1. the start and end callbacks can now return
a upb_flow_t and set a status message.
2. clarified some semantics around passing an
error status back from the callbacks.
Sources and sinks communicate by means of a
upb_handlers object, which encapsulates a set of
handler callbacks and will possibly offer richer
semantics in the future like giving specific
fields different callbacks.
The upb_handlers protocol supports delegation, so
sets of handlers can be written in reusable ways.
For example, if a set of handlers is written to
handle a specific .proto type, those handlers can
be used whether that type is at the top level or
whether it is a sub-message of a higher-level type.
Delegation allows the streaming protocol to
properly compose.
Joshua Haberman