Continue this connection on a new thread. A handler may call
http_spawn/2 to start a new thread that continues processing the
current request using Goal. The original thread returns to the
worker pool for processing new requests. Options are passed to
thread_create/3, except for:
- Interfaces to library(thread_pool), starting the thread
on the given pool.
If a pool does not exist, this predicate calls the multifile
hook create_pool/1 to create it. If this predicate succeeds
the operation is retried.