[Plasma] Build error on macOS

[asfimport]

Since the upgrade to flatbuffers 1.9.0, I'm seeing this error on the Ray CI:

arrow/cpp/src/plasma/format/plasma.fbs:234:0: error: default value of 0 for field status is not part of enum ObjectStatus

I'm planning to just remove the '= 1' from 'Local = 1'. This will break the protocol however, so if we prefer to just put in a 'Dummy = 0' object at the beginning of the enum, that would also be fine with me. However, the ObjectStatus API is not stable yet and not even exposed to Python, so I think breaking it is fine.

Reporter: Philipp Moritz / @pcmoritz
Assignee: Philipp Moritz / @pcmoritz

Related issues:

• Segfault when plasma client goes out of scope before buffer. (Is contained by)

_{Note: This issue was originally created as ARROW-2506. Please see the migration documentation for further details.}

[asfimport]

Robert Nishihara / @robertnishihara:
I think it makes sense to just remove the = 1.