Workaround #834, which was causing partitioning to infinite loop when

there are disconnected railway segments dangling around

map_model/src/objects/block.rs

@@ -268,6 +268,13 @@ impl Perimeter {
     pub fn collapse_deadends(&mut self) {
         self.undo_invariant();
 
+        // TODO Workaround https://github.com/a-b-street/abstreet/issues/834. If this is a loop
+        // around a disconnected fragment of road, don't touch it
+        if self.roads.len() == 2 && self.roads[0].road == self.roads[1].road {
+            self.restore_invariant();
+            return;
+        }
+
         // If the dead-end straddles the loop, it's confusing. Just rotate until that's not true.
         while self.roads[0].road == self.roads.last().unwrap().road {
             self.roads.rotate_left(1);

tests/goldenfiles/blockfinding.txt

@@ -12,3 +12,5 @@ data/system/gb/bristol/maps/east.bin
     1061 single blocks (1 failures to blockify), 7 partial merges, 1 failures to blockify partitions
 data/system/gb/london/maps/camden.bin
     3519 single blocks (9 failures to blockify), 33 partial merges, 0 failures to blockify partitions
+data/system/gb/london/maps/southwark.bin
+    3487 single blocks (5 failures to blockify), 42 partial merges, 1 failures to blockify partitions

tests/src/main.rs

@@ -256,6 +256,7 @@ fn test_blockfinding() -> Result<()> {
         MapName::new("gb", "leeds", "north"),
         MapName::new("gb", "bristol", "east"),
         MapName::new("gb", "london", "camden"),
+        MapName::new("gb", "london", "southwark"),
     ] {
         let map = map_model::Map::load_synchronously(name.path(), &mut timer);
         let mut single_blocks = Perimeter::find_all_single_blocks(&map);