory *ch,
             shift_fork_entry_nodec(ch);
             DP_message_decref(peeked_msg);
             if (used == 1) {
-                HISTORY_DEBUG("Fork fallbehind cleared");
-                ch->fork.fallbehind = 0;
+                clear_fork_fallbehind(ch);
             }
             if (type == DP_MSG_UNDO || type == DP_MSG_UNDO_POINT) {
                 // Undo operations aren't handled locally.
@@ -1497,7 +1513,7 @@ bool DP_canvas_history_handle_local(DP_CanvasHistory *ch, DP_DrawContext *dc,
     DP_PERF_BEGIN_DETAIL(fn, "handle_local", "type=%d", (int)type);
 
     if (!have_local_fork(ch)) {
-        set_fork_start(ch);
+        set_fork_start(ch, type == DP_MSG_UNDO_POINT);
         make_save_point(ch, find_save_point_index(ch), false);
     }
     push_fork_entry_inc(ch, msg);
@@ -1577,7 +1593,7 @@ void DP_canvas_history_handle_local_multidab_dec(DP_CanvasHistory *ch,
     DP_PERF_BEGIN_DETAIL(fn, "handle_local_multidab", "count=%d", count);
 
     if (!have_local_fork(ch)) {
-        set_fork_start(ch);
+        set_fork_start(ch, false);
         make_save_point(ch, find_save_point_index(ch), false);
     }