Why does HTTPFields._Storage have a lock?

[Joannis]

Since it's handled as CoW through isKnownUniquelyReferenced, I would not expect to need a lock.

[guoye-zhang]

The lock is necessary for the index which is built lazily upon access. It’s possible that multiple immutable fields instances share the same storage without an index, and multiple threads call methods that need the index in parallel.