* UPB_STOP -> UPB_BREAK, better represents breaking
out of a parsing loop.
* UPB_STATUS_OK -> UPB_OK, for all status codes, more
concise at no readability cost (perhaps an improvement).
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.
Unfortunately my previous detailed commit message was
lost somehow by git or vi. Will have to explain in more
detail at a later date the rationale for this change.
The build will be broken until I port the old decoder
to this new interface.
Joshua Haberman