The S2 data structures S2EdgeIndex and S2Loop have mutable state that can be changed by const methods. Previously this was thought to be innocuous because it only affected integer counters. However, map and multimap data structures are also affected, which can result in a server crash when accessed concurrently.

• S2Loop::vertext_to_index_
• S2EdgeIndex::mapping_

The issue with S2Loop was explicitly suppressed in TSAN builds.