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While the inbound path of a netty pipeline is thread-safe, the outbound
path is not. That means that multiple threads can compete to write messages
to the next stage of the pipeline.
The network library sometimes breaks a single RPC message into multiple
buffers internally to avoid copying data (see MessageEncoder). This can
result in the following scenario (where "FxBy" means "frame x, buffer y"):
T1 F1B1 F1B2
\ \
\ \
socket F1B1 F2B1 F1B2 F2B2
/ /
/ /
T2 F2B1 F2B2
And the frames now cannot be rebuilt on the receiving side because the
different messages have been mixed up on the wire.
The fix wraps these multi-buffer messages into a `FileRegion` object
so that these messages are written "atomically" to the next pipeline handler.
Author: Marcelo Vanzin <vanzin@cloudera.com>
Closes #5234 from vanzin/SPARK-6578 and squashes the following commits:
16b2d70 [Marcelo Vanzin] Forgot to update a type.
c9c2e4e [Marcelo Vanzin] Review comments: simplify some code.
9c888ac [Marcelo Vanzin] Small style nits.
8474bab [Marcelo Vanzin] Fix multiple calls to MessageWithHeader.transferTo().
e26509f [Marcelo Vanzin] Merge branch 'master' into SPARK-6578
c503f6c [Marcelo Vanzin] Implement a custom FileRegion instead of using locks.
84aa7ce [Marcelo Vanzin] Rename handler to the correct name.
432f3bd [Marcelo Vanzin] Remove unneeded method.
8d70e60 [Marcelo Vanzin] Fix thread-safety issue in outbound path of network library.
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