Marian Seitner created SSHD-249:
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Summary: Data race in AbstractSession.close() may lead to NPE and
blocks during shutdown
Key: SSHD-249
URL: https://issues.apache.org/jira/browse/SSHD-249
Project: MINA SSHD
Issue Type: Bug
Affects Versions: 0.9.0
Reporter: Marian Seitner
Priority: Critical
The issue will be described in more details because a) it was enthralling to
debug and reproduce b) it might be interesting for others to fully understand
c) the probability to discover it is quite low, nonetheless it happened during
an aggressive integration test.
The symptoms were a NullPointerException in AbstractSession.close() line 335
and a blocking thread which was supposed to shut down a server instance. After
looking at the code line 331 was identified as critical section as it was the
only place where a null value could have sneaked into the array.
In order to run into this problem two preconditions have to be met: a) the
server is currently shutting down and thus trying to close all channels and
sessions b) one or more channels are unregistered at the same time.
What happens in line 331:
In case of unlucky timing the (first) call to channels.values() returns a
larger set (whose size is used to initialize the array) than the second call,
which is used to actually populate the array, so one or more array elements are
null.
This single problem illustrates three issues:
* Atomic treatment of non-atomic operations
* Unguarded access to the channels map through (un)registerChannel() etc.
* Incorrect exception handling (line 340 never gets called due to missing
latch.decrementAndGet() calls, so the IoSessionCloser is never executed (which
then leads to the blocking thread))
If somehow manageable a patch will be provided.
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