Steven Adams created POOL-424:
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Summary: GenericObjectPool.invalidateObject() can leave other
threads waiting to borrow hanging
Key: POOL-424
URL: https://issues.apache.org/jira/browse/POOL-424
Project: Commons Pool
Issue Type: Bug
Affects Versions: 2.12.1
Reporter: Steven Adams
Attachments: PoolTest.java
If multiple threads are waiting to borrow an object from a GenericObjectPool,
and some other thread invalidates a number of existing objects already borrowed
from the pool, the waiting threads can be left hanging waiting for new idle
objects that never materialise.
I've attached a simple standalone test program that demonstrates the issue. It
requires two runtime arguments: a test case (either '1' or '2') and a boolean
value indicating if objects should be returned to the pool normally ('false')
or invalidated instead ('true').
The test uses a pool configured with a max size of 2. Test 1 borrows 2 objects
from the pool, then in the same thread either returns or invalidates them, then
tries to borrow another 2 objects.
Test 2 is similar, but starts two concurrent threads to each do the second set
of borrows before the main thread returns or invalidates the first set of
borrowed objects.
Test 1 in either case, and test 2 in the "return normally" case all work as
expected. However test 2 in the invalidate case almost always fails with one
of the borrow threads hanging until the borrow timeout expires:
{noformat}
$ java -classpath .:commons-pool2-2.12.1.jar PoolTest 2 true
main: test-string-0
main: test-string-1
Thread-0: getting
Thread-1: getting
Thread-0: test-string-2
Thread-0: got
Exception in thread "Thread-1"
java.lang.RuntimeException: Exception in borrowObject() at
PoolTest.get(PoolTest.java:101)
at PoolTest.lambda$test2$0(PoolTest.java:71)
at java.base/java.lang.Thread.run(Thread.java:829)
Caused by: java.util.NoSuchElementException: Timeout waiting for idle object,
borrowMaxWaitDuration=PT4.999997S
at
org.apache.commons.pool2.impl.GenericObjectPool.borrowObject(GenericObjectPool.java:309)
at
org.apache.commons.pool2.impl.GenericObjectPool.borrowObject(GenericObjectPool.java:231)
at PoolTest.get(PoolTest.java:92)
... 2 more
{noformat}
If the commented out code in the get() method at the end of the test class is
enabled, to retry the borrow if a timeout occurs, then a new object is added to
the pool then and able to be borrowed. But this requires the timeout to expire
first:
{noformat}
$ java -classpath .:commons-pool2-2.12.1.jar PoolTest 2 true
main: test-string-0
main: test-string-1
Thread-0: getting
Thread-1: getting
Thread-0: test-string-2
Thread-0: got
failed to obtain object: java.util.NoSuchElementException: Timeout waiting for
idle object, borrowMaxWaitDuration=PT4.999994S
trying again
Thread-1: test-string-3
Thread-1: got{noformat}
I believe the issue is the "ensureIdle(1, false)" call at the end of
GenericObjectPool.invalidateObject(T, DestroyMode). This only ensures that at
most one idle object remains in the pool, regardless of how many threads are
waiting on the pool. In the test case, if a thread invalidates multiple
objects in quick succession before waiting threads get a chance to wake up and
take, then only one new object is created to replace all those that have been
invalidated.
The expected behaviour, in the test case, is that both waiting threads should
be able to borrow a pooled object as soon as the existing objects have been
invalidated.
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