Sergio Bossa created CASSANDRA-14507:
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Summary: OutboundMessagingConnection backlog is not fully written
in case of race conditions
Key: CASSANDRA-14507
URL: https://issues.apache.org/jira/browse/CASSANDRA-14507
Project: Cassandra
Issue Type: Bug
Components: Streaming and Messaging
Reporter: Sergio Bossa
The {{OutboundMessagingConnection}} writes into a backlog queue before the
connection handshake is successfully completed, and then writes such backlog to
the channel as soon as the successful handshake moves the channel state to
{{READY}}.
This is unfortunately race prone, as the following could happen:
1) One or more writer threads see the channel state as {{NOT_READY}} in
{{#sendMessage()}} and are about to enqueue to the backlog, but they get
descheduled by the OS.
2) The handshake thread is scheduled by the OS and moves the channel state to
{{READY}}, emptying the backlog.
3) The writer threads are scheduled back and add to the backlog, but the
channel state is {{READY}} at this point, so those writes would sit in the
backlog and expire.
Please note a similar race condition exists between
{{OutboundMessagingConnection#sendMessage()}} and
{{MessageOutHandler#channelWritabilityChanged()}}, which is way more serious as
the channel writability could frequently change, luckily it looks like
{{ChannelWriter#write()}} never gets invoked with {{checkWritability}} at
{{true}} (so writes never go to the backlog when the channel is not writable).
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