Albertas Vyšniauskas created ARTEMIS-4646:
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Summary: Unacknowledged MQTT message ID is reused after ID
generator wraparound
Key: ARTEMIS-4646
URL: https://issues.apache.org/jira/browse/ARTEMIS-4646
Project: ActiveMQ Artemis
Issue Type: Bug
Components: MQTT
Reporter: Albertas Vyšniauskas
Hello,
MQTT message IDs are currently generated by incrementing *ids* variable in
*{color:#000000}{color:#000000}OutboundStore{color}{color}* class and do not
perform any additional checks before trusting that it is unique. This works
great until one or more MQTT messages are not acknowledged in timely manner
while receiving and acknowledging other messages. A different message with the
same ID will be sent after 32767 additional messages are received and
acknowledged.
There are also more issues with MQTT message ID generation:
# *AtomicInteger* is used for *ids* variable, but all accesses and
modifications are done in synchronized context, so there is no need for it to
be atomic.
# Maximum allowed *ids* value before wraparound is {*}Short.MAX_VALUE{*}, but
MQTT protocol allows values in the range of 1 to 65535 (all unsigned short
values except 0).
# Wraparound is done to value 1 and is immediately incremented thus skipping
message ID 1 and using 2 as first ID after wraparound.
# There is a harmless but wrong attempt to remove from *mqttToServerIds* hash
map by non-MQTT message ID in the following code:
\{_}mqttToServerIds.remove(p.getA());{_}. It is harmless because _p.getA()_
returns *Long* value and will never match any key in *mqttToServerIds* hash map
which uses *Integer* keys.{*}{*}
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