Fix wrong handeling of empty fetch response. #323
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Empty fetch response happens only in incremental fetch sessions. The current parsing code is a copy-paste from kafka_protocol test_lib code which is fine. But the comment is wrong: It's fetch session not transactional session which is a entirely different thing. And the handling is even more wrong: In case empty fetch response is received, it should not advance begin_offset, but keep retrying the one used in the last fetch request, because essentially an empty response implies no new offsets and no partition metadata change either.
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k32
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mikpe
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So to discover version discrepancy early
k32
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Empty fetch response happens only in incremental fetch sessions.
(which we do not support yet)
The current parsing code is a copy-paste from kafka_protocol test_lib code
which is fine.
But the comment is wrong: It's fetch session not transactional session
which is a entirely different thing.
And the handling is even more wrong: In case empty fetch response is
received, it should not advance begin_offset, but keep retrying the one
used in the last fetch request, because essentially an empty
response implies no new offsets and no partition metadata change either.
Noticed this when investigating #321, but it should not be the cause of it,
because the bug is reported on brod 3.4,
the highest fetch request supported in 3.4 is version 3 (session is introduced in version 7).
we do not support incremental fetches in latest brod either.
Unless there is a bug in kafka which may take a random fetch request as
a part of some session.